Chapter nine: Output and its use II

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Digested slurry: the profit lies in the use of the effluent
Biomass uses without anaerobic digestion
Biomass uses following anaerobic digestion
Land application of effluent
Algae production
The use of anaerobic fermentation treatment in livestock breeding
Nutritional value of effluent in livestock diets
Fish feeding with digested cow manure
Effluent as a substrate for growing plants and crops
Uses of effluent for mushroom production
Composting processes
Process alternatives for composting
Is the composting profitable?
Composition and digestibility of different sized fractions in cattle slurry

Digested slurry: the profit lies in the use of the effluent

Economic evaluation studies have shown the importance of using the digested slurry after the anaerobic digestion process, as well as the biogas. Marchaim et al. (1981ab, 1983), as well as other groups in other countries, described the main uses of digested slurry, before and after separation. The economic importance of the digested slurry is becoming more acceptable in recent years in the Developing Countries as well, and this concept is presented in many publications of China, India and other countries. What follows is a summary of some of the commoner uses of the digested slurry, and the main research done in this topic, with the economic emphasis on its uses.

The slurry discharged from a digester contains 1 - 12% solids and consists of refractory organics, new cells formed during digestion, and ash. The slurry can be used in its liquid or solid fractions, dried or as total slurry.

Components of slurry which provide fertilizer and soil conditioner properties are soluble nutrients and trace elements, insoluble nutrients, and the organics present in the solids (humic materials). The components of a specific digested material are similar in content, despite other differences, to the raw material used for the digestion process, and must be examined according to the original materials uses and value. The uses of slurries without anaerobic digestion is still very common in many countries, and its value can not be ignored (Vetter et al. 1988).

Biomass uses without anaerobic digestion

The effect of use on the nitrogen present in the biomass is below.discussed

There are many possible ways in which biomass resources can be used. The most efficient way of using cattle manures is to provide fertilizer, soil conditioner and/or fuel from a given amount of biomass. The processes described occur to a lesser extent with vegetative waste materials and other biomass resources. In most cases attention is paid especially to nitrogen content since this element is usually important in terms of both quantity and effect on crops.

Biomass can be used for:

a. burning;
b. applying to the field surface;
c. applying to the field and ploughing under;
d. comporting and applying to the field;

The effect of use on the nitrogen present in the biomass is discussed below.

Option A: Burning is common in many developing countries, and results in the complete loss of nitrogen, through volatilization and mineralization. Phosphorus, potassium and the trace elements remain in the ash. The biomass is often burned in traditional three-stone fires which have a thermal efficiency of 10 - 15%. Recently (especially in China and India) an improved stove is used, and the efficiency can reach 30%. Burning leaves virtually no fertilizer, and the traditional fuel efficiency is considerably lower than for biogas produced from the same amount of biomass. This use of biomass meets acute energy needs of people, but ignores global needs for fertilizers.

Option B: Applying biomass directly to the field surface is practiced in most countries (Vetter et al. 1988), and the fate of the nitrogen depends on the composition of the biomass. Nitrogen is present in animal manures in two forms: organic and ammonia. In most other sources of biomass it exist in smaller quantities, and as organic matter. Most organic nitrogen is in the form of proteins and nucleic acids, while the ammonia nitrogen is present as either the ion, NH₄, or free ammonia, NH₃. For fresh cattle manures ammoniac nitrogen can vary from a low of 3% (Idnani and Varadarajan 1974) to 20% (Hamilton Standard 1980), or even as high as almost 40% (Hashimoto et al. 1981a). For dairy manure, equivalent figures are 24% (Hart 1963) and 37.6% (Jewell et al. 1976); for pigs, around 18% (UNEP 1981); and for fresh chicken manure, 8%.

When fresh manure is spread on the surface of a field, almost all the ammonium nitrogen is lost through volatilization. Direct field application is not an efficient use of biomass resources (Vetter et al. 1988).

Option C: Ploughing fresh manure into the field prevents loss of ammonia through volatilization, and almost all the nitrogen is conserved. However, under certain conditions, organisms can nitrify free ammonia to nitrite (NO₂) and nitrate (NO₃). These ions are relatively soluble and can be leached from the soil. Implementation of this option is relatively time-consuming, especially if the biomass is manure that i" produced daily, and its application is infrequent. Storage of the biomass is necessary in winter, and a large percentage of the nitrogen can be lost to the atmosphere if storage conditions are not suitable.

