Anaerobic fermentation of poultry manure

ANAEROBIC

FERMENTATION

OF

POULTRY

MANURE

A. E. ADDERLEY, I. E. SMITH and S. D. p r o B l r t

School of Mechanical Engineering, Cran[l"eld Institute ~! Technology, Cranfield, Bedford MK43 OAL (Great Britain)

SUMMA R Y

Although both in nature and in conventional anaerobic decomposition process plants, acidification and gas production occur simultaneously, the rates of methane production and waste stabilisation can be enhanced by arranging that the process occurs in two stages. Initially, the waste is fermented at an optimal temperature of 25°C and a 2"5 per cent, by weight, total solids dilution to organic acids, and is subsequently transferred to a tank and held at optimal conditions .for the methane producing bacteria.

INTRODUCTION

The production of combustible gases from agricultural wastes using anaerobic micro-organisms is already a feasible proposition: it would become attractive to the farmer provided the process were reliable and economically viable. Although interest is principally in the fuel end-product, the biological plant must also be considered as a waste stabiliser and as a system that conserves the fertilising properties of the waste input. The increased use of intensive-production techniques in agriculture has led to problems concerning the disposal of excrement. The land area available locally is often insufficient to accommodate the large volumes of untreated waste being produced and excessive application to the same area of land may cause organic pollution of water courses, create undesirable conditions in the soil, taint the herbage and liberate intestinal pathogenic bacteria into the environment, as well as generate offensive odours. These problems are particularly acute for the poultry industry in which a high percentage of the enterprises are operated by 'landless stockholders', and most others have a very limited spreading area. 163 Applied Energy (Z) (1976)--© Applied Science Publishers Ltd, England, 1976 Printed in Great Britain

164

A. E. ADDERLEY, I. E. SMITH, S. D. PROBERT

CHEMISTRY AND MICROBIOLOGY

In nature there are two main agents through which organic waste material is decomposed: (a) obligate aerobes or (b) obligate anaerobes. There is, in addition, a third agent--faculative anaerobes--which can grow with or without oxygen. These three categories can be further subdivided into three major physiological groups: thermophiles, mesophiles and psychrophiles (see Fig. I). Each group has its own optimal growth temperature, although the three groups can survive over ranges of temperatures by the production of bacterial endspores.

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in the first step of anaerobic digestion, complex organics are converted by enzymatic hydrolysis to simpler, soluble, organic compounds: sugars, alcohols, peptides, amino-acids, glycerol and fatty acids. These are further fermented to volatile fatty acids, principally acetic, propionic, butyric and valeric: acetic is generally the dominant acid produced. During the second stage, which occurs simultaneously with the first, the volatile acids are reduced to methane, carbon dioxide and other gases in smaller concentrations. Acetic acid and carbon dioxide provide the main sources of methane by the following reactions: CH3CO2H --~ CH 4 + CO 2 CO2 + 8H ~ CH4 + 2H20

ANAEROBIC FERMENTATION OF POULTRY MANURE

165

Longer-chain volatile acids (e.g. propionic acid) decompose, firstly to gases and a shorter-chain length volatile acid, according to: 4CH3CH2CO2H + 2H20 ---, 4CH3CO2H + 3CH4 + CO2 (1) 4CH3CO2H ~ 4CHa + 4CO2 (2) The fastest-growing methane bacteria are those that utilise the single-carbon organic compounds (methanol and formic acid); the bacteria fermenting acetic and propionic acids grow less rapidly and are much more sensitive to environmental conditions such as acidity and carbon/nitrogen ratio. The most rapid growths of micro-organisms occur within narrow pH bands: for example 6.5 < pH < 7.5 in the case of anaerobic methanogenic bacteria. The pH of a slurry undergoing digestion may be controlled by several different acid-base chemical equilibria, but at pH = 7 the major chemical system controlling pH is the carbon dioxide-bicarbonate system, which can be expressed in the form: [H +][HCO3-] Kc [H2CO3]

where K¢ is the ionisation constant for carbonic acid. The requirement for carbon by the bacteria is about 30 times greater than that for nitrogen. Higher ratios than this slow down the process and lower ratios result in nitrogen loss from the waste in the form of ammonia, which reduces the fertilising properties of the effluent. Poultry manure has a C/N ratio of 7.5:1, which is reasonable for digestion. The efficiency of sewage digestion can be measured in terms of the reduction of biochemical oxygen demand (BOD) or chemical oxygen demand (COD). These oxygen demands are expressed as mg of COD or BOD per mg of volatile, rather than total, solids. Volatile solids (often abbreviated to VS) are defined as those that can be removed by heating to 600°C. For poultry manure, volatile solids represent 72 to 77 per cent by weight of the total solids present. Hart' gives figures of 0.278 mg BOD/mg VS and 1-11 mg COD/mg VS for fresh poultry manure. Waste stabilisation is directly related to the rate of methane production : the BOD or COD is removed by conversion of organic matter to methane gas and only that portion of waste converted to methane is actually stabilised.

