Journal of
Waste Management and Environmental
Issues
Original Research ARTICLE
PUBLISHED: 14 April 2017
Feasibility study of power generation from waste water
treatment with patchy data
John P.T. Mo1* and Emosi Koroitamana2
RMIT University, Australia.
Fiji National University.
1
2
Reviewed by:
Dr. Elsaied Mohamed
Dr. Kriveshini Pillay
*Correspondence:
John P.T. Mo,
[email protected]
Abstract
It is well known that the treatment of waste water produces biogas as a by-product
that is harmful to the environment. The Kinoya waste water treatment plan has been flaring
the biogas to eliminate the risk to the society. A feasibility study was carried out to study
the possibility of converting the biogas energy to useful power using technologies such as
a gas turbine. However, due to historical reasons, it is often impossible to obtain necessary
data of the waste water treatment plant to support system design. This paper focuses on
estimating the available energy capacity and associated means of disseminating the energy
either through the plant’s own power requirements or to be sold to the power grid.
Keywords: Biogas, powergeneration, waste watertreatment, fossil fuels
Introduction
in handling biogas produced from waste water.
Modern societies generate large volumes of waste water
from both domestic and commercial sources. Waste water
should be treated before discharge to the environment to
protect public health and the environment. For a rural
community, due to isolation and lack of infrastructure support, water treatment was legislated to be provided on the
individual facility (Van Hulle et al, 2012). In this situation,
the effluent quality and treatment performance should be
assessed and regular maintenance of the system should
be arranged. For community-based waste water treatment
systems, more sophisticated analysis tools including linear
programming should be used to optimise system cost and
rate of discharge (Ito et al, 1979).
Many waste water treatment plants are flaring the biogas
to eliminate the risk of gas pollution to the society. However,
flaring was found to be one of the sources of increasing
pollutants in the atmosphere because of production of carbon dioxide and other heat absorbing gasses (Diaz, 2004).
Routine flaring with incomplete combustion generates
pollutants which is a threat to human and animal health.
Similar study by Edino et al (2010) also showed that political tension and economic adversity were prevalent in areas
where flaring took place regularly. Soltanieha et al (2016)
suggested that regulatory requirements should be in place
for managing the impacts of gas flaring to the environment.
Unfortunately, the impacts of flaring were difficult to study
due to changing environmental conditions such as wind
It is well known that the treatment of waste water pro- direction and site locations (Anejionu et al, 2015).
duces biogas as a by-product. The gas is rich in methane
due to anaerobic digestion of organic matters (Stagner,
On the other hand, biogas is regarded as a form of renew2016). This type of emission is harmful to the environment. able energy that does not rely on fossil fuels. However, the
Research in pollution management suggested that most process of converting biogas energy to useful power is not
emissions were related to human consumption (Stigliani, straightforward. Due to the massive investment requirements,
1990). Syri et al (2002) presented a model to study the im- it is necessary to carry out a comprehensive design evaluapacts of climate change mitigation strategies on air pollution. tion study to identify all issues prior to actually putting the
Amon et al (2001) analysed a specific source of pollution idea to practice. A formal design framework is required to
from farmland and found that greenhouse gas emissions guide the design process. Small scale decentralized waste
were very high and ecologically harmful. However, infor- water treatment are good for small and rural communities.
mation for identifying and quantifying the pollution due An integrated design methodology based on mass balance
to emissions was largely lacking. Plans to manage pollution was used (Thomas, 2009). The methodology was embedded
due to waste water have been largely ad hoc, particularly in a design tool that allowed analysis of different compowww.clytoaccess.com
1
John P.T. Mo et al., (2017)
nents with site conditions to produce a design satisfying
the treatment goals. For large scale development, Liu et al
(2010) presented an overview of an engineering framework
for planning and design of energy systems. This framework
was generic and would depend on the availability of data
to support design evaluation. Svanes et al (2010) used a
holistic design methodology that incorporated a number of
indicators and integrated several evaluation methods. The
methodology was regarded as resource and data intensive.
cally, biogas has a calorific value between 21-23 MJ/m3
as compared to natural gas which has a calorific value of
39MJ/m3 (Kazagic et al, 2016).
