SW Aerobic Anaerobic Treatment

Biochemical methods of conversion • 839

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/litreCOD utilized = 6 kg/m3 × 100 m3/d = 600 kg COD/dayMethane 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 thepresence or absence of oxygen. Three common methods of processinginclude composting, vermicomposting, and biomethanation/anaerobicdigestion.

Composting

Composting is a biological process in which the organic matter present inwaste is converted into enriched inorganic nutrients. The manure obtainedhas high nitrogen, phosphorus, and potassium content. Heterotrophic micro-organisms act upon the organic matter and by the action of enzymes, convertorganic compounds first into simpler intermediates like alcohol or organic ac-ids and later into simple compound like sugars. This produces humic acid andavailable plant nutrients in the form of soluble inorganic minerals like nitrates,

RB-14.p65 30/11/2007, 4:47 PM839

840 • Renewable energy engineering and technology

sulphates, and phosphates. The quality of compost depends upon the wastebeing composted. The presence of high nitrogen, phosphorus, and potassiumcontents in the organic waste facilitates production of high-quality manure af-ter composting. The average composition of different constituents in thecompost is given in Table 14.13. Table 14.14 gives concentration limits of heavymetals in compost.

There are different methods of composting various kinds of organicwastes such as agro residues, animal waste, household waste, and so on. Someof 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 vegetableand fruit peelings – papers, and so on, can be converted into compost by the

Table 14.13 Characteristics of compost

Constituent Typical value (%)

Total nitrogen 1.3Total phosphorus 0.2–0.5Total potassium 0.5Organic phosphorus 0.054pH 8.6Moisture 40–50Organic matter 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 10.00Cadmium 5.00Chromium 50.00Copper 300.00Lead 100.00Mercury 0.15Nickel 50.00Zinc 1000.00

Note *C/N rati not to exceed 20–40 and pH not to exceed 5.5–8.5Source The Gazette of India notification (2000)

RB-14.p65 30/11/2007, 4:47 PM840


action of earthworms. An aerobic condition is created due to the exposure oforganic waste to air. Many Asian countries are adopting the process of vermi-composting for waste disposal. Although there are thousands of species ofearthworms, Eisenia foetida and Eisenia andrei are effective in decomposition oforganic wastes. Certain biochemical changes in the earthworm’s intestine re-sult in excretion of cocoons and undigested food known as vermicastings thatare an excellent manure due to the presence of rich nutrients—vitamins andenzymes, nitrates, phosphates, and potash. The enzymes produced also facili-tate the degradation of different biomolecules present in solid waste intosimple compounds for utilization by micro-organisms. The different stages ofvermicomposting are shown in Figure 14.22.

Anaerobic digestion systems for solid waste treatment

Anaerobic digestion involves breakdown of organic matter in biomass suchas animal dung, human excreta, leafy plant materials, and so on, by micro-or-ganisms in the absence of oxygen to produce biogas, which is a mixture ofmethane and carbon dioxide with traces of hydrogen sulphide. The optimumtemperature for the anaerobic digestion process is 37 ºC and the optimum pHis 7. In addition to waste treatment, the process of anaerobic degradation isadvantageous due to the generation of biogas that can be used for various ther-mal applications or for power generation. Besides, digested sludge can be usedas manure in place of chemical fertilizers. Biogas is a clean fuel as it is smoke-less and thus does not cause health hazards of eye, throat, and lung. In somestates in India like West Bengal, biogas slurry is also used for fish feed inpisciculture.

In India, biogas technology is more than five decades old. Differentmodels of biogas plants in use are discussed below.

Figure 14.22 Different stages of vermicomposting

RB-14.p65 30/11/2007, 4:47 PM841


Biogas plants for animal wastes

KVIC (Khadi and Village Industries Commission) biogas plantThe KVIC has been a pioneering agency in the field of biogas in India. Thefirst plant model, Gramalakshmi, was developed in 1950. This served as a pro-totype for the KVIC floating dome model that is being disseminated in thecountry since 1962. The KVIC biogas plant is a composite unit of a masonrydigester and a metallic dome that acts as a gas holder wherein gas is collectedand delivered at a constant pressure through a pipeline. A constant pressure ismaintained due to the upward and downward movement of the lid of thedigester along the central guide pipe fitted in a frame in the masonry(Figure 14.23).

