Centre for Rural Development & Technology, I.I.T., Delhi 1100016, India Department for Biochemical Engineering & Biotechnology, I.I.T., Delhi 1100016, India c Department of Mechanical Engineering, I.I.T., Delhi 1100016, India Received 31 July 2003; received in revised form 18 August 2003
Abstract Biogas, a clean and renewable form of energy could very well substitute (especially in the rural sector) for conventional sources of energy (fossil fuels, oil, etc.) which are causing ecological–environmental problems and at the same time depleting at a faster rate. Despite its numerous advantages, the potential of biogas technology could not be fully harnessed or tapped as certain constraints are also associated with it. Most common among these are: the large hydraulic retention time of 30–50 days, low gas production in winter, etc. Therefore, efforts are needed to remove its various limitations so as to popularize this technology in the rural areas. Researchers have tried different techniques to enhance gas production. This paper reviews the various techniques, which could be used to enhance the gas production rate from solid substrates. Ó 2004 Published by Elsevier Ltd.
Keywords: Biogas production rate; Additives; Anaerobic filters; HRT
1. Introduction In today’s energy demanding life style, need for exploring and exploiting new sources of energy which are renewable as well as eco-friendly is a must. In rural areas of developing countries various cellulosic biomass (cattle dung, agricultural residues, etc.) are available in plenty which have a very good potential to cater to the energy demand, especially in the domestic sector. In India alone, there are an estimated over 250 million cattle and if one third of the dung produced annually from these is available for production of biogas, more than 12 million biogas plants can be installed (Kashyap et al., 2003). Biogas technology offers a very attractive route to utilize certain categories of biomass for meeting partial energy needs. In fact proper functioning of biogas system can provide multiple benefits to the users and the community resulting in resource conservation and environmental protection. Biogas is a product of anaerobic degradation of organic substrates, which is one of the oldest processes used
for the treatment of industrial wastes and stabilization of sludges. Since it is carried out by a consortium of microorganisms and depends on various factors like pH, temperature, HRT, C/N ratio, etc., it is a relatively slow process. Lack of process stability, low loading rates, slow recovery after failure and specific requirements for waste composition are some of the other limitations associated with it (Van der Berg and Kennedy, 1983). Anaerobic fermentation being a slow process, a large HRT of 30–50 days is used in conventional biogas plants. This leads to a large volume of the digester and hence high cost of the system. The decrease in gas generation during winter season has been reported which, poses a serious problem in the practical application of this technology. Kalia and Singh (1996) found that biogas production reduced from around 1700 l/day in May–July to around 99l/d in January–February. All this has resulted in restricted popularization of biogas technology in rural areas. Thus there is a need to improve the overall efficiency of anaerobic digestion process in the biogas plants. This could be done by several methods such as optimizing the various operational parameters, satisfying the nutritional requirements of microbes (Lettinga et al., 1980; Wilkie and Colleran, 1986), using different biological and chemical additives and by manipulating the feed pro-
0960-8524/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.biortech.2004.02.010
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portions (Sanders and Bloodgood, 1965; Nyns, 1986). Recirculation of digested slurry (washed out microbes) back into the reactor and modification in the design of existing biogas plants are some of the other ways to improve the gas production in biogas plants. Recently, efforts have been made to either reduce the HRT or enhance biogas production for the same HRT by incorporating fixed film matrices in the reactors, which help to retain microbes in the reactors. Recently ultrasonification of wastewater has been found to enhance the removal of COD by almost 10% (McDermott et al., 2001). A review of Indian advances in biogas technology was prepared by Singh and Maheshwari (1995). This paper presented a comprehensive view of the various methods, which could be used to enhance the gas production rate from the solid substrates.