LITERATURE REVIEW

Kaplovsky (as reported in reference 2) studied the production of volatile acids during the digestion of seeded, unseeded and limed-fresh solids. Before and during the time of active gasification, acetic and butyric were the predominant acids present. Pohland and Bloodgood 3 investigated individual volatile-acid concentrations. Several continuous digesters were operated in the mesophilic and thermophilic

166

A. E. ADDERLEY, 1. E. SMITH, S. D. PROBERT

ranges under different loading conditions. During retarded digestion, the rate of gas evolved decreased, whereas volatile acid production was enhanced. Acetic acid was the most important volatile acid intermediate during digestion as well as the primary source of gas produced from volatile acids. McCarty e t al. 4 examined individual volatile acids during the fermentation of pure organic compounds and compared them with those identified in solution under conditions of retarded digestion. The main intermediate volatile acid was acetic which was formed directly from the fermentation of carbohydrates, proteins and fats, and was also produced as a product in the methane fermentation of propionic and butyric acids. 3, • Propionic acid was formed primarily from carbohydrates, but also from the fermentation of proteins. Several other acids in lesser amounts were normally present also. McCarty and Brosseau 5 considered the effects of high concentrations of individual volatile acids on the anaerobic process. Any sudden increase in the concentration of acetic and butyric acids up to 6 gl- ~ (both individually and in combination) stimulated, rather than hindered, the digestion process, provided a neutral pH was maintained. A sudden increase in the concentration of propionic acid alone to levels of 3 to 8 gl- 1 hindered gas production for up to two weeks, after which time acclimation occurred and the rate of gas evolvement returned once again to normal. Any increase of the propionic acid concentration did not affect the major methane bacteria responsible for the fermentation of acetic acid, but inhibited the bacteria responsible for the propionic/acetic conversion. Anaerobic digestion is essentially a two-stage process comprising: (i) acid formation and (ii) methane production. The methane stage has erroneously been considered to be the rate-limiting step, and so most previous investigations have been focused on the combined acid-methane process. The purpose of the present investigation was to discover the optimal conditions for acid and methane formers when operated in isolation from one another.

EXPERIMENTAL PROGRAMME

Poultry manure was diluted to give five samples varying between 1.4 per cent and 13 per cent total solids present by weight. Each sample was loaded into fifty 250 ml digestion bottles; ten bottles of each sample were placed in thermostatically controlled water baths set at temperatures of 4, 17, 25, 30 and 35°C. Acidity and pH were measured over a thirty-two day period. The acid production, measured as acetic, was expressed as a fraction of the mass of original total solids--this enabled an optimal dilution and temperature to be established. The bottles were frequently agitated in order to maintain a uniform mixture. In the second series of experiments, only the optimal dilution specimen was

ANAEROBIC FERMENTATION OF POULTRY MANURE

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acidified over the full range of temperatures (see Figs. 2 and 3). In addition to titrations for acidity, gas samples were collected in balloons and subjected to analysis with a gas chromatograph. The gas collection bottles contained 480 ml samples. In the final experiment acidified waste was mixed with seed sludge from a working methane digester. The gas output from this waste, held at 35'C, was compared with that of a similar quantity of seeded, fresh waste maintained at the same temperature.

DISCUSSION OF RESULTS

Figure 3 illustrates the maximum acid production from samples diluted to 1.4, 2-4, 4.7, 7.0, 9-1 and 13 per cent total solids. Although the highest rate of production was achieved with the 1.4 per cent samples, the improvement over the 2.4 per cent was only marginal in t h e b e s t case at 25°C. Also a 2-5 per cent dilution would be the most practical in terms of digester size, heat input and handling costs. Although

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ANAEROBIC FERMENTATION OF POULTRY MANURE

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efficiently until the conditions became favourable to the methane bacteria. A temperature of 25°C was found to be the optimum for acid production (see Fig. 3). Gas analysis of the 2.5 per cent samples indicated that the volume of gas and the percentage methane rose as the temperature was increased up to 35°C. Acid production, however, increased with increasing temperature up to 25°C. Decreasing

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acid concentrations above 25°C were not due to inefficiency in biological conversion, but rather the depletion was the result of the action of methane bacteria reducing the acids at a faster rate than that at which they were formed (Fig. 4). An apparent optimal temperature of 25°C for the acid producers was due to a compromise between the rate at which acid was produced and the rate at which the

A. E. ADDERLEY, I. E. SMITH, S. D. PROBERT

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acid was converted to gas. Figure 5 shows the manner in which the acid concentration was reduced as both the volume and percentage of methane in the gas increased. The methane concentration rose rapidly after three days at 35°C and then temporarily halted. This could have been due to the growth of bacteria utilising the single carbon organic compounds--methanol and formic acid.