The quality of the sludge going into the anaerobic digester determines the quality of gas produced. Activated
sludge or return sludge is most often used in large wastewater treatment plants in situations where the effluent
should be of high quality and there is limited space (Santos
et al, 2016). There are five major groups of microorganIn practice, data scarcity has been a major scientific isms generally found in the aeration basin of the activated
challenge for accuracy and precision assessment of envi- sludge process (Li et al, 2014). The anaerobic digestion by
ronmental systems, especially in climate-stressed devel- bacteria and algae produces methane and other gasses
oping countries (Bhowmik and Costa, 2015). It is often which can be collected for generating heat.
impossible to obtain necessary data of the waste water
treatment plant to support system design. To overcome
The typical activated sludge process incorporates mixing
data scarcity issue, Beykikhoshk et al (2015) proposed an screened water with the recycled mixture from the secapproach for targeted knowledge exploration on Twitter. ondary clarifier tank, due to its high content of organisms.
Shabbir et al (2015) relied on statistical extrapolation to The mixture produced is called mixed liquor. The return
estimate the total power demand at peak hour. Gampe et sludge is taken from the clarifier and returned back into
al (2013) used remote sensing technique to complement the aeration tank where the mixture is injected with air
data scarce areas. These methods require extensive infra- provide oxygen and causing the solids to remain suspended.
structure support. This paper uses a systems engineering
approach to develop the system design model that can Quality of biogas
manage data scarce components and focus on estimating the available energy capacity and associated means
Biogas produced in an ideal anaerobic digester could
of disseminating the energy either through the plant’s contain 80% methane, which is the result of utilising
own power requirements or to be sold to the power grid. methanogenic bacteria (Diak et al, 2013). However, the
quality of the biogas is highly dependent on the quality
Literature Review
of the residual water and biological digesters (bacteria).
At the end of digestion, both biogas and a digested moist
Sludge is a kind of biomass material that is deposited solids are formed which is usually dewatered to produce
from the treatment of waste water. From the sludge, biogas compost.
is generated from breaking down sludge organic matter
using anaerobic digestion process. The biogas is rich in
In a typical anaerobic digestion facility, the gas composimethane and has the potential for power generation. Unlike tion is about 15% of the total output stream and the liquid
solar photovoltaic cells or wind power generators, biomass and solid components of the residual compost share the
power generation technology is significantly less dependent remaining 85% in an equal manner (Ostrem et al, 2004).
on the weather conditions (Zhang et al, 2007). However, The gas itself, however mainly consists of methane and CO2.
due to its toxicity and corrosive content, the biogas can- The lower amount of methane in the biogas compared to
not be used immediately in power engines. This literature the natural gas (90-95% and 55-65% respectively) classireview, therefore, tries to understand the complete path of fies biogas as the “low-grade natural gas” (Grande, 2011).
waste water treatment to power generation and possible
options. Based on the categories knowledge, a theoretical
The anaerobic sludge digestion process produces methframework can be developed to guide the design process. ane gas that has heat value and capacity with the potential
to generate power, at an unknown capacity. The overall
Production of biogas
recovery of energy is a combination of factors such as the
effectiveness of the digestion process, the efficiency of enIn activated sludge systems, anaerobic digestion of the ergy recovery and the type of treatment process. Table 1
sludge produces biogas. The calorific value is defined by shows the range of gas quality measured by a research
the amount of energy produced by complete combustion team in Switzerland studying two sewage digestion plants
of a standard amount of material. The calorific value of over a few months (Van Herle et al, 2004). The amount of
biogas from waste water depends on the inflow waste gas produced for waste water treatment generally ranges
water and effectiveness of the fermentation process. Typi- from 0.8-1.1 m3/kg of volatile solids destroyed.
