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V = Effective liquid volume of the reactor = 100 m3/day ÷ 8 kg COD/m3 ×
18 kg COD/m3/day = 44.4 m3
Total liquid volume of reactor, Ve = V/effectiveness factor = 44.4/0.8 = 55.5 m3
Area of the reactor = feed rate/upflow velocity
= 100 m3/day/(1.5 m/h × 24 h/day) = 100 m3/day/36 m/day = 2.77 m2
A = 3.14 × D2/4 = 2.77 m2
or D2 = 3.53 m2
D = 1.88 m
Liquid height of the reactor = Ve/A = 55.5/2.77 = 20 m
Total height = 20 + 2.5 = 22.5 m
(ii) HRT = liquid volume of reactor/Feed rate
= (55.5 m3/100 m3/day ) × 24 h/day = 13.32 h
(iii) Amount of soluble COD degraded = 8000 × 0.75 = 6000 mg/litre
= 6 kg/m³
Effluent COD concentration = 8000 – 6000 = 2000 mg/litre
COD utilized = 6 kg/m3 × 100 m3/d = 600 kg COD/day
Methane produced = 600 kg COD × 0.35 m3/kg COD (removed) = 210 m3/day
Aerobic and anaerobic systems for solid waste treatment
The treatment of the organic solid wastes can be carried out either in the
presence or absence of oxygen. Three common methods of processing
include composting, vermicomposting, and biomethanation/anaerobic
digestion.
Composting
Composting is a biological process in which the organic matter present in
waste is converted into enriched inorganic nutrients. The manure obtained
has high nitrogen, phosphorus, and potassium content. Heterotrophic microorganisms act upon the organic matter and by the action of enzymes, convert
organic compounds first into simpler intermediates like alcohol or organic acids and later into simple compound like sugars. This produces humic acid and
available plant nutrients in the form of soluble inorganic minerals like nitrates,
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sulphates, and phosphates. The quality of compost depends upon the waste
being composted. The presence of high nitrogen, phosphorus, and potassium
contents in the organic waste facilitates production of high-quality manure after composting. The average composition of different constituents in the
compost is given in Table 14.13. Table 14.14 gives concentration limits of heavy
metals in compost.
There are different methods of composting various kinds of organic
wastes such as agro residues, animal waste, household waste, and so on. Some
of these are heap, pit, lagoon, chamber, and Berkeley and Nadep methods
(Agarwal and Saxena 2001).
Vermicomposting
This is a process whereby food materials, kitchen wastes – including vegetable
and fruit peelings – papers, and so on, can be converted into compost by the
Table 14.13 Characteristics of compost
Constituent
Typical value (%)
Total nitrogen
Total phosphorus
Total potassium
Organic phosphorus
pH
Moisture
Organic matter
1.3
0.2–0.5
0.5
0.054
8.6
40–50
30–70
Sources US Composting Council (2002); Bhardwaj (1995)
Table 14.14 Limits of heavy metals in compost
Constiuents
Concentration not to exceed (mg/kg dry basis)
Arsenic
Cadmium
Chromium
Copper
Lead
Mercury
Nickel
Zinc
10.00
5.00
50.00
300.00
100.00
0.15
50.00
1000.00
Note *C/N rati not to exceed 20–40 and pH not to exceed 5.5–8.5
Source The Gazette of India notification (2000)
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action of earthworms. An aerobic condition is created due to the exposure of
organic waste to air. Many Asian countries are adopting the process of vermicomposting for waste disposal. Although there are thousands of species of
earthworms, Eisenia foetida and Eisenia andrei are effective in decomposition of
organic wastes. Certain biochemical changes in the earthworm’s intestine result in excretion of cocoons and undigested food known as vermicastings that
are an excellent manure due to the presence of rich nutrients—vitamins and
enzymes, nitrates, phosphates, and potash. The enzymes produced also facilitate the degradation of different biomolecules present in solid waste into
simple compounds for utilization by micro-organisms. The different stages of
vermicomposting are shown in Figure 14.22.
