Agricultural Waste.~ 4 (1982) 231 239

A N A E R O B I C D I G E S T I O N O F S I L A G E E F F L U E N T U S I N G A N U P F L O W F I X E D B E D R E A C T O R

MARY BARRY ~ EMER COLLERAN*

Department of Microbiology, University College, Galway, Ireland

ABSTRACT

Anaerobic digestion of silage eJfluent was achiered by a three-day liquid-retention period in a laboratory scale, upflow, fixed-bed reactor. The arerage percentage decrease in pollution potential obtained was 75.70~i with respect to COD and 85,~, with respect to VFA. The methane content of the gas produced averaged 84 °/o during the 48 day trial period. At an input COD of 14460 mg litre- 1, the average yield of biogas was 5.1m 3 per cubic metre feed or 0.49m 3 biogas per kilogramme COD removed by the digester. It is apparent from the results obtained that a fixed-bed reactor which is in routine use jbr pig slurry digestion can be switched to a dilute silage-eJ.[tuent feed without any upset in digestion ej~'ciency.

INTRODUCTION

The ensilage of moist green crops is normally accompanied by the production of a large volume of liquid effluent. The loss of dry matter in the effluent is considerable and constitutes a serious loss ofensiled crop. Losses in the order of 10 ~o of the total silage dry matter have been reported by many workers (Allred, 1955; Zimmer, 1964: Sutter, 1957: Mo & Fyrileiv, 1979).

The quantity of effluent produced is influenced by a number of factors including moisture content (Jones & Murdoch, 1954; Woolford, 1978); the degree of compression (Greenhill, 1964); the extent of chopping (Watson & Smith, 1956) and the use of acid additives (Bastiman, 1975). Since moisture content is the most important of these factors, the total quantity of effluent produced may be directly related to the moisture content of the crop at ensiling (Castle & Watson, 1973).

* To whom all correspondence should be addressed.

231 Agricultural Wastes 0141-4607/82/0004-0231/$02.75 ~ Applied Science Publishers Ltd, England, 1982 Printed in Great Britain

232 MARY BARRY, EMER C O L L E R A N

Although effluent can flow from a silage silo for 8-10 weeks or longer, the high initial flow rates ensure that the bulk of the effluent is discharged during the first seven days after ensiling (Hamilton, 1960; Gross, 1972; Bastiman, 1976). During this period, the BOD of the effluent is extremely high and can exceed 90000mg l i t re - ' (Spillane & O'Shea, 1973). Consequently, silage effluent is one of the major agricultural pollutants of watercourses in the north-western region of Europe (Patterson & Walker, 1979).

Currently, silage effluent is collected in specifically constructed tanks or channelled to cattle slurry collecting tanks for eventual disposal by land-spreading. Dilution of the effluent with water is necessary in order to reduce the risk of scorch occurring in grass crops (Woolford, 1978). Disposal by this method ensures some utilisation of the plant nutrient constituents, namely nitrogen, phosphorus and potassium, but the energy yielding constituents and protein content are wasted (Patterson & Walker, 1979).

This paper reports the results of an initial investigation of the suitability of silage effluent as a substrate for anaerobic digestion. Biomethanation of silage effluent is attractive as a treatment system since it combines pollution reduction with methane gas production and does not markedly diminish the fertiliser value of the raw effluent. The reactor used in this study was a laboratory scale, upflow fixed-bed reactor of the design developed in Galway by Newell, Dunican and Colleran (Newell, 1981; Colleran et al., 1981) and based on the anaerobic filter design pioneered by Young & McCarthy (1969).

MATERIALS A N D METHODS

Apparatus A fixed-bed reactor was constructed from wavin piping to an outside diameter of

155mm and a height of 125cm (Fig. 1). Conventional wavin sewer fittings of 150mm nominal size were used as end pieces. The fittings consisted of a wavin coupler and plain wavin stopper, the latter forming an airtight joint with the former by means of a lubricated rubber sealing ring.