Journal of Waste Management and Environmental Issues
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John P.T. Mo et al., (2017)
A study in Bangladesh shows the average biogas to electricity conversion rate (Ali and Islam, 2014). Each cubic
meter (m3) of biogas with average sewage gas composition
as shown in Table 1 contains the equivalence of 6 kWHr
of calorific energy. When biogas is converted to electricity,
in a biogas powered electric generator, about 2 kWHr of
useable electricity will be generated, and the rest of the
original energy content will be converted into heat which
is either disposed of through the cooling system or can also
be used for other lower temperature heating applications.
Table 1. Sewage gas composition.
Max
Methane %
65.22
Carbon dioxide %
37.43
Nitrogen+Oxygen %
1.3
Water (steam) %
3
Hydrogen Sulphide (ppm) 3.77
Min
60.1
32.1
0.1
0
0
Average
62.835
35.11
0.59
1.685
1.47
in power engines.
Water Scrubbing Technology
Compressed raw biogas is fed into a scrubbing tower
where the physical absorption of CO2 and H2S in water occurs at high pressure. The dissolved H2S and CO2 is then collected at the bottom of the tower (Horikawa et al, 2004). This
is considered the easiest and cheapest purification method.
Chemical Absorption process
High concentrations of hydrogen sulphide H2S is corrosive to reactors and engines when producing electricity
from biogas. Complete elimination of H2S is necessary for
robustness of biogas fuelled engines. Reduction of H2S in
biogas can be done by methods such as oxidation, external
chemical treatment.
Chemical Absorption is usually an extension of the water
scrubbing process. During chemical absorption, around
Parameters in an anaerobic digester 97-99% of the H2S content is removed and leaves most of
the CO2 for the water scrubbing to process. The H2Sremoved
Some of the parameters affecting the anaerobic digester by sodium hydroxide (NaOH) washing physical solvents
include pH, temperature, carbon/nitrogen (C/N) ratio, re- consume less energy so hence are more energy efficient,
tention time. These factors need to be kept within a desirable making it the most commonly used chemical. Different
range. A lower C/N ratio cause’s ammonia build-up and ph types of physical absorbents are methanol, selexol, rectisol,
value exceeding 8.5 which is toxic for the methanogenic genosorb, morphysorb (Petersson and Willinger, 2009). If
bacteria. According to Balat and Balat (2009), pH level a small amount of oxygen is present then one column can
within digestion system is from 5.5 to 8.5, but methano- operate but loading is limited. In some cases, a two column
gens function only in pH range 6.7 and 7.4. A decline in system is implemented to combat this limitation generally
pH would suggest an acid build up and digester instability.
To maintain optimal C/N ratio the intake must remain in Pressure-Swing Absorption process
desirable range. In an anaerobic digester, microorganisms
utilize carbon 25-35 times faster than nitrogen; a high
Pressure swing absorption process uses the technolC/N ratio indicates a rapid consumption of nitrogen by ogy of separating certain gasses from other gasses under
methanogenic bacteria, resulting in a lower gas production. pressure. Small scale plants with a flow rate of 10m3/hr of
biogas are currently operational, this technology is still
Retention time is the amount of time the sludge remains feasible to be applied to flow rates of 10000m3/hr for biogas.
inside the anaerobic digester. Minimum solids retention Initial the raw biogas is compressed to 4-10 bar which is
times are in the range of 2-6 days depending on the tem- then fed to a vassal containing an adsorbent material to
perature for anaerobic digestion system usually, Retention retain the CO2. Adsorbent materials are commonly are
time ranges from 30-60 days (Ezekoye et al, 2011). The Zeolites, activated carbon and molecular sieves are used
retention time is determined by the average time it takes as a trap, in absorbing at high temperature. The enriched
to break down organic material, this is measured by the CH₄ is collected at the top of the vessel while the adsorbent
amount of Chemical Oxygen Demand (COD) and Bio- becomes saturated with CO2. The H2S adsorbed into the
logical Oxygen Demand (BOD) of the existing effluent. material is usually irreversible so a system should be placed
before the PSA to remove H2S (Clair and Fordham, 2009).
