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Anaerobic Digestion: Industrial Applications
D.Nally, S. Smith, F. Hackett, H. Mooney
Introduction
Anaerobic Digestion may be defined as a waste treatment in which liquor or slurried
organic wastes are decomposed biologically under strictly anaerobic conditions.
(Diaz et al, 1982)
Anaerobic digestion is used worldwide for the treatment of industrial, agricultural and
municipal wastewaters and sludges.
Recently, it has also been applied to the
treatment of solid municipal wastes.
Anaerobic treatment alone will usually be
insufficient to meet the effluent discharge standards and should therefore be combined
with a post treatment, which in most cases is an aerobic process. The anaerobic
process is often used as the first stage in the treatment of high strength organic wastes,
with the purpose being to reduce COD loads so that conventional aerobic processes
can accommodate the waste.
Anaerobic processes involve converting the organic matter in wastewater to methane
and carbon dioxide through a series of reactions involving a combination of
facultative and obligate anaerobic microorganisms. Although anaerobic digestion
involves a complex consortium of microbes, the biochemical processes as well as the
microbial species can be divided into three main categories.
A schematic
representation of the reaction steps is outlined in Figure 1.0, appendix 1.
(Droste et al, 1997)
1. Hydrolysis
Hydrolysis is the breakdown of large complex soluble and insoluble molecules, for
example, starch, fatty acids, proteins and alcohols. The substrates are broken down
through enzymatic hydrolysis to form sugars, alcohols and amino acids.
2. Acetogenisis and Acid Formation
The same microorganisms that perform the hydrolytic reactions carry out the
fermentation through this stage. The end products of hydrolysis are fermented into
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organic acids, other low molecular weight compounds, hydrogen and carbon dioxide.
The primary product of this fermentation is acetic acid. The bacteria responsible for
this stage in the process are known as acetogenic bacteria. Such microbes are quite
resilient, having an optimum pH of 5-6. (Droste et al, 1997).
3. Methanogenisis
Methanogenisis is the production of methane, which is the ultimate product of
anaerobic digestion and occurs via two major routes.
The primary route is the
fermentation of the products from the acid forming phase, ie. acetic acid to methane
and carbon dioxide.
It has been estimated that approximately 70% of methane
production is derived from acetic acid. Bacteria that utilise the acetic acid are known
as “acetoclastic-acetophylic bacteria” and the overall reaction is as follows:
CH3COOH
CH4 + CO2
Secondly, some methogens are capable of utilising hydrogen to reduce carbon dioxide
to methane (hydrogenophyilic methanogenisis) with an overall reaction of;
4H2 + CO2
CH4 + 2H2O
It should be noted that the methane formers are considered the most environmentally
sensitive in a digester population. (Casey et al, 1997). Their rates of metabolism are
lower than that of the acid former and therefore methane production is often the ratelimiting step in anaerobic digestion.
The optimum pH for methanogens is
approximately 7.0 but the activity drops to very low values when the pH falls outside
the range of 6-8.
Thus the fermentation of methane in anaerobic digestion reduces carbon based
pollution of wastewater as expressed by BOD and COD, and at the same time the
process produces useable energy in the form of biogas (CH4 and CO2).
Fundamentals
Both physical and chemical variables influence the habitat of the microorganisms in
the reactor. Such variables include:
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•
Temperature
It is common knowledge that temperature affects the rate of most reactions, ie. as
temperature increases, so too does the rate of reaction. In all, there are two
optimal temperature ranges in which methane can be produced: 30-40oC
(mesophilic) and 50-60oC (thermophilic). Although methane can be produced at
temperatures of less than 10oC, it is preferable, in order to obtain reasonable
methane levels, to keep temperatures above 20oC. Rates of methane production
double for each 10oC rise in temperature within the mesophylic range.
(Droste et al, 1997)
•
pH
The most important process control parameter is pH. As stated previously, the
optimum pH range for all methanogenic bacteria is between 6 and 8, but the
optimum pH for the microorganisms in the reactor as a whole is near 7. As a
result, the lower growth rates of the methanogens require that the process be run
at conditions most favourable to them.
As such, the pH required for good
performance and stability in anaerobic systems ranges from 6.5-7.5, although
stable operations have been observed outside this range. The system must also
contain adequate buffering capacity so as to facilitate the production of volatile
acids and carbon dioxide that will dissolve at the operating pressure. As a result,
excess alkalinity or ability to control pH must be present to prevent against the
accumulation of excess volatile acids. The three major chemical sources of
alkalinity are lime, sodium carbonate and sodium bicarbonate.
•
Mixing
Mixing is an important factor not only in pH control but also in the maintenance
of uniform environmental conditions.
In the absence of adequate mixing,
undesirable microenvironments can develop.
As mixing distributes buffering
agents within the reactor, the build up of high concentrations of intermediate
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metabolic products is prevented. Such metabolic products could otherwise inhibit
the activity of methanogenic bacteria.
•
Ammonia and Sulphide Control
Inhibition of anaerobic metabolism at high concentrations can occur due to free
ammonia (NH3). Acclimatisation of anaerobes to high ammonia concentrations
can be achieved but large fluctuations can obstruct the process. At a high pH,
free ammonia is more toxic than the ammonium ion (NH4+). The addition of acid
can control elevated levels of pH and ammonia which would otherwise lead to
system failure.
•
Nutrient Requirements
Anaerobic digestion requires a number of trace elements such as Nickel and
Cobalt, which have been shown to promote methanogenisis. Excessive amounts
of these trace elements are found in all wastes. The low growth yield of anaerobes
from a given amount of substrate results in lower nutrient requirements compared
to aerobic organisms.
•
Solids Retention Time
The most important advancement in the field of anaerobic digestion has been the
recognition of the central role of sludge age or solids retention time (SRT) in
controlling the process. The SRT is the average time that a solid particle stays in
the reactor. In suspended growth processes, the SRT is the same for all solid
particles but in other processes it may differ. In a suspended growth reactor,
without recycling and where the SRT and Hydraulic Retention Time (HRT) are
the same, a severe constraint is placed on the anaerobic process due to the slow
growth rates of the anaerobes and this factor results in the need for very large
reactors. The new anaerobic modifications on the other hand either control the
SRT independently of the HRT, or, by process design, promote the occurrence of
very high SRT’s.
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•
Hydraulic Retention Time (HRT)
The HRT has been shown to be the single most important factor affecting the
treatment performance of upflow anaerobic filters. The HRT, which depends on
wastewater characteristics and environmental conditions, must be long enough to
allow metabolism by anaerobic bacteria in digesters. Digesters based on attached
growth have a lower HRT (1-10 days), than those based on dispersed growth (1060 days). The retention times of mesophilic and thermophilic digesters range
between25-35 days but can be lower. (Bitton et al, 1994)
The characteristics of recently developed anaerobic treatment processes will now be
examined: (see Figure 2.0, appendix 1)
1. Conventional Anaerobic Treatment
Conventional treatment consists of a well-mixed reactor without solids recycle.
The SRT is equal to the HRT but the reactor must provide a minimum HRT of 10
days at 35oC. Before the 1950’s, anaerobic digesters were frequently not supplied
with mixing systems and neither were they operated at elevated temperatures.
Modern Conventional Systems do however incorporate these features and are
often referred to as high rate digesters when compared to the older (conventional)
systems. Examples of Conventional Anaerobic Processes include septic tanks,
municipal sludge digesters and stirred tank reactors.
2. Floc Based Digesters
a) Contact Processes
To maintain biomas density, sludge recirculation was originally introduced
into anaerobic digestion as the “anaerobic contact process”. These systems are
collectively known as floc based digesters, whereby sludge is recirculated
from a settling tank back into a fully mixed digester with the aim of increasing
the SRT. Suitable HRTs’s range from 1-7 days, depending on the waste
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characteristics. The process was originally developed for the treatment of
biological wastes ie. the food industry.
b) Upflow Anaerobic Sludge Blanket Reactors (UASB’s)
The UASB process was developed in the Netherlands and is the most
commonly used high rate anaerobic process.
The reactor relies on the
development of a dense, active sludge mass in the lower portion. The UASB
is reported to be more sensitive than other processes to waste composition,
with startup being more difficult and requiring special considerations to
develop the sludge blanket. (Droste et al, 1997).
3. Fixed Film Reactors
In these reactors, biomass is retained by attachment to an inert support medium
and hence remains in the reactors for times longer than the HRT. (Foster et al,
1987).
a) Upflow Fixed Film Reactors (UFF’s)
UFF reactors are “packed biological reactors”, filled with rocks or plastic
modules that provide a multitude of random channels and a large surface area.
Upflow reactors retain some solids in suspension in the pore spaces and the
treatment is derived from a combination of both suspended and fixed film
growth. (Droste et al, 1997)
b) Downflow Fixed Film Reactors
With downflow reactors, the downflow geometry has the advantage of the
biogas rising against the flow, aiding effective distribution without the
expense or complexity of such a distribution arrangement. (Foster & Wase et
al, 1987).
Both of the afore mentioned result in the rapid rate biomethanogenic
conversion of soluble wastes. Suspended solids concentration of up to 3% can
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be accomodated by the downflow reactor. (Ghywoweth & Isaacson et al,
1987).
4. Expanded and Fluidised Bed Reactors
The fluidised or expanded bed reactor is the most recent innovation in anaerobic
treatment technology. (Droste et al, 1997).
Waste water flows upwards through a sand bed which provides a surface area for
the growth of bacterial biofilms. The flow rate is high enough to obtain an
expanded or fluidised bed. (Bitton et al, 1994).
These processes have been applied to; a) the reduction of BOD and COD in both
weak and strong wastewaters, b) denitrification and most importantly, c) the
pretreatment of industrial wastewater to sewer acceptance standards. (Foster &
Wase et al, 1987).
5. Two-Phase Anaerobic Digestion
This system adapts the two stage nature of anaerobic metabolism.
Separate
reactors are designed for acidogenesis and methanogenesis, which means that the
conditions in each reactor can be optimised for either group of microorganisms.
As it is impossible to optimise the conditions for both groups of bacteria, phase
separation reduces the instability of performance caused by fluctuations in feed
stock loading, pH and toxic feed components. (Chywoweth & Isaacson et al,
1987).
Anaerobic metabolism is inherently more unstable than aerobic metabolism and in
general, anaerobic treatment is more unstable than aerobic treatment.
However,
aerobic treatment is not without problems, and some of the new anaerobic processes,
particularly the fixed film options, are virtually as stable as conventional aerobic
systems.
Anaerobic systems will generally require more time to recover from
poisoning by toxic waste than aerobic systems because of the lower synthesis rates of
the anaerobes. (Droste et al, 1997).
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In general, comparison between aerobic and anaerobic treatment processes should be
undertaken with caution. Each individual case has peculiarities that may make only
certain processes feasible. In many cases, both aerobic and anaerobic should be used
together for optimal treatment. (Droste et al, 1997).
As an individual unit, an anaerobic digestion system displays the following
advantages:
•
Low production of biological sludge
•
High treatment efficiency
•
Low capital costs
•
No oxygen requirements
•
Methane production (a potential source of fuel)
•
Low nutrient requirements
•
Low operating costs
(Wheately et al, 1990).
The following report deals with the industrial applications of anaerobic digestion and
will be discussed under the following headings:
1. Anaerobic Digestion in the Treatment of Municipal Waste
2. Anaerobic Digestion Technology in the Food Industry
3. Anaerobic Digestion in the Treatment of Wastewaters from the Pharmaceutical
Industry.
4. Applications of Anaerobic Digestion in the Treatment Wastewaters from the
Chemical Industry.