3. Techniques for enhancing biogas production Different methods used to enhance biogas production can be classified into the following categories: ii(i) Use of additives i(ii) Recycling of slurry and slurry filtrate (iii) Variation in operational parameters like temperature, hydraulic retention time (HRT) and particle size of the substrate (iv) Use of fixed film/biofilters 3.1. Use of additives Some attempts have been made in the past to increase gas production by stimulating the microbial activity using various biological and chemical additives under different operating conditions. Biological additives include different plants, weeds (Gunaseelan, 1987), crop residues, microbial cultures, etc., which are available naturally in the surroundings. As such, generally these are of less significance in terms of their use in the habitat, however if used as additives in biogas plant could improve its performance significantly. The suitability of an additive is expected to be strongly dependent on the type of substrate. 3.1.1. Green biomass Powdered leaves of some plants and legumes (like Gulmohar, Leucacena leucocephala, Acacia auriculiformis, Dalbergia sisoo and Eucalyptus tereticonius) have been found to stimulate biogas production between 18% and 40% (SPOBD, China, 1979; Chowdhry et al., 1994). Increase in biogas production due to certain additives appears to be due to adsorption of the substrate on the surface of the additives. This can lead to high-localized substrate concentration and a more favourable environment for growth of microbes (Chandra and Gupta, 1997). The additives also help to maintain favourable conditions for rapid gas production in the reactor, such as pH, inhibition/promotion of acetogenesis and methanogenesis for the best yield, etc. Alkali treated (1% NaOH for 7 days) plant residues (lantana, wheat straw, apple leaf litter and peach leaf litter) when used as a supplement to cattle dung resulted in almost twofold increase in biogas and CH4 production (Dar and Tandon, 1987). Partially decomposed ageratum produced 43% and Euphorbia tirucalli L. produced 14% more gas as compared to pure cattle dung (Kalia and Kanwar, 1989; Rajasekaran et al., 1989). Trujillo et al. (1993) found that the addition of the tomato-plant wastes to the rabbit wastes in proportion higher than 40% improved the methane production. Crop residues like maize stalks, rice straw, cotton stalks, wheat straw and water hyacinth each enriched with partially digested cattle dung enhanced gas production in the range of 10–
2. Process and mechanism of biomethanation The anaerobic biological conversion of organic matter occurs in three steps. The first step involves the enzyme-mediated transformation of insoluble organic material and higher molecular mass compounds such as lipids, polysaccharides, proteins, fats, nucleic acids, etc. into soluble organic materials, i.e. to compounds suitable for the use as source of energy and cell carbon such as monosaccharides, amino acids and other simple organic compounds. This step is called the hydrolysis and is carried out by strict anaerobes such as Bactericides, Clostridia and facultative bacteria such as Streptococci, etc. In the second step, acidogenesis, another group of microorganisms ferments the break-down products to acetic acid, hydrogen, carbon dioxide and other lower weight simple volatile organic acids like propionic acid and butyric acid which are in turn converted to acetic acid. In the third step, these acetic acid, hydrogen and carbon dioxide are converted into a mixture of methane and carbon dioxide by the methanogenic bacteria (acetate utilizers like Methanosarcina spp. and Methanothrix spp. and hydrogen and formate utilizing species like Methanobacterium, Methanococcus, etc.).The three stages of methane fermentation are shown in Fig. 1.
H2 COMPLEX ORGANICS ACETIC ACID
HYDROLYSIS AND FERMENTATION ACIDOGENESIS AND DEHYDRO-GENATION METHANE FERMENTATION
CH4
Fig. 1. Different stages of methane fermentation.
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80% (El Shinnawi et al., 1989; Somayaji and Khanna, 1994). Babu et al. (1994) observed improvement in biomethanation of mango processing wastes by several folds by the addition of extracts of seeds of Nirmali, common bean, black gram, guar and guargum at the rate of 1500 ppm. Mixture of Pistia stratiotes and cowdung (1:1) gave a biogas yield of 0.62 m3 /(m3 day) (CH4 ¼ 76.8%, HRT ¼ 15 days) (Zennaki et al., 1998). Recently Sharma (2002) observed an increase of 40–80% in biogas production on addition of 1% onion storage waste (OSW) to cattle dung in a 400-l floating drum biogas reactor. 