Fish feeding with digested cow manure

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When digester slurry is used in ponds, the nutrients stimulate the growth of both phytoplankton (algae) and zooplankton (daphnia and crustaceans), which the fish harvest. Alviar et al. (1980) investigated the growth of fish in an integrated farming scheme in the Philippines. The average yield of Tilapia nilotica was 25 kg/m² every two months (19 tonnes/ha/year).

In southern China, cultivation of fish in ponds is common. Normally the fish are fed wheat bran pellets. In recent years digester slurry has been used as a feed supplement, increasing fish production and decreasing costs for feed (National Office for Biogas, 1982).

The use of untreated manure for feeding fish has been a common usage in the Far East for many years. The use of cow manure for the enrichment of fish ponds, however, is relatively uncommon (Hepher and Schroeder 1977). This is because cows are allowed to roam freely, while pigs and poultry, which are raised in larger numbers than cows, are kept in feedlots associated with the ponds, and their manure is utilized for feed in the ponds. Studies on the use of organic fertilizer in aquiculture have been undertaken in Israel (Moav et al. 1977; Rappaport and Sarig 1978) and the United States (Buck et al. 1978). The effect of liquid manure on growth in polyculture of several varieties of fish was studied in Israel by Moav et al. (1977).

Anaerobically digested cow manure (mesophyllic) was used as feed in fish ponds in 1976 (Schroeder et al. 1976; Marchaim and Criden 1981). These experiments were conducted in polyculture, with common carp, tilapia and silver carp. The ponds were divided into 3 groups, according to differences in treatment: the first group was fed pellets; the second group, liquid cow manure; and the third, digested cow manure. The tilapia grew at the same rate in all three groups, but the carp fed on pellets grew faster than the other two groups. The researchers were unable to determine the effect on the manure on each species in this experiment, though it did indicate that tilapia is probably one of the species best suited to feeding on slurry.

Dissolved oxygen was also measured in the early morning hours, and varied between 1 - 8 ppm, remaining at over 3 ppm for 80% of the time. No correlation was found between feed treatment and dissolved oxygen. Primary production studies were conducted to estimate the effects of organic materials on fish yield. Rates of photosynthesis were found to be only slightly higher in slurry- fed ponds than in chemically fertilized ponds. In ponds fed with slurry plus feed, a higher proportion of zooplankton was found in the total plankton population than in non-manured ponds.

As a result of the three seasons of experiments, it was felt that digested slurry can be used in fish ponds, thereby saving 50% of pellets used, with a considerable influence on fish pond economics. Several other experiments have been conducted by the "NEFAH" group at kibbutz fish ponds, using different slurries and diet compositions. In the Migal Laboratories, growth rates of tilapia were examined with (a) high protein pellet diet (28%); (b) 50% low protein pellets (21%) + 50% wet untreated effluent; (c) 50% low protein pellets + 50% sun-dried effluent (dry matter basis) by Degani et al. (1982).

The results of this study show that digested slurry can replace 50% of the food in fish ponds, but that the different kinds of slurry are not equal in their effects. In a study of the influence of the liquid fraction of the digested slurry on tilapia culture, it was found that the low level of carbohydrates was replaced by growing algae, to balance the ratio of metabolic energy to protein in the diet. It was found that the liquid fraction of the slurry may be important in improving the oxygen level, raising primary production and the concentration of chlorophyll a.

Marchaim et al. (1983) showed that in fish ponds, substituting 25% of the food with cow manure gives the same production from the pond as regular feed. The level of oxygen is higher in the ponds fed with slurry than in control ponds fed with regular feed. In intensive growth ponds (22,000 - 30,000 fish/ha of 350 g fish) growth rates of carp were much higher in ponds fed with a 30% substitution of feed by the liquid fraction after sieving through a vibrating screen, than in control ponds.

Fang Xing and Xu Yiz Hong (1988) found that the big-manure, when put in the fish pond, can be used to breed plankton in the water for feeding fish, achieving good results. The Chinese way of doing it is to spread the slurry into the fish pond at 400 kg per mu every three days. Under this management the breeding of fish with big-manure is best. It gives five benefits:

(1) There are many nutrients in biogas manure. It can breed plankton in water for fish nourishment;
(2) It has been fermented completely, and therefore it can not consume more dissolved oxygen and does not reduce the quality of the water;
(3) It change the colour of water into a drab tea-colour, contributing to absorption of heat from the sun. The temperature of water is raised, and contributes to the fish growth;
(4) After anaerobic fermentation, bacteria and eggs of parasites in the big-manure have been killed: it therefore reduces fish diseases;
(5) The pH of big-manure is neutral, improving fish growth.

