Chapter 6
CONVERSION OF MANURE INTO PROTEIN BIOMASS BY FERMENTATION

6.0 GENERAL

Several research groups have introduced systems for converting animal wastes by biotechnological processes into products having a greater nutritive and thus commercial value. The magnitude and complexity of this problem from the scientific and technological points of view are enormous, because animal wastes are high in complex populations of wild micro-organisms concerned with eating each other more than the biological waste itself.

According to Brown (1972), anaerobic processes are generally easier and cheaper, but yield less profit and are hampered by the discharge of effluent, or even solid waste, which is incapable of anaerobic conversion. On the other hand, aerobic processes are necessary when manures (cattle manure for example) are rich in ligno-cellulosic constituents digestible only by aerobic action.

Based on mesophilic fermentation, bacteria offer a wider range of micro-organisms and require less controlled conditions. Thermophilic fermentation, however, offers a higher degree of safety through prolonged exposure of the biomass to higher temperatures, eliminating pathogens and pasteurizing the product. In addition, the thermophilic process yields more biomass, as it also utilizes the ligno-cellulosic constituents. For this reason, most scientists turn to thermophilic organisms that offer high protein substrate (50 to 60%) and are rich in nutritionally important amino acids (lysine, methionine, cystine and tryptophane) (Brown, 1972), usually limiting in livestock rations.

The degradation of organic matter can be accomplished by psychrophilic, mesophilic and thermophilic micro-organisms. Coulthard and Townsley (1973) prefer the following temperatures:

6.1 AEROBIC FERMENTATION PROCESSES

Several aerobic processes (Bellamy, 1969a,b; Anon., 1972a,b; Brewer, 1975) have been developed for converting animal wastes into single-cell protein (SCP), yeasts or other types of protein biomass. Aerobic processes produce, from 1 tonne of manure (DM), about 500 kg of dry product (SCP and yeast) containing 50% crude protein, with amino acid patterns similar to those of soya. The value of such a protein concentrate would be in the range of $180–$220 per tonne. Lower protein yields (35–50% crude protein) have been obtained from materials such as feedlot waste.

Based on the processing of 18,000 tonnes of feedlot waste from 25,000 head of cattle, the cost of plant was estimated in 1973 at $5,5 million, producing 3,600 t/year of 100% protein, or 8,182 tons of protein concentrate similar to soybean meal (44%).

Operating costs per year (Brewer, 1975) can be established for specific conditions from the following data:

This plant represents a minimum economic scale for production of feed supplement.

A commercial system for manure (and other organic, agriculturally related waste) conversion into protein feed supplement has been developed by Parsons Brinckerhoff Quade and Douglas, Inc., (1976).

The process involves size reduction of waste by grinding, hydrolysis of the volatile organic fraction, separation of the ligno-cellulosic fraction, submerged fermentation of the cellular biomass, and dehydration. The process yields SCP biomass containing 45–50% crude protein and discards the lignocellulosic fraction after separation in a primary, mechanical separator. Data from commercial applications are not available.

6.2 ANAEROBIC FERMENTATION PROCESSES

A batch process for fermentative conversion of feedlot manure filtrate into protein feed supplement for ruminants has been developed by Erdman and Reddy (1978). The process involves dilution of feedlot wastes with water (1:1), agitation and filtration through a No. 8 sieve. The filtrate is fortified with a carbohydrate source (in this case, cheese whey) and fermented under anaerobic conditions. The indigenous microflora inherited from the filtrate and whey is used as a productive micro-organism. A constant pH is maintained by continuous addition of ammonia as a buffer to neutralize volatile fatty acids generated during the anaerobiosis. The best yields were obtained at pH 7 and 43°C, at a concentration of fermentable carbohydrates of 3.5 to 10.0%. The process yielded biomass rich in CP, of which however 60–70% was in the NPN form (ammonium salts or organic acids), while the residue consisted of microbial cells. In view of the simplicity of the process, the authors claim it can be easily adapted to the farm level.

More and Anthony (1970), fermenting cattle manure under anaerobic conditions for 3 days, found a significant increase of protein, from an original 16.99% in fresh manure to 43.26% in fermented manure with a net increase of amino acids more than 20% (see Table 100).

The level of volatile fatty acids indicates that during the anaerobic process a very intense synthesis of organic acids occurred, and the authors reported that in 16½ hours of incubation at 37°C, the pH of the fresh manure dropped from 6.25 to 4.00.

Yeast has been produced from separated solubles (from fibres) of cattle manure (Singh, 1968). The recoverable, soluble portion of cattle manure represented 69% of the DM. The process elded 40.9% CP (on DM) in the final product.

Table 100
MANURE: COMPOSITION CHANGE BY FERMENTATION

Source: Moore and Anthony, 1970.

6.3 THERMOPHILIC PROCESS

Recent Nebraska researchers (Hashimoto et al., undated; Prior et al., 1978) have reported the development of a successful process for converting animal wastes into methane and protein concentrate by thermophilic anaerobic fermentation and centrifugation. Protein yields were as follows:

The protein biomass of fermenter effluent contained a very high level of nutritionally important amino acids: lysine, threonine and methionine. The level of these amino acids in centrifuge cake was also relatively high as compared with soybean meal or lucerne hay (see Table 101).

Feeding trials showed that the fermenter effluent can be favourably compared with soybean meal. The economics of the process have been evaluated for a feedlot of 10,000 head. In summary, the following parameters were obtained:

Table 101
DRIED FERMENTER EFFLUENT, DRIED CENTRIFUGE CAKE, LUCERNE HAY AND SOYBEAN MEAL:
ESSENTIAL AMINO ACID COMPOSITION
(g AA per 16 g organic N except as other wise noted)

^a Data from Atlas of nutritional data on United States and Canadian feeds.

Source: Hashimoto et al., undated.

The centrifuge cake value depends upon the capture efficiency of the centrifuge.

The thermophilic anaerobic system appears to be an economically feasible process for a larger operation.

Thermophilic processing of pig waste conducted by Canadian researchers (Coulthard and Townsley, 1973) involved batch processing of pig sludge. A stabilized, odourless sludge, when dried, was considered by the authors as suitable for feeding.

Development status

All fermentation processes are still in the stage of pilot plants or small commercial operations. Thermophilic types appear to be most promising.

Health hazards

All fermentation processes are completely safe.

6.4 FUNGI AND MOULDS

The use of fungi and moulds as potential production micro-organisms has recently been reviewed by Calvert (1977). Current studies indicated that some types of fungi grow satisfactorily on animal wastes, but the problem of toxicity affects the otherwise promising results.