Purification Processes
It is clear from the foregoing review that biogas from
waste water has a lot of unwanted substances that reduce
the system efficiency and cause corrosion. A purification
process is required to pre-condition the biogas if it is used
Journal of Waste Management and Environmental Issues
Membrane Purification process
Membrane purification process for gas cleaning utilizes
a membrane-porous material that let some gasses permeate
through its structure, while other are retained. The most
3
John P.T. Mo et al., (2017)
common materials are a hollow fiber made from different
polymers. Gasses impurities with small molecular size, low
affinity, and a low permeability can be retained. Impurities
such as CO₂, O₂, and H₂O pass through the membrane as
permeate, while low permeable CH₄ is retained and collected at the end of the hollow column. The gas is forced
at pressure through the membrane structure. Impurities
are being drawn out by the membrane leaving only the
desired purified biogas (Li et al, 2015).
Siloxane removal
Siloxane is organo-silicons added in personal care
products. Over 1 million tonnes of siloxane is produced
annually by the personal care industry worldwide. This
substance is not eliminated in the water treatment process
or digesting process and remains in biogas. Siloxanes in
biogas can greatly reduce the efficiency of energy recovery
from biogas (Scott, 2016). During combustion, siloxanes
are converted into silicon dioxide deposits, leading to abrasion of engine parts or the build-up of layers that inhibit
essential heat conduction or lubrication. The technology
from the removal of siloxane includes absorption (fixed
bed and fluidised bed), gas and deep chilling, biological
removal, catalytic process, membrane technology and inengine removal approach (Abatzoglou and Boivin, 2009).
Biogas flaring characteristics
Biogas flaring is used to safely burn excess biogas being produced. When designing an appropriate flare the
aspects of air requirement, exhaust gas flow rate, stack exit
velocity, residence time and energy balance are considered
(Caine, 2000). The temperature of the flare is determined
by the amount of air added to the biogas. The relationship
of excess air added and the flame temperature is dependent on the heat released from combusting the methane
(Stone et al, 1992).
Methane auto ignites at temperature 537.2oC. The
amount of oxygen needed to oxidise methane has been
defined as lower explosive level (LEL) and upper explosive
limit (UEL). These limits are known as flammability limits.
The LEL for methane is 5.0% and the UEL is 15% (Baukal,
2012). This means a volume of air with a concentration
between the specified limits will be flammable.
Methods of power generation from
biogas
a lot of heat. If a power generation system is available, the
biogas can be captured and converted to a useful form of
energy. Biogas can potentially be used in many types of
equipment as methods of electricity and/or heat production.
Production of heat and stream
A basic application for biogas is thermal energy (heat).
Small biogas systems can provide enough energy for cooking and heating water. Conventional gas burners with a
simple adjustment of the air-to-gas ratio can adjust to
use biogas. The quality of biogas needed for a gas burner
is low, and only requiring a pressure of 8-25mbar and
maintaining H2S levels beneath 100ppm.
Electricity Generation or Combined
Heat and Power (CHP)
Combined heat and power systems produce primarily electricity and use the inevitable waste heat for other
purposes. The systems of producing both electricity and
heat together have an overall combined efficiency greater
than just producing either power or heat (Coelho et al,
2006). Internal combustion engines are most commonly
used for CHP production. Gas turbines can also produce
both CHP with a comparable efficiency of a spark ignition
engine and also have low maintained.
Internal combustion engines
Internal combustion engines require a very clean fuel,
otherwise, engine wear and low power output will occur.
Methane is the most valuable component for using biogas
as a fuel. Other components that don’t contribute to the
calorific heating value of biogas are usually removed during the purification system (Jenkins, 1998).
Internal combustion engines use a higher compression
of fuel to ignite the fuel rather than using spark plugs, i.e.
using Otto cycle. Otto engines can be operated on biogas.