Effluent as a substrate for growing plants and crops

Uses for horticulture
Growth and rooting experiments

It is widely accepted that an improved growth medium for horticulture and mushroom production is obtained when organic materials are included. The most common organic component is peat moss (Chen et al. 1984) which may also serve as the sole component of growth medium. The use of peat is, however, accompanied by some problems:

(1) The price of horticultural peat is high, and shipping it long distances considerably increases its price;
(2) Peat resources throughout the world are limited and nonrenewable (ibid.);
(3) In some cases, sterilized peat serves as an enrichment medium for various phytopathogenic fungi species, such as Pitum sp. It seems, therefore, that finding substitutes for peat is an important task for soil scientists and horticulturists.

Liquid digested slurry was found to be an unsuitable growth medium, completely inhibiting the germination of seeds. Experiments were therefore conducted with thermophilic effluent, after sieving through a vibrating screen and using the coarse fibre fraction (called "Cabutz"), after leaching the water to reduce salinity.

Raviv et al. (1983) testing herb plants on 5 different media, for development and regenerative ability under outdoor conditions, found that Cabutz gave better results than peat-moss or rock wool. Similar results were obtained with most (but not all) house-plants, and Cabutz is now sold as a substitute for peat- moss to many commercial greenhouses and plant nurseries in Israel.

Table 9.8: Physical comparison of "Cabutz" and other substrates

The chemical compositions of Cabutz and enriched Finland peat are also similar, though Cabutz is not as stable as peat, and chemical changes occur during storage.

Sieved, dried effluent from thermophilic digestion in the Kaplan Industries plant in Florida (built by Hamilton Standard, Inc. of Connecticut) is also sold as soil conditioner to nurseries (Coe and Davenport 1981).

The intensive work done on thermophilic digested slurry has given rise to the suggestion that some special characteristic is gained during fermentation at 55°C. This is being examined by the Volcani Institute, Israel. One of the results of growing crops on Cabutz was a faster growth of the root system than on other substrates; and Cabutz also had a positive effect on the growth rates of corn and wheat. Raviv et al. (1982) examined the possibility of achieving higher yields of crops on Cabutz, on the basis of ideas mentioned above ("Fish feeding with digested cow manure"). The purpose was to harvest the foliage after a short period, for direct feeding or silage. The possibility of resowing on the same substrate was also examined. In spite of a relatively low percentage of germination, it appeared that, after the period of germination, an increase of approximately 80 g/m /day in weight was achieved in the foliage alone. To this, the high rate of root growth has also to be added, probably the main contributing factor to the high growth rate. After 4 seasons of growing corn on the same Cabutz, winter wheat was also grown on it. It was estimated that 10 - 20 kg/m dry matter could be grown in one year, but this very high yield has yet to be confirmed in field experience. It is also noteworthy that the liquid fraction can be used as a fertilizer, and that the quantity of water needed per ton of dry weight was considerable less, though the irrigation program had to be carefully controlled.

The amount of pesticides and herbicides required was also very small, since most of the pests and herbs are destroyed by thermophilic digestion (see below). This phenomenon is of special importance in mushroom cultivation. Results from Migal Laboratories in Israel (Levanon et al. 1983) showed an increase of almost 20% in yields of mushrooms grown on Cabutz, instead of casing soil, with an acceleration of growth in the first 3 cuttings.

Uses for horticulture

Chen et al. (1984) have described the main physical and chemical properties of Cabutz, as compared to sphagnum peat-moss. The similarity in major physical properties is Striking: The bulk density of Cabutz ranges from 0.08 - 0.12 g/cm³ compared to 0.09 g/cm³ for peat; particle density is the same (1.6 g/cm³ ); the porosity for Cabutz was calculated to be 93 - 95% compared with 95 - 97% for peat-moss; the hydraulic conductivity value for Cabutz and peat is 120 - 150 cm/in and 150 cm/in, respectively. However, a significant difference in the water/air ratio should be noted. One of the most important requirements of a growth substrate is its ability to hold and supply large quantities of water, while at the same time it should be structurally adapted to entrap large volumes of air. The minimum air space in peat should be around 15% of volume, while the ideal value is around 20-30%. When saturated Cabutz was allowed to drain for two hours, the air space was found to be 32%, while in peat moss it reached about 18%. After 24 hours of drainage, the air space was 43% and 24%, respectively. Cabutz is well aerated, because of its comparatively large particles. As a result, it also requires more frequent irrigation.