5. Sources and Uses For Biogas
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Bibliography
DROSTE, R.L, (1997), Theory & Practice of Water and Wastewater Treatment,
Wiley & Sons, New York
WHEATLY, A., (1990), Anaerobic Digestion: A Waste Treatment Technology,
Critical Reports on Applied Chemistry Volume 31, Elsevier Applied Science, London
CHYWOWETH, D.P., R, ISAACSON, (1987), Anaerobic Digestion Of Biomass,
Elsevier Science Ltd., New York
CASEY T.J, (1997), Unit Treatment Processes in Water & Wastewater Engineering,
Wiley & Sons, Chichester
FORSTER C.F., D.A. WASE, (1987), Environmental Biotechnology, Ellis Horwood
Limited Publishers, Chichester
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Section 1
Anaerobic Digestion
Of
Municipal Waste
By
Sandra Smith
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Municipal Solid Waste Digestion
The treatment of municipal solid waste (MSW) has been one of the major problems of
modern society for some time. In the United States alone, output of solid waste now
totals over 220 million milligrams per year and is increasing by about 1% per year. As
this waste is associated with pollution, bad odours and high expenditure the idea of
digesting the organic fraction of MSW as an alternative to landfill disposal has been a
subject of research and discussion for some years (Wheately 1990). In general MSW
is all waste collected from domestic, commercial and some industrial (non-hazardous)
sources, however as the exact composition of municipal solid waste varies from
Country to Country, solid waste is non-standard and so typically no two wastes are
the same (Kiely 1997), see table 1, page 27.
The simultaneous digestion of the organic fraction of municipal solid waste
(MSWOF) together with sewage sludge under mesophilic conditions is regularly used
in several Countries, including the U.S.A. This process is routinely performed
conveying the MSWOF, after rough screening, to the wastewater treatment plant
where it is partially treated. The resulting mixture of screened material and sewage
sludge containing most of the organic fraction of household waste is then submitted to
digestion. This additional supply of organic substances into the digester often requires
the application of higher residence times or, alternatively, enlarging the reactor
volume, or adding additional digesters. The only alternative which would allow use of
the pre-existing plants without any substantial modification, is to carry out the
digestion under thermophilic conditions (Del Borghi et al 1999).
The earliest experiments on municipal waste digestion which considered mixing the
solid material (about 4% total solids) with sewage sludge or water to make a slurry
that could be digested in a continuous flow stirred-tank digester, in the same way as
sewage sludges was established in Florida, USA in 1978 and was known as the
RefCom process (Wheately 1990). Here, the shredded refuse was made into a slurry
of between 4 to 10% total solids (TS) with recycled water, obtained by dewatering the
digestive effluent. This slurry was then digested in a thermophilic (600C), stirred tank.
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Various sizes of tanks have been used during the laboratory, pilot and large-scale
tests; the most recent plant had two 350,000-gallon digesters. Mixing was by topdriven paddles, which were designed to circulate the slurry up and down the tank.
Although this digestion process was successful: in that biogas contained
approximately 50% methane and volatile solids (VS) destruction was also about 50%,
there were a number of problems encountered for example with waste separation and
preparation of the feed and as a result ceased operation in July 1985 (Wheately 1990).
The first European demonstration plant treating MSW by anaerobic digestion (AD)
was built by Valorga in France in 1984. It has since been developed to full scale. At
this plant a dry method in which the sorted MSW is digested at a dry matter content of
30-35% using a continuous flow tank digester is operated. The digester contents are
mixed at intervals by gas injected into the bottom of the tank, as is the case in gas
mixed sludge digesters. Although this process can be operated at the thermophilic
range, that is, 450 to 550C it is generally run at 370C. Following the success of this
plant at La Buisse, a second large installation was built at Amiens, with a capacity of
110,000 tonnes of refuse per annum. The construction of this plant which has three
2,400m3 digesters, was supported by the European Energy Demonstration Scheme of
the CEC (Wheately 1990).
In Belgium, another "dry" process, the Dranco process, has also been running on a
large scale for a number of years. The solid material is mixed with sewage sludge or
some other liquid returned from the digester effluent, to give a feedstock of about 3035% TS. The process is a continuous digestion with an overall retention time (RT) of
18-21 days at 550C. The effluent material is dewatered in a screw press and the liquid
can be returned to make up the feed. Gas is collected in a separate gasholder and is
used for electricity generation. The biogas of methane content 55% is produced at the
rate of 125 to 185 L per kg organic matter, added to the unstirred-tank digester. This
rate can vary however depending upon the composition of the waste being treated
(Wheately 1990).
Due to the high solids content of the feed, gas production rates in both the Valorga
and Dranco solid-digestion systems are high; over 3m3 methane per m3 digester
volume, per day compared with about1 to 1.5 biogas volumes from the usual sludge
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digesters. The digested solids, after dewatering, can be sold as fertiliser (anaerobic
compost) or combusted for heat purposes or used as inert landfill.
In Sweden, a process (The Biomet) in which AD of MSW at an intermediate TS
concentration of 7-10%, has recently been developed on a pilot scale (Szikriszt et al
1988). During this process a mix of sorted municipal solid waste and sewage sludge is
fermented to methane in the pilot scale reactor. The waste is ground in a hammermill
and sorted. RDF and magnetic material are removed and the remaining fraction is
mixed with the sludge from a municipal sewage treatment plant. Before digestion, the
mixed substrate is disintegrated in a Novel pre-treatment stage involving a mechanical
hydropulper. In this pre-treatment, a substantial size reduction is achieved as well as a
separation of heavy particles. The mixed pre-treated material is digested at an inlet TS
concentration of 8%. The reactor is operated at two different volumetric loading rates
with RTs of 27 and 19 days.
The biogas yield from this pilot scheme was surprisingly high: 0.23-0.29 m3 Ch4 (tons
VS)
–1
added, with a methane content of 53-57% (V.V. -1). Moreover samples of the
reactor effluent have been dewatered and analysed for nutrient content and
concentration of heavy metals and according to Swedish regulations, the digested
material would be allowed as a soil conditioner (Szikriszt et al 1988).
It should be noted that although the ancillary equipment for utilising the biogas and
for preparing the organic fraction from the unsorted waste is the same in continuous
flow and batch digestion systems, batch fermentation of the organics of municipal
waste has been suggested to be a simpler and cheaper process than continuous
digestion. Brummeler et al (1988; cited in Wheatley 1990) described a batch process,
which as yet has only been tested only on a small scale for mechanically sorted
municipal waste. The initial TS content of the waste was set at 35%. The waste was
packed into a cylindrical, unmixed, digester tank and incubated at 30oC, with an
inoculum of digested sewage sludge. Initial tests indicated that the waste had to be
buffered by sodium bicarbonate to prevent acidification of the material and to allow a
good start-up. As alkali addition to a large scale system would be too costly, it was
found that a partial aerobic composting of the waste prior to digestion prevented
acidification. It was documented that about 15% of the waste solids had to be
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destroyed in the composing to get a good digestion, these 15% solids being easily
degraded and so causing a rapid acid formation under aerobic conditions. The overall
time of the batch digestions was about 4 months and the methane yield about 801kg
organic waste.
Although AD processes have been commercially available for solid waste reduction
since the mid 1980s most of this initial commercial activity was confined to Europe.
Though for more than 50 years AD has been used extensively in the U.S. for sewage
sludge reduction, commercial digesters have only recently been promoted in the U.S.
for the processing of MSW. Europe, on the other hand, has been a leader in the
commercialisation of various solid waste digester designs including dry anaerobic
composting, high concentration slurry reactors and 2-phase digestion (Hayes et al
1993). Europe's lead over the U.S. in this process has occurred for a number of
reasons, including:
•
A higher dependency on foreign oil which has elevated the strategic importance of
alternative fuels in Europe
•
Higher fossil fuel prices which have made alternative energy sources more
competitive in Europe
•
Severe land space constraints for new landfills coupled with negative reactions in
many areas of Europe to MSW incineration.
Other successful applications of AD processes in Europe include at Vaasa in Finland,
where since 1990, 25,000 tonnes of household waste and sewage sludge is processes
per year. Likewise the city of Salzburg in Austria has installed an anaerobic digester
which processes 20,000 tonnes per year of separately collected organic and putresible
wastes from households. Also eleven AD plants have been constructed in Denmark,
of which seven process municipal wastes. Of these seven, one plant in Herring,
established in 1993, processes 166 tonnes per day (Prism 1999).
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In Britain, Worcestershire is set to become the first local authority to invest in AD for
treating household waste, if planning obstacles can be overcome (Anon 1999a).
Although in 1993 the County of Kent in southeast England had previously prepared
contracts for a 40,000 tonne per year AD plant near Ashford, in January of last year
Kent county council announced that plans for this AD scheme had been abandoned
due to financial reasons.
Kent County Council said that the project would have lifted Ashford's household
waste recycling rate to 66% because it included a sorting plant to remove recyclables
and separate the organic fraction. Worcestershire now appears to be the strongest
contender to build the UK's first household waste AD plant. The £500 million
contract, spanning 25 years involves privatisation of the existing landfill operation the
development of a new kerbside collection service and three material recycling
facilities ( Anon 1999a).
Meanwhile, Southampton City Council is hoping to revive a proposed AD scheme,
which was to be featured in Hampshire's waste disposal contract. The project,
originally to be delivered by French contractor Onyx Aurora, was dropped on cost
grounds. Elsewhere, AD on a small scale is to be tested in South Shropshire with help
from a £95,000 DTI grant. Greenfinch plans to commission the pilot plant in March to
digest kitchen waste from 1,500 households. Heat from the biogas and liquid fertiliser
produced will be used in aquaculture. The company believes that the technology
would be commercially viable for projects covering 5,000 or more households (Anon
1999a).
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Anaerobic Digestion of Domestic Sewage Sludge
Although municipal solid waste digestion is a fairly recent innovation, the first direct
application of AD to sewage solids is attributed to Louis H. Mouras of Vesoul,
France, who developed a cesspool in approximately 1860 in which it was claimed that
sewage solids were liquefied (Casey 1997). Anaerobic digestion processes were
subsequently incorporated in sewage treatment systems using dual-purpose tanks
combining sedimentation and digestion. The Travis hydrolytic tank (Hampton,
England, 1904) and the Imhoff tank (Germany, 1904) are examples of such systems.
The Imhoff tank in particular, was installed at most small to moderate sized treatment
plants and are still in wide use today in Ireland, although few have been constructed
since 1950 (Gray 1989).
In 1990, more than 7 million dry tonnes of sewage sludge was produced within
Europe, 23,000 tonnes of this being produced in Ireland (Killilea et al 1999). The
options traditionally used in many European countries for the disposal of such sludge
included landfilling, landspreading and marine dumping. Only in more recent years
have composting, incineration and other thermal processing systems been used to any
significant extent. In 1990, marine dumping and disposal to landfills accounted for the
bulk of annual Irish sewage sludge production, with landspreading making up less
than 10% (Colleran 1993). With the closure of the first option in 1998, a significant
change in sludge disposal routes in Ireland occurred.
Anaerobic digestion was first applied to sewage sludge treatment in England in 1895
and for the past 50-60 years has been widely used throughout the world (Colleran
1993). AD is well suited to the stabilisation of primary aerobic sludges because the
large amount of non-microbial organic matter present can be reduced substantially
with a minimal production of biomass. It also releases the bound water, thereby
making the treated sludge easier to thicken. Secondary aerobic sludges can also be
treated anaerobically, but their relatively low solids concentration and high proportion
of microbial solids, often make them more amenable to AD (Grady et al 1980). The
benefits of anaerobically digesting both primary and secondary sewage solids include
the significant reduction in solids handling, net usable energy generation in the form
of biogas, pathogen destruction by at least 90% and odour reduction (Gray 1989).
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These benefits are highly significant particularly if the final disposal outlet is
landfilling or landspreading.