3.1.2. Microbial strains Strains of some bacteria and fungi have also been found to enhance gas production by stimulating the activity of particular enzymes. Cellulolytic strains of bacteria like actinomycetes and mixed consortia have been found to improve biogas production in the range of 8.4–44% from cattle dung (Tirumale and Nand, 1994; Attar et al., 1998). All the strains exhibited a range of activity of all the enzymes involved in cellulose degradation, viz. C1 enzyme, exglucanase, endoglucanase, bglucosidase. It seemed that endoglucanase activity was of central importance for the hydrolysis of cellulose. Geeta et al. (1994) found that sugarcane bagasse pretreated with Phanerochaete chrysosporium for 3 weeks under ambient temperature conditions produced higher gas with cattle excreta. Dohanyos et al. (1997) examined the use of cell lysate as a stimulating agent in anaerobic degradation of municipal raw sludge, excess activated sludge and their mixture. The effect of lysate is caused by the still remaining activity of released enzymes and by the stimulating properties of other compounds that are present inside the cells. The improvement of CH4 yield from thickened activated sludge ranged from 8.1% to 86.4% while in case of a mixture of thickened activated sludge and primary sludge it was found to vary from 0% to 24%. 3.1.3. Inorganic additives Several inorganic additives that improve gas production have also been reported. Shimizu (1992) claimed that higher concentration of bacteria could be retained in the digester by the addition of metal cations since cations increase the density of the bacteria, which are capable of aggregating by themselves. Wong and Cheung (1995) found that the plant with a higher content of heavy metals (Cr, Cu, Ni and Zn) had a higher CH4 yield than the control. The addition of iron salts at various concentrations [FeSO4 (50 mM), FeCl3 (70 lM)] have been found to enhance gas production rate (Wodzinski et al., 1983; Patel et al., 1993; Rao and Seenayya, 1994; Clark and Hillman, 1995). Nickel ions (2.5 and 5 ppm) enhanced biogas up to 54% due to the activity of Ni-dependent metallo-enzymes involved in
biogas production (Geeta et al., 1990). Addition of rock phosphate (RP) proved superior to single super phosphate (SSP) while digesting rice straw in batch fermenters (Bardiya and Gaur, 1997). Malik et al. (1987) obtained an increase of 8–11% by the addition of urea and diammonium phosphate (DAP). Certain adsorbents are also reported to improve gas production for example Madamwar and Mithal (1986) obtained a maximum enhancement of over 150% with higher CH4 content (65% CH4 ) on addition of 10 g/l commercial pectin. According to Kumar et al. (1987) commercial charcoal Darco G-60 resulted in 17% and 34.7% increase in biogas in batch and semi continuous fermenters, respectively. Also, the locally produced wood charcoal (16% enhancement in biogas) was found as good as the commercial charcoal in batch digesters. Patel et al. (1992) found a trend of enhanced gas production with high CH4 content and lower effluent BOD and COD with increasing doses of different adsorbents (gelatin, polyvinyl alcohol, powdered activated charcoal, pectin, kaolin, silica gel, aluminium powder, bentonite and tale powder) on anaerobic digestion of water-hyacinth-cattle dung. They observed (Patel and Madamwar, 1994) a twofold increase in gas production on addition of 4 g/l silica gel, with CH4 content of 72.8% as compared to control (62%). Process stability increased with increasing levels of silica gel, indicating that volatile acids were consumed at a faster rate in the presence of an adsorbent. Using Ca and Mg salts as energy supplements, CH4 production was enhanced and foaming was avoided (Mathiesen, 1989). Dhawale (1996) found 25–35% enhancement in anaerobic digestion of manure by the addition of Eosin blue dye at 0.1 lM concentration. Gaddy (1994) found a new method for improving the performance of anaerobic digestion of solid substrate. It involved the addition of at least 1-chelating agent (preferably 1–100 lM, especially 10 lM) 1:2 diaminocyclohexane-N,N, tetraacetic acid, EDTA, citric acid or nitrilotriacetic acid (NTA)) and at least one nutrient (preferably 1–5000 lM (10 lM)) of iron, sulfide, selenium or nickel, especially FeSO4 , FeCl2 , SeO2 or NiCl2 ) to a solid substrate for solubilizing solid nutrients to enhance bacterial growth. Methane production can be increased or smaller digesters can be used to achieve the same methane production. Faster start up, greater stability and more rapid recovery from upsets were possible by using this new method. 3.2. Gas enhancement through recycling of digested slurry/slurry filtrate The recirculation of digested slurry back into the reactor has been shown to improve the gas production marginally, since the microbes washed away are reintroduced back into the reactor, thereby providing an