The chemical properties of Cabutz are close to those of enriched sphagnum peat, although generally higher values for N. P. K were observed in Cabutz. Experiments with sensitive plants grown on Cabutz have shown response to fertilization with iron chelates (Raviv et al. 1983).

Growth and rooting experiments

Trials on growing cucumbers in different Peatrum treatments were conducted in a nursery in Israel. The control consisted of the standard growth medium consisting of 50% sphagnum peat moss (either Finnish or Dutch) and 50% perlite no. 4 or fine tuff (0 - 8 mm). The experimental substrates, after 3 h of leaching, replaced the peat-moss. Each replicate consist of 48 seeds. The experiment was designed in randomized six blocks. Irrigation by spraying was applied for 30 sees each hour. Plants were fertilized, from the sprouting stage, with 3 1 Sheffer 737 (Israel Chemicals, Ltd.: mixture of 7.3% N. 3.2% P₂O₅, 6.5% K₂O and micro-elements) to a tray daily. 20 days after sowing, plants were harvested and dried at 65°C for a week.

In the comparison of Peatrum (the slurry of digested rumen content) with Cabutz and peat-moss for growing cucumbers, rooting rates were initially similar, but the growth-rate of plants in Peatrum was slower, and there was a clear lack of balance in the nutrient supply. The beneficial effect of organic materials as a component in rooting media can be partly explained by the presence of root-promoting materials in the products of decomposition, and of humic acid compounds. These materials are more abundant in peat moss and Cabutz than in Peatrum, which may explain the lower growth-rate in the latter. Composting probably generates some of these compounds, and a suitable nutrient supply (iron) must be added. Peat-moss has a higher fibre content than Cabutz, and the higher the fibre content, the longer the period of comporting required.

The use of composted and uncomposted Peatrum as a growth medium for seedlings and plants in greenhouses showed that it is of lower quality than Cabutz, but this could be the result of not adequate composting and of some deficiencies in oxygen and nitrogen.

Uses of effluent for mushroom production

Standard chemical and physical methods were defined by Levanon et al. (1984) for the analysis of substrates for mushroom production. These parameters were set according to the local conditions in Israel, and they enabled the development of a quality control system for the production of substrates for mushrooms.

Most of the substrates for mushroom production are organic wastes or by-products. Agricultural by-products, as well as industrial and municipal wastes, are used, mainly via composting, to produce raw materials for mushroom production. There is a need for standard chemical and physical parameters that will allow production of the proper substrate for each mushroom variety in every country, with local wastes or by-products. Of supreme importance is the knowledge of the nutritional needs of every mushroom variety. In the present study, standard chemical and physical methods were defined for the analysis of substrates for mushroom cultivation. The use of these methods allows manipulation of the substrate composition, according to the needs of the growing fungi in important stages of its life cycle. These parameters enable the development of quality control systems, for the production of proper substrates for mushroom cultivation.

Chemical and physical parameters for qualification of substrates (composts and casing material) for mushroom production were selected after a series of experiments. Those of critical importance in various stages of substrate preparation and mushroom growth are: moisture, ash, organic matter, total and ammonia N. crude protein, lignin, cellulose and hemicellulose. Physical parameters are essential, especially in the qualification of substrates for casing soils, because their main role is to provide the proper physical conditions for fruit bodies development. The most important parameters are: electrical conductivity, volume weight, specific density, porosity and water holding capacity.

An investigation was conducted on the suitability of Cabutz as a casing soil in the production of mushrooms (Agaricus bisporus). In recent years, peat moss has been used as the main component of the casing material. The casing material has to fit the following requirements:

a) Sufficient water holding capacity to serve as a water reservoir for the growing mushroom crop;
b) good structure - porosity, to maintain gas exchange between the compost surface and the growing rooms air;
c) pH values in commercial casing are usually 7.3 - 7.8;
d) the presence of soluble ions (electrical conductivity) must be low;
e) the casing must also be clean of soil pests and pathogens, therefore pasteurization (heat or chemical treatment) is usually needed.