Anaerobic digestion has not been applied, as yet, to any significant extent in sewage
treatment plant in Ireland. The first high-rate digestion system installed in Ireland was
at the sewage treatment plants in Tullamore, County Offaly, which caters for a
population equivalent of 16,000 (Killilea et al 1999). Like all of the existing anaerobic
digesters treating sewage sludge in Ireland, the digester is a cylindrical gas mixed
reactor following the traditional continuously stirred tank reactor (CSTR) design. The
digester operates at mesophilic temperatures and treats primary settled sludge and
settled mechanically thickened waste activated sludge (WAS). Treatment includes
primary settlement with biological filtration and anaerobic sludge stabilisation of
supernatant. Primary and WAS, including chemically precipitated sludge, is sent to
the 330m3 prefabricated digester at a rate of 17m3/day. The biogas produced (400600m3/day) is stored in a gas bell tank until required for heating purposes. After
digestion, solids are collected in a storage tank and further stabilised before being
spread on agricultural land or further dewatered to 17% dry solids (DS) and sent to
landfill (Killilea et al 1999).
Table 2 (see page 27) summarises the performance of the anaerobic digester at
Tullamore over a four-month period. From this it can be determined that the high
alkalinity and gas yields (400-600m3/day) suggest excellent process performance.
Unlike the anaerobic digester installed at Tullamore, which is operating very
efficiently, recently the same system was decommissioned for upgrading at the
Buncrana sewage treatment plant in County Donegal. The sewage treatment plant at
Greystones, County Wicklow, also has an AD installed. Here primary and secondary
WAS are also treated. The sludges are individually thickened before being pumped
forward to mixing tanks. From there, the blended sludge is transferred to the CSTR,
where it is digested for a minimum of 12 days at 35oC. Biogas generated in the
process, is recovered and stored in a gas holder, until burnt in a gas engine to generate
electricity, meeting 20-30% of the plant's electricity requirements. Heat recovered
from the electricity generation is used to heat the sludge and also to provide space
heating for buildings (Anon 1999b). Although information was requested regarding
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the operating efficiency of this anaerobic digester, unfortunately it was not obtained in
time so as to be submitted in this report.
In 1992, the Department of the Environment in Ireland commissioned a national
sludge strategy study for 25 of the counties of Ireland (excluding Dublin) (Killilea et
al 1999). In 1992, there were only two AD plants treating sewage solids in Ireland.
The resulting study, produced by the western FTA Ltd company in 1993,
recommended a further 23 viable sites for the AD of sludge. As a result, by 2001 it is
reckoned that seven AD plants will be in full operation in Ireland. Currently eleven
sites are at the design stage and are expected to undergo construction within the next
2-5 years. A decision on the process design for 4 additional sites has yet to be
finalised. It is expected that approximately 20 anaerobic digestion plants treating
sewage solids in Ireland will be in operation by the year 2010 (Killilea et al 1999).
Presently in Ireland there are 5 anaerobic digesters treating sewage solids, all of which
are of the mesophilic (30oC-38oC) CSTR design. However, new designs are likely to
incorporate thermophilic technologies as mesophilic treatment by itself, is considered
insufficient for removing pathogens and the resulting solids are unsuitable for direct
spreading on land (Killilea et al 1999).
In the UK, AD has been widely used in sewage plants for a great number of years,
particularly those treating population equivalents of greater than 50,000 people
(Grady et al 1980). Sewage treatment in the UK produces 1.1 million tonnes dry
weight of sewage sludge per year, but this is expected to double by 2006. Around
70% of this sludge is treated by heated AD (Killilea et al 1999). In the UK, AD is
carried out at approximately 240 sewage treatment plants (see table 3, page 28). AD at
small sewage treatment plants (STPs) is rare. It is more common at medium size
works (serving 10-100,000 people); 164 of these 683 STPs have digesters whilst 80%
of the 270,000 tonnes per year produced by the nine largest plants in the UK, serving
500,000 people or more, is digested (Meadows 1993).
Not all the biogas produced in the UK from AD of sewage sludge is accurately
metered, so there are no exact figures for the quantity produced (Meadows 1993).
However, it is possible to roughly estimate the volume on a per capita basis, for
primary sludge typical yields range from between 15-22 m3 per 1000 persons, whereas
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secondary sludges can produce up to 28m3 per 1000 persons per day (Gray 1989).
Thus estimates for total biogas production within the UK range from 221-260 million
m3 per year. Within the UK, biogas has been used for many years. The priority is
using the gas to maintain digesters at 30o-35oC.
There are two ways of heating digesters. One is using a hot water boiler, fuelled by
digester gas. The second heat source is a combined heat and power (CHP) system
where the gas fuels, dual fuel, spark ignited (SI) or gas turbine engines. About half of
all UK digester gas is used in combined heat and power schemes (Meadows 1993). In
the UK the technology of CHP from digester gas is well proven with a long history of
application.
In recent years also the emphasis in the larger treatment plants in the UK has focused
not only on a more efficient biogas use within the works, but also on recycling/reuse
of the treated solids (Colleran 1993). The Modgen sewage works, which serves 1.4
million inhabitants in an area near Heathrow airport in London, is a prime example of
this. On a daily basis the plant receives a sewage volume of 480,000 m3 which yields
3,700m3 of settled sludge per day with a dry matter content of 4%. The sludge is
digested in 20 anaerobic digesters at a RT of approximately 20 days. Biogas output is
58,000m3 per day. This gas meets almost 100% of the energy needs of the treatment
plant and the digested sludge is subsequently dewatered, stabilised and used in either
a wet or dry form as fertiliser or soil conditioner (Colleran 1993). Likewise a similar
type of plant, near Paris, that is Acheres treatment plant, treats the sewage from 8
million people in 30 digesters, which have a total operational volume of 300,000 m3.
The biogas is used for co-generation of electricity and heat and the digested slurry is
thermally treated at 200oC prior to filter press separation to yield a solid fraction,
which is then sold as a soil conditioner, and a liquid fraction which is recycled back to
the overall system. Although the thermal step ensures kill-off of all possible
pathogens that might be present in the digested product, it is expensive in energy
terms and as a result the produced biogas satisfies only 50% of the overall energy
requirements of the treatment works (Colleran 1993).
Page 23 of 76
For more than 60 years AD has been used for stabilisation of sewage sludge in
Denmark (Prisum 1993). In 1993, more than 100 digesters with volumes ranging from
300 to 6000 m3 had been constructed (Prisum 1993). In the last decade, the AD
process has also been introduced in centralised biogas plants treating organic waste
from households, industry and agriculture. Such plants have become an integrated
part of the Danish waste management structure (Colleran 1993). The plants known as
joint biogas plants or collective biogas plants were originally designed to treat the
manure surpluses in certain regions of Denmark. All plants now treat a mixture of
manures and industrial wastes. In some instances, source separated MSW and sewage
sludge is also co-digested. At the linkogas plant in Lintrup, the daily supply of raw
material consists of 300 tonnes of animal manure from 62 farmers in the region, 15
tonnes of intestinal contents from slaughterhouse operations, 31 tonnes of fish waste,
1-2 tonnes of marmalade production waste and 8-9 tonnes of domestic sewage sludge.
The plant treats 131,000 tonnes of waste per annum and the biogas produced (apart
from the 10% used for the heating requirements of the plant) supplies the adjacent
village of Roddinge with district heating (Colleran 1993).
There are currently eleven full-scale collective biogas plants in Denmark with digester
volumes ranging from 800 to 3x 2500 m3. There are also a further eight at the design
or construction stage (Colleran 1993).
Page 24 of 76
At this stage in the report, it is important that the different treatment systems available
for the digestion of sewage sludge are detailed. As previously discussed, in the UK
and Ireland conventional digestion of sewage sludge is usually a two stage process
with the primary digester heated to the desired temperature and continually stirred in
order to allow optimum anaerobic activity, with acid formulation and gas production
to occur simultaneously. The secondary digester is unheated and can be used for two
functions: either to continue digestion under psychophilic conditions or it can be used
for sludge separation (Gray 1989). This type of digester is referred to as a
continuously stirred tank reactor. Recently fixed film reactors, in particular fluidised
bed reactors, have also shown to be capable of treating domestic wastewater at
ambient temperatures (25oC) on a lab scale and along with the UASB (upflow
anaerobic sludge blanket reactor), hold the most promise for future full scale domestic
sewage treatment (Droste 1997). The application of the UASB for the treatment of
sewage sludge will be discussed later.
The traditional use of reinforced concrete for constructing the shell of digestion tanks
for sewage works is used in most countries and indeed it is still considered essential
for tanks of over about 2000m3 capacity (Ferranti et al 1987). Capacities of individual
digesters on large works can range up to 12,000m3. Steel tanks have been used to a
limited extent for many years but a more dramatic departure from the "concrete
tradition" has occurred in the last six years in the UK where the use of prefabricated
glass coated steel tanks has now become the most common form of construction for
new digesters. The traditional "low form" digestion tank, that is diameter greater than
height, employed in the UK and North America for many years is now considered to
be less conductive to efficient mixing than tanks with a high aspect ratio, that is height
greater than diameter. Although the low form tanks often perform quite well they tend
to be difficult to mix and deposit grit. The taller form of the digestion tank (aspect
ratios >1) used traditionally at sewage works in mainland Europe is seen in its most
extreme form in the "egg-shaped" digesters used for some very large plants in
Germany. This shape is said to be economical to construct and to be efficient but
clearly it is only appropriate for very large plants (Ferranti et al 1987).
In the strict sense, there is no absolute need for mixing in order for the AD process to
function. But the mixing system of a digester does serve several important purposes,
Page 25 of 76
which include uniform dispersal of raw sludge feed throughout the tank, maintenance
of uniform temperature conditions and prevention of stratification. The trend over the
years has been away from mechanical mixing systems and towards gas mixing.
Although gas mixing is in some cases, less efficient in terms of energy requirements it
reduces maintenance problems and can be effective (Ferranti et al 1987).
Virtually all heated anaerobic digesters at sewage works in Europe are designed to
operate in the mesophilic temperature range (32o-37oC). The numbers of thermophilic
(45o-55oC) anaerobic digesters are very small, though they have been reported to be
operating satisfactorily in Canada and the USA (Ferranti et al 1987). Most digesters
are heated by hot water boilers or by waste heat from gas engine with a water/sludge
heat exchange system either internal or external to the digester. The use of direct
injection of live steam has been successfully revived recently in the UK and it is also
used at a few plants in mainland Europe. A major advantage of steam is the direct
heat transfer to the sludge and elimination of the need for heat-exchange equipment.
For sewage sludge digestion plants there has been a gradual reduction in the accepted
design retention period from around 30 days to 15 days. In most countries now, a
period of 15-20 days is considered quit adequate to give full digestion (Ferranti et al
1987).
With regards the UASB reactor, it has been applied successfully in the treatment of
municipal sewage sludge, in several regions with hot climates. Schellinkhout et al
(1985; cited in Wheately 1990) and (Malina et al 1992) described a 64m3 UASB
digester treating raw domestic sewage in a town in Columbia during a period of five
and a half years, where the ambient temperature was a fairly constant 25oC. The high
temperature caused the sewage to become septic in the sewers and was more dilute
than the usual sewage from a city with a temperate climate; but the treatment was
successful in reducing BOD and COD by about 80%, at a hydraulic retention time of
3-3.5 hours; coping with the usual diurnal variations in sewage flow and composition.
However, some further treatment of the digester effluent might be needed depending
on local regulations for discharge. In a further study Grin et al (1985; cited in
Wheately 1990) found that treatment efficiency of raw sewage dropped off
considerably at temperatures below 12oC.
