Chapter seven: Anaerobic processes, plant design and control

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Digester types
Sizing of digesters
Comparison of alternative design approaches
Problems and solutions of feedstocks and effluents in full-scale biogas plants (based on hobson, 1987)
Control device in an anaerobic digestion process
Process control

 

Digester types

Carrying out anaerobic digestion in a closed reactor, with sufficient volume for the biological reactions to occur without stress, comprises the primary technical requirement. Based on external limitations, such as capital outlay, treatment efficiency, net energy yields and operational skill, the technology available ranges from very rudimentary to quite sophisticated systems for both family scale and full commercial scale. The fact that anaerobic digestion has been used in practical situations for over 100 years demonstrates that it is a viable technology. Problems can arise, however, when there are external constraints, such as limited capital and low operational skills.

Many biogas officers in Developing Countries and commercial companies in Developed Countries have the optimum technology to tailor a plant to specific situation. The following is a summary of the main types of digester in common use.

Batch and Dry Fermentation: This is the simplest of all the processes. The operation involves merely charging an airtight reactor with the substrate, a seed inoculum, and in some cases a chemical (regularly a base) to maintain almost neutral pH. The reactor is then sealed, and fermentation is allowed to proceed for 30 - 180 days, depending on ambient temperature. During this period, the daily gas production builds up to a maximum, and then declines. This fermentation can be conducted at "normal" solids content (6 - 10%) or at high concentrations (>20%), which is then known as "dry" fermentation. Its main components are shown in Figure 7.1.

Figure 7.1: Batch Digester.

One of the most successful biogas programs using batch systems has been that of Maya Farms in the Philippines (Maramba 1978). Using a 1:1 dilution of swine manure (12.5% total solids, 10.0% volatile) and a residence time of 30 days at around 30C, average volumetric efficiencies of around 1.0 0% were obtained. This was achieved with a seed inoculum of 20% by weight of the total digester slurry, which resulted in maximum gas production rates. By using more than 30 reactors extensively, emptying and recharging one each day, a constant supply of biogas is ensured.

As evident from the description of anaerobic digestion up to now, the "Batch" system is inefficient, but cheap to build.

Considerable interest has been shown in "dry" fermentation, a process which Jewell et al. (1981) have worked on for a number of years. They found that fermentation can proceed at total solids concentrations up to 32%. With a . of grass mixed with manure at 25% total solids and 35C, using a manure inoculum of 30% by weight, they obtained volumetric gas productions of 0.79 l\l\d over 60 days, which increased to around 3.0 l\l\d at 55C. They concluded that such a reactor would have to be started only once a year. The unloading and use of the digested slurry can therefore be planned in advance.

The stage of development of "dry" fermentation and the process parameters need further work and research. However, even at this stage it appears to be a viable technology, and its gas production rates are competitive with semi-continuously fed reactors.

Fixed Dome (Chinese): A fixed dome biogas digester was built in Jiangsu, China as early as 1936, and since then considerable research has been carried out in China on various digester models. The water pressure digester was developed in the 1950s. In one variation, the displaced effluent flows on to the roof of the reactor, enabling the roof to withstand the gas pressure within more easily.

In terms of absolute numbers, the fixed dome is by far the most common digester type in Developing Countries. This reactor consists of a gas-tight chamber constructed of bricks, stone or poured concrete. Both the top and bottom of the reactor are hemispherical, and are joined together by straight sides. Some new structural Considerations have been published lately (Tentscher 1989). The inside surface is sealed by many thin layers of mortar to make it gas tight, although in the old type digesters gas leakage through the dome was often a major problem. This was changed in the new Chinese type of design. The digester is fed semi-continuously (i.e. once a day), the inlet pipe is straight and ends at mid-level in the digester. There is a manhole plug at the top of the digester to facilitate entrance for cleaning, and the gas outlet pipe exits from the manhole cover.

The gas produced during digestion is stored under the dome and displaces some of the digester contents into the effluent chamber, leading to gas pressures in the dome of between 1 and 1.5 m of water. This creates quite high structural forces and is the reason that the reactor has a hemispherical top and bottom.