There is some association between microflora and mushroom mycelium which supports its transition to formation of fruit bodies. Hence, pasteurization must be limited to avoid elimination of beneficial micro-organisms (Hayes 1978; Levanon et al. 1983, 1984). As in horticulture, a shortage of peat moss has led to a continuing search for alternative raw materials which could serve as casing soil. In order to check if Cabutz could serve as a raw material for casing soil, its chemical and physical properties were compared with those of peat moss (Table 9.9), with similar results to the experiments described above. There is a great similarity in the physical properties (specific density, volume weight, water holding capacity and porosity) between peat moss and Cabutz, although the water holding capacity per volume, calculated on a saturated matter basis, is higher in Cabutz. Despite its high water holding capacity, the physical structure of Cabutz (especially its high porosity) allows the high rate of aeration required for mycelium growth and fruit bodies development.

Table 9.9: Chemical and Physical Properties of Cabutz and Sphagnum Peatmoss

*Measured on a wet matter basis - other parameters measured on a dry matter basis.

The chemical composition of Cabutz differs from that of peat moss in several parameters. In Cabutz, higher pH, nitrogen and phosphorus content were found. The pH of the Cabutz (8.3 - 8.1) is close to the optimal value for easing soil, which makes the usual supplementation of peat moss with limestone (in order to raise pH value) unnecessary. On a recent commercial-scale trial, we found a high yield (22.0 ± kg mushrooms/m ) with Cabutz as a casing layer. As a waste material product, the price of Cabutz (which is produced commercially) is constantly lower than imported peat moss. The benefits of using Cabutz commercially are therefore both ecological (recycling agricultural waste with pollution potential) and economic.

Fig 9.1: Cumulative mushroom yield in a commercial farm using 17 square meters for each treatment in which Cabutz, Peatrum and comporsted Peatrum were used as easing soil. Only one experiment was performed because of the limitations of commercial conditions.

It was shown by Levanon et al. (1983, 1984) that mushroom yields, using Cabutz as casing soil, reached those in the standard uses of peat-moss, with some higher yields in the first two flushes of mushroom growth. It was also found that the Cabutz contains less harmful moulds than peat-moss. The laboratory experiments were then scaled up to a commercial mushroom farm and high yields (2.5 kg mushroom/m²) with leached Cabutz as casing soil were achieved. The price benefit of using Cabutz is high, and is now in widespread use in northern Israel mushroom farms (Marchaim 1991).

Mushrooms were grown in commercial rooms with automatic climate control (temperature and humidity) with the addition of forced fresh air circulation, and control of carbon dioxide concentrations by means of Siemens IR CO₂ detectors). Each room contained a total effective growing area of 170 m² , in two rows of five beds of 17 m² each, one on top of the other. The experiment was performed in 3 beds of 17 m² each, with fresh or comporsed Peatrum for 2 months and a control of 50% peat moss with 50% Cabutz, the standard mixture used on farms. The beds were filled with compost' according to the commercially accepted ratio of 115 Kg compost/m² . Compost was obtained from a commercial plant nearby. The casing materials were treated with formaldehyde, the disinfectant commonly used on farms (0.5 l formaldehyde (40%) in 15 l of water for each m³ of casing material.) Fresh Peatrum was compared to composted Peatrum and to the control substrate. The Peatrum was additionally washed with water to a conductivity of less than 4 mmohos and limestone was added according to local procedures. All environmental treatments were performed as for Cabutz/peat treatment.

The results of using Peatrum as a casing soil for mushroom production were encouraging in comparison to Cabutz and peat-moss (Fig. 9.1). The comported material showed a better performance as casing soil, but the environmental conditions for using "Peatrum" have yet to be established. Peatrum was also examined, fresh and after 2 months of composting, as a substitute to Cabutz. Peatrum was used as 100% casing soil, while the control was 50% peat + 50% Cabutz. Cabutz is probably richer in suitable nutrients, and effects on the morphogenesis of the fungus were better than in high straw content Peatrum (Levanon 1988).

Fresh Peatrum has lower quality than Cabutz as a casing soil and growth medium, but reaches almost the same quality after 2 months of composting.

At the same time, it is possible to adapt conditions to use Peatrum as a casing material, without loss of quality.

Composting processes

Several ways of treating the slurry were examined by different groups, one of them separation on a vibrating screen, as described by Marchaim (1983), and washed with water. The liquid phase is usually used as a liquid fertilizer for irrigation, or is discharged.