They can also work with gasses with a low mass flow rate
(Porpatham et al, 2012). Usually, a small amount of petrol
(gasoline) is used to start the engine. An Otto cycle engine
producing 18kW of power demands around 5.6m3 of biogas
an hour. Biogas-powered engines based on Otto principles
require biogas higher than 45% methane content. The
electric efficiency varies from 34% to 45%. The duel fuel
engine (diesel and biogas) works with a lower electrical
efficiency of between 30% and 45%.
Biogas is about 20% lighter than air and has an ignition Gas turbine
temperature range of 650 to 750 degrees. It is also a colourless and odourless gas that produces a clear blue flame when
The US tends to use biogas in Gas turbines, more often
burned similar to natural gas. Flaring of biogas produces than other countries. Biogas gas turbines are rarely used
Journal of Waste Management and Environmental Issues
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John P.T. Mo et al., (2017)
in developing countries or for small-scale applications due
to its high price. Small biogas power turbines produce a
power output of approximately 30-75kW and are readily
available to be purchase. The maintenance performed on
gas turbines also requires specialised skills. The quality of
gas entering the system must be purified as a harmful gas
or a high hydrogen sulphide level will shorten the lifespan
of the engine and cause significant damage.
Stirling engine
is high, it is important to follow a well thought through
methodology. The aim is to pursue a proven pathway that
helps to systematically research, collect, generate, analyse and apply the data that can be made available in the
course of the system’s design. Figure 1 shows the research
methodology structure.
In Figure 1, it is assumed that there are a few possible
routes to success, i.e. provide a blueprint for a biogas power
generation plant. Irrespective of the routes to system design,
the system design methodology provides a guide to the
essential elements and processes that will give the highest
probability of successful development of the waste water
to power generation system.
Atkins et al (1990) developed a Stirling engine which
converted waste wood to power. The engine was designed
with air-charged and self-pressurizing to make it efficient for
its size and cost. In a Stirling engine biogas is combusted externally, thus heating the Stirling motor through the heat exThe system design methodology can be roughly segchanger. Stirling engine has a heavy tolerance of fuel compo- mented in three sections. The modelling section is located
sition and quality. They are expensive and characterised by at the top part of Figure 1 where the physical components
low efficiency, consequently limiting the application of use. are modelled mathematically to represent the behavior
of the components during work situations. Once data
Direct combustion steam turbine
is obtained, the analysis phase consists of several major
engineering determinations, including selection, verifiSteam turbine technology system consists of a boiler cation, and numerical computation. Verification of the
or heat exchanger which is used to generate steam at a sub-system components is also essential. Ultimately, the
temperature above boiling point and at a given pressure. project outcome is a set of drawings and design that can
The steam produced passes through a multistage steam be given to contractors to work on.
turbine which in turn drives an electrical generator. In
the case of biomass utilization biomass is burned using Case Study–Kinoya Waste Water
direct combustion system in a combustor or furnace to Treatment Plant
generate a hot gas, which is fed into a boiler to generate
steam, which is further expanded through a steam turbine
Kinoya Waste Water Treatment Plant (WWTP) treats
or steam engine to produce mechanical or electrical energy. incoming sewage using conventional aerobic process
Steam turbines follow a thermodynamic cycle known as including trickling filters, clarifiers and sequential batch
Rankine cycle. This cycle converts heat to work externally reaction process that is meant for producing treated water.
via a closed loop, generally using water a working fuel.
These processes allow separation of suspended solids from
the water in the form of sludge that is fed to anaerobic
Biogas fuel cells
digesters for the sludge stabilization. There is a provision
of belt filter press on site for dewatering of sludge to proFuel cell technology offers the potential to converted duce sludge that contains low moisture content. The low
Biogas directly into electricity, some facilities demonstrat- moisture content sludge can be sold to other applications.
ing this are now operational in Europe and North America.