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additional microbial population. The recycling of the digested slurry along with filtrate has also been tried out to conserve water and to enhance biogas production (Malik and Dahiya, 1990; Santosh et al., 1999). Kanwar and Guleri (1994) reported that about 60–65% more biogas production can be obtained by simply recycling the digested slurry in 1 m3 plug flow type pilot plants. They suggested that recycling of digested slurry along with fresh dung might help in overcoming the problem of underfed biogas plants as well as in maintaining higher gas production in winter season. They encountered no other problems like precipitation of substrate, increase in acidity/alkalinity and ammonia toxicity. Brummeler et al. (1992) suggested that leachate recycling resulted in faster digestion rate while anaerobically digesting solid organic waste in a biocell (pilot scale, 35 °C). Scaling did not affect the rate of process, provided leachate recycling was done at a rate of 0.3 m3 /(m3 day). An increase of up to 18.8% in gas production (CH4 ¼ 80%) was observed by Malik and Tauro (1995) when predigested slurry was used along with 10% effluent slurry recycling in a 1 m3 daily fed floating drum biogas digester (pilot plant, HRT ¼ 30 day). A 10-fold and threefold increase in the degradation rate of mannitol and lactic acid was observed by Jarvis et al. (1995) when liquid recirculation (LR) was initiated in a silage-fedtwo-phase biogas plant. The number of hydrogenotrophic methanogens increased 10-fold while there was an increase of ninefold in their activity. Liquefaction of cellulose and hemicellulose was low from the start of recirculation (3% and 20% reduction respectively) and was not affected by Santosh et al. (1999) carried out experiments on a plant of 1 cu.m capacity and found that recycling of 50% slurry filtrate along with 10% digested slurry can lead to about 50% water conservation and 10% increase in gas production. 3.3. Variation in operational parameters The performance of biogas plant can be controlled by studying and monitoring the variation in parameters like pH, temperature, loading rate, agitation, etc. Any drastic change in these can adversely affect the biogas production. So these parameters should be varied within a desirable range to operate the biogas plant efficiently. 3.3.1. Temperature 3.3.1.1. Effect of temperature on biogas production. Temperature inside the digester has a major effect on the biogas production process. There are different temperature ranges during which anaerobic fermentation can be carried out: psychrophilic (<30 °C), mesophilic (30–40 °C) and thermophilic (50–60 °C). However, anaerobes are most active in the mesophilic and thermophilic temperature range (Mital, 1996; Umetsu et al., 1992; Maurya et al., 1994; Takizawa et al., 1994; Desai
and Madamwar, 1994; Zennaki et al., 1996). The length of fermentation period is dependent on temperature. Angelidaki and Ahring (1994) observed that when the NH3 load was high, reducing temperature below 55 °C resulted in an increase of biogas yield and better process stability, as shown by the reduced VFA concentration. Garba (1996) observed that methanogens were very sensitive to sudden thermal changes, therefore, any drastic change in temperature should be avoided. Nozhevnikova et al. (1999) proposed a two step anaerobic treatment of cattle dung i.e. (i) acidogenic fermentation at high temperature (55–82 °C), and (ii) separation of solid and liquid fractions and treating the liquid manure under low temperature conditions (5–20 °C). Long term adaptation of active psychrophilic microbial communities was found to be essential for efficient treatment of cattle dung at low temperature (Nozhevnikova et al., 1999; Meher et al., 1994). Recently a review paper on biomethanation under psychrophilic conditions has been published (Kashyap et al., 2003).
3.3.1.2. Installation technique for getting optimum temperature conditions. Most of the remedies mentioned in the literature to enhance biogas production are aimed at increasing the digester temperature to mesophilic range (i.e. optimum temperature). It is noted that systematic studies on biomethanation by psychrophilic microflora are lacking (Kashyap et al., 2003). Some precautions taken during the installation of biogas plants and coating them with insulating materials also helps in keeping the temperature in the digester within the desired range (Molnar and Bartha, 1989). In order to increase gas yield, it is preferred to construct biogas plants sun-facing and in a manner as to protect them from cold winds. Biogas plants should be covered with locally available crop residues for minimizing heat losses from the plants. A simple technique of charcoal coating of ground around the digester had been found to improve gas production in KVIC biogas plant by 7–15% (Anand and Singh, 1993). Installation of PVC greenhouse type structure over a biogas plant allowed solar heating of the substrate from 18 to about 37 °C. It was possible to obtain substantial increase in gas yield on a typical winter day by covering the gas holder with a transparent polyethylene sheet during sunshine hours and using a movable insulating material during the off-sunshine hours (Bansal, 1988; Tiwari et al., 1988). Desai (1988) found that if the temperature of digester content could be maintained at 40 °C then it was possible to reduce the HRT by over 40%. They found solar ponds to be helpful in preventing heat losses during night and in maintaining digester temperature at desired level. Solar assisted biogas plants achieve higher gas yield particularly during winter months (Tiwari and Chandra, 1986). Hot