In the anaerobic digestion treatment of slaughterhouse wastes, the sieved fibrous material, the Peatrum, is the result of the above process, and was used almost immediately (Marchaim 1991). The Peatrum was mixed with a shovel every 2 - 3 days to accelerate comporting, by exposing it to as much air as possible. Samples were taken for analysis and growth experiments during the composting process

Fig. 9.2: Changes in organic matter and ash content in the Peatrum from a slaughterhouse after thermophilic digestion, with composting time (from Marchaim 1991).

Composting is an exothermic, aerobic, microbial process of stabilization of organic material in heaps. The microorganisms in this process derive from the atmosphere, water and soil; it is an indigenous mixed population. The organisms belong to the microflora (bacteria, actinomycetes, fungi, algae) and microfauna (protozoa). Controlled environmental factors are the requirement for microbial metabolism, and the function of the process technique for composting is to optimize and to maintain these factors. The most important environmental factors are water, oxygen, nutrients, pH level, temperature.

Water: Microbial metabolism needs free water. Therefore, theoretically the optimal moisture content of the organic material is 100%, which excludes water lack during the composting process. In the three phase system (solid, liquid, gaseous) the part of the pores filled with gas and their permeability and communication to the atmosphere must be at a level to allow the exchange of the respiration gases.

Oxygen: Because aerobic microorganisms are responsible for the composting process, their oxygen demand has to be supplied by atmospheric air. At the same time, the carbon dioxide of the microbial respiration must be removed. For this, the minimum of the oxygen content and the maximum of the carbon dioxide content should be about 10%. The oxygen demand depends on the temperature: the maximum is between 50° - 60°C.

Nutrients: For the metabolism of the microorganisms, water, oxygen, a carbon source (the organic material), macro-nutrients (nitrogen, phosphorus, potassium) and certain trace elements are necessary in water soluble form. Special requirements belong to the C/N ratio and the availability of the carbon. Because the C/N ratio of the microbial substance is between 4 - 9, the C/N ratio of the organic material should be no higher than 20 - 25, and for a spontaneous start of the composting process no higher than 15. If the C/N ratio is wider, there will be also biodegradation and microbial growth, but retarded by nitrogen lack. If the C/N ratio is lower, the microbial development is undisturbed, but the losses of nitrogen as gaseous ammonia are relatively high, because the microorganisms cannot use it rapidly. The availability of the carbon of plant material is specially influenced by the degree of lignification.

Temperature: During composting the microbial metabolism release heat (34 to 42 kJ/g C), which results in a heat accumulation in large heaps due to the insulating effect of the material. The temperature can rise up to over 80°C. Depending on the temperature, different groups of microorganisms are active. Because the highest rate of degradation is achieved by thermophilic microorganisms, at a temperature between 50° - 60°C, this temperature is the optimum for a quick composting process. From the viewpoint of safe hygienization, the temperature should be higher than 60°C.

pH-level: The optimum of the pH-level for bacteria is in the neutral to alkaline range, for fungi in the acid range, and for actinomycetes between both. In general, a pH-level of 7 is required, but decomposition is possible at a pH-level of 3 - 9 of the substrate. The microorganisms are able to change the pH-level by their metabolism to an optimal range, but they can also change the pH-level to a toxic range.

There are only few reports about composting of separated solid matter. Two examples can show the composting behaviour of characteristic substrates. The first example demonstrates the composting of solid matter from liquid manure (Terre et al. 1987; Raviv et al. 1987). The solid matter had a crumbly structure and an air volume of 44% for the fresh manure, compared with 39% for the thermophilic digested manure, and 31% for the two-stage fermented manure. The temperatures during the self-heating process were high enough to kill pathogenic organisms and weed seeds. The product from this composting process is commercially marketed in Israel, and is the Cabutz, referred to above.

The second example shows the problems when composting separated solid matter. The substrate from thick stillage and grass silage, separated after hydrolysis, was crumbly but smeary with fine fibres of the grass. At the start of the composting process, the smell of the substrate was distasteful because of the fatty acids (pH 5.3). Although the pH value was low, the temperature rose up to 71°C within 5 days. When the substrate was turned after 6 days it had lost the distasteful acid smell. During the following weeks of composting the substrate became softer and softer, and lost the air volume, so that anaerobic conditions predominated.