Fuel cells are expensive and the process requires a very
Based on the research methodology in Figure 1, this
clean (Nehrir, 2009). A fuel cell located in Renton, WA case study focused on the available data on the plant inflow,
consumes 4360 m3 of biogas and produces approximately process, and the gas flaring capacity. The data and analysis
1MW per day. It is being used to power the plant but is outcome of this investigation is intended to support a
equivalent to powering 1,000 households. Biogas to fuel prospective proposal of a system incorporating a proposed
cell boosts net output of electricity by a minimum 60%, gas turbine or applicable power generating equipment.
compared to reciprocating engines at 30% and turbine This proposal can be used as a base to develop a profile
engines at 40%. Fuel cells also have lower carbon emission of present and future R&D projects of sewage treatment
per unit of electricity.
power generation on the Kinoya WWTP.
System Design Methodology
Since the investment for a power generation system
Journal of Waste Management and Environmental Issues
Inflows
In deciding what data to collect, the research team has
5
John P.T. Mo et al., (2017)
Research
governing
equations of biogas turbine
Research
governing
equations of biogas spark engine
Develop model of
biogas to power
generated in both
types of generators
Develop model of
waste water to
biogas generated
(vol., compo., etc)
Research
governing
equations of other
bio-gas generators
Investigate the
optimum
configuration of
biogas generation
for Kinoya WWTP
Research the amount
and type of inflow to
the plant
Research the effects
of trade waste to the
inflow and resulting
biogas composition
Experiment with a
small gas turbine on
site to collect data
Verify/modify
model of testing
configuration with
actual data
Select purifier and
define quality of
biogas
Receive and analyse
data for the flow rate
of biogas being flared
Scale up data in
the model to full
scale
Recommend
testing
configuration
(with predicted
performance)
Develop testing
procedure
Provide a blue print
for a biogas power
generation plant
Figure 1. System design methodology.
spent some time to observe the type of data that could be
readily captured. One of the data types was inflows. On 10
June 2015, the plant was monitored on an interval of 15
mins for 24 hrs. The data can be found in Figure 2. The
trend of usage was peak during day time. A mean flow
of 32.2 MLD (app. 1342.42 m3/hr or 372.9 L/s, Approx.
161,090 EP) was recorded. This data indicates that the
measurement was taken on a sunny day.
The infiltration, illegal connection, industrial waste and
government waste are factored in this inflows projection.
Therefore, the current plant capacity is capable of treating
the ADWF and slight overload during wet weather (PDWF)
but will not be very effective during PWWF.
Furthermore, the effect of inflow due to rainfall is investigated by checking independent information from the
Bureau of Meteorology. From Fiji Meteorological Service
This gives a benchmark for understanding a realistic (2006), the rainfall varies in the year. Figure 4 shows a
possible amount of biogas. To determine the future supply, typical annual rainfall days’ pattern for Suva and Nadi. The
Figure 3 shows the projected sewer in the next 20 years. Suva area has more rainy days than Nadi and the pattern
The data set in Figure 3 incorporates the information from seems to be more uniform over the year. From another
the master planning unit with liable development on the study jointly conducted by Fiji Meteorological Service
plant and likewise the growth in sewer demand. Inflows (2011) and the Australian Commonwealth Scientific and
Research Organisation, there is no clear trend of annual or
are differentiated into three categories:
seasonal rainfall at Suva and Nadi Airport from since 1950.
1. Average dry weather flow (ADWF)
This information indicates that the increasing inflows
2. Peak dry weather flow (PDWF)
in the next 20 years as shown in Figure 2 will be due to
3. Peak wet weather flow (PWWF)
Journal of Waste Management and Environmental Issues
6
John P.T. Mo et al., (2017)
Figure 2. Kinoya WWTP inflow observed on 10 June 2015.