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water used in the slurry preparation also helps to improve the gas production. 3.3.2. pH pH is an important parameter affecting the growth of microbes during anaerobic fermentation. pH of the digester should be kept within a desired range of 6.8–7.2 by feeding it at an optimum loading rate. The amount of carbon dioxide and volatile fatty acids produced during the anaerobic process affects the pH of the digester contents. For an anaerobic fermentation to proceed normally, concentration of volatile fatty acids, acetic acid in particular should be below 2000 mg/l. Jain and Mattiasson (1998) found that above pH 5.0, the efficiency of CH4 production was more than 75%. The twophase anaerobic reactor using cheese whey and dairy manure as substrate operated as a single-phase reactor when the pH was not controlled while when pH of whey was controlled in the methanogenic stage, it operated as a two-stage two-phase reactor (Ghaly, 1996). The major problem related to drastic reduction in pH due to rapid acidification of onion storage waste (OSW) was overcome by Sharma (2002) by mixing cattle dung with OSW in a suitable ratio so that medium is well buffered to take care of acid accumulation. 3.3.3. Pretreatment Feedstocks sometimes require pretreatment to increase the methane yield in the anaerobic digestion process. Pretreatment breaks down the complex organic structure into simpler molecules which, are then more susceptible to microbial degradation. Pretreatment could be done in any of the following ways: ii(i) i(ii) (iii) (iv) i(v) Pretreating the feedstock with alkali or acid Predigestion of fresh substrate Thermochemical pretreatment Ultrasonic pretreatment Ensilage of feed
helped in the pretreatment of polymeric constituents and conversion of major components of carbohydrates into volatile fatty acids. It produced 58% more gas as compared to control (Madhukara et al., 1993). 3.3.4. Particle size Though particle size is not that important a parameter as temperature or pH of the digester contents, it still has some influence on gas production. The size of the feedstock should not be too large otherwise it would result in the clogging of the digester and also it would be difficult for microbes to carry out its digestion. Smaller particles on the other hand would provide large surface area for adsorbing the substrate that would result in increased microbial activity and hence increased gas production. Sharma et al. (1988) found that out of five particle sizes (0.088, 0.40, 1.0, 6.0 and 30.0 mm), maximum quantity of biogas was produced from raw materials of 0.088 and 0.40 mm particle size. Large particles could be used for succulent materials such as leaves. However, for other materials such as straws, large particles could decrease the gas production. The results suggested that a physical pretreatment such as grinding could significantly reduce the volume of digester required, without decreasing biogas production (Gollakota and Meher, 1988; Moorhead and Nordstedt, 1993). 3.3.5. C:N ratio It is necessary to maintain proper composition of the feedstock for efficient plant operation so that the C:N ratio in feed remains within desired range. It is generally found that during anaerobic digestion microorganisms utilize carbon 25–30 times faster than nitrogen. Thus to meet this requirement, microbes need a 20–30:1 ratio of C to N with the largest percentage of the carbon being readily degradable (Bardiya and Gaur, 1997; Malik et al., 1987). Waste material that is low in C can be combined with materials high in N to attain desired C:N ratio of 30:1 (Barnett, 1978; Fry and Merill, 1973; Gotass, 1956; Singh, 1974). Some studies also suggested that C:N ratio varies with temperature. According to study conducted by Idnani and Laura (1971) biogas production from 0.5 kg of cow dung was almost doubled from 17.2 to 31.5 l by addition of 200 ml of urine. Use of urine soaked waste materials is particularly advantageous during winter months when gas production is otherwise low. 3.3.6. Agitation Stirring of digester contents needs to be done to ensure intimate contact between microorganisms and substrate which ultimately results in improved digestion process. Agitation of digester contents can be carried out in a number of ways. For instance daily feeding of slurry instead of periodical gives the desired mixing effect. Stirring can also be carried out by installing certain
Dar and Tandon (1987) observed an improvement of 31–42% in microbial digestibility and an almost twofold increase in biogas when alkali treated (1% NaOH for 7 days) plant residues were used as a supplement to cattle dung. Predigestion of fresh cattle slurry in a batch system for 1–2 days at 30–35 °C increased acetate production and the use of this slurry as a feed material for anaerobic digesters increased biogas production by 17– 19% and CH4 content from 68–75% to 75–86% (Singh et al., 1983). Patel et al. (1993) found that thermochemical pretreatment of water hyacinth improved biomethanation and the best results were obtained when water hyacinth was treated at pH 11.0 and at 121 °C. Ultrasonic pretreatment of waste activated sludge for 30 min resulted in a 64% increase in methane production (Wang et al., 1999). Ensilage of mango peel for 6 months