Process alternatives for composting

The process alternatives for the composting of separated solid matter depend primarily on its water content, consistency and structure. For composting in windrows higher than 0.5 m the total solid content should be at least 25% and the air volume should be at least 30%, or better 50%, to provide aerobic conditions. At this air volume, self-aeration of the fine-crumbly or fine-fibrous substrate is possible, up to a height of 1.0 - 1.5 m. For a composting process in boxes, with a filling height above 1.5 m, aeration is necessary. If the total solid content is higher than 25%, and the air volume more than 30%, the solid matter is suitable for direct composting in windrows, with heights up to 1.0 - 1.5 m; If the total solid content is between 15 - 25% the air volume is often lower than 30%, and aeration insufficient, even though the substrate has a crumbly structure. A composting process in windrows without additives is possible when the height is less than 0.5 m and the substrate is mixed from time to time, to keep or to produce air pores; When the total solid content of the solid matter is 15 25%, and the consistency pasty, viscous or crumbly, an addition of water- absorbing and structure-forming stuffs is necessary (e.g. straw, wood chips, sawdust); At total solid contents lower than 15%, the solid matter has a pasty or liquid consistency and an addition of dry matter is essential; At low total solid contents of the solid matter, chopping of the straw allows quick absorption of the water.

Is the composting profitable?

The costs for separating out the solid matter from biogas plants are composed of a separator, a tractor shovel or tractor with frontloader and a passable bed-rock for the storage of the solid matter. These costs arise in all processes, with separation of solid matter as an essential part of the process. If the aim of the separation is the production of compost material, additional costs for the composting process result from a windrow turning machine (self-powered or tractor-powered), a passable surface for the windrows, and a roof for rain-protection for composting during the whole year. If the contents of total solids of the separated solid matter are too low for the composting process additional investments are necessary for chopping or crushing of water absorbing dry matter, storage of the dry matter, and a tractor with a trailer for the transport of the dry matter to the windrows. If the consistency of the solid matter is liquid or pasty, so that a mixing of the components with the turning machine is impossible, a mixer-dosing device for the solid matter and the dry matter is required. If the intention is to sell the compost in plastic bags, investment is necessary for a dryer for compost, or storage of water-absorbing additives (e.g. Perlite or peat), sacking equipment and storage of the bags.

The content of total solids after the composting process is in the range of 55 - 70%, too high for storage and transport in plastic bags without trouble with surplus liquid. The content of total solids should be less than 50%.

Composting of separated solid matter for utilization in horticulture or in hobby gardens offers a high return for compost production. The allowable limit of costs depends on the price of the compost at the market. Examples from Israeli practice show satisfactory profits for selling high-value composts (free of pathogenic organisms and weed seeds, smelling of earth, crumbly, favourable to growth of plants). However, compost production of some thousand m³ per year seems to be necessary, because of the high investment. Composting of separated solid matter for utilization in agriculture as fertilizers seems not to be viable, because of the high costs, and the alternative possibility of utilization without comporting. If a comporting process is necessary under for environment protection, or degradation of odoriferous components of the solid matter after hydrolysis (fatty acids), storage in heaps, with a maximum height of 0.5 m, is possible.

Composition and digestibility of different sized fractions in cattle slurry

Animal slurries may be passed through a separator, a common item of farm machinery, to give a liquid fraction which is more easily pumped. This separated liquid gives fewer problems of blockage and scum formation in subsequent anaerobic digestion, and may also be capable of digestion in anaerobic filters (Peck and Hawkes 1987). Higher gas yields have been reported from the digestion of separated cattle slurry when compared with whole cattle slurry (Rorick et al. 1984; Lo et al. 1983, Peck et al. 1985). Peck et al. (1989) showed that passage of whole cattle slurry through a commercial roller press separator alters the composition of the waste as well as its size distribution. The composition and digestibility of fractions of various sizes obtained from whole cattle slurry have been compared. The smallest particles have a higher lignin: holocellulose ratio, produce gas richer in methane, and give lower gas yields per volatile solids added. A reduction in particle size after digestion was observed in all fractions, with the >1700 µm fraction least affected.

It was found (Peck et al. 1988) that the biogas from the smallest fractions was richer in methane. This is presumably related to the different composition of the solids destroyed, since lipids and proteins should give a higher percentage of methane than carbohydrates. After digestion, the digester contents were again sieved, and a reduction in particle size was observed in all fractions. The >1700 µm fraction was least affected, only 22% of the particles by weight being smaller after digestion, while in the remaining fractions 31 - 49% of the particles (by weight) were smaller.

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