Anaerobic digestion systems for solid waste treatment
Anaerobic digestion involves breakdown of organic matter in biomass such
as animal dung, human excreta, leafy plant materials, and so on, by micro-organisms in the absence of oxygen to produce biogas, which is a mixture of
methane and carbon dioxide with traces of hydrogen sulphide. The optimum
temperature for the anaerobic digestion process is 37 ºC and the optimum pH
is 7. In addition to waste treatment, the process of anaerobic degradation is
advantageous due to the generation of biogas that can be used for various thermal applications or for power generation. Besides, digested sludge can be used
as manure in place of chemical fertilizers. Biogas is a clean fuel as it is smokeless and thus does not cause health hazards of eye, throat, and lung. In some
states in India like West Bengal, biogas slurry is also used for fish feed in
pisciculture.
In India, biogas technology is more than five decades old. Different
models of biogas plants in use are discussed below.
Figure 14.22 Different stages of vermicomposting
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Biogas plants for animal wastes
KVIC (Khadi and Village Industries Commission) biogas plant
The KVIC has been a pioneering agency in the field of biogas in India. The
first plant model, Gramalakshmi, was developed in 1950. This served as a prototype for the KVIC floating dome model that is being disseminated in the
country since 1962. The KVIC biogas plant is a composite unit of a masonry
digester and a metallic dome that acts as a gas holder wherein gas is collected
and delivered at a constant pressure through a pipeline. A constant pressure is
maintained due to the upward and downward movement of the lid of the
digester along the central guide pipe fitted in a frame in the masonry
(Figure 14.23).
Figure 14.23 Schematic of a KVIC biogas plant
Source Eggeling, Guldager, Hilliges, et al. (1979)
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Janata biogas plant
The first fixed dome plant was (originated in China) was introduced in India
in the form of Janata biogas plant by Gobar Gas Research Station, Ajitmal (a
wing of Planning, Research, and Action Division, Lucknow) in 1978. In addition to the low installation cost as compared to the KVIC plants, the
Janata model plants have the advantages of using locally available building materials and skills in construction. The design consists of a masonry structure,
which integrates the reactor and gas-holding space. The inlet and outlet tanks
are connected to the digester through displacement chambers. The gas outlet
is located on top of the dome and the digested slurry is withdrawn from the
opening in the outlet displacement chamber (Figure 14.24).
Deenbandhu biogas plant
The Deenbandhu plant was developed by AFPRO (Action for Food Production), New Delhi, in the early 1980s. In 1981, AFPRO constructed eight
biogas plants of 2 m3 capacity of different designs and shapes as part of a
training programme in collaboration with FAO/UNDP. The design was
standardized after minor modifications in 1984. The design consists of segments of two spheres of different diameters joined at their bases, which acts
as the digester as well as the gas storage chamber. To prevent shortcircuiting, sufficient distance between the inlet and outlet tanks is maintained
by incorporating segments of spheres with base diameters not less than the diameter of conventional Janata plants of the same capacity (Figure 14.25).
Figure 14.24 Schematic digram of Janata biogas plant
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Figure 14.25 Schematic diagram of Deenbandhu biogas plant
Reproduced with permission from The McGraw-Hill Companies
Source Singh, Myles, and Dhussa (1987)
T E R I biogas plant
The T E R I biogas plant, first develop ed at its field research unit in
Pondicherry, was introduced in 1985 in the village Dhanawas, Gurgaon district (Haryana). The model was introduced as a field prototype, and after
improvements and modifications, the final design has been disseminated
in different parts of India since 1987. About 173 T E R I model biogas
plants are installed in 46 villages spread over seven states of India (Figure
14.26).
Figure 14.26 T E R I fixed dome biogas digester
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The TERI spherical type of biogas plant had evolved in an effort to
reduce the construction cost and to improve biogas yield in the fixed dome
biogas models. Some salient features of the TERI plant are listed below.
 Completely spherical shape, requiring lowest material consumption and
amenable for fast construction
 Increased gas storage, prevention of slurry from entering gas pipes due to
sufficient distance between slurry level and gas outlet
 Reinforcement of dome with tiles for better durability
 Costs and performance comparable to the Deenbandhu model
 Tangential input or a diffuser box results in an effective residence time of
32 days compared to an HRT of 40 days for conventional plants (Kishore,
Raman, and Ranga Rao 1987; Kishore, Raman, Pal, et al. 1998).
In addition to the above there are several other models such as the VINCAP, Pragati and Krishna models which are popular in some regions.