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mixing devices like scraper, piston, etc. in the plant. It is possible to achieve mixing effect by incorporating a nozzle for flushing slurry as provided in the German design of Schmidt–Eggersgluss type biogas plant. Gas recirculation has also been found to enhance mixing and thus gas production (Mohanrao, 1974; Aubart and Farinet, 1983; Van and Faber, 1996). Baier and Schmidheiny (1997) used mechanical disintegration (wet milling) to physically disrupt cellular material and observed that net biogas production was enhanced. 3.3.7. Seeding of biogas plant It is often necessary to introduce enriched seeding bacteria into the digester for starting up the anaerobic fermentation process. Generally digested sludge from a running biogas plant or a municipal digester, material from well-rotted manure pit, or cow dung slurry is used as seed. If during the operation volatile fatty acids are accumulated due to overloading, this can be corrected by reseeding and temporarily suspending the feeding of digester or by adding lime in requisite quantities. Addition of inoculum tends to improve both the gas yield and methane content in biogas. It is possible to increase gas yield and reduce retention period by addition of inoculum (Dangaggo et al., 1996; Kanwar and Guleri, 1995; Kotsyurbenko et al., 1993). 3.3.8. Organic loading rate (OLR) Gas production rate is highly dependent on loading rate. Methane yield was found to increase with reduction in loading rate (Vartak et al., 1997a). In an another study carried out in Pennsylvania on a 100 m3 biogas plant operating on manure, when OLR was varied from 346 kg VS/day to 1030 kg VS/day, gas yield increased from 67 to 202 m3 /day. There is an optimum feed rate for a particular size of plant, which will produce maximum gas and beyond which further increase in the quantity of substrate will not proportionately produce more gas. According to Mohanrao (1974), a daily loading rate of 16 kg VS/m3 of digester capacity produced 0.04 0.074 m3 of gas/kg of dung fed. A lab-scale digester operating at different OLRs produced a maximum yield of 0.36 m3 /kg VS at an OLR of 2.91 kg VS/ m3 /day (Sundrarajan et al., 1997). Based on pilot plant studies (1 m3 capacity), maximum gas yield was observed for a loading rate of 24 kg dung/m3 digester/day although percent reduction of VS was only 2/3rd of that with low loading rate (Mohanrao, 1974). 3.3.9. Hydraulic retention time (HRT) HRT is the average time spent by the input slurry inside the digester before it comes out. In tropical countries like India, HRT varies from 30–50 days while in countries with colder climate it may go up to 100 days. Shorter retention time is likely to face the risk of washout of active bacterial population while longer
retention time requires a large volume of the digester and hence more capital cost. Hence there is a need to reduce HRT for domestic biogas plants based on solid substrates. It is possible to carry out methanogenic fermentation at low HRT’s without stressing the fermentation process at mesophilic and thermophilic temperature ranges (Zennaki et al., 1996; Singh et al., 1995; Garba, 1996). On the other hand Sanchez et al. (1992) found improvement in organic matter removal on increasing HRT while anaerobically treating cattle dung. Desai and Madamwar (1994) observed maximum gas production of 2.2 l/l/day (CH4 ¼ 62%) at an HRT of 10 days having a loading rate of 6 gTS/l while treating a mixture of cattle dung, poultry waste and cheese whey in the ratio of 2:1:3. Baserja (1984) observed that at a TS concentration of 7%, the duration of digestion could be reduced to 10 days without compromising the stability of the process, but the optimum period was 16–20 days. 3.3.10. Solid concentration The amount of fermentable material of feed in a unit volume of slurry is defined as solid concentration. Ordinarily 7–9% solids concentration is best-suited (Zennaki et al., 1996). The biogas yield increased, reaching 0.46 m3 /(m3 day) at 37 °C and 0.68 m3 /(m3 day) at 55 °C respectively. Baserja (1984) reported that the process was unstable below a total solids level of 7% (of manure) while a level of 10% caused an overloading of the fermenter. 