At present there are about 5 million family-sized fixed dome plants of 6, 8 and 10 m3 digester volume operating in China, and the target is 400,000 to 500,000 of the small size household digesters and 25,000 for medium and large scale (farms, distilleries, etc.) annually. Although China has the largest construction activity of all countries in the region, it is slow in relation to the huge potential. The new attitude is to develop positively, pay attention to both construction and to management, and seek for practical benefits. Sound work and quality in construction are being emphasized. Construction of family sized fixed dome digesters is well advanced and already standardized at national level. Solid experience is available with regard to the properties of construction materials, technique and design. This is outlined in training material. Construction material and technique is selected at the site (e.g. brickwork, lime-mortar, cement-mortar, concrete cast-in-place, etc.) to keep costs low. A brick dome may be constructed on an umbrella-shaped framework or a concrete cast-in-place digester used.

The design of fixed-dome digesters has been developed in China in respect of (a) the four important horizontal lines; (b) gas pressure; (c) average rate of gas production; (d) gas storage; (e) digester size; (f) geometric forms, loads and forces. The inside water level at ambient pressure is at 95% of total volume. The gas pressure in fixed-dome digesters is equal to, or below, 120 cm of water. Ratios of key dimensions are kept constant, e.g. diameter to height of the cylinder is 2:1. The HRT, for both cow and pig manure, is 35 - 40 days at total solids concentrations of 5 8% an] 4 - 7%, respectively. Gas production varies between 0.15 0.6 m per day, depending on ambient temperature. The state of development of fixed dome digesters is quite advanced and much is known about material, methods of construction, cost, suitable digester feedstock and gas production rates.

Figure 7.2: Fixed Dome (Chinese) Digester

Floating Dome (Indian or KVIC Design): In India, the history of biogas technology has developed since 1937. In 1950, Patel designed a plant with a floating gas holder which caused renewed interest in biogas in India. The Khadi and Village Industries Commission (KVIC) of Bombay began using the Patel model biogas plant in a planned program in 1962, and since then it has made a number of improvements in the design.

The Floating Dome digester is disseminated by KVIC and workshops recognized by KVIC. Those most commonly constructed are of 6 and 8 m3 gas production capacity. The digester is designed for 30, 40 and 55 days' retention time: the lowest time applies to the hot southern States, the highest to the cooler northern States. Construction costs vary according to ambient temperatures, for which partial compensation is allowed for by subsidies. The main material fed is cattle manure. At community plant level, nightsoil is digested in a mixture with cattle dung, and at large farm level other types are being introduced, to digest materials such as water hyacinth. The drum was originally made of mild steel, until fiberglass reinforced plastic (FRP) was introduced successfully, to overcome the problem of corrosion. Nearly all new digesters are equipped with FRP gas-holders.

The cost of a mild steel gas-holder is approximately 40 50% of the total cost of the plant. FRP gas-holders are 5 - 10% more expensive than the steel drum. The following Table gives the cost of FRP drums in two different enterprises visited.

Table 7.1. Comparison of Cost of FRP Gas-holders

For Digester of
m
3 gas/day

KVIC turnkey worker
near Bombay

Industrial Enterprise
near Coimbatore

Weight (kg) Cost (Rs) Weight (Kg) Cost (Rs)
3 - 3,200 48 4,000
4 - 3,400 - -
6 - 4,200 70 5,800
8 - 5,200 85 7,150
10 - - 193 10,500
No. of fiberglass layers 2   3
From Tentschner (1989).

The daily production capacity of FRP drums by a KVIC turnkey worker near Bombay was higher than steel drum production capability: 3 FRP drums can be made in 5 work days, while 2 steel drums require 8 work days. The floating dome design, upon which the KVIC model is based, is used extensively throughout the world. A typical KVIC design is shown schematically in Figure 7.3. The reactor wall and bottom are usually constructed of brick, though reinforced concrete is sometimes used. The gas produced in the digester is trapped under a floating cover, which rises and falls on a central guide. The volume of the gas cover is approximately 50% of the total daily gas production. The pressure of the gas available depends on the weight of the gas holder per unit area, and usually varies between 4 - 8 cm of water pressure.

Figure 7.3: Floating Dome (Indian) Digester

The reactor is fed semi-continuously through an inlet pipe, and displaces an equal amount of slurry through an outlet pipe. When the reactor has a high height: diameter ratio, a central baffle is included to prevent short circuiting.