Page 26 of 76
Similarly, the Sao Paulo State in Brazil, where living conditions are poor and there is
a lack of sanitation services, led to the development of the use of a UASB reactor for
sewage treatment (Vieira 1988). Initially tests were made with a 106 litre capacity
UASB reactor, which showed the process feasibility. As positive results were
obtained with this system at HRTs of 4 hours and ambient temperatures, that is
effluent values of 57mg BOD/L, 155mg COD/L and 59mgSS/L, the technology was
tried out at a demonstration scale. A 120m3 UASB reactor was thus designed and
constructed and its operation confirmed the results initially obtained in the pilot plant;
i.e. a HRT of 6.5 hours gave effluents with 113mg COD/L and 48 mg BOD/L. As
these results clearly showed that the UASB concept as a sewage treatment system
offered a new, simple and economical technological option for facilitating the
improvement of sanitary conditions of the Sao Paulo State, several reactors were
subsequently planned for various other localities within the country (Vieira 1988).
Finally, although the AD process has been successfully applied for the treatment of
various other industrial wastewaters (which will be discussed throughout other
sections within this report) it is the anaerobic treatment of domestic and municipal
sewage that has received most attention and from which most of the expertise in
digestion has been developed (Gray 1989). Moreover, it is not just in developed
Countries that the process of AD has been successfully applied, simplified versions of
fully mixed reactors have been applied in many developing countries, particularly
India and China. Alaa El-Din (1980; cited in Gray 1989) estimated that 5 million
individual anaerobic digesters had been set up in China by 1978. About 5m3 of gas per
day is generated by one 10m3 biogas plant. This is normally sufficient to supply three
meals to a Chinese family of five, with the sludge and effluent produced used as
fertiliser and irrigant respectively (Forster 1985).
Page 27 of 76
Conclusion
In conclusion, the state of development of AD technology for handling MSW sludges
is at a relatively high level, with many improvements in the technology that have
extended the capability of the process to handle a wider range of feedstock types and
concentrations. In the 1960s the majority of textbook applications of the AD process
mainly dealt with industrial and municipal sludges of fairly homogeneous consistency
with concentrations of between 1-4% (Hayes et al 1993). Today, however because of
breakthroughs made in research, anaerobic processes can now operate on feed streams
with organic concentrations as low as .01% to more than 40%.
Page 28 of 76
Page 29 of 76
Page 30 of 76
BIBLIOGRAPHY
Anon (1999a)
Faltering progress on anaerobic digestion of household waste
Ends report 288 pp. 14-15
Anon (1999b)
Anaerobic Digestion at Greystones sewage works
Wicklow County Council
Casey, T.J. (1997)
Unit treatment processes in water and wastewater engineering
J. Wiley and sons, Chichester
Colleran, E. (1993)
An overview of the treatment, usage and disposal of sewage sludge. In European
seminar on European efficient options for the treatment of sewage sludge, (1993) ed.
R. Ward
Forbairt-Opet, Dublin pp.13-24
Del Borghi, A.; Converti, A.; Palazzi, E.; Del Borghi, M.
Hydrolysis and thermophilic anaerobic digestion of sewage sludge and organic
fraction of municipal solid waste
Bioprocess Engineering 20 (1999) 553-560
Droste, R.L. (1997)
Theory and practice of water and wastewater treatment
J. Wiley and Sons inc, New York
Ferranti, M.P.; Ferrero, G.L.; L'Hermite, P. (1987)
Anaerobic Digestion: results and demonstration projects
Elservier Applied Science, London
Forster, C.F. (1985)
Biotechnology and wastewater treatment
Cambridge Uni. Press, Cambridge
Grady, C.P. Jnr.; Lim, H.C. (1980)
Biological wastewater treatment, theory and applications
Marcel Dekker Inc, New York
Gray, N.F. (1989)
Biology of wastewater treatment
Oxford University Press, Oxford
Page 31 of 76
Hayes, T.D.; Frank, J.R. (1993)
The production of biogas from biomass and wastes. In Oppurtunities for Innovation:
Biotechnology, (1993) ed. R.M. Busche
Technomic, Pennsylvania pp.191-217
Killilea, J.E.; Colleran, E.; Scahill, C. (1999)
Establishing procedures for design, operation and maintenance of sewage sludge
anaerobic digestion plants
(In press)
Kiely, G. (1997)
Environmental Engineering
Mc Graw Hill Companies, London
Malina, J.F. Jnr.; Pohland, F.G. (1992)
Design of a process for the treatment of industrial and municipal wastes
Technlomic Publishing Company, Pennsylanvia
Meadows, M. P. (1993)
Anaerobic Digestion, the experience in the UK. In European seminar on European
efficient options for the treatment of sewage sludge, (1993) ed. R. Ward
Forbairt-Opet, Dublin pp. 61-70
Prism (1999)
Infosheet (Anaerobic Digestion)
http://www.wrf.org.uk/AD-IS .html
Prisum J.M. (1993)
Anaerobic Digestion, the Danish experience. In European seminar on European
efficient options for the treatment of sewage sludge, (1993) ed. R. Ward
Forbairt-Opet, Dublin pp. 49-50
Szikriszt, G.; Frostell, B.; Norrman, J.; Bergstrom, R. (1988)
Pilot-Scale anaerobic digestion of municipal solid waste after a novel pre-treatment.
In Anaerobic Digestion 1988, ed. E.R. Hall and P.N. Hobson
Pergamon, Oxford pp. 375-382
Vieira, S.M.M. (1988)
Anaerobic treatment of domestic sewage in Brazil – research results and full-scale
experience. In Anaerobic Digestion 1988, ed. E.R. Hall and P.N. Hobson
Pergamon, Oxford pp. 185-196
Wheately, A. (1990)
Anaerobic Digestion: waste treatment technology Critical reports on applied
chemistry vol. 31
Elservier applied Science, London
Page 32 of 76
Section 2
Anaerobic Digestion Technologies in the
Food Processing Industries
By
Dawn Nally
Page 33 of 76
Introduction
Anaerobic wastewater treatment has been applied for most agro- food and beverage
industries such as distilleries; sugar factories; potato processing industries; dairies;
fish,
potato
and
coffee
processing
industries;
slaughterhouses;
yeast-
factories;canneries; fruit processing plants; soft drinks industries;yeast production and
sugar production.'Anaerobic treatment by itself will usually be insufficient to meet the
effluent discharge standards and should therefore be combined with a post- treatment
such as an aerobic process''.(gtz 1999)
For the purpose of this project it would not be possible to demonstrate the
performance of Anaerobic digestion in all of the above industries and so an insight
into a selected number will be given. Details of the performance of the systems
involved in each will be provided.
Food Processing - The Industry
''The food industry produces large volumes of wastes, both solids and liquids,
resulting from the production ,preparation ,and consumption of food. These wastes
pose increasing disposal problems and represent a loss of valuable biomass and
nutrients. In many cases food processing wastes might have a potential for recycling
raw materials, or for conversion into useful products of higher value as by-products ,
or even as raw materials for other industries, or for use as food or feed after biological
treatment''.(Martin 1991 )
Characteristics of Food Processing Waste
''The composition of wastes emerging from food processing factories is extremely
varied and depends on the nature of the product and the production technique
employed. Certain characteristics of particular industries are useful in the selection of
suitable methods for waste treatment. In general, wastes from the food-processing
industry have the following characteristics (Litchfield 1987 cited in Martin 1991):Large amounts of organic materials such as proteins, carbohydrates and lipids,
Page 34 of 76
Varying amounts of suspended solids depending on the source, and high BOD or
COD.'' (Martin 1991)
The Biotechnological Aspect of Food - processing waste treatment
''The food industry may be recognized in many aspects as the largest ,oldest and most
potentially valuable form of biotechnology. In particular, the production of complete
different products from waste, usually by applying biotechnological methods has
gained great attention.
Under anaerobic conditions food processing wastes containing carbohydrates, lipids
and proteins can be digested to yield biogas.
Anaerobic digestion combines
detoxification of waste materials by upgrading them to their use as animal feed or
fertilizer and the production of biogas. Biogas varies in composition according to the
composition of the waste treated. The content of methane generally lies between 60%
and 80% with carbon dioxide and hydrogen sulphide as major remaining
substances.(Hacking 1988 cited in Martin 1991) The anaerobic treatment processes
can be applied to all segments of the food processing industries with particular
reference to concentrated waste streams.
Technical Aspects
It has been said that ''anaerobic systems are more sensitive for disturbances and the
start-up phase of a newly installed reactor may demand a long period of time.''(gtz
1999 ) Also, ''All modern high-rate anaerobic treatment processes are therefore based
on the effective retention of the anaerobic sludge. In this way the sludge retention is
uncoupled from the hydraulic retention time and the biomass in this way is offered
sufficient time for multiplication. Solids retention times of over 100 days can be
obtained at HRT's of only a few hours and high loading rates can be applied.''(gtz
1999 )
For the purpose of this project it would not be possible to demonstrate the
performance of anaerobic digestion technologies in each of the above industries so an
Page 35 of 76
insight into some will be given. Details of the performance of the systems employed
will be given, along with some advantages, benefits and disadvantages.
Coffee-Processing Industries
The ‘'San Juanillo Coffee Mill'’, Narajo, Costa Rica adapted the principals of
conventional anaerobic treatment technology.
The Costa Rican Coffee Institute
ICAFE ,charged a Dutch consultancy firm ''BTG Biomass Technology Group'' to
develop solutions tot he treatment of the wastewater's from coffee mills. A process
was thus invented called the RANBI (Reactor anaerobic de baja inversion) process,
which adapts the principals of conventional anaerobic treatment technology to a lowcost process,adapted to the specific conditions of Costa Rican coffee mills.
Conventional treatment has been dealt with in the main introduction . The reactor in
this process must provide a minimum HRT of about 10days @ 35Oc.(Droste 1997)
Details of the process involved were as follows;
The process:-Wastewater's from coffee mills contain high concentrations of sugars
and other organic components, which originate from pulp and mucilage.
Ata
reasonable cost these soluble components cannot be removed and so are removed by
biological processes. Here, the bacteria remove the dissolved organic matter. In an
anaerobic process these bacteria live in conditions without air. Their excretion is in
the form of a gaseous product called ''biogas''. In San Juanillo it is burned in the
ovens which generate the heat for drying the coffee, substituting fuelwood. The
article also states that since energy production is important in relation to anaerobic
digestion it should be noted that for this particular process air injections are not
required.
Therefore, power consumption is very low, implying another positive
contribution to the environment. The treatment capacity of the plant is 4000kg
COD/day,and the treatment efficiency is 80% COD removal.
generated biogas per day is 1000m3/day.
The maximum
Page 36 of 76
See Fig 1,of Flow Diagram of San Juanillo Coffee mill System.
Page 37 of 76
Sugar Refining Industries
Sugar Refining is another process incorporating anaerobic digestion technology. A
large sugar refinery belonging to the Egyptian Sugar Integrated Industries group,
called ''The Hawamdia complex'' is located near the Giza on the river Nile. Effluent
at the plant is highly contaminated with a maximum BOD of 14,000 mg/l and
maximum COD of 25,000 mg/l. The Belgian group ''Seghers'' have designed and
installed a two-stage anaerobic -aerobic effluent treatment plant called ''Unitank'', to
treat the sugar refinery effluent.
The process:- Wastewater's is initially collected in a pump-pit where screening
occurs. The wastewater is then pumped through a cooling unit and greases/oils and
sand are removed. Effluent from the cooling unit flows by gravity to a biological
conditioning tank. Due to the acidic nature of the effluent a correction tank is
required, then the pretreated wastewater is directed to a methane reactor where
degradation of the organic compounds to CH4 takes place. Wastewater is then stored
in the anaerobic tank. Because anaerobic treatment is only partial an aerobic posttreatment i.e. a two-stage aerobic treatment step is included. Tertiary treatment is
applied by adding activated Carbon in the low-loaded aerobic stage. This continues in
the coagulation tank. FeCl3 is added to form big flocs. Flocculated wastewater flows
to a Sedimentation tank. Finally, Wastewater flows to a chemical tank where any
residual COD is oxidized by strong oxidants(Cl2 or H2 O2)
Sehers have had to extend the ''Hawamdia ETP'' to cope with increasing hydraulic and
organic capacity.