3.4. Biofilters/fixed film reactors Fixed film reactors have been used since long for the treatment of wastewater where they have helped to reduce the HRT from 30–40 days to a few hours (Kloss, 1991). These reactors come under the category of advanced reactors like UASB, fluidized bed, upflow anaerobic filters, etc. They help in enhancing the performance of wastewater treatment systems by providing an increased surface area for attached growth of the microbes in the form of a fixed film on an inert medium leading to increased population of microbes in the reactor and their retention in the digester even after the digested slurry flows out (Van der Berg and Kennedy, 1983). Fixed film technique has been used commonly for substrates of very low solids content where filters of very large surface area are used. However, but the studies are scanty with substrates of high solids content like cowdung slurry. Many criteria need to be considered for selection of suitable materials for long life of the fixed film matrix (Young and Song, 1984). The material should be non-biodegradable. The structure of the fixed film matrix should also be mechanically stable. Materials should be easily available in the local market at a reasonable cost. Different materials like nylon sponges,
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PVC, clay pipes, etc. had been used as support medium for fixed film reactors (Wilkie et al., 1984). Fixed film reactor packed with sponge nylon as support performed well in terms of specific biogas production rate as compared to conventional reactors. The results showed good digester productivity as well as satisfactory sludge stabilization in fixed film digester (Solicio and Del, 1987; Meier et al., 1993). Vartak et al. (1997b) found the performance of polyester medium with its high porosity and surface to volume ratio to be best both at 37 and 10 °C. It also yielded the maximum reduction in volatile solids (VS) and COD at 37 °C. Weiland and Peters (1992) obtained 75% and 25% reduction in HRT and reactor volume respectively using plastic support for anaerobic digestion of screened cattle excrement as compared to conventional system. It had high accumulation of biomass during the 2-year study and could be operated in a broad range of loadings with a constant COD removal efficiency and high process reliability. Raju and Ramaligaiah (1997) observed a high biogas yield of 0.70 m3 /kg VS added in PVCP packed reactors (20-day HRT) and COD reduction was three times more as compared to conventional reactor. Henry (1985) achieved high OLRs and a considerable reduction in the HRT utilizing random oriented plastic supports. This single stage reactor with recycle had many advantages like easy operation, homogeneous distribution within the reactor, maximum agitation, low risk of clogging or foaming and ease of control of biological activity by monitoring sludge activity. Sanchez et al. (1995) used PVC plastic pipes and ceramic raschig rings and found that anaerobic fixed bed reactor could work at a high OLR without clogging. The efficiency of more than 60% in VS reduction and 55% in COD reduction were obtained at HRTs as low as 6 days. Raju and Ramaligaiah (1997) used burnt coconut shells and obtained a high biogas yield of 0.72 m3 /kg VS added (20 day HRT) while Ganesh Kumar et al. (1996) achieved an increase of about 40% in gas production by the addition of broken burnt bricks as carriers for immobilizing microbes. They suggested that larger lumps of bricks might be used to avoid clogging in practice. Sorlini et al. (1990) obtained highest biogas production (144.0 l/kg VS fed) in the digester with wood chips while production was almost nil in the digester with expanded clay. The number of anaerobic cellulolytic bacteria was considerably lower in the bottom sediment of the wood chip digester than in that of the expanded clay digester, whereas the number of methanogens was significantly higher. On the other hand Lomas et al. (1999) found lab and bench scale digesters packed with straight vertical channels of potter’s clay to present large biodegradation efficiency and allowed large organic loads compared with continuously stirred tank reactor.
Sanchez et al. (1994) obtained a predominant presence of filamentous methanogenic forms closely resembling Methanosaeta (Methanothrix) located on the outer layer and in the bacterial framework of the biofilm, when they used six different support materials (polyurethane, bentonite, diabase, diatomaceous earth, sepiolite and PVC) for digestion of domestic sludge. The enhancement of CH4 production was rapid and occurred within 24 h of sludge incubation. Meier et al. (1993) found that energy substrates stimulated the attachment of P. aeruginosa and Citrobacter amalonaticus significantly as compared to the fixation behaviour in basal medium without substrates. This mechanism may be important for retaining methanogenic biomass in anaerobic biofilms and thus could help to reduce the start-up period of biofilm digesters and to enhance methanogenesis. Recently, preliminary work on this concept at pilot scale (400 l, HRT ¼ 30 day) has been carried out by Rana et al. (2002) at IIT, Delhi using stone chips and iron mesh biofilters. For the entire year, the biogas production from the reactor with iron mesh was consistently higher (17%) than that from the conventional reactor. However, certain difficulties such as clogging of the reactor and decay of iron mesh after one year were encountered during the study.