Figure 14.23 Schematic of a KVIC biogas plantSource Eggeling, Guldager, Hilliges, et al. (1979)

RB-14.p65 30/11/2007, 4:47 PM842


Janata biogas plantThe first fixed dome plant was (originated in China) was introduced in Indiain the form of Janata biogas plant by Gobar Gas Research Station, Ajitmal (awing of Planning, Research, and Action Division, Lucknow) in 1978. In ad-dition to the low installation cost as compared to the KVIC plants, theJanata model plants have the advantages of using locally available building ma-terials and skills in construction. The design consists of a masonry structure,which integrates the reactor and gas-holding space. The inlet and outlet tanksare connected to the digester through displacement chambers. The gas outletis located on top of the dome and the digested slurry is withdrawn from theopening in the outlet displacement chamber (Figure 14.24).

Deenbandhu biogas plantThe Deenbandhu plant was developed by AFPRO (Action for Food Pro-duction), New Delhi, in the early 1980s. In 1981, AFPRO constructed eightbiogas plants of 2 m3 capacity of different designs and shapes as part of atraining programme in collaboration with FAO/UNDP. The design wasstandardized after minor modifications in 1984. The design consists of seg-ments of two spheres of different diameters joined at their bases, which actsas the digester as well as the gas storage chamber. To prevent short-circuiting, sufficient distance between the inlet and outlet tanks is maintainedby incorporating segments of spheres with base diameters not less than the di-ameter of conventional Janata plants of the same capacity (Figure 14.25).

Figure 14.24 Schematic digram of Janata biogas plant

RB-14.p65 30/11/2007, 4:47 PM843


T E R I biogas plant

The T E R I biogas plant, first develope

i

s

s

ed at its field research unit in

Pondicherry, was introduced in 1985 in the village Dhanawas, Gurgaon dis-trict (Haryana). The model was introduced as a field prototype, and afterimprovements and modifications, the final design has been disseminatedin different parts of India since 1987. About 173 T E R I model biogasplants are installed in 46 villages spread over seven states of India (Figure14.26).

Figure 14.25 Schematic diagram of Deenbandhu biogas plantReproduced with permission from The McGraw-Hill CompaniesSource Singh, Myles, and Dhussa (1987)

Figure 14.26 T ER I fixed dome biogas digester

RB-14.p65 30/11/2007, 4:47 PM844


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 VIN-CAP, Pragati and Krishna models which are popular in some regions.

Design calculations for fixed dome biogas digesterThe 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 design-ing 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 (m

3) Volume of spherical segment corresponding to height H2 V2 (m

3) Volume of spherical segment corresponding to height H3 V3 (m

3)The volumes can be expressed as

...(14.39)...(14.40)

...(14.41)


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 re-quired 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 = 0.3H3 – 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 2where 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 h2 h3 Ao

1 0.98 0.3 0.78 0.98 1.242 1.18 0.3 0.78 1.07 2.523 1.34 0.3 0.83 1.17 3.384 1.43 0.3 0.7 1.14 5.93

Source Kishore, Raman, Pal, et al. (1998)


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 solidwastes after pre-sorting and segregation have a high organic and moisturecontent. Thus anaerobic digestion is a suitable process of disposal. However,the digester design is not as simple as that for liquid effluents and animaldung. This is due to the heterogeneous nature of the substrate, which af-fects the efficiency of the reactor during digestion. The use of conventionalbiogas plant design described above would result in operational problemssuch as use of large amounts of water for slurry preparation, extensive pre-treatment/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 ofdifferent composition at different concentrations are described below.

Single-stage system

In this system, the entire process of hydrolysis, acidification, andmethanogenesis occurs simultaneously in a single reactor. The single-stage sys-tem can be subdivided into low-solid (<15%) and high-solid (>20%) systems onthe basis of the total solid content in reactor. The low solid process requirespre-treatment to prepare a homogeneous slurry through screening, pulping,and so on. To maintain homogeneity and to prevent the settling of heavier par-ticles and floating of the lighter layer, which can affect the mechanical parts,mixing and periodical removal of scum may be required. Although simple inoperation, due to low-solid content, slurry preparation requires the additionof water, which results in increased volume and cost, in addition to the in-creased drying and maintenance. Short-circuiting of feed material is one of theproblems faced in this system.