Option D: Composting is a common way of recycling biomass in Developing Countries, as well as in Developed Countries. The biomass is piled in a heap (agricultural residues are mixed with animal manure) and left to decompose aerobically. The pile is occasionally turned over or otherwise aerated. Compost may be stored for an long period of time before it is applied to the field. The composted biomass has few degradable organics, is essentially inoffensive to handle, is reduced in volume, and does not attract flies or other insects. However, there is a loss of nitrogen during composting and storing. Data on nitrogen loss reported by Gunnerson and Stuckey (1986) is listed in Table 9.1.

Table 9.1: Nitrogen Loss due to Composting or Digestion

For an optimal use of slurry, it is very important to calculate the correct nutrient content. Under optimal conditions the efficiency of pig and poultry slurry nitrogen application can be as high as 60 - 80% of an equivalent amount of commercial nitrogen fertilizer in the first year of application. Cattle manure efficiency is normally at the range of 30 - 50%. Since part of the organic nitrogen is released in subsequent years, the nitrogen efficiency, in the long run, reaches almost 90% of inorganic nitrogen. Detailed attention must be paid to problems of hygiene and odours.

Biomass uses following anaerobic digestion

Anaerobic digestion provides both fuel and fertilizer, while options A - D above provide either one or the other, but not both. Nitrogen can be lost during digestion only by reduction of nitrates to nitrogen gas and volatilization of ammonia into the biogas. Since there is very little nitrate present in manure, such loss through reduction is insignificant. Loss of nitrogen through volatilization of ammonia can occur from the slurry if not handled correctly.

Since organic matter is degraded during digestion to produce biogas, the percentage of nitrogen in the slurry rises, compared with solid content. Nitrogen is conserved during anaerobic digestion. For example, a 23% reduction in total solids concentration is accompanied by a corresponding increase in the nitrogen content of the remaining solids. This may create an illusion of "new" nitrogen, if only the total Kieldahl nitrogen (TKN) is considered. Jewell et al. (1976) found that the TKN for dairy manure increased from 5.2% - 6.9% of the solids during digestion and Hart (1963) found increases from 3.7% to 3.9% of the solids. Rajabapaiah et al. (1979) also carried out detailed mass balances on a KVIC digester and found that nitrogen was conserved.

The ammonia fraction of the TKN in digester slurry has an important influence on its fertilizer value, since ammonia is the form of nitrogen most easily taken up by plants. In its organic form the nitrogen is released more slowly, and some fraction may not be degraded, thus being unavailable to plants. With animal manures, the ammonia nitrogen concentration increases during digestion: Jewell et al. (1976) found that the ammonia nitrogen in dairy manure increased from 37.6 - 44.6% of the TKN during digestion. Similarly, Hart (1963) found an increase of from 24.0 49.0% during digestion.

The chemical composition of materials loaded into the anaerobic methanogenic thermophilic digestion system, and of the digested slurry obtained from this continuous process, is presented in Table 9.2. The breakdown of organic material was over 25%.

Table 9.2: Chemical composition of input materials and digested slurry out-put of the methanogenic fermentation system at a slaughterhouse

*Average of 12-15 analyses; analysis was carried out regularly, once every 2 weeks (Marchaim et al. 1991).

Land application of effluent

The direct application of manure to the land is the commonest single technique for its disposal and use in the world. It improves filth, increases water-holding capacity, lessens wind and water erosion, improves aeration, promotes the growth of beneficial organisms and maintains soil fertility. The economic value of manure as a fertilizer is calculated from its available nitrogen, phosphorus and potassium content, and as a soil conditioner: the same criteria are applied to the effluent generated by the biogas plant.

Manure contains many salts that are included in the cattle ration or consumed in the water. Heavy application of manure can increase the accumulation of soluble salts in the soil (i.e. its salinity), especially in arid regions, and these must be leached from the crop root zone, normally through under-drainage. The greater the amount of manure applied to the land, the greater the quantity of water needed for leaching, without which the salinity of the soil will be enough to inhibit plant growth and lessen yields. Salts are, in fact, the principal limiter in the application rate of manure to crop land, and salinity has become an acute problem in heavily cultivated areas.