Most of the KVIC type digesters are operated at ambient temperatures, so that retention times depend on local variations. Typical retention times are 30 - 40 days in warm climates, such as Southern India, where ambient temperatures vary from 20 - 40C, 40 - 50 days in moderate climates, such as the Central and Plains areas of India, where minimum temperatures go down to 5C, and 50 - 80 days in cold climates, such as the hilly areas of Northern India, where minimum temperatures go below 0C.

Typical feedstock is cattle dung, although substrates such as agricultural residues, nightsoil and aquatic plants have been used. Cattle manure, generally about 20% solids, is diluted to 10% total solids before feeding, by adding an equal quantity of water. The daily average gas yield varies from 0.20 to 0.60 volume of gas per volume of digester ratio in cold to warm climates.

Many laboratories, universities, and industries throughout the world, and especially in India, continue to improve the KVIC design. Efforts are being made to optimize the design parameters, to improve the volumetric efficiency, and make the facilities economically and structurally sounder. Heating, mixing and insulation have been introduced on an experimental basis, as well as modifications in geometric configurations and locations of inlets and outlets.

Janata Model. This type of digester is disseminated by the Indian NGO network of AFPRO and many government agencies. It is 20 - 30% cheaper than the floating drum model, and uses local materials to a much greater extent. It has the limitation that only good quality materials and expert construction produce a satisfactory plant.

Nearly all sizes of Janata digesters are designed for 60 days' retention time. This is a clear disadvantage compared to the KVIC type. A standardization of three different retention times has yet to be approved by DNES. A wide range of sizes, from 2 - 30 m3 daily gas production are made; the most common sizes are 2, 3, 4 and 6 m3 gas/day. Discussions are being held to change and improve the design of the Janata model.

Table 7.2: Janata Model: Loading and Production Characteristics.

Gas prod-
uction
(m /day)
Daily
fresh dung
(Kg)
Volume of dig-
estion chamber
(m
3)
Gas prod-
uction
(m
3 /m3 /day)
2 50 6.0 0.34
3 75 9.5 0.33
4 100 12.0 0.33
6 150 18.5 0.32

Source: Khandelwal and Mahdi (1986)

Up to 1986, a total to 642,900 digesters had been built in India. Community and Institutional Biogas Plants (IBP) are also being constructed. Poor farmers and low castes are supposed to be involved and to participate in operation of Community Plants (CBP).

Table 7.3: Total Number of Digesters Constructed in 1985/6 at Family Level and Approximate Share of Executing Agencies.

Agency Number % Type
KVIC 20,000 10.8 Floating drum
Governmental 156,000 84.4 mainly Janata
AFPRO/NGO 9,000 4.8 Janata
Total 185,800 100.0  

Table 7.4: Installation of Family-Sized Biogas Plants by KVIC

Period No. per Period Cumulative Total
Up to March 1975 13,508 13,508
During Vth Plan 65,905 80,113
1980/81 7,964 88,077
1981/82 9,180 97,257
1982/83 11,033 108,290
1983/84 15,029 123, 319
1984/5 18,224 141,543
1985/86 (target) 20,000 161,543

Source: KVIC (1984)

Table 7.5: Completed Community and Institutional Biogas Plants up to End 1985

Period Up to 31.3.85. During 1985/86 Total
Digester type CBP IBP CBP IBP  
Number 48 53 24 9 134

Source: Annual Report 1985/86. DNES, Ministry of Energy.

Bag Design (Taiwan. China): The bag digester is essentially a long cylinder (length: diameter 3:14) made of PVC, a Neoprene coated nylon fabric, or "red mud plastic" (RMP), a proprietary PVC, to which wastes from aluminum production are reported to be added. Integral with the bag are feed and outlet pipes and a gas pipe (see Figure 7.4). The feed pipe is arranged so that a maximum water pressure of approximately 40 cm is maintained in the bag. The digester acts essentially as a plug flow (unmixed) reactor, although it can be stored in a separate gas bag (Park et al. 1979).

Figure 7.4: Bag-Red Mud (Taiwan, China) Digester.

