Anaerobic digestion of piggery waste

Anaerobic digestion is a biological process by which organic matter is converted to methane and carbon dioxide by microbes in the absence of air (oxygen). In nature, anaerobic conversions occur at all places where organic material accumulates and the supply of oxygen is deficient, e.g. in marshes and lake sediments. Microbial formation of methane also plays a role in the ruminant digestion.In digestion units, the external conditions acting upon the process can be regulated to speed it up as compared with that occurring in nature. Moreover, the gas produced can be collected and used as a fuel. In this way, anaerobic digestion for waste treatment allows us to reduce the concentration of polluting organic substances and produce useful energy. So there is increasing interest in anaerobic digestion, especially as fossil energy threatens to give out and people are concerned about environmental pollution.By the end of the 19th Century, anaerobic digestion was used to stabilize excess sewage sludge. The process was also utilized for energy production from agricultural wastes (manure, straw) whenever fossil energy was in short supply, e.g. during and shortly after World War II in France, Algeria and Germany. However, after the War energy from digestion of agricultural wastes could not compete with cheap fossil energy and direct interest in the process diminished. The main objectives of using digestion for disposal of animal manure are the abatement of malodour nuisance and the recovery of energy from wastes. The change-over from traditional animal husbandry to intensive systems has resulted in the production of large amounts of manure in certain regions of Western Europe. Disposal of the manure creates severe environmental pollution.The wastes produced in intensive animal production often consist of combined droppings: faeces, urine and spillage water. During storage of the slurry, low-molecular malodorous compounds such as volatile fatty acids, phenol, p -cresol, 4-ethylphenol, indole and skatole are formed by microbial activity (Spoelstra, 1978). The spreading of these slurries often causes malodour nuisance, especially in more densely populated areas. Upon subjecting the wastes to anaerobic digestion under controlled conditions, most of the biologically degradable material is converted to methane, thus producing a stabilized residue without offensive odour. Any malodorous compounds given out during digestion form components of the digester gas and will not lead to malodour nuisance in the surroundings unless the gas be discharged unburned.The gas produced is an useful fuel either for heating or for combustion engines, e.g. to generate electricity. Farms with a high energy consumption can largely cover their farm needs with the gas produced. Examples are pig and poultry breeding units and dairies. However, in spite of the favourable prospects on individual farms, manure digestion can play only a minor rolein the national energy supply, since the maximum production is about 1% of present energy consumption.According to present knowledge of the microbiology and biochemistry of anaerobic digestion (Zeikus, 1980; McInerney et al., 1980) the conversion of complex organic material to methane results from the activity of at least 4 distinct trophic groups of bacteria: fermentative bacteria, hydrogen-producing acetogenic bacteria, methanogenic bacteria and hydrogen-consuming acetogenic bacteria. As a result of the interaction between the contributing groups, anaerobic digestion as a whole is a stable process.However, the classification of the microbial population does not provide an adequate method of analysing the course of the digestion process in practice. So a simplified scheme was proposed, based on routine chemical analysis. In the scheme, anaerobic digestion was split up into three steps: (1) hydrolysis of undissolved compounds, (2) acid formation from dissolved organic compounds and (3) methane formation. The course of these separate steps of the digestion process can be estimated from routine data, such as the gas production, gas composition, Chemical oxygen Demand and volatile fatty acids (Chapter 1).The feasibility of anaerobic digestion processes depends on the availability of a suitable seed material. In Chapter 3 experiments are described on the influence of organic loading rate, in the range 0.07 - 0.125 kg COD kg -1VS day -1, on the adaptation of digested sewage sludge to piggery waste. Digested sewage sludge was a fairly suitable seed material, since methane formation started immediately after adding piggery waste. However, all experiments were characterized by a period of imbalance, which might be attributable to an inhibition of the methanogens by ammonia nitrogen at concentrations over about 1700 mg l -1. In a period of 2-3 months, dependent on the loading rate, the methanogenic population was adapted to these concentrations of ammonia nitrogen. The adapted populations were also capable of forming methane at far higher ammonia nitrogen concentrations, viz. up to 5000 mg l -1. The idea that methanogenesis was temporarily inhibited by ammonia nitrogen was supported by the results of two series of batch-type experiments, on the digestion of volatile fatty acids by two types