During the biogas digestion process, water-soluble salts are dissolved into the aqueous solution. Here they are evenly distributed, and only approximately 30% of the solution accompanies the coarser fraction ("cabutz" or "biosolids") after separation. Hence, the solids have a lower salt content than the original manure, the rest being concentrated in the liquid fraction.

In most countries where biogas plants were constructed, the effluent was used as a fertilizer. The use of the effluent was extensively studied by institutes in the Republic of China, and they found chemical changes in the organic substances during fermentation. According to studies in Sichuan Province (1979), the nutrient contents of the effluent increased yields by 6 - 10%, regardless of kinds of soil; the same results have been reported by groups in other parts of the world. In long-term experiments, it was shown that the chemical and physical properties of the soil were improved markedly, after a few years of applying digester effluent, while total yields of several crops were 11 - 20% higher than controls. The NEFAH group (Marchaim and Criden 1981; Marchaim 1983) found that there was no clear difference between compost and effluent treatments, but that slurry did not increase the salinity of the soil, and reduced residual effects in the long term. Digestion followed by drying results in the loss of great amount of the ammonia. Jewell et al. (1981) found that 35% of the ammonia nitrogen was lost during drying over 72 days (see Table 9.3). The amount of ammonia nitrogen lost during drying will depend on a number of factors such as its concentration in the slurry, the pH of the solution, and the temperature of drying. This is also true sun-drying.

Table 9.3: Ammonia losses from Stored Mesophyllic Effluent (g/l)

Reference: Jewell et al. (1981)

Analysis of the benefits of anaerobic digestion based on nitrogen alone tends to neglect humus, micro-nutrients, trace elements and water in the slurry. Taking these factors into account, the value of digested slurry may be considerably higher than an analysis based only on nitrogen indicates.

Table 9.4: Estimated quantities of manures or fertilizers needed to supply 1 kg nitrogen to any given area of cropland

The nitrogen present in inorganic fertilizers is assumed to be potentially 100% available to plant=. For comparative purposes, the availability of nitrogen in organic manures i" assumed to range from 25% (e.g. Idnani and Varadarajan 1974) to 100%. Both inorganic fertilizers and organic manures often contain plant nutrients in addition to nitrogen, and organic manures provide important soil conditioning factors. Although important for sustained maintenance of soil fertility and plant growth, these are not presented in this Table, for the sake of simplicity. Nitrogen values of manures are based on Rajabapaiah et al. 1979.

The application of digested sludge over a period of years has led to a continuous increase in crop production (Marchaim 1983 and others). This may be due to the effect of slow release nitrogen compounds and improved soil structure. In order to utilize low grade phosphorite, a new type of fertilizer - biogas sludge phosphohumate - has been developed in China. This is made by mixing the sludge with phosphorite powder in ratios of 10:1 to 20:1, and comporting for 1 - 3 months. In soils lacking phosphorus the use of this material may increase yields by over 20%.

Typical compositions of manures after anaerobic digestion are shown in Table 9.6. Note that the three fertilizer elements, nitrogen, phosphorus and potassium, are each present in the range of 1 - 1.5%.

Algae production

Digester effluent has been added to a number of experimental ponds to evaluate its effect on algae production. In Taiwan, Hong et al. (1979 grew the blue-green algae Spirulina platensis in the effluent from a swine manure digester. The algae were harvested from the surface with nets, and productions of 7.3 and 9.7 g/m3 (equivalent to 1.9 x 2.5 tonnes/ha/year) were achieved during winter and summer respectively. The harvested algae contained 57.5% protein.

Table 9.6: Chemical Composition of Organic Digested Manures (oven Dry Basis)

Filtering, collection and drying of unicellular algae is costly and requires large areas of land and volumes of water. The addition of chemical coagulants, such as alum, increases costs and reduces the acceptability of the dried protein as an animal feed. Boersma et al. (1981) concluded that the production of algae from digested swine manure was not the best use of the slurry. Maramba (1978) point out that soybean oil meal is a less expensive protein source. This does not take into account the potential of algae as a source of phycochemicals.

The use of anaerobic fermentation treatment in livestock breeding

There are many nutrients in anaerobically digested manure and it can be used not only for raising the fertility of soil, improving soil, increases agricultural production, for feeding fish, finless eel, earthworm and pigs, but can also be used for breeding silkworm and hatching chickens. The use of Anaerobic fermentation treatment in breeding is a comprehensive utilization of both the biogas and the slurry. Many farmers in the villages of China have successfully made use of biogas technology (Fang Xing and Xu Yiz Hong 1988) in breeding silkworms with biogas, since this requires suitable heating and lighting. Lighting with biogas lamps enables the cocoons to be formed 4 - 6 days earlier, the quality of cocoons is good, and the output is increased by about 30% over that without using biogas lamps in otherwise the same conditions.