The Unitank at Hawamdia consists of an improved anaerobic
upflow reactor (UASB). Unitank uses the same principals as conventional anaerobic aerobic systems and is characterized by continuous operation. The most important
difference to the conventional system is the cyclical operation of Unitank, thus
resulting in aeration and sedimentation being carried out in one compartment.( Allison
P. 1999). The benefits of this system are that BOD is reduced from 14,000mg/l in the
influent to 230mg/l in the effluent. Also, COD from 25,000mg/l to <40mg/l in the
effluent.
Page 38 of 76
Food & Beverage Industries
''The food and beverage industries tend to produce wastewater's containing high
preparations of soluble or colloidal material of 'natural origin', often with high
concentrations of carbohydrates.''(Foster & Wase 1987). Special reactors that treat
these wastewaters are of relatively recent design and employ conventional processes.
In full scale operation the reactors can rarely exceed 4kg COD or volatile solids
M3'day, but in laboratory or pilot plant studies two or three times these loadings can
be achieved.
In industry, sugar beet pulps contain a high proportion of cellulose and are
pretreated with Trichoderma horzianum to hydrolyze the cellulose enzymatically.
The pretreated pulps could then be degraded at space loading of 1kg volatile
solidsm3/day. The feed contained 20g volatile solids/l and yielded 1.74m3 of biogas
per kg of volatile solids applied.
Palm oil effluent is generated in the process of oil palm fruit. A tonne of palm oil
produced, gives rise to 2-3 tonnes of wastewater with a BOD of approximately
2500mg/l. In this case, laboratory scale reactors were able to produce effluents with a
BOD of less than 2000mg/l up to 4.5kg COD m3/day loadings. Other achievements
of this system were biogas yields of 0.7-0.8m3/kg volatile solids for 80-90% volatile
solids reduction.
Wastes from the soft-drinks bottling industry contain high
concentrations of readily degradable soluble organic materials, often with significant
amounts of sucrose.
The have COD concentrations of 50-150,000mg/l.
The
conventional laboratory scale digestion achieves satisfactory treatment at a loading of
1.28kg volatile solidsm3/day for 10 days with HRT. Volatile solids concentration
was maintained at less than 2000mg/l
Distilling Industry
Anaerobic digestion technology in the wine industry was described in the studies
carried out at Universite Montpellier ll, France. The process of 'down-flow fluidized
bed ''technology was employed.
According to (Campbell1977), Wastewater's
produced by the brewing and distilling industries are derived from carbohydrates, are
produced by fermentation and are almost wholly organic in nature.
Page 39 of 76
The process of ''down-flow fluidized bed'' technology presents advantages over other
processes like high organic loading rates and short hydraulic retention times. ''The
aim of the work done was to determine the feasibility of a down-flow fluidized bed
reactor for the anaerobic digestion of a red wine distillery effluent, with a carrier
material that allows low energy requirement for fluidization, providing also a good
surface
for
biomass
Caleron,Moletta 1996)
attachment
and
development.''(Buffiere,Elmaleh,Garcia-
Droste (1987)writes that media characteristics affect the
operation and performance of the reactors. Ideal media have more surface area and
high porosity. Higher porosity material allows the accumulation of more biomass.
Organic loading rates can be increased ,which decreases required reactor volume and
HRT and also increases the upflow velocity through the reactor. Higher upflow
velocities retard clogging but the structure of highly porous media prevents significant
biomass loss at higher velocities. In a fully developed reactor ,gas production from the
film provides a considerable degree of mixing. This mixing keeps the solids in
suspension for some time rather than settling directly to the bottom. Suspended solids
concentrations up to 3% can be accommodated by downflow fixed film reactors.
Page 40 of 76
A schematic diagram of the experimental set-up for ‘down-flow fluidized bed
technology’ used , can be seen in Fig 2.
Page 41 of 76
Ground perlite , an expanded volcanic rock was the carrier employed. The system
achieved 85% Total organic carbon removal at an organic loading rate of 4.5kg Total
organic carbon m3/day.(Buffiere et al 1996) The carrier material was found to be a
very important parameter, because biomass accumulation brings about changes in
particle volume and density, thus affecting the whole system. '' Perlite allowed a high
biomass hold-up, with minimum particle washout because of its density.
Other
advantages of the down-floe fluidized bed system were no clogging and a low energy
requirement because of the low fluidization velocities required''(Buffiere et al.1996).
A high-rate reactor based on the ''Upflow anaerobic sludge blanket (UASB
) was installed recently by Heineken Breweries in Ho Chi Minh City to treat their
wastewater.
The HRT is single most important factor affecting treatment
performance of upflow anaerobic filters, with COD removals varying in relation to
HRT.( Jerzis E. Dr., )
Confectionary / Ice-cream Industry
Yet another application of anaerobic digestion technologies was studied at ''Institute
grasa-Spain''. It was entitled ''The evaluation of the instability and performance of the
treatment of high-strength ice cream waste-water using an up-flow anaerobic sludge
blanket (UASB) reactor.''
Foster & Wase (1987) agree that in anaerobic digestion sludge retention time, (SRT)
is a significant factor, particularly in view of the relatively long mean generation
times of the methanogens and acetogens.
From the analysis carried out by Banks CJ & Borja R ,(1996) at ''Institute grasa'' the
substrate concentration and Ph under different operating conditions were determined.
From the concentrations of substrates measured at various levels above the bottom of
the reactor, two reaction stages were distinguished. These were acidogenesis and
methanogenesis. Instability of this system was recognized as being caused by high
influent concentration, due to the accumulation of volatile fatty acids in the
acidogenic stage of the process. These fatty acids were beyond the assimilative
Page 42 of 76
capacity of the methanogenic stage. A range of stable operating conditions was
obtained from the profile of the reactor. The optimal influent COD concentration was
between 11 and 23 g/l at a HRT of 5 days for system stability.
Although anaerobic digestion is used mainly for treatment of liquid food industry
effluents, preliminary laboratory work has been carried out on the anaerobic digestion
of solid and liquid wastes from biscuit and chocolate production. These wastes
were collected from the mixing and packaging units and were rich in easily biodegradable carbohydrates and fats.(Ranade et al 1989) The data collected showed
that the waste is amenable to anaerobic digestion and the biogas produced could be
used in the factory for the production of the biscuits and chocolate.
Cheese Whey Industry
The technology of anaerobic digestion can also be seen in the Cheese whey Industry.
At ''Sardar Patel university ,Gujarat ,India this technology was studied under the
process of an Anaerobic upflow fixed-film bioreactor for the biomethanation of salty
cheese whey. Fixed film reactors employ a fixed medium that provides a surface on
which bacteria attach and grow. The medium immobilizes the bacteria; consequently
these reactors can achieve an SRT in excess of 100 days.(Droste 1997)
The Process;- In order to develop a suitable reactor for the biomethanation of highstrength salty cheese whey, the performance of anaerobic upflow fixed-film reactors
packed with different support materials was studied. The support materials consisted
of charcoal, gravel,brick pieces, pumice stones and PVC pieces. Results of the
studies showed that the charcoal bedded reactor gave the best performance, with
maximum gas production of 3.3l/l digester/day and an enriched methane content of
69% CH4. This maximum gas production was achieved at a HRT of 2 days @ 40
oC.(Madamwar D, Patel P,C, 1999)
Clanton and Goodrich (1985), Backus,Fox and Morris ,in their literature on
''Anaerobic digestion of Cheese Whey" say that improvements in treating cheese
whey would allow cheese manufacturing facilities to produce a product at lower cost.
Pretreating the whey using anaerobic digestion before advanced wastewater treatment
Page 43 of 76
can reduce energy and operating costs. This presents obvious advantages when this
process is used.
Meat & Fish processing Industries
The Meat and Fish processing industries are both important when considering
anaerobic digestion technologies. According to Hansen(1983); Meat packaging plants
may consider anaerobic digestion processes for the production of biogas, especially
methane from their waste materials. ''Slaughterhouse blood, which is a protein-rich
residue of meat production, can be utilized for the recovery of applicable protein
prepares. About 90% of the thick blood protein can be recovered, and the ironcontaining haem fraction can be used as a dietetic ingredient for specialized products
and as a food additive.''( Holley & Kobald 1989 cited in Martin 1991)
Dairy Industry
The new technology of anaerobic digestion is also applicable to the Dairy Industry.
Studies were carried out by Ince_O,(1998) at Istanbul Technical University on ''The
Potential energy production from anaerobic digestion of dairy wastewater.''For a
period of 3 months the performance of a pilot-scale upflow anaerobic filter(UFAF)
,otherwise known as an upflow fixed film bioreactor. treating a dairy wastewater was
studied. Results showed that the UFAF system achieved over 85% COD and 90%
BOD removal efficiencies, at an organic loading rate (OLR) of 6kg COD /m3.day,
with a HRT of 20 hours. Methane produced was in the range of 75-85% with the
corresponding methane yield of 0.32-0.34 m3 CH4/kg COD removed. The system
produced approximately 770 litres CH4 /day.
This would maintain all energy
requirements of the feed and recirculation pumps and mixing.
Chywoweth and Isaacson (1987)in their description of the ''Attached film digester'',of
which one type is the anaerobic filter, suggest that; the anaerobic filter consists of a
filter bed filled with inert support material such as gravel,rocks, charcoal or plastic
media. This type of system results in rapid-rate biomethanogenic conversion of the
soluble wastes. According to Foster & Wase(1987) a high proportion of biomass
(50%) is not strictly attached to the support media but is rather held in the interstitial
Page 44 of 76
spaces of the filters. The hydraulic flow through the filters is low, so this interstitial
biomass is retained. Wastewater's, which contain a high concentration of suspended
solids or a high degradable organic concentration, tend to cause blockage or
channeling within the filters. Foster & Wase(1987) confirm that anaerobic filters
have been used mainly for wastewaters, which contain moderate organic
concentrations (BOD 5000mg/l) and low concentrations of suspended solids.
Provided the filters are not overloaded they produce biogas at the standard rate of
0.5m3 kg COD.
''Effluent composition depends on the design of the dairy and the type of
production,for example; the quantity of milk fo rconsumers, butter and cheese or other
milk-based products. Characteristic for dairy effluents is ht ehigh content of the
carbohydrate lactose (often 500mg/l),which as a result of bacterial decomposition
forms organic acids and low pH values. Generally dairy effluents do not contain
measureable amount of solids and flotable matter.''(Mortensen,1977).
Anaerobic Digestion Technologies in Ireland
Research was carried out by myself on what industries in Ireland are encorporating
anaerobiv digestion technology in the treatment of their wastes. Surprisingly it was
found that not many at all are involved with anaerobic digestion. Economically, it
would not be viable for the industries to have this system as mostcompanies would
not be large enough and production would not be as great as that of neighbouring
countries.
For example ''Heinz, Dundalk, Co.Louth employs a waste managemant system based
on a balancing system. Here, the effluent is held in a holding tank, after which the pH
is adjusted to above 6.7. Following the adjustment of the pH the effluent is sent to
Louth County Council for further treatment.
One Industry which is considering the technology of anaerobic digestion is
''Batcholars, Dublin''. The following information was obtained from Mr Ian Jones
(Laboratory technician).