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In the mesophilic region, the best temperature for the methane-producing bacteria from the acidification experiments appeared to be 35°C. This was confirmed by combining an acidified slurry with one containing an established colony of methane bacteria in a 1:1 ratio. Comparative tests were then undertaken at 35°C and 40°C. Higher gas outputs could have been obtained by operating in the thermophilic range but, in practical plants, the increased output of gas (and hence

ANAEROBIC FERMENTATION OF POULTRY MANURE

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energy) does not justify the greater heat input necessary to maintain the higher temperature. The experiments were therefore restricted to mesophilic temperatures (see Fig. 6). After seven days, the 35°C and 40°C specimens had generated 148 ml and 133 ml, respectively, establishing 35°C as the more desirable temperature for

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methane production from acidified waste. The acidified waste taken from the 25°C bath produced over six times more gas during the first twenty-four hours of digestion than the fresh slurry. This surge fell sharply, showing that the methane bacteria had outstripped their food supply and there was a four-day delay before the bacteria became balanced and gas production was re-established.

172

A. E. ADDERLEY, I. E. SMITH, S. D. PROBERT

The products of biological action are usually toxic to the bacteria that produce them above certain levels, but the level at which the volatile acids become toxic to the micro-organisms is unknown. It was apparent that not all of the waste was converted to acid during the first stage of the process owing to this toxic effect and the bacteria were not able to operate again until the acid concentration had been reduced. After eleven days the acid concentration began to rise in the digester containing the previously acidified waste, gas production fell and it was necessary to add sodium bicarbonate to restore the pH to approximately neutral. Gas production ceased after 24 days. A satisfactory pH was maintained in the seeded fresh waste sample throughout the experiment. Gas production reached a maximum after the first four days, but this was less than half the maximum attained with the acidified waste. The volatile solids content of the samples was estimated to be 6.9 g for the acidified waste plus seed and 8.04 g for the fresh waste plus seed. When the total gas production was calculated, the acidified waste had the highest output at 2.4 ml/g VS/day compared with 1.65 ml/g VS/day for the fresh waste.

CONCLUSIONS

The optimal temperature and dilution for fermenting chicken manure to volatile acids were 25°C and 2.5 per cent total solids by weight, respectively. The methane bacteria, apparently sensitive to temperature changes, preferred a temperature of 35°C and 2.5 per cent total solids present. Thus the rates of methane production and waste stabilisation can be improved by arranging that the anaerobic fermentation of poultry manure occurs in two stages at 25°C and 35°C. Further investigation is necessary in order to establish the commercial viability of this process.

ACKNOWLEDGEMENTS

We wish to thank Mr H. B. Meek of Wick End, Bedfordshire, whose chickens unstintingly supplied the raw material for this investigation.

REFERENCES I. S. A. HART, Sludge digestion tests of livestock manures, Journal Water Pollution Control Federation, 35 (1963) pp. 748-57. 2. P. L. McCARTY and C. A. VATH, Volatile digestion at high loading rates, lnternationalJournalof Air and Water Pollution, 6 (1962) pp. 65-73. 3. F. G. POHLAND and D. E. BLOODGOOD, Laboratory studies on mesophilic and thermophilic anaerobic sludge digestion, Journal Water Pollution Control Federation, 35 (1963) pp. 11-42. 4. P. L. McCARTY, J. S. JERISand W. MURDOCH, Individual volatile acids in anaerobic treatment, Journal Water Pollution Control Federation, 35 (1963) laD. 1501-16.

ANAEROBIC FERMENTATION OF POULTRY MANURE

173

5. P. L. McCARTY and M. H. BROSSEAU,Effect of high concentrations of individual volatile acids on anaerobic treatment, Proceedings 18th Industrial Waste Confi, rence, Purdue University, 1963, pp. 283-96. 6. P. L. McCAa-r¥, Fundamentals of anaerobic treatment, Public Works (November, 1964), pp. 91-4.