Design calculations for fixed dome biogas digester
The biogas digester design is based on quantity of dung available. The
important parameters in designing the digester are gas production rate G,
active slurry volume Vs, and dome radius. The biogas plant consists of the
digester, inlet, and outlet. The digester consists of active slurry volume, gas
storage space, and buffer space.
Referring to Figure 14.26, the following parameters are considered for designing the biogas plant.
 Radius of the plant R (m)
 Distance between the gas outlet and slurry exit level of the outlet tank
H1 (m)
 Distance between the gas outlet and initial slurry level (level at nil pressure)
in the biogas plant H2 (m)
 Distance between gas outlet and final slurry level (level at full pressure) in
the biogas plant H3 (m)
 Volume of buffer space in dome V1 (m3)
 Volume of spherical segment corresponding to height H2 V2 (m3)
 Volume of spherical segment corresponding to height H3 V3 (m3)
The volumes can be expressed as
...(14.39)
...(14.40)
...(14.41)
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The volume of slurry displaced within the digester is equal to the
volume of slurry displaced in the outlet tank, which is in turn equal to the required gas storage space. Cooking is generally done twice a day. Hence, the gas
storage space would be roughly equal to half the volume of gas produced per
day.
...(14.42)
where Ao is the cross-sectional area of the outlet tank (m2).
Assuming a gas storage space of 60% of the daily gas production, G (m3),
(V3 – V2) = 0.6 G
...(14.43)
Based on the field experience, to prevent gas line choking, the following values
are chosen
H1 = 0.3
H3 – H1 ≥ 0.5
For an HRT of 40 days, the biogas production from 1 kg of cattle dung is
about 0.04 m3 of gas. With a dilution factor of 1:1 for slurry preparation from
dung, the active slurry volume is (G/0.04) × (2/100o) × 40 or 2G m3.
Hence
...(14.44)
The ratio L/B for the outlet tank is taken as 2
where L is the length and B is the breadth.
Substituting for L = 2B
B = √(A0/2)
The equations are solved using the EUREKA software resulting in various
dimensions for different gas production volumes (Table 14.15).
Table 14.15 Design values for different gas production rates
Gas volume G, m3
Radius R
h1
h 2
h3
Ao
1
2
3
4
0.98
1.18
1.34
1.43
0.3
0.3
0.3
0.3
0.78
0.78
0.83
0.7
0.98
1.07
1.17
1.14
1.24
2.52
3.38
5.93
Source Kishore, Raman, Pal, et al. (1998)
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Bioreactors for other organic wastes
Organic wastes such as fruit and vegetable wastes, leafy wastes, food wastes,
food-processing industry wastes, and organic fraction of municipal solid
wastes after pre-sorting and segregation have a high organic and moisture
content. Thus anaerobic digestion is a suitable process of disposal. However,
the digester design is not as simple as that for liquid effluents and animal
dung. This is due to the heterogeneous nature of the substrate, which affects the efficiency of the reactor during digestion. The use of conventional
biogas plant design described above would result in operational problems
such as use of large amounts of water for slurry preparation, extensive pretreatment/shredding and pre-processing, handling of large amounts of slurry,
and clogging of the gas outlet due to floating of partially digested material.
The different types of biodigester designs used for handling solid wastes of
different composition at different concentrations are described below.
Single-stage system
In this system, the entire process of hydrolysis, acidification, and
methanogenesis occurs simultaneously in a single reactor. The single-stage system can be subdivided into low-solid (<15%) and high-solid (>20%) systems on
the basis of the total solid content in reactor. The low solid process requires
pre-treatment to prepare a homogeneous slurry through screening, pulping,
and so on. To maintain homogeneity and to prevent the settling of heavier particles and floating of the lighter layer, which can affect the mechanical parts,
mixing and periodical removal of scum may be required. Although simple in
operation, due to low-solid content, slurry preparation requires the addition
of water, which results in increased volume and cost, in addition to the increased drying and maintenance. Short-circuiting of feed material is one of the
problems faced in this system.
Another classification is dry or wet system depending on the total solid
concentration of slurry. Different configurations in each category are briefly
described below.
Dry systems
Dranco (dry anaerobic composting) process
This comprises a thermophilic, single-phase anaerobic fermentation step,
which is followed by the aerobic maturation phase. A wide range of solid
wastes can be handled at different total solid concentration. Complete
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digestion of the residue is ensured due to the post-aerobic process
(<http://www.ows.be/dranco.htm>).