Page 45 of 76
Batcholars is a food-processing company which employs a waste management system
based on filtration. Waste produced is generally of low solids content and consists of
beans, peas and other vegetable matter, along with various sauces. They also produce
juice and some fruit wastes. The filtration system employs a 2mm mesh to prevent
solid wastes from entering th eeffluent. The prevention of suvh solids from entering
effluents discourages microbial growth. Therefore no finer mesh is employed to filter
microorganisms from waste produced. Waste sources are diluted to produce neutral
pH levels of 5 to 6 which will not encourage microbial growth. The BOD, COD and
suspended solids of the final effluent is also assessed before discharge. General
figures for these parameters are 100mg/l, 300mg/l and 20mg/l respectively. These
parameters are mearured at least six times a year.
Teagasc set specifications for microbial levels within effluent and should tougher
levels be set in the future, Batcholars may well consider incorporating anaerobic
digestion in future waste management. Anaerobic digestion would also allow for an
increase in waste output. However, at present, due to the low solid content of the
wasteeeproduced by Batcholars, anaerobic digestion is seen as not being
economically viable.
‘’Guinness Ireland’’ do not encorporate the Anaerobic digestion system in the
treatment of their waste products.
Guinness have a plant in Spain where the
technology is in place, but Mr Noel Deering (Guinness Ireland) assured me it would
be implemented in Guinness Ireland in the future. The need for such technology at
the moment is not vital.
Conclusion
As seen from the above illustrated examples, many food-processing industries around
the world employ Anaerobic digestion technologies. It is likely that factories will
have to reduce their water consumption in the future. There will be an increased
recycling of process water which will result in lower amounts of water and higher
concentrated effluents. Because of this, an increased demand for anaerobic processes
Page 46 of 76
can be expected.(Gtz 1999) Processes based on conventional methods are still in use
today, with ‘’Fluidized bed reactors’’ being the most recent innovation.
It is evident also, that the Anaerobic technologies of today are slowly making their
way into industry in Ireland,with ‘’Batcholars’’ being the most promising. A high
organic solids content of wastes in Irish industries will provide a definite need for the
incorporation of anaerobic digestion technologies in the near future.
Page 47 of 76
Bibliography
• Allison P, (1999) Sugar refinery effluent treatment Worldwater
And Environmental Engineering. 22,3 p.28
• Anon,(1999) Anaerobic wastewater treatment system.
http://btg.ct.utwente.nl/projects/405/405_en.html
• Banks CJ & Borja R (1996) Evaluation of instability and
performance of an upflow anaerobic sludge blankeet (UASB)
reactor treating high strength ice-cream wastewater.
Biotechnology & applied Biochemistry, 23,1, 55-61
• Buffiere P, Elmaleh S, Garcia- Caleron D, Moletta R. (1998)
Anaerobic Digestion of Wine Distillery Wastewater in down-flow
fluidized bed.
Elsevier Science Ltd. Great Britain.
• Campbell N.(1977), Wastewater Treatment in Brewing and
Distilling. Process Biochemistry.
• Chywoweth D.P.,Isaccson R. (1987) Anaerobic Digestion of
Biomass. Elsevier Science Ltd. New york.
• Clanton CJ, Goodrich P.r.,(1985) Proc. 5th International
Symposium on Agricultural wastes, p.475
• Droste R.l. (1997) Theory & Practice of Water and Wastewater
treatment. John Wiley & Sons.
Page 48 of 76
• Foster F, Wase D.A. john,(1987) Environmental Biotechnology.
Ellis Hornwood limited, Chicester, England.
• Gadre R.V, Godbole S.H., Meher K.K., Ranade D.R., Yeole
T.Y.,(1989) Biological Wastes,28,157.
• Gtz (1999) Promotion of Anaerobic Technology.
http://www.gtz.de/anaerob/Appliccca.htm.
• Hacking A.J. (1988), Food Biotechnology 2 , p. 249 Elsevier
Applied Science, London & New york
• Holley M., Kobald W. (1989) Paper presented at the 5th
International Congress on Engineering and Food, Koln,Germany.
• Ince- O (1998) Potential energy production from anaerobic
digestion of dairy wastewater. Journal of Environmental science
& health, 33,6, 1219-1228.
• Jervis E.Dr, Anaerobic Treatment of Industrial effluent using the
anaerobic Baffled Reactor. Dept. of the Environment Science,
University of Bradfore, U.K.
• Litchfield J.H. (1987) Food Biotechnology,1 ,1 29.
Page 49 of 76
• Madamwar D, Patel P.C. (1999) Anaerobic upflow fixed-film
bioreactor for biomethanation of salty cheese whey. Applied
biochemistry & Biotechnology, 76,3,193-201
• Martin A.M. (1991) Bioconservation of Waste Materials to
Industrial Products, Elsevier Applied Science New York.
• Mortensen B.F. (June 1977), Effluent Comtrol in Food Processing
Industries. Process Biotechnology.
Page 50 of 76
Section 3
Anaerobic Digestion
Of
Pharmaceutical Waste
By
Hilary Mooney
Page 51 of 76
Introduction
For pharmaceutical processing companies increased cost of dealing with wastewater
has led to more precise control and integration processes to reduce wastage.
Concentrated wastes found in pharmaceutical companies are ideal for anaerobic
digestion. Anaerobic digestions efficient waste treatment programme and reduction in
energy consumption, sludge production and volumetric capabilities of the digestor
was found and hence significant reduction in both the total operational costs were
found. Anaerobic pre-treatment is most effectively applied to wastewaters with high
concentrations of readily degradable organic constituents. The general concept is
based on achieving effluent discharge limits of 20 mg/l soluble BOD5 and 30 mg/l
TSS. (Linsey and Franzni 1982).
The anaerobic degradation of chemically hazardous substances in wastewaters is
successful if microbial pre-treatment, as the first stage of the process, has been carried
out. By using semi-aerobic conditions, under established optimal parameters, the
microbial pre-treatment of various wastewater’s from the production of cellulose
containing lignosulhonates, from the pharmaceutical industries, containing antibiotics
and quaternary ammonium salts, as well as from chemical industries, containing
substitute naphthalenesulphonic acids, proved to be extremely successful. (W.W.
Eckenfelder et al.)
By means of the selected mixed culture of facultative anaerobes, depolymerizaton,
detoxification and the biotransformation of the structure of the hazardous substances
in the wastewaters are achieved. These pre-treated wastewater’s are much more
favourable substrates for further anaerobic degradation.
The anaerobic degradation of hazardous substances has the advantage of using
microbial mixed culture, which in many cases is defined. The origin of these mixed
cultures is active sludge, soil, and water sediments. By using the technique, of
enrichment, it is possible to prepare the adapted mixed cultures, which are able to
degrade anaerobically, the soluble fraction of lignin as well as a great number of
organic substances.
Page 52 of 76
The hazardous substances in the wastewaters are accompanied by a great number of
simple organic compounds such as carbohydrates, volatile organic acids, alcohols and
simple aromatic compounds.
The concentration of the simple compounds in
wastewaters may be two to three times higher than that of hard to degrade hazardous
substances, which results in high COD value of wastewaters. ( Margareta Glancer and
S.N. Ban).
For the microbial pre-treatment of wastewaters, the mixed cultures of bacteria are
used in different concentrations. The anaerobic degradation of the pre-treated
wastewaters is carried out at 35 to 37 oC and at pH value 6.8 to 7.2. The heat content
of methane is approx. 36,000 KJ/m3 that results from this process. The changes in the
composition of the wastewater and of the synthetic media, used for monitoring the
degradation as well as the composition of biogas are determined by analytical
methods. (M. Glancier et al).
The fast and complete biodegradation of hazardous compounds in the wastewaters of
pharmaceutical industries was possible to be achieved by a number of processes.
Due to increased water charges smaller volumes of more concentrated waste are ideal
for anaerobic digestion. For some industries with very strong wastewaters, namely
pharmaceutical distilling and fermentation for example, there are very few
alternatives. (W.W. Eckenfelder, J.B. Patoczka and G.W. Pulliam). There is a need
for further developments of efficient waste treatment processes. Anaerobic digestion
fulfils the requirement as well as offering significant advantages in wastewater
treatment through reducing the energy consumption, sludge production and
volumetric capacity of the digesters, hence a significant reduction in both the total and
operational costs. The technological challenge in making anaerobic processes more
competitive with that of aerobic treatment lies with increasing the rate of digestion of
the waste and improving the stability of the reactor. (Michael Henry and Christine
Varaldo).
The high rate digesters used in the pharmaceutical industry are more efficient and
often require fewer columns than singe stage digesters. The content are mechanically
mixed to ensure better contact between the organics and the microorganisms and the
unit is heated to increase the metabolic rate of the microorganisms, thus speeding up
Page 53 of 76
the digestion process. Optimum temperature is around 350C. Factors affecting the rate
of breakdown in anaerobic digesters include the following: organic loading rate, food
to micro-organisms ratio, solids retention time, and mixed liquor volatile suspended
solids to name a few. Figure 1 gives a summary of the design and operating
parameters for treating wastewaters using different anaerobic processes.
The main anaerobic digestion systems used in the pharmaceutical industry which are
mentioned in this report are:
•
Fixed film anaerobic digestion
•
Methane up flow reactor
•
Inter Circulation reactor
•
Biobed
•
Biothane
•
Anaerobic baffle reactor
•
Hybird ABR’s (HABR)
Page 54 of 76
Fixed Film Anaerobic Digestion
SGN a French company has designed a fixed film anaerobic digestion process for
treating pharmaceutical wastewaters. SGN developed the process for treatment of
pharmaceutical plant wastewater’s containing organic compounds that are degradable
by anaerobic digestion. SGN supply full-scale industrial anaerobic digestion units to
process approximately 350m3 per day of pharmaceutical plant wastewaters with a
total pollution loading of 16 tons per day of COD.
Anaerobic fixed film reactors were developed in 1968 and have grown to represent
advanced technology that has been used effectively for treating a variety of industrial
wastes. A number of variations have been developed in the intervening years,
including fully packed up flow anaerobic filters, hybrid up flow anaerobic, down flow
anaerobic filters, fluidised bed reactors and up flow sludge blanket reactors. Over 400
full-scale up flow anaerobic fixed film systems have been constructed in the U.S.,
Canada, Asia Europe and South America. These processes are most suitable for
treating high strength industrial wastewaters having COD concentrations above 3000
mg/l. (Waste Water Handbook).
Fixed film reactors are basically contact processes in which wastes pass over or
through a mass of biological solids contained within the reactor, which is attached to
the surfaces of the media matrix as a thin bio film, entrapped within the media matrix
and held as a granulated or flocculated biomass within the reactor by the action of the
media or a gas solids separation device. Soluble organic compounds passing in close
proximity to this biomass diffuse into the surfaces to the attached or granulated units
where they are converted to intermediates and to end products, specifically methane
and carbon dioxide. Design and operation features of the anaerobic fixes film reactor
are included in this report. Ref. Figure 1.0.
Page 55 of 76
Mineral Content and Toxicity of Waste Waters
Pharmaceutical wastewaters differ from agriculture and industrial substrates by the
often absence of mineral ions.
This is a dual problem in anaerobic digestion.
Methanogenic bacteria have specific nutritional requirements that include certain
mineral elements such as nickel, cobalt and iron. Recent progress in understanding
the biochemical and microbiological aspects of anaerobic digestion allows these needs
to be identified and precisely quantified.
As well as the usual nitrogen and
phosphorus additives the effluent to be treated is supplied with the necessary mineral
nutrients in trace quantities of 10-2 to 10+2 PPM depending on the element.
In the case of some concentrated acid condensates the waste waters must be adjusted
with alkaline agents such as lime or soda to maintain a buffer zone in the bioreactor
around an optimal pH value close to neutral. The continuous recycling that is a
feature of the SGN fixed film processed minimises this need for effluent
alkalinization by the recirculation of biological buffers to the bioreactor.
Toxicity
It is very unusual for pharmaceutical wastewaters to be entirely free of toxic agents
such as disinfectants and solvents.