The basic design originated in Taiwan, China, in the 1960s (Hao et al. 1980), due to problems experienced with brick and metal digesters. The original material used, a Neoprene coated nylon, was expensive and did not weather well. In 1974 a new membrane, RMP, was produced from the residue from aluminum refineries. It was inexpensive and has a life expectancy of up to 20 years (Hong et al. 1979). Due to its availability, PVC is also starting to be used extensively, especially in Central America (Umana 1982). The membrane digester is extremely light (e.g. a 50 m3 digester weighs 270 kg), and can be installed easily by excavating a shallow trench, slightly deeper than the radius of the digester. Due to its simple construction, and the fact that it is prefabricated, the cost of this digester is low, around $30 per m3.

The Taiwanese evolved the bag digester primarily to treat swine manure, which is also the most common substrate in Korea and Fiji. Due to its low cost and excellent durability, the Chinese have also started producing these digesters, and claim that the cost is as low as $25 to $30 per m3. Depending on the availability of the plastic, a rapid expansion in the use of bag digesters is expected in China, and in time it may replace the fixed dome as the preferred type in China.

Typical retention times in bag digesters, for swine waste, vary from 60 days at 15 - 20C, to 20 days at 30 - 35C. One advantage of the bag is that its walls are thin, so the digester contents can be heated easily if an external heat source, such as the sun, is available. The Chinese have found that average temperatures in bag digesters, compared with dome types, are 2 - 7C higher. Hence specific yields can be from 50 - 300% higher in the bag (0.235 - 0.61 volumes of gas per volume of digester per day). Park et al. (1979) also found this to be true in Korea, and obtained specific yields varying from 0.14 in winter (8C) to 0.7 in summer (32C) for swine manure.

In their present state of development, bag digesters appear to be very competitive, because of their low cost. However, more data are needed on their durability, with regard to weather and mechanical failure (e.g., sharp objects piercing the bag). The potential for increasing their performance by heating with solar tents should also be explored.

The RMP or semi-plastic digesters in China are commonly batch type dig esters. They are filled with straw and manure and operated for 6 - 8 months. Discharging is easy because the RMP gas-holder can simply be removed. Even when cracks in the wall occur, it does not affect gas production. The film is fixed with bricks in a water jacket and seals the digester completely. The RMP material is also used as a gas barrier, for pipes and hoses and for many other applications outside the biogas sector. At present there are about 50,000 RMP digesters of over 10 m3 operating in China.

Plug Flow Design: The plug flow reactor, while similar to the bag reactor, is constructed of different materials and classified separately. A typical plug flow reactor consists of a trench lined with either concrete or an impermeable membrane (see Figure 4.5). To ensure true plug flow conditions, the length has to be considerably greater than the width and depth. The reactor is covered with either a flexible cover gas holder, anchored to the ground, or with a concrete or galvanized iron top. In the latter type, a gas storage vessel is required. The inlet and outlet to the reactor are at opposite ends, and feeding is carried out "emi-continuously, with the feed displacing an equal amount of effluent at the other end.

Figure 7.5: Plug Flow Digester.

The first documented use of this type of reactor was in South Africa in 1957 (Fry 1975), where it was insulated and heated to 35C. Specific yields (vol. gas/volt digester/day) of 1 - 1.5 were obtained, with retention times of 40 days and loading rates of 3.4 kg total solids per m / day.

Jewell and his colleagues at Cornell University have carried out a considerable amount of work on this design over the last 8 years. Hayes et al. (1979) described a comparison between a rubber lined plug flow reactor and a completely mixed digester. Both had a total volume of 38 m , and were fed on dairy manure at 12.9% total solids. Their results are summarized in Table 7.6. Digester temperatures were not stated, but it is assumed that both were maintained at 35C.

Table 7.6: Comparison of Completely Mixed Digester with Plug Flow Digester

  Completely Mixed Plug Flow
HRT (d) 15 30 15 30
Specific volume (m3 gas/m3 reactor/days) 2.13 1.13 2.32 1.26
Specific gas production (m3 /kg VS Added) 0.281 0.310 0.337 0.364
Gas composition (%CH4) 55 58 55 57
Volatile solids reduction (%) 27.8 31.7 34.1 40.6
Reference: Hayes et al. (1979).        