A dispersion plate of 150 mm diameter and 3 mm in thickness and containing six holes of 6 mm diameter was inserted in the base of the reactor above the input feed line. Rubber dispersion rings were fixed at intervals to the inside wall of the reactor to ensure against short circuiting of the liquid influent feed. The reactor was filled with washed limestone chips graded between ¼ in and 36 in (6-35 mm and 4.75 ram) giving a final liquid volume of 8.4 litres. The reactor was fed continuously by a gravity feed system controlled by a peristaltic pump located on the effluent line. The operating temperature was maintained at 2 8 ° + 3°C throughout the run. Gas output was monitored using a laboratory built wet gas meter which recorded the passage of gas in 750 cm 3 batches or by a simple, liquid displacement siphon system

D I G E S T I O N O F S I L A G E E F F L U E N T 233

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involving l0 litre aspirators filled with tap water. Gas samples for analysis were removed using a syringe needle which was inserted through a bung in the gas sampling port indicated in Fig. 1.

Analytical methods Chemical oxygen demand (COD) was determined by standard methods

(Standard Methods, 1975). Volatile fatty acids (VFA) were measured as mg acetic acid litre -1 by the column partition chromatographic technique (Standard

234 MARY BARRY, EMER COLLERAN

Methods, 1975). Total settleable solids (TSS) were determined by centrifugation of samples at 3000 rpm for 20 min followed by heating of the pellet to constant weight at 103°C (approximately 12h). The percentage ash was determined after in- cineration in a muffle furnace for 2 h at 550 °C.

Gas samples, removed daily from the gas sampling port, were analysed immediately for CH 4 content using a 2-M Poropak S or N column in a Pye-Unicam 204 gas chromatograph fitted with a gas-sampling port and a flame-ionisation detector. Nitrogen was used as carrier gas with a flow rate of 20 ml min 1 and the oven temperature was 120 °C.

Digester-start-up The reactor had previously been seeded with anaerobic sludge taken from the

bottom of a pig slurry lagoon and fed with the supernatant fraction of pig slurry for a period of 6 -8 months. Feeding had then been discontinued for about 3 -4 months. Since previous experiments (Colleran, 1980) had shown that fixed-bed reactors retained microbial activity during quite lengthy shut-down periods, the reactor was started up by simply connecting the feed lines and pumping pig slurry through on a three-day liquid retention basis.

Digester feed Pig slurry was obtained from a local pig-fattening unit. The slurry had been

allowed to stand for a minimum of 3 weeks prior to collection. During this period, much of the polymeric material in the waste is hydrolysed and converted into volatile fatty acids (Colleran, 1980; Newell, 1981) and residual solid materials settle out by gravity. The supernatant liquid contains up to 80 ~;, of the total COD and has a solids content of approximately 0.8 1 o//,,. The liquid supernatant was collected from the slurry holding tank on the farm and stored in 300-gallon containers. The slurry supernatant was not preheated prior to passage through the fixed-bed reactor which was located in a temperature-controlled room.

Silage effluent was obtained from a local farm. The effluent draining from the silo was collected in a concrete holding tank which did not receive any effluent from cattle or pig units on the farm. It did, however, receive water run-off from the roof of adjacent sheds.

RESULTS

Start-up The reactor was started with pig slurry supernatant at an influent rate of 2.8 litre

per day corresponding to a liquid retention time within the reactor of 3 days. The slurry supernatant had been diluted by rainfall in the holding tank and had an average COD content of 15000 mg litre-1 and a VFA content of 3800 mg litre a Gas production commenced immediately on start-up although the methane content

DIGESTION OF SILAGE EFFLUENT 235

(60-75 %) was lower than the average value of 85 "jjo obtained routinely with stabilised fixed-bed reactors treating pig slurry supernatant (Colleran, 1980). By the end of the 3-week initial feeding period, the percentage methane in the gas had maximised between 80 and 90 % (Fig. 2). The average COD removal during this period was 66% (Fig. 3). This is considerably lower than the value of 85 90% obtainable on well-stabilised fixed-bed reactors treating pig slurry supernatant (Newell, 1981 ; Colleran, 1980). It is, however, in agreement with previous results (unpublished) which indicated that reactors which had been inoperative for 2 3 months or longer required about 3 4 weeks feeding before maximal COD removal was obtained with slurry supernatant feed.

The progressive increase in digestion efficiency during the three week initial feeding period is clearly seen from the increase in VFA removal rates presented in Fig. 4. The percentage VFA removal increased steadily from an initial 72";i to 82-5 % by the end of the 3-week period.