Figure 3. Kinoya WWTP projected inflow in the next 20 years.
population growth rather than affected by the amount
of rainfall.
and dissolved materials from the incoming sewage. The residuals removed from the unit operations are called sewage
sludge that needs further treatment before disposal. This
Treatment process
treatment is, in general terms, a type of anaerobic digestion that kills pathogens and reduces the mass of sludge
The treatment process of Kinoya WWTP uses a conven- by decomposing organic matter through the breakdown
tional aerobic process including trickling filters, clarifiers of volatile solids. The anaerobically digested sludge is rewhich has being upgraded and developed over a decade. ferred to as bio-solids, which can be applied to amend the
These processes allow separation suspended solids from soil or disposed of at landfills. The plant has two existing
the water in the form of sludge that is fed to anaerobic anaerobic sludge digester with one under repair at the time
digesters for the sludge stabilization.
of this study. The gas flaring is found to be conducted for 8
hours daily. The combined volume of the digester is 3,050
The sewage treatment plants are meant to remove solids m3 and the estimated retention time is 23 days.
Journal of Waste Management and Environmental Issues
7
John P.T. Mo et al., (2017)
Figure 4. Number of rainy days in the month for Suva and Nadi.
Energy content in biogas
From the literature review, it was estimated from other
information that about 2 kWHr of electricity form one m3
of biogas can be generated. Information of the biogas over
the first half of June has a composition as shown Table 2.
The methane content is much lower than the published
data and there is a very high hydrogen sulphide content.
High hydrogen sulphide content hinders methane production. The data indicates that the bio-digester condition is
still not optimum.
Table 2. Recorded biogas composition in first half of June.
Methane (%) Carbon
Hydrogen
Oxygen
dioxide (%) sulphide (ppm) (%)
46.8
41.0
4979.4
12.3
If we take the current flaring rate of 8 hours per day
at a flow rate of 257 m3 per hr, the total amount of biogas
is 2,056 m3 per day. This can then be turned into 4,112
kWHr or 4.1 MWHr of useable electricity.
The Bio-chemical Oxidation Demand (BOD) is an
indication whether Kinoya WWTP is treating waste effectively or not. The final effluent BOD should be within
the standard range of 40mg/L. Figure 5 shows the level of
effluent over the years 2007 to 2014. In 2011, data recorded
by WAF shows a lot of fluctuation. It is noted that during
this period, WAF has been grading Trickling Filter No 1,
Journal of Waste Management and Environmental Issues
restored Primary Clarifier No 1 and Secondary Clarifier
No 2 and upgraded sludge pumps.
Since the level of BOD is still to be improved, a number
of bio-solids will increase in the future once the treatment processes in Kinoya WWTP are rectified to the
as-designed level.
Gas production analysis
According to information from the Operation team,
it is believed to be flaring at an average rate of 257 m3/hr.
In order to obtain statistical data for analysis, the SCADA
system was studied to see if information could be downloaded from the SCADA system’s database (Figure 6).
However, this information has not been made available due
to various reasons. Hence, the gas flaring data collected
so far are manually recorded from the display.
Figure 7 shows the relation between the current flow
rate of gas flaring with time on a result batch, the measuring meter captures this result with a time interval of 10
minutes and currently flaring an 8 hours daily. Over the
recorded period, the gas flare flow rate seems to quite
steady at 272 m3/hr within 1% variation. This measurement somehow validates the ability of the system to flare
at least the designed capacity of 257 m3/hr.
Furthermore, the temperature of the gas flaring was
recorded against the immediate gas flaring flow rate during
the data logging session. If we assume that the volume of
8
John P.T. Mo et al., (2017)
Figure 5. BOD of raw sewage & final effluent.
Figure 6. Sample screen of SCADA system.
Journal of Waste Management and Environmental Issues
9
John P.T. Mo et al., (2017)
Figure 7. Recorded rate of gas flaring on 1/6/2015.
gas remains constant during the combustion in the flare,
the temperature can be approximated to some kind of
relationship with the gas flaring flowrate. The “regression”
line in Figure 8 is inserted to show a possible modelling
approach but it is still early to conclude. In addition, it
can also be seen that at some instances such as the point
at the most right-hand end of the graph, the relation of
these deviated points does not exhibit the same pattern.