The plug flow reactor gave higher gas production rates than the completely mixed one. The high specific yield, compared with figures for typical fixed dome and floating dome designs, of 0.1 0.3, are due to higher temperature and higher loading rates. At 20C the plug flow reactor yields about 0.42 volumes of gas per volume of digester per day. At typical lower loading rates (9% versus 12.9% total solids) this figure would decrease to around 0.29.

Anaerobic Filter: Apart from the batch digester, all the designs discussed above are known as suspended growth systems, and when there is no recycling of solids, the hydraulic retention time (HRT) is equal to the biological solids retention time (f ). Due to the slow growth of anaerobic organisms, f has to be of the order of 20 - 60 days, depending on the temperature, in order to prevent the active organisms from being washed out, and process failure occurring. HRT is also high, and reactor volumes are substantial, leading to specific yields.

In order to reduce reactor volume, a unit known as the immobilized growth digester has been evolved. One of the earliest and simplest types of this design was the anaerobic filter. This typically consists of a tall reactor (H/D = 8 - 10) filled with media on, or in which the organisms can grow or become entrapped (see Figure 7.6). Media used have varied from river pebbles (void volume = 0.5) to plastic media (0.9), although any material which provides a high surface area per unit volume is suitable. The choice of media depends on considerations such as cost, void volume, availability and weight. The waste to be treated is usually passed upward through the filter, and exits through a gas syphon, although downflow configurations can be used. The organisms growing in the filter consist of two types: those attached to the media, and those entrapped in a suspended form, within the interstices of the media. At low hydraulic loading rates, both sorts are prevalent, while at high hydraulic loads the suspended organisms are washed out, leaving only the attached forms. Due to entrapment and attachment, high biological solids retention times (f ) are possible, at very low HRTs.

Additional information is presented in Chapter 6, concerning wastewater treatment by anaerobic filter systems.

Figure 7.6: Anaerobic Filter.

Because of the physical configuration of the filter, only soluble wastes can be treated without blockage, although diluted pig waste has been treated successfully, with a total solids content of 2% (Chavadej, 1980). Waste strengths from 480 ppm COD up to 90,000 ppm COD have been treated in filters, and retention times as low as 9 hours, based on void volume, are possible with COD removal of 80% (Young and McCarty 1969). However, more typical retention times are in the order of 1 -2 days (Arora and Chattopadhya, 1980), and at these times over 90% COD removals are possible. Loading rates as high as 7 kg COD per m /day are possible, and under these conditions specific yields of 4.0 have been measured (Xinsheng et al. 1980).

Anaerobic Baffled Reactor: This design, which is very recent, was evolved by Bachmann et al. (1982) at Stanford University. The reactor is a simple rectangular tank, with physical dimensions similar to a septic tank, and is divided into five or six equal compartments, by means of partitions from the roof and bottom of the tank (see Figure 4.7). The liquid flow is alternately upward and downward between the partitions, and on its upward passage the waste flows through an anaerobic sludge blanket, of which there are five or six. Hence the waste is in intimate contact with active biomass, but due to the design, most of the biomass is retained in the reactor.

With a soluble waste containing 7.1 g/l COD and a retention time of 1 day at 35C, Bachmann et al. (1982) obtained 80% removal efficiencies of COD, with a specific yield of 2.9. Similar tests have been carried out with diluted wastes (0.48 g/l COD) and similar performance we. obtained at 25C. Due to its physical configuration, this type of reactor appears to be able to treat wastes with quite high solids contents, and hence may be an alternative to anaerobic filters. Since the process is new, little developmental work has been done on it, but it could be applicable in Develanina countries under certain circumstances.

Figure 7.7: Anaerobic Baffled Reactor (ABR).

Additional information is presented in Chapter 6 concerning wastewater treatment by anaerobic filter system.

Anaerobic Contact Process: This process in similar to the aerobic activated sludge process, in that cell recycling is used to maintain high (f ) at low HRT. Hence, good removal efficiencies can be obtained with small reactors. Since the anaerobic sludge is still actively producing gas when it exits from the digester, problems have been experienced in getting it to settle quickly. Various methods have been used to get around this problem, including thermal shock and vacuum degasification (see Figure 7.8).