Methanogenic bacteria adapt, after
acclimatisation, to relatively high concentrations of certain toxic substances with no
significant abatement performance. In the event of temporary exposure to higher
concentrations, the toxicity can be reversed within a period of a few hours to a few
days.
Page 56 of 76
SGN Fixed Film Technology
Efficient anaerobic digestion processes with short hydraulic retention time require that
biomass be held back in the reactor. This result can be obtained by immobilising the
active bacteria on an inert media, for example, leading to the class of systems known
as fixed or stationery bed reactors. SGN had more than ten years experience with
biological fixed film in plastic media aerobic bio filters when it decided to apply the
same basic technological approach to anaerobic digestion.
Acidification and
anaerobic digestion take place at the same time in the digestor. The methanogenic
agents therefore hold the pH close to neutral through immediate consumption of fatty
acids. Effluent flows downwards in the digester.
This has several advantages:
•
The formation of crusts and foaming are avoided by continuous sprinkling of the
surface. Down flow also permits easy uniform feeding of the effluent, leading to a
uniform distribution throughout the entire volume of the digester.
•
Continuous liquid recycling and sequential expansion of the plastic media by
conterflowing biogas permit close control of the biomass trapped in the digester
and hence easy control of biological activity.
•
Treated effluent is removed from the digester by a spillway and is charged
through a hydraulic seal. This maintains a constant level in the digester notably
for gas venting and assures leak tightness.
Page 57 of 76
SFJ Project Data
The anaerobic digestion unit at the SFH chemical plan at Cuisse-Lamotte (Oise) in
France processes pharmaceutical plant wastewaters with:
PH:
1 to 2
Soluble COD:
45 to 48 grams/litres, in the form of short-chain acids
Salinity:
0
Suspended solids:
0
Temperature:
20 degrees Celsius
The unit is sized to handle 16,000 kg of COD per day in 335 to 355 m3 per day of
wastewater’s: i.e. 5,280 tonnes per year of COD.
Its main characteristics are:
Liquid volume of digester:
1,900 m3
Pollution abatement:
90% reduction in soluble COD (i.e. 14.4
tonnes/day of COD)
Biogas production:
5,000 Nm3 of methane per day (i.e. 4.2 TOE/day
of energy)
The digester is made of glass-coated steel to allow processing of wastewater’s with
non-zero salinity.
For optimal operation:
•
Continuous on-line chromatographic analysis of the wastewaters identifies toxic
substances before routing to anaerobic digestion.
•
The pH of recycled effluent is continuously measured.
In the event of
unacceptable variation, effluent feed is temporarily interrupted until the digester
has automatically restored correct pH conditions.
•
The produced biogas is routed directly, without treatment, to a pre-existing boiler.
The treated wastewaters are routed to the pre-existing aerobic treatment facilities.
•
The total capital investment, including installation and start-up costs, is about 10
million French francs.
In view of the project’s innovative features, the
commission of the European Communities has covered part of the costs through a
grant of 3.4 million francs
Page 58 of 76
Biothane process
The following company GB Biothane International have set up two processes, which
are used for wastewater processing in the pharmaceutical industry, known as Biothane
and Biobed. The biothane process is a biological process in which the organic
pollutants present in wastewater are converted into energy rich biogas by bacteria in
an anaerobic environment.
This process takes place in a so-called UASB up flow anaerobic sludge blanket.
Wastewater is fed to the bottom of the reactor by means of a specially designed
distribution system and passes upwards through dense granular sludge bed. The
pollutants are rapidly converted into methane rich biogas and treated effluent flows
over a weir at the top of the settlers for discharge. The settlers are of a special
patented design and ensure a good separation of treated water, biogas and sludge. (Ref
fig. 2.0 and 3.0)
As a result of the special construction of the reactor and the layout of the process it’s
possible to maintain high bacterial sludge concentrations. These high concentrations
allow for high organic reactor loadings up to 15 kg COD/m3 reactor volume per day.
Due to the availability of sufficient granular seeding sludge these loadings can be
achieved within only six to eight weeks following system start up. COD removal
efficiencies of 80-95% are achieved inmost applications. The process has shown itself
to be resistant to upset within a wide range of imposed conditions, and the biomass
has demonstrated extensive storage stability. The following summarises the
advantages:
•
Low investment and operating costs
•
Stable, largely automated process control
•
Compact construction
•
Low energy consumption
•
Low nutrient requirement
•
Biogas production 0.4- 0.5 m3 for each kg of COD input
•
Loading up to 15 kg COD /m3 reactor volume per day
•
COD removal efficiency up to 95%
Page 59 of 76
•
Quick system start up
•
Reactor shut down for prolonged periods is no problem
•
Limited production of stabilised, easily dewaterable excess sludge
•
Absence of odour nuisance: the installation is completely enclosed
•
Absence of noise nuisance
Biobed process
The fundamentals of anaerobic conversion are basically the same for the Biobed and
Biothane processes: microorganisms present in granular sludge convert the organic
pollutants in the wastewater into energy rich biogas.
It has a vertical construction. This relatively high height to diameter ratio of the
reactor vessel results in high up flow velocities of water and gas. During operation a
granular sludge develops with excellent settling properties that can be sustained in the
reactor by virtue of the specially designed and patented three-phase separator, which
is integrated in the top part of the reactor.
Special features of the Biobed system
1. The bacteria immobilised in granules have a high sedimentation rate and are not
flushed out in spite of the high liquid and gas velocities.
2. A high concentration of biomass throughout the reactor volume can develop
owing the excellent performance to the special three-phase separator. The
purification capacity is therefore greater than with conventional systems 15-30 kg
COD /m3 reactor volume per day).
3. Separate per sedimentation tanks are no longer needed. Inert material is flushed
through the reactor.
4. The construction of a Biobed system the cost is lower than those for a comparable
conventional system.
5. The reactors have a relatively small volume due to the brief liquid residence time.
They are completely enclosed and can be made of non - corrosive materials.
6. The vertical construction requires only a fraction of the surface needed by
conventional installations.
7. The enclosed construction prevents air pollution and smell through leakage of
waste gasses.
Page 60 of 76
8. For relative impacts of wastewater strength on the individual cost are as flows.
Capital costs of aerobic treatment, aerobic systems are more sensitive to increased
wastewater strength. This is a result of proportionately increasing aeration volume
required for anaerobic systems the reactor volume for relatively weak waste water
is controlled by hydraulic considerations, particularly for the upflow anaerobic
sludge blanket reactor used in the pharmaceutical waste treatment.
A laboratory study comparing the treatment efficiency on pharmaceutical waste
compared two anaerobic processes namely the Anaerobic baffle reactor ABR and
hybrid ABR’s. Both the HABR and the ABR have been shown to successfully treat a
phenolic wastewater. At loading of 11292 mg COD/l , over 95% was removed by the
HABR down flow compared to the HABR up flow with only 82%. Microscopic
observations revealed differences in the bacterial populations over the length of the
reactor, though no predominant species appear in any individual chamber. The results
suggest an improved stability of the reactor with media in down flow over the other
systems. The greatest proportion of COD was removed in the initial chambers over
the other systems. The greatest proportion of COD was removed in the initial
chambers indicating a financial offset exists between the required number of
chambers in series and the desired treatment efficiency. (G.B. Biothane International).
Page 61 of 76
Page 62 of 76
Page 63 of 76
Bibliography
DR. Jerizis E.
Anaerobic Treatment of Industrial Effluent using the Anaerobic Baffle Reactor
Department of Environmental Science, University of Bradford, West Yorkshire, BD7
1DP,
W.W.Echenfelder, J.B Patoczka and W.G.Pulliman
Anerobic V’s anaerobic treatment in the U.S.A.
Anaerobic Digestion 1988.
Page 105-111
Biothane International Handbook
Process of Anaerobic Digestion 1999
G.B. Biothane International,
Wateringseweg 1, PO Box 1,
2600 MA Delft, The Netherlnads.
Bruce A.M., Kouzeli – Katsiri, and A Newman
Anaerobic Digestion of Organic Agricultural Wastes 1986
Elseverier Applied Science Publishers LTD.
M Glanceier and SN Ban:
Anaerobic Treatment of Chemically Hazardous Wastewaters 1988
Anaerobic Digestion 1988
Page 465-471
MJ Parker
Waste Water Collection System Maintenance 1996
Published: Lancaster Base Technomic
Wheatly A
Anaerobic Digestion A Waste Treatment Technology
Critical Reports in Applied Chemistry,
Volume 31,
Elsevier Applied Science. London.
Page 64 of 76
Section 4
Industrial Applications of Anaerobic
Digestion in the Treatment of Chemical
Waste
By
Fiona Hackett
Page 65 of 76
Introduction
For many years, anaerobic digestion was mainly used for the purpose of treating
municipal wastes, as well agricultural and agroindustrial wastes. However, much
research has been carried out in recent years to investigate the possibility of applying
anaerobic processes to the treatment of chemically hazardous wastes, and, at present,
many chemical companies continue to use such systems successfully.
Because of the vast differences between the various chemical processing industries
worldwide, much research has been necessary to investigate the suitability of an
individual system for the treatment of a particular waste in terms of:
• Process requirements
• Capital cost
• Maintenance cost
• Quantities of sludge production
• Types of microorganisms necessary for the biodegradation of the chemicals in
question
• Possible toxic elements within a system
• Energy yield (methane)
• Additional pre and post-treatments
Anaerobic Digestion of Industrial Sludges
A wide analysis of possible applications of anaerobic processes to the treatment of
organic chemical industrial wastes has been done by Campagna et al. (1982) and
Pieroni and Campagna (1986). An evaluation of anaerobic digestibility of primary
and secondary sludges from various industrial processes has been carried out in a
mesophilic environment, to verify possible inhibition effects. The sludges used for
the investigation were derived from the petrochemical industries and from the
production of acrylic resins and pharmaceuticals.
Strong inhibition effects were observed in all of the primary sludge samples used in
the experiment.
In particular, it was noted that absolutely no reduction in the
Chemical Oxygen Demand (COD) of the sludge from the petrochemical industry was
obtained, even after the sludge had been washed.
Page 66 of 76
Strong inhibition effects were still noticed in the secondary sludge from the
petrochemical factories, but this time, washing of the sludge to reduce the salt
concentration aided the process greatly. Further studies found that inhibition could be
completely avoided by mixing the petrochemically derived sludge with a municipal
sludge in a 1:1 ratio.
In the case of the secondary sludge from the production of acrylic resin, fermentation
was carried out in a continuous mixed digester, with COD reduction and methane
yields similar to those observed for secondary domestic sludges.
One very important result of the above investigations was the observation that codigestion of secondary sludges from industrial and domestic treatment plants resulted
in a methane production higher than expected, most likely due to the more balanced
fermentation environment. (Sanna et al. 1986)
Anaerobic Digestion of Liquid Industrial Wastes
For the purpose of studying the effects of anaerobic digestion on the treatment of
industrial wastewaters of a chemical nature, Pieroni and Campagna (1986) chose 13
samples on which to experiment. The samples ranged in origin, from petrochemical
factories to industrial units producing dyes and pigments, pharmaceuticals,
polyolefins and polyacrylates, all of which had COD values ranging from 0.290kg/m3. It was found to be necessary to dilute the original wastewaters, where COD
was quite high, as this factor seemed to be having an inhibitory effect on the
anaerobic process.
Five of the wastewaters were chosen to be tested in continuous anaerobic reactors and
those five samples were as follows; one from the petrochemical industry, two samples
from different polyolefins productions, one from pharmaceutical intermediates and
one from acrylic resins. The anaerobic treatment systems used in the experiments
were up-flow anaerobic filters and ABR reactors. With hydraulic retention times
ranging from 12.5 to 25 hours and volumetric loads from 0.5-1.9 kg COD/m3 reactor
per day, COD reductions from 40% to 80% were observed, along with methane yields
Page 67 of 76
of 0.12 to 0.25 Nm3/kg COD. These figures relate to a methane concentration in the
resulting biogas of 55-80%.