4. Innovations in digester designs Limited efforts have been made to improve the designs of reactors to enhance gas production. A high performance biogas plant was designed and studied by Aili et al. (1991) for treating chicken manure. The coarse particles and feathers of the raw chicken manure were removed in the pretreatment tank. Clear supernatant was pumped into heating tank and when its temperature reached to 38–40 °C, the warm liquid was drained into No. 1 Anaerobic fermenter, then overflowed into No. 2 Anaerobic fermenter (modified UASB) for fermentation to yield biogas. System started quickly under mesophilic temperature. Gas production rate was 3.27–3.87 m3 / m3 day with a maximum value of 4.041 m3 /m3 day. Methane content was also good. In another study carried out by Wanjun (1992) to overcome the weak points of cylindrical biogas digesters like difficult discharging, low gas production rate, etc., a new biogas digester was designed which doubled the gas yield rate. The gas yield had a gradual increase every year and it was about 3–5 times of the cylindrical biogas digester. Based on field measurement, a 6 m3 biogas digester of this design could produce 1692.6 m3 biogas per year, with a volumetric gas production rate of 0.98 m3 /m3 day and gas yield rate amounted to 0.49 m3 /m3 kg TS. Recently Tumchenok (1996) tested a new bioreactor for anaerobic digestion of animal or poultry waste as substrate. The digester re-
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sulted in increased methane percentage in biogas. Kumar (1997) developed a new cost effective family type biogas plant (Konark Model) with high efficiency by changing its shape to spiral. Gas storage volume was increased by 50% compared to 33% that of Deenbandhu and Utkal Model most widely used for their cost effectiveness. Here construction cost was reduced by 10–15% (with respect to Deenbandhu) if constructed with brick masonry and by 30–35 % if constructed using ferrocement technology. It was structurally sound due to its spiral shape and covers least surface area. Its gas storage capacity had been increased and short-circuiting had been prevented by providing a baffle wall with holes inside the digester in between inlet pipe and outlet bottom tank.
availability and mechanical strength in long run. It would help to reduce HRT considerably resulting in cost reduction of biogas plant, without compromising on quantity and quality of biogas. Surprisingly in this promising research area, using high solids content substrate like cow dung slurry most of the studies have been carried out at laboratory level only, whereas these techniques have been successfully tried in the treatment of wastewater under field conditions. An extensive study on this aspect is warranted.
Acknowledgements Authors are grateful to Ministry of Non-conventional Energy Sources (MNES) and CSIR for providing financial support for carrying out this work.
5. Conclusions and future R&D areas A critical analysis of literature reveals that there is a strong possibility to enhance the biogas production under field conditions. Use of certain inorganic, organic additives seems to be promising for enhancing biogas production. Among different types of biomass (plant and crop residues) used as additives, some have been found to enhance the gas production manifolds. However their utility is limited due to the seasonal availability in different regions. Also, clogging of the reactor in the long run is another problem observed under field conditions. Practical aspect of using pure microbial culture as additives should be looked into, in view of certain problems especially human health and ecodynamics. Further techno-economics of using additives on daily basis needs to be worked out by further extensive experimentation at field level. Recirculation of effluent slurry on daily basis and stirring of the digester’s contents by using simple techniques for enhancing biogas production seems to be quite viable under rural conditions. Keeping various parameters within the desired range also improves gas production but the practical difficulty lies in maintaining and monitoring these regularly. It is a crucial point which needs due consideration since a slight change in pH or temperature could otherwise result in reduction of gas production. Similarly formation of volatile fatty acids beyond a particular range hinders the methane production. Loading rate and solid concentration should be properly balanced and continuously maintained. As for the fixed film technique, it has certain merits over the above mentioned methods. Once the material to be used as carrier has been installed in the reactor, the reactor can be operated under normal conditions without any daily addition or monitoring. Different materials (stone chips, iron mesh, clay, wood chips, etc.) had been tried for fixed film reactors depending on their local References
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