Nutritional value of effluent in livestock diets

Considerable interest and effort have been directed towards the use of animal wastes as fodder. The results of early experiments were reviewed by Smith and Wheeler (1979), and suggested that manure could be reefed with some nutritional benefit, and with little adverse effect on animal health or the wholesomeness of animal food products.

Many studies have been performed to evaluate the chemical composition of biomass resulting from thermophilic anaerobic fermentation of cattle wastes (Prior and Hashimoto 1981, Marchaim et al. 1981). Because of high capital costs involved in building the fermentation plant, preliminary analyses showed that a reasonable return for the feeding value of the effluent biomass is essential to the profitability of the fermentation process (Hashimoto and Chen 1981). While research has shown that biosolids have nutritional value for beef cattle fodder, the supporting data regarding the quantity and quality of protein, fibre, ash and energy are subject to a wide range of interpretations.

Total ash and total nitrogen (N) in the influent and effluent do not change significantly during fermentation. However, the proportion of total N that is in the form of ammonia- N increases from » 27% to » 48% (Prior and Hashimoto 1981). Assuming that all the non-ammonia-N is in the form of protein, the protein content is enriched from » 25% - » 32% (dry matter basis) during the fermentation process, on the basis of amino acid composition of the influent and effluent. The amino acid content of the dry matter is approximately doubled. Thus, the fermentation process enriches the protein content of the dry matter. If all the N in the effluent could be recovered and used as a diet supplement, the latter would have a high crude protein equivalent; however, making all the N in the effluent available is difficult, due to the finely divided bacterial cell size and the solubility of ammonia-N.

Normally, when effluent is centrifuged, over half the amino acid-N is lost in the centrate. This probably represents bacterial protein (intact cells, etc.), which would be a more digestible amino acid fraction than amino acids that may be trapped in the more lignified biosolids captured during solid/liquid separation. In centrifugation and belt filtration, most of the ammonia-N is lost in the liquid waste. Ammonia-N in the effluent represents » 48% of the total N. This level of ammonia in the effluent should not present a problem as an N supplement in ruminant rations.

Re-feeding of digested animal wastes to cattle, pigs and poultry has been demonstrated to be a potential use of the effluent product. When organic materials are digested anaerobically, a significant fraction is reduced to ammonia, some of which is taken up by growing bacterial biomass and converted to new amino acids. With cattle waste, increases of 230% of total amino acids have been measured after digestion (Table 9.7). In addition, considerable quantities of vitamin B12 are synthesized during digestion, and preliminary results from work at Maya Farms (Maramba 1978) indicate concentrations of over 3,000 mg B12 per kg dry sludge. In comparison, the main sources of B12 in animal feeds, fish and bone meal, contain 200 and 100 mg/kg respectively. Digested sludge thus has potential as an animal feed supplement and, due to the high costs of these supplements ($200/MT for cottonseed meal), could enhance the financial viability of biogas plants.

Table 9.7: Comparison of Amino Acid Composition of Cattle Wastes, Dried Centrifuged Fermenter Biomass, Fermenter Influent and Fermenter Effluent Cattle Centrifuged Fermenter Fermenter

Note: Data, expressed as mg amino acids per g DM, obtained following 72 hours of acid hydrolysis in evacuated flasks. Values represent mean of three determinations on composite material during two separate weeks.

Reference: Prior and Hashimoto (1981).

At Maya Farms in the Philippines (Judan 1981), solids are recovered in settling tanks and dried in the sun. The feed material from the sludge provides 10 - 15% of the total feed requirement of pigs and cattle, and 50% for ducks. At this concentration, it was found that weight gains for pigs were slightly higher than a control group (Maramba 1978). Alviar et al. (1980) also found that dried sludge could be substituted in cattle feed with satisfactory weight gains and savings of 50% in the feed concentrate used. The apparent gross energy of the dry matter does not significantly differ between influent and effluent, but available energy is lower in the effluent. Volatile fatty acids serve as primary sources of energy for the ruminant: they decrease from 9 - 10% to 3 - 4% during anaerobic fermentation. This is because the process converts available energy into gas. Hence, the effluent, apart from the bacterial debris, has a low feed value for energy, though retaining its value as a roughage source to stimulate rumination.