Another classification is dry or wet system depending on the total solidconcentration of slurry. Different configurations in each category are brieflydescribed below.

Dry systems

Dranco (dry anaerobic composting) processThis comprises a thermophilic, single-phase anaerobic fermentation step,which is followed by the aerobic maturation phase. A wide range of solidwastes can be handled at different total solid concentration. Complete

RB-14.p65 30/11/2007, 4:47 PM847


digestion of the residue is ensured due to the post-aerobic process(<http://www.ows.be/dranco.htm>).

KompogasThe process involves thermophilic fermentation for microbial conversion oforganic substance present in the material into compost and biogas. The proc-ess 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 designsThe Valorga process was developed and patented by the French companySteinmuller Valorga to treat mixed solid waste. It is a single-stage plug flowtype process without any mechanical mixing for the treatment of mixedmunicipal 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 processSource <www.kompogas.ch/en>, last accessed on 24 July 2006

RB-14.p65 30/11/2007, 4:47 PM848


Wet systemsSingle-stage wet systemsWAASA process

The process operates at both thermophilic and mesophilic temperatures,with the thermophilic process having an HRT of 10 days compared to 20days in the mesophilic design. It has been used for various types of wastes, in-cluding municipal solid waste and bio-solids, and the concentration range is10%–15% of total solids. The digester consists of a pre-digestion chamber andthe contents are mixed by biogas circulation.

Linde processThe process involves automatic separation of contaminants in the wet prepa-ration stage. The digesters are designed such that there is gas recirculationresulting in high biogas yield. The digested residue has a high compost valuedue to complete decomposition. The process can also be used for combineddigestion 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 wastesThe ASTRA centre at the Indian Institute of Science, Bangalore, has beenworking on various designs to convert leafy biomass to biogas. Based on expe-rience with various digester designs, the plug flow digester design, where thesolid waste movement occurs horizontally, was favoured. A 5 m3/day biogasplant based on the plug flow design was set up for digestion of leafy wastes

Figure 14.28 Valorga processSource <www.valorgainternational.fr/en>, last accessed on 10 October 2006

RB-14.p65 30/11/2007, 4:47 PM849


(Figure 14.30). Feeding of 50 kg/day resulted in gas production varying be-tween 45 and 100 litres/kg of biomass with an average production of 50 litres/kg (Jagdish, Chanakya, RajaBapaiah, et al. 1998)

Two-stage systemsHITACHI design

The process includes a thermochemical pre-treatment of waste under alka-line conditions at 60 oC for three hours and a two-phase digestion processconsisting of a liquefaction phase at a temperature of 60 oC. This results in re-duction in the processing time to eight days.

Figure 14.29 Linde processReproduced with permission from ElsevierSource <http://62.27.48.13/en/P0052/P0054S>, last accessed on 24 July 2006

Figure 14.30 Plug flow digester for biogas generationReproduced with permission from ElsevierSource Jagdish, Chanakya, Raja Bapaiah, et al. (1998)

RB-14.p65 30/11/2007, 4:47 PM850


IBVL designThis was developed by the Institute for Storage and Processing of AgricultureProduce, the Netherlands. In this process, the first stage is a liquefactionphase followed by a high-rate methane-producing reactor.

TEAM digesterTERI has developed a bi-phasic process for treating different types oforganic solid wastes (Lata, Rajeshwari, Pant, et al. 2001; Rajeshwari, Lata,Pant, et al. 2001; ). The process, called TEAM (TERI’s Enhanced Acidificationand Methanation) process, is a two-stage anaerobic digestion process designedspecially for biomethanation of organic solid wastes that are fibrous and havelight floating materials. Operating between 35 ºC and 40 ºC, the first stage ofthe process extracts the organic content from the solid wastes while thesecond stage generates biogas. The system has six acidification reactorsoperating in series and a single UASB reactor for methane production fromvolatile fatty acids. The retention time is six days for the acidification processand one day for the UASB reactor. The process does not involve any agitationmechanism resulting in low maintenance requirement. The digested sludgehas a high NPK content and the treated effluent from the UASB is reused forextraction of the organic contents in the acidification phase (Figure 14.31).

Figure 14.31 TEAM process for organic solid wastes

RB-14.p65 30/11/2007, 4:47 PM851

SW Aerobic Anaerobic Treatment

Documents

Transcript of SW Aerobic Anaerobic Treatment