The first recorded instance of use of the anaerobic contact process occurred in 1955 (Schroepfer et al. 1955) where waste from a meat packing house (BOD 1.6 g/l) was treated successfully at retention times of only 12 hours at 35C. BOD removals of 95% were obtained, at loading rates of 3.2 kg BOD per m3 /day and, even at 25C, removals of 95% were achieved. Many food wastes can be treated efficiently using this process. With rum stillage (COD 54.6 g/l) removals of 80% were obtained, at loading rates as high as 8.0 kg COD per m3 /day (Roth and Lentz 1977). Raw sewage (COD 1.2 g/l) has been treated at 20C, with low retention times (22 hours) in a contact process, and high removals (90%) were obtained (Simpson 1971).

While some full scale plants are currently operating in Developed Countries, there are no known plants in Developing Countries. With high- strength industrial wastes, it would appear that other anaerobic processes (e.g., filter, ABR) would be just as efficient, easier to operate, and require less capital outlay.

Figure 7.8: Anaerobic Contact Digester

Upflow Anaerobic Sludge Blanket (UASB): This process is quite recent, and was developed by Lettinga et al. (1979, 1980) in the Netherlands. The reactor consists of a circular tank (H/D = 2) in which the waste flows upward through an anaerobic sludge blanket comprising about half the volume of the reactor (see Figure 4.9). An inverted cone settler at the top of the digester allows efficient solid-liquid separation. During start-up, the biological solids settle poorly, but with time a granular sludge develops that settles extremely well, and the active biomass is retained within the reactor.

With predominantly soluble industrial wastes 3 (potato processing wastewater) loading rates as high as 40 kg/m /day COD are possible, with retention times as low as 3.5 hours. Under these conditions volumetric gas production figures of 8.0 are possible (Lettinga et al., 1980). Since the process does not use media to maintain the active biomass, total solids content in the feed can be as high as 3%. Operation requires a relatively high degree of sophistication, especially during the critical start up phase. In most cases, alternative designs (filter, ABR) are available with a lower degree of complexity.

Inclined Tubular digesters: The effects of aspect ratio, digester inclination to the horizontal and retention time were investigated (Chapman et al. 1988) and its performance on laboratory scale investigated (Floyd and Hawkes 1986). The digester is a modified form of the horizontal displacement digester: the digestion vessel is tubular, but inclined at an acute angle to the horizontal. Thus, the main advantages of a horizontal displacement digester are retained, while the exposed surface area of the digester contents, where scum and crusts can form, is minimized. It is also mechanically simpler to remove any scum and crust which does form. Operation of the digesters was reliable, and there were no recurring problems. Gas yields ranged from 0.282 - 0.318 m /kg VS added, with whole slurry at 35C. The greater gas yields from the inclined tubular digesters are attributed to longer retention of particulate material within the digester than in CSTR systems. The main applications of this design are likely to be for treating particulate waste" of <8% TS concentration, where some settling will occur.

Figure 7.9 Upflow Anaerobic Sludge Blanket (UASB).

 

Sizing of digesters

As discussed in Chapter 4, anaerobic digestion depends on the biological activity of relatively slowly reproducing methanogenic bacteria. These must be given sufficient time to reproduce, so that they can replace cells lost with the effluent sludge, and adjust their population size to follow fluctuations in organic loading and the generation of volatile acids and other substrates from the earlier steps. If the rate of bacteria lost from the digester with the effluent slurry exceeds the methanogen growth rate, the bacterial population in the digester will be Uwashed out" of the system. Washout is avoided by maintaining a sufficient residence time for solids, and thus bacterial cells remain in optimal concentration within the digester.

Designing a properly sized digester to obtain the maximum biogas production per unit of reactor volume is important in maintaining low capital construction costs. The digester should be sized to achieve desired performance goals in both winter and summer, and must be large enough to avoid "washout."

Design goals could be maximal gas production with minimal capital investment, achieving pollution control and reduction of pathogens, or simply the production of a reasonable amount of gas with a minimum of operational attention. The uses of the slurry after the digestion process is a critical consideration since the main income to the plant can come from that material. The differences in uses also determine the digestion retention time. Criteria must be established, prior to design, since not all goals can necessarily be achieved with a single design. Assuming that adequate performance data are available for the feedstock under anticipated operating conditions, the digester can be designed for size and other features, such as the degree of heating and mixing, to meet the desired criteria.