For the chemical wastewaters in question, anaerobic digestion appeared to be an
economically viable pretreatment process to be utilised before the final aerobic
purification stage. (Sanna et al. 1986)
Anaerobic Digestion of Mono- & Di-Phenols and Aromatic Organic
Compounds
Much study into the treatment of the above mentioned chemicals has been carried out
by G. Vallini and co-workers in association with the Department of Environmental
Science, University of Venice, Italy. In order to design efficient anaerobic systems,
with the ability to successfully treat industrial wastewaters of a chemical nature, a lot
of research was carried out on the anaerobic degradation of lignin and relevant
aromatic compounds.
The first step was the identification of the anaerobic bacteria, which would be
responsible for degradation of the organic compounds. A consortium of anaerobes
was found which would utilise activated carbon as the carrier, in a phenol-fed,
expanded-bed anaerobic reactor.
This consortium was characterised as being
composed of three microorganisms: Methanobacterium formicium, Methanotrix
soehngenii and a coccobacillus. This combination of anaerobic microbes was able to
satisfactorily degrade the compounds in question, producing 3.5 parts methane for
every 1 part phenol (by volume). Other advantages to this consortium of bacteria was
the ability if the microbes to gradually adapt to high concentrations of phenol
(1,400mg/l), as well as the fact they were able to degrade p-cresol and 2,4-dimethylphenol.
Pulp Bleaching Wastewater
The treatment of wastewater from pulp mills has become one of the most researched
areas in terms of anaerobic digestion.
Such investigations have been deemed
necessary due to the fact that the process of pulp production is characterised by a very
high consumption of water, with severe contamination of the water at various stages.
The process gives rise to a number of persistent and extremely toxic chlorinated
Page 68 of 76
organic compounds such as chlorolignins and chlorophenols, in addition to low
molecular chloroaliphatics such as chloroform, chloroethanes, tetrachloroethylene and
chloroacetones.
One of the main characteristics of the microorganisms which degrade persistent
compounds is the fact that, like those used for certain “special” biochemical reactions
such as nitrification and biomethanisation, they not only have relatively low growth
rates but lack a strong tendency to form easily sedimented flocs. It is due to this fact
that it is extremely difficult to retain the microbes in the reactor so that the
concentration of the biomass is kept at an optimum level. To overcome this problem,
the most effective method is immobilisation of the biomass on carrier materials. The
higher biomass concentration obtainable in the reactor when a carrier material is used
allows considerably greater space-time yields than those achieved in comparable
systems equipped with gravity sedimentation units. Additionally, the use of carrier
materials with buffering properties can considerably increase the stability of the
process. Primary anaerobic trials showed that the presence of carrier materials,
especially if they are cationically modified and have adsorptive effects, intensifies the
anaerobic degradation of the constituents of bleaching plant wastewater.
Continuous optimisation trials were also carried out in several parallel-operated lab
scale plants.
A wastewater stream using organochlorine compounds was used and
had the following properties;
Wastewater stream:
beech pulp, alkali extract
COD:
6,000-6,500 mg/l
Organochlorine concentration:
52-60 mg/l
Duration of trials:
50 days
Reactor volume:
1.6 L
Biomass solids content (pellets):
15.6g/l
COD space loading:
1.7 to 3.2g COD/l.d
Reaction Temperature:
35oC
Carrier Filling:
400ml carrier suspension (25% volume) per
reactor
Carrier:
- Lignite coke
- PUR foam pads
- Modified PUR granules
Page 69 of 76
The ionically modified and surface modified PUR carriers were colonised very
rapidly by the microorganisms that cause biodegradation, with their performance
exceeding that of the other systems, and, remained constant under constant conditions.
Another important discovery which was made in the course of these trials was the fact
that two-stage treatment is far more effective than single-stage treatment. This is due
to increased stability of a two-stage process which is necessary for the anaerobic
degradation of such organic compounds.
After approximately five months of these trials, it was possible to apply the successful
installations to the actual wastewater treatment facility at the pulp mill. It was
confirmed again that it is preferable, both as regards the space-time yield and as
regards the process stability, to operate with immobilised anaerobic organisms,
especially in the case of the two-stage system. It was not possible to separate the
biomass from the contact sludge reactor by sedimentation at reasonable cost. The
solution to this problem was to retain the biomass in the reactor by charging it. This
was done with the modified PUR carrier, which in effect increased the biomass
concentration as well as the space-time yield. The aerobic after-treatment was carried
out in a large laboratory installation with an aerated reactor and a final clarifier with
vertical flow.
Further Conclusions
As the constituents of the wastewater were degraded quite slowly, the methane stage
of the process was subjected to only a low load. Therefore, an expanded bed reactor,
but not a fluidised bed reactor was included among the types of reactor considered for
selection. Also, in order to reduce the energy requirements of the system, a lighter
carrier material was chosen, ie approximately 85g solids per litre of suspension.
Nitrogenous Aliphatic & Aromatic Compounds
Aliphatic and aromatic organic nitrogenous compounds make up an important fraction
of waste material of both natural and industrial origin. Because of the nature of these
compounds, they are highly toxic and create excessive odour problems when
degraded aerobically and are therefore considered a factor in air pollution. It is for
this reason that the anaerobic treatment of these substances is desirable. Extensive
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studies were carried out on such wastes to investigate the effects of anaerobic
digestion.
Putrescine was fermented by pure cultures of highly specialised strict anaerobes to
acetate, butyrate, hydrogen and ammonia. Anthranilic acid was degraded in stable
methanogenic enrichment cultures, via benzoate as an intermediate, to methane,
carbon dioxide and ammonia. Pyridine was completely degraded in methanogenic
enrichment cultures also.
The anaerobic degradation of the above compounds has many advantages over
aerobic treatment such as:
• The substrates may react with molecular oxygen and form radical-catalysed
polymers
• The substrate lowers the surface tension of aqueous solutions and therefore causes
enormous foam formation in aerated basins
• The substrate is degraded faster or more efficiently in the absence of oxygen
• The substrates are volatile and toxic (ill-smelling) and create air pollution
problems
Due to the problem of the slow growth rates of the microorganisms required for the
biodegradation of these compounds, it is necessary to use a fixed or fluidised bed
reactor. The use of such a system will ensure the retention of the microbial biomass
in the reactor. (Shink et al. 1988)
Anaerobic Pretreatment
As stated previously, many high strength industrial wastewaters have significant
concentrations of recalcitrant organic compounds combined with high concentrations
of highly degradable organic matter. One such wastewater which fits this description,
is that which results from the esterification process used in the manufacture of
polyester fibres. Such a wastewater would have a typical COD concentration of
approximately 29,000 to 45,000 mg/l, with the bulk of the COD loading being
contributed by acetaldehyde and ethylene glycol, both of which are amenable to
anaerobic biological treatment. In addition to those compounds mentioned above, this
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type of wastewater is also likely to contain biorecalcitrant cyclic ethers such as 1,4dioxane and 2-methyl-1,3-dioxolane. It is possible to remove these compounds from
the wastewater using Advanced Oxidation Processes (AOP’s) or aerobic
biodegradation using a specialised bacterial culture. In the event of using either of
these processes, it is necessary to have subjected the wastewater to an anaerobic
pretreatment system. (Purdue et al. 1994)
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Bibliography
HALL, E.R, P.N HOBSON, (1988) Anaerobic Digestion 1988, Pergamon Press,
Oxford
G. BITTON, (1994), WASTEWATER MICROBIOLOGY, Wiley-Lis & Sons, New
York
Puridine University, (1995), Proceedings of 49th Industrial Waste Conference 1994,
Lewis Publications
N.F GRAY, (1989), Biology of Wastewater Treatment, Oxford University Press,
Oxford
SHINK, B. (1986) Environmental Aspects of the degradation potential of anaerobes,
in: Biology of Anaerobic Bacteria, H.C DUBOURGIER, Elsevier, Amsterdam
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Section 5
Biogas:
Sources & Uses
By
Fiona Hackett
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Introduction
Methane production is a common phenomenon in several diverse natural
environments ranging from glacier ice to sediments, marshes, termites, rumen and
oil fields.
Anaerobic digestion also produces this useful gas which contains
approximately 90% energy and has a calorific value of roughly 9,000 kcal/m3.
Methane can be burned on site to provide heat for digesters or to generate electricity.
The production of methane in the anaerobic process also leads to a reduction in the
BOD of the waste being treated. (Bitton et al. 1994)
Uses for Methane
There are three main options for the use of biogas and they are as follows:
• Burning to produce heat
• Generation of electricity
• To fuel vehicles
Each of the options requires intermediate processing after the gas has been produced.
Such processing can range from simple storage to cleaning and compression (in the
production of fuel gas – LPG). (Gray et al. 1989)
Of all the methane produced by anaerobic digestion processes, two thirds is derived
from acetotrophic methanogens, with the remaining third being the result of carbon
dioxide reduction by hydrogenophilic methanogens. (Bitton et al. 1994)
Methane Fuelled Vehicles
One of the most exciting and cost effective uses of biogas to date has been its use in
the fuelling of vehicles. In a pilot study at the Modesto wastewater treatment plant in
California, compressed biogas has been used to fuel both cars and lorries. A similar
study has also been carried out in the UK in the Avonmouth Sewage Treatment Plant
of Wessex Water Authority. At this plant, biogas had, for several years, been used
for the production of electricity and it was then used in a series of trials, to run eight
vehicles. (Gray et al. 1989)
Before using biogas to fuel a high-efficiency internal combustion engine, the carbon
dioxide content of the gas must be removed, followed by the compression of the
purified gas. For the purpose of the above trials, the raw gas, which is a mixture of
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methane and carbon dioxide is scrubbed with the clarified effluent, which acts to
remove the carbon dioxide. The purified gas is then compressed and stored in
cylinders. Each of the cylinders can hold 240 litres of compresses gas which has a
methane concentration of 99%. The cylinders are connected to vehicle filling bays
with standard LPG valves and snap-on hose connectors. Approximately 0.7m3 of
gas is equivalent to 1 litre of petrol and the Avonmouth plant can produce 23 litres of
gas per hour, which meets 95% of their fuel requirements.
One of the main
disadvantages to the system in the beginning was the fact that the plant was unable
to produce sufficient quantities of gas during the Winter months, with a wasted
surplus in the Summer. Modifications to the system have however ruled out this
problem. (Gray et al. 1989)
The performance of the gas is claimed to be as good as petrol, with claims of a
cleaner engine which requires less maintenance, prolonged life of the engine oil, oil
filter and the spark plugs as well as a cleaner exhaust. The cost of the project at
Avonmouth was £30,000, ie £25,000 for the plant and £5,000 for the engine
modifications. The Anglian Water Authority has also been working on a similar
project, with the same cost implications as the Avonmouth plant. (Gray et al. 1989)
Biogas On The Farm
In January of 1999, plans were made for the first centralised biogas plant to be used
in the treatment of farm slurry. The proposed plant in Holsworthy would treat a
possible 350 tonnes of slurry per day from up to 50 local farms, with the biggest
source of slurry being that from some 3,500 dairy cows. British Biogen, a trade
association for bioenergy businesses, believes that there is room for 100 centralised
biogas plants such as that mentioned above in the UK, generating a total of 75MW
of electricity by 2010. (ENDS, Jan 1999)
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Bibliography
G. BITTON, (1994), WASTEWATER MICROBIOLOGY, Wiley-Lis & Sons, New
York
N.F GRAY, (1989), Biology of Wastewater Treatment, Oxford University Press,
Oxford
Anon, Signs of life from farm biogas projects, ENDS Report 288, January 1999