of methanogenic sludge, viz. digested sewage sludge and digested piggery waste, at increasing ammonia nitrogen concentrations. In the experiment with digested sewage sludge adapted to 815 mg l -1ammonia nitrogen, methane formation occurred at all ammonia nitrogen concentrations investigated (in the range 730 - 4990 mg l -1), but an increasing lag phase was observed at increasing concentrations. This was not so for the experiment with digested piggery waste adapted to 2420 mg l -1ammonia nitrogen, Here the methane formation started immediately without any significant lag phase for the propionic acid degradation. The maximum rate of methane formation slowly decreased with increasing ammonia nitrogen concentrations. Daily fed experiments with piggery waste (loading rate 4.1 kg TS m -3day -1) demonstrated successful digestion at all ammonia nitrogen concentrations imposed, viz. 2070, 3670 and 5290 mg l -1. The gas production only slightly decreased with increasing ammonia nitrogen concentration.From the combined results it appears that the presumed inhibitory effect of ammonia nitrogen concentrations in excess of approx. 1700 mg l -1is only temporary. Consequently the use of anaerobic digestion may also be considered for wastes having a N-content far exceeding this level.Chapter 4 deals with the influence of the piggery waste concentration and the detention time on the digestion at a temperature of 30°C. At all manure concentrations (4%, 6% and 9% TS) and detention times investigated (in the range 10 - 40 days) a stable digestion could be achieved and maintained except with 9% TS manure at a 10-days detention time (space load approx. 9 kg TS m -3day -1). At fixed detention times an increase in the manure concentration effected a decrease in both the gas yield per kg TS added and the reduction of malodorous compounds. The gas production improves significantly in increasing the detention time from 10 days to 15 days, but beyond a 15-days detention time there is only a slight further increase. Although a stable digestion could be maintained at space loads up to 6 kg TS m -3day -1, the maximum load for a satisfactory reduction of objectionable piggery waste odours was about 4 kg TS m -3day -1, provided this loading rate was achieved by digesting 6% TS piggery waste at a 15-days detention time and not by digesting 9% TS manure at a 22.5-days detention time. The experimental results further indicate that hydrolysis is the rate-limiting step in the digestion of piggery wastes.Experiments concerning the effect of temperature in the range 13°C - 55°C on the digestion of piggery waste are described in Chapter 5. The experiments were performed in daily fed laboratory digesters at a loading rate of approx. 4 kg TS m -3day -1. No methane was produced at a digestion temperature of 13°C but an active methane fermentation took place at 20°C. In the mesophilic temperature range 20°C. - 40°C) the methane production increased sharply with temperature in the range 20°C - 25°C, but in the range 25 - 40°C there was only a slight further increase. Hydrolysis of undissolved manure components turned out to be the rate-limiting step of the digestion process.Under thermophilic conditions (55°C) the methane production decreased by about 25% as compared to mesophilic digestion, in spite of a somewhat higher degree of hydrolysis. Digestion under thermophilic conditions seems to be more sensitive to high ammonia nitrogen concentrations than mesophilic digestion, which presumably is caused by an increase in the fraction of free ammonia with temperature (at constant pH and total ammoniacal nitrogen concentration).From the results it can be concluded that digestion under mesophilic conditions is most adequate for piggery waste stabilization. Furthermore the results of the mesophilic experiments indicated that at a load of approx. 4 kg TS m -3day -1the optimum temperature with respect ot the net energy recovery is 27°C - 30°C. when high-grade fuel is required to elevate the temperature. When sufficient waste energy is available the process preferentially should be conducted at a temperature of 35°C - 40°C.Chapter 6 contains a general discussion on the application of the anaerobic digestion process and provides information about the features of piggery waste as a substrate for anaerobic digestion. Items discussed include the biodegradability, the degree of stabilization, the process stability and the economical feasibility of the process. Data are provided which demonstrate that the gas production from piggery wastes can be increased by petreating the wastes. However, in view of the accompanying consumption of energy and/or chemicals it is doubtful whether or not these pretreatment methods are effective in increasing the net energy production.The Appendix deals with preliminary batch-type experiments on the elimination of two aromatic malodorous compounds, viz. phenol and p -cresol, in anaerobic digestion. The results demonstrate that both these compounds are converted into methane by the microbial population in piggery waste digesters. Furthermore indications were obtained that phenol is an intermediate in the p -cresol degradation and that the first step in the microbial breakdown of p -cresol apparently is a demethylation of p -cresol to phenol.