Kompogas
The process involves thermophilic fermentation for microbial conversion of
organic substance present in the material into compost and biogas. The process occurs at a temperature of 55–60 ºC and the retention time is 15–20 days
(<http://www.kompogas.ch/en/The_Kompogas_process/the_kompogas_
process.html>) (Figure 14.27).
Valorga designs
The Valorga process was developed and patented by the French company
Steinmuller Valorga to treat mixed solid waste. It is a single-stage plug flow
type process without any mechanical mixing for the treatment of mixed
municipal waste resulting in energy production. (Singh 2002) (<http://
www.undp.org.in/programme/GEF/dec%2002/dec02/article-3.htm>)
(Figure 14.28).
Figure 14.27 Kompogas process
Source <www.kompogas.ch/en>, last accessed on 24 July 2006
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Figure 14.28 Valorga process
Source <www.valorgainternational.fr/en>, last accessed on 10 October 2006
Wet systems
Single-stage wet systems
WAASA process
The process operates at both thermophilic and mesophilic temperatures,
with the thermophilic process having an HRT of 10 days compared to 20
days in the mesophilic design. It has been used for various types of wastes, including municipal solid waste and bio-solids, and the concentration range is
10%–15% of total solids. The digester consists of a pre-digestion chamber and
the contents are mixed by biogas circulation.
Linde process
The process involves automatic separation of contaminants in the wet preparation stage. The digesters are designed such that there is gas recirculation
resulting in high biogas yield. The digested residue has a high compost value
due to complete decomposition. The process can also be used for combined
digestion of bio-waste and sewage sludge and/or agricultural waste (manure)
(<http://62.27.58.13/en/p0052/p0054/p0054.jsp#1>) (Figure 14.29).
Plug flow digester for leafy biomass wastes
The ASTRA centre at the Indian Institute of Science, Bangalore, has been
working on various designs to convert leafy biomass to biogas. Based on experience with various digester designs, the plug flow digester design, where the
solid waste movement occurs horizontally, was favoured. A 5 m3/day biogas
plant based on the plug flow design was set up for digestion of leafy wastes
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Figure 14.29 Linde process
Reproduced with permission from Elsevier
Source <http://62.27.48.13/en/P0052/P0054S>, last accessed on 24 July 2006
(Figure 14.30). Feeding of 50 kg/day resulted in gas production varying between 45 and 100 litres/kg of biomass with an average production of 50 litres/
kg (Jagdish, Chanakya, RajaBapaiah, et al. 1998)
Two-stage systems
HITACHI design
The process includes a thermochemical pre-treatment of waste under alkaline conditions at 60 oC for three hours and a two-phase digestion process
consisting of a liquefaction phase at a temperature of 60 oC. This results in reduction in the processing time to eight days.
Figure 14.30 Plug flow digester for biogas generation
Reproduced with permission from Elsevier
Source Jagdish, Chanakya, Raja Bapaiah, et al. (1998)
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IBVL design
This was developed by the Institute for Storage and Processing of Agriculture
Produce, the Netherlands. In this process, the first stage is a liquefaction
phase followed by a high-rate methane-producing reactor.
TEAM digester
TERI has developed a bi-phasic process for treating different types of
organic solid wastes (Lata, Rajeshwari, Pant, et al. 2001; Rajeshwari, Lata,
Pant, et al. 2001; ). The process, called TEAM (TERI’s Enhanced Acidification
and Methanation) process, is a two-stage anaerobic digestion process designed
specially for biomethanation of organic solid wastes that are fibrous and have
light floating materials. Operating between 35 ºC and 40 ºC, the first stage of
the process extracts the organic content from the solid wastes while the
second stage generates biogas. The system has six acidification reactors
operating in series and a single UASB reactor for methane production from
volatile fatty acids. The retention time is six days for the acidification process
and one day for the UASB reactor. The process does not involve any agitation
mechanism resulting in low maintenance requirement. The digested sludge
has a high NPK content and the treated effluent from the UASB is reused for
extraction of the organic contents in the acidification phase (Figure 14.31).
Figure 14.31 TEAM process for organic solid wastes
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