This means there may be an unforeseen reaction that needs
to be investigated further. Factors such as the quality of
gasses and the control of flow rate can affect the chemical
reaction in the flare and are unknown at this stage.
Preliminary Gas Turbine Assessment
The gas turbine is the most unused technology in Fiji in
Figure 8. Temperature for different gas flaring flow rate.
terms of renewable energy. With high electricity expenses,
the research team will need to tap on such ingenious way to
supplement electricity with the availability of by–product
to biogas which is sludge in this instance.
Based on the information collected so far, if a consistent
sewage increase is assumed, treatment, biogas production,
gas control, after taking into account the loss of power
generation from the gas turbine to the electricity generator, the amount of electricity energy available can be seen
in Figure 9. At the current ADWF inflow, the estimated
power available is 4.1 MWHr per day, increasing to 9.1
MWHr per day in 2033. However, the PWWF inflow data
shows current power at 9.8 MWHr per day to 26.4 MWHr
per day in 2033. The average power available would lie
somewhere around 5 MWHr per day at present to 11~15
Figure 9. Projected electricity power available.
Journal of Waste Management and Environmental Issues
10
John P.T. Mo et al., (2017)
MWHr per day in 2033.
Kinoya WWTP and allows scope for scaling up to the future.
Since the energy content of biogas is estimated at 4.1
Preliminary System Design
MWHr, the gas turbine will only run for a maximum of 4
hours per day. However, the power demand for the Kinoya
The research team proposes to carry out a pilot study WWTP will vary during the day (max. 0.7 MW during
of a smaller scale gas turbine installation and monitored peak inflows but will be less at other times), this amount
operations using the biogas produced by the Kinoya Waste of energy can be spread over a 24 hours period. Therefore,
Water Treatment Plant. An important part of this proposed research on how to operate the gas turbine power at nonresearch is to set up a comprehensive SCADA system peak times to preserve biogas fuel is required.
that will form the backbone of future automated Kinoya
WWTP. Due to lack of system information and develop- Conclusion
ment options, we are unable to obtain automated logs of
The treatment of waste water produces biogas as a bydata required for analysis. The specific parameters are:
product that is harmful to the environment. This paper
reviews literature to define the essential elements in the
1. Plant inflow flow meter
2. Gas flow meter from the Digester to the biogas holder design of a power generation system from biogas produced
3. Gas flow meter from the Gasholder to the flaring furnace from wastewater. Due to historical reasons, many waste4. Quality (or composition control) of biogas
water treatment plants are developed on an ad hoc basis
5. Kinoya WWTP operating parameters such as plant
and are unable to provide sufficient data to support large
scale system development such as power generation from
load, sewage level fluctuations, etc.
biogas. A system design methodology is proposed in this
Figure 10 shows a preliminary concept of a biogas power paper to form a guide to the design of the wastewater to
generator and feed system to the Kinoya WWTP.
power generation system in the case of Kinoya wastewaThe following operating criteria for the design of the ter treatment plant. Preliminary information fitting into
system are required:
the system design methodology seems to suggest that a
gas turbine power generation system is possible but care
should be taken to maintain a close to consistent waste
1. 24 hours continuous operation
water flow and effectiveness of biogas conversion.
2. Gas purification to enhance power efficiency and
minimise unexpected impact to the gas turbine
3. Load monitoring system to balance loading to the gas Acknowledgement
turbine with grid power if necessary.
The authors would like to thank the support of Water
The proposed gas turbine will have a capacity of 0.5 MW Authority of Fiji on this research work and in particular,
to 1 MW depending on more accurate data being collected Mr. Opetaia Ravai, for allowing the research team to colin the initial stage of the proposed R&D project. This level lect data from the Kinoya wastewater treatment plant in
of power generation will meet the current demand of the this research.
Load feedback control
Bio Digester
Purifier
Gas
storage
Gas
Turbine
Elect
Gen
Plant
load
Figure 10. Preliminary system concept.
Journal of Waste Management and Environmental Issues
11
John P.T. Mo et al., (2017)
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