The mineral content of the dry matter also remains constant during the process of fermentation, but the concentration is increased, due to the loss of dry matter during the process. Factors that influence the mineral content of the waste are the type of installation from which the wastes are obtained and the amount of supplementary minerals added to the diet. Of particular interest is the high ash content of the dried centrifuge cake and the corresponding high silica content. Silica (SiSO₄) is an important factor in digestibility, particularly of the cell wall constituents of more mature forages. Plant metabolic silica causes an average decline of 3 units in digestibility per unit of silica. When there is sand or soil contamination, a factor of 1.4 units decline in digestibility per unit of silica is applicable. It is likely that silica in the fermenter effluent results, to a large extent, from solid contamination, particularly in wastes derived from dirt lots.

It was found by Prior et al. (1981) that the use of fermenter biomass a" a feed ingredient for livestock appears to have merit, although some technical problems must be solved. Dried, centrifuged biomass can be fed at a level up to 10% of dietary dry matter, without markedly affecting the utilization of diet components. The disadvantages of feeding it are that considerable quantities of nutrient are lost by centrifugation, while the capital and energy costs of installation and operation of centrifuge and drying systems are extremely high. While elimination of the drying process would retain additional N. storage of the wet centrifuged biomass would be a problem, particularly during extreme weather. The incorporation of the total fermenter effluent into a ration has the advantage of retaining a higher proportion of the nutrients, but the amount of water in the effluent limits the quantity that can be incorporated into the ration. The major effects that have been observed from feeding the effluent have been an apparent decrease in the digestibility of dry matter, N. ash and gross energy (in sheep) and decreased total ruminal volatile fatty acid concentration in cattle (Prior et al. 1981).

Feeding trials have also been conducted by Pacific Gas and Electric and Southern California Gas Companies (1981). Their biogas pilot facility, near Brawley, California, provided a supply of effluent, and feeding trials were conducted at El Centro, California. They found that there was no apparent reduction in ration palatability and consumption by cattle, but at 10 - 12% of the ration, centrifuged additions to it had a very pronounced effect on cattle performance. In both the research and the feedlot trials, feed consumption increased, rate of gain decreased, feed conversion rate decreased, and the cattle did not exhibit the degree of finish at slaughter of cattle not consuming biosolids. At this rate of feeding, the biosolids appear to be a negative energy input, requiring energy from the digestive system to pass the material through. At this feed intake rate, there wan still expected to be a nutrient contribution from mineral- and nitrogen containing ingredients (both protein and non-protein nitrogen). The high ash component imposed the principal penalty to the biosolid, since it diluted the energy in the ration, and required additional energy to pass the silica (from plant silica and soil) through the gut.

Because of the low efficiencies and the high capital and operational costs associated with centrifuging (Hashimoto and Chen 1981), other methods to recover the nutrients in the biomass were investigated. Studies were undertaken, in which the effluent was mixed directly with corn and roughage source. The advantages of this are the use of 100% effluent and the retention of the ammonia in the diet. The disadvantage is a high dietary moisture, which can reduce "bunk life" and consumption. While Prior and Hashimoto (1981) found negative effects on consumption, Marchaim et al. (1981), when using "NEFAH" process effluent, which is of much higher solid content (up to 12%), found that 25% of the total dry matter in the diet of Holstein heifers can be replaced, while retaining normal performance (Marchaim 1983). Experiments with feeding calves were continued in Israel, and showed a saving of 20 - 30% (ibid.). This could be done only when a cheap source of metabolic energy is used (e.g. grain dust): when corn or grains have to supply the missing energy, the saving is lower. This was more pronounced when beef cattle were fed up to 25% dry matter of digested slurry, and gained a little less weight (ibid.). The main reason for that is probably the adaptation period of the cattle to the diet, influencing performance.

On the other hand, it was found that one year after stopping feeding the calves with effluent, the 30 calves fed on slurry for over 14 months produced significantly more milk than the 30 calves of the control. This was observed in the first two lactating periods: no explanation for the phenomenon was offered.
To solve the main problem, the deficiency of metabolic energy in the effluent, an attempt was made to regenerate the energy by photosynthesis on the separated fibre fraction, the "Cabutz".

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