Optimal methane production per m3 digester capacity must allow a margin of safety in size, equal to several days' additional retention beyond "optimum", to ensure that occasionally stressful environmental conditions will not upset the maintenance of a viable methanogenic bacterial population. The extremely large safety factor used in conservatively designed sewage sludge digesters, to enhance pathogen destruction and pollution control of even toxic feedstock is not the most cost efficient for methane production. Desired results must therefore be determined prior to making sizing decisions.

A number of empirical methods have been employed in the design of conventional sewage sludge digesters, where emphasis has been pollution control and ecological reasons, rather than maximum biogas production. This situation has not been common in Developing Countries until recently, but in Indonesia, for instant, the anaerobic system that digests palm oil was designed and erected only because of ecological and legislative reasons. Conservative design parameters applicable to sewage sludge treatment also result in digester sizes of up to 50% larger than those needed for plants designed for maximum methane production. The ecological and environmental issues are receiving more importance in many countries, and will lead in the near future to different types of digesters.

Design of large installations, particularly those with a typical feedstocks, is based on a fundamental understanding of the anaerobic processes which achieve the desired performance goals. Large installations can also afford considerable attention to operation, allowing refinements in the process, such as control of temperatures, volatile acids and feedstock pumping, to improve biogas production rate per unit digester volume.

A major criterion for size in the design of anaerobic digesters is the mean cell (or solids) residence time (c) . The mean cell residence time is defined as the mass of bacterial cells in the digester divided by the mass of cells removed from the digester per day. For a conventional digester without solids recycle, is equivalent to the hydraulic retention time, (HRT), and is thus directly related to digester volume. It has been found that at a given temperature, most digester performance parameters of interest can be correlated with c, and that washout can be avoided if is maintained above a critical minimum value, cm.

In industrialized countries, heating of digesters is common practice, and the trend has been toward thermophilic digestion, while in Developing Countries digesters are usually operated at ambient temperature". Because the anaerobic digestion process essentially stops at 10C, the digester contents must be maintained at a temperature higher than this for significant gas production. Therefore design is based on critical temperature periods of the year, using anticipated temperature within the digester rather than ambient air temperature.

It is evident from Yeoh (1988) that utilization of biogas from the anaerobic treatment of palm oil brings about significant economic gains, which are particularly convincing in thermal conversion systems whose efficiencies have been reported to range from 0.7 - 0.9, compared with conversion efficiencies of 0.25 0.35 in electrical and mechanical systems. However, it is to be stressed that the assessment assumes high utilization of the biogas produced (80%), which may not be easily achievable at the present stage of development, considering the fact that the palm oil milling operation is largely self-sufficient, in terms of energy, through the use of fibre and shell; and therefore biogas produced from its effluent treatment plant represents an energy source mainly in excess of its own requirements. The economics of the anaerobic treatment system, as a revenue investment, therefore depends very much on the extent and the way that the biogas is used, particularly off-site.

Economic evaluation should be viewed only as a broad guide to the comparative cost-benefits of resource recovery from the anaerobic treatment of palm oil. Although it does provide a macro- view of the effects of digestion temperature and mode of biogas utilization on the bio-methanation system, through the translation from technical terms to tangible financial terms, the figures quoted should be taken as estimates. This is because important factors, such as operational variability and inflationary effects on costs and revenues were not considered. Furthermore, the benefits arising from biogas utilization are very sensitive to the actual value of the energy form for which it is substituted, which may vary considerably from case to case.

Anaerobic digesters can utilize a large number of organic materials as feedstocks. These include animal manures, human wastes, crop residues, food processing and other wastes, or mixtures of one or more of these residues and wastes. Animal manures exhibit good nutrient balances, are easily slurries and are relatively biodegradable. The range of biodegradability reported varies from 28 - 70%. This variation is partly due to the diet of the animals and amount of bedding to the animal that is also used digestion. For example, Hashimoto and Chen (1981) showed that as the percentage of silage is increased, at the expense of ground corn, the degradability of the manure decreases, since silage contains a high percentage of lignocellulose materials. Thus, in Developing Countries, where cattle are fed agricultural wastes, the manure is less biodegradable than where cattle are fed ground grains or commercial feed.

Fresh manure is much more biodegradable than aged and/or dried manure because of the substantial loss of volatile solids over time.

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