Digestion of silage effluent The silage effluent obtained from the holding tank had a COD content of

14460 mg litre l, a VFA content of 4270 mg litre 61 and contained 3 g of solids per litre. The low strength of the effluent is due to dilution by rainwater run-off to the holding tank. Feeding of silage effluent commenced on day 21. The data illustrated in Figs. 1 3 indicate that the fixed-bed reactor adapted immediately to the change in feed. No decrease in COD removal rates was observed (Fig. 3). On the contrary, a progressive increase in COD removal was observed. The COD content of the digested effluent ranged from 4200 to 2100 mg litre 1 and averaged 3510 mg litre 1 during the 48 day feeding period. This value represents an average of 75.7 % reduction in COD as listed in Table 1. The pH of the silage effluent on entry to the digester was 5.7 whereas after digestion the pH had risen to 8.4.

TABLE 1 SUMMARY OF DIGESTER PERFORMANCE

Substrate: Silage effluent liquid retention time (days); 3 liquid capacity o f reactor (litres); 8.4 loading rate (kg COD per cubic metre per day) 4.7

Average C O D removal (%) 75.7 (Range) (69 85)

Average VFA removal (%) 84.5 (Range) (79 90)

Average Methane Content of Biogas (%) 84 (Range) (79 90)

m 3 biogas m -3 digester day-~ 1.68 m 3 biogas m -3 input feed 5.1 m 3 biogas kg-~ C O D removed 0.49 m 3 methane kg - 1 C O D removed 0.39

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DIGESTION OF SILAGE EFFLUENT 237

The VFA content of the input silage effluent was 4270 mg litre- ~ and the range of the digested effluent was 720 to 336 mg litre-~ with an average of 662 mg litre 1 (Table 1). The percentage removal of volatile acids increased on commencement of silage effluent feeding and averaged 85 'Voo removal during the trial period (Fig. 4). The methane content of the biogas produced fluctuated between 78 and 90 "/o and averaged 84 ~o during the 48 day period.

The total settleable solids of the raw silage effluent was 3 g litre-~ with an ash content of 39 °,i. After digestion, TSS decreased slightly to 2.7 g litre- ~ and the ash content increased dramatically to 67 ~o as a result of the removal of volatile solids by conversion into methane and CO z.

As noted in Table 1, the average yield of biogas obtained was 5.1 m 3 per cubic metre of input feed or 0.49 m a of biogas per kilogramme COD removed in the digester.

Recycling of digested eJfluent On day 70, refeeding of the digested effluent was commenced. An immediate drop

in COD and VFA removal rates was observed. The average COD removal rate during the next 10 days of feeding was 15 o, which increased to an average of 22 o

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during the final 16 days of the trial (Fig. 3). VFA removal decreased initially to a low of 12~,; and subsequently increased during the final 20 day period (Fig. 4). Considerable fluctuation in the methane content of the gas produced was observed (Fig. 2).

DISCUSSION

Normally, start-up of an anaerobic fixed-bed reactor is accomplished by initial feeding on a long liquid retention basis (approx. 10 days) followed by a progressive increase in feeding rate until the minimum liquid retention time which maintains maximum COD removal is attained, Once the reactor is properly seeded and shown to function maximally, it may be switched off for quite lengthy periods without dramatic loss in microbial activity. Although the reactor used in this study had been switched off for 3 -4 months, recommencement of slurry supernatant feeding on a three-day retention basis resulted in an immediate start-up of digestion. The data presented in Figs. 2, 3 and 4 shows that digestion efficiency increased steadily over the initial three-week period both in terms of COD and VFA removal rates and in the methane content of the biogas produced. This stability towards stoppages of feed input is one of the advantages of the anaerobic fixed-bed reactor design and is particularly attractive in the context of farmer-operated slurry digesters.

It is clear from the data presented in Figs. 2, 3 and 4 and in Table 1 that silage effluent, at the dilution used in this study, is very amenable to anaerobic digestion using the fixed-bed reactor design. It would appear from this initial study that a

238 MARY BARRY, EMER COLLERAN

reactor which is in routine use at farm level for the digestion of pig slurry may be switched to a silage effluent feed without any apparent consequent loss of digestion efficiency. Since silage effluent is a seasonal substrate produced over a relatively short time scale, its digestion by existing apparatus designed for year-round substrates, such as pig slurry, is an economic necessity. Passage of silage effluent through the reactor does not result in any decrease in slurry digestion capability either since the reactor used in this study continued to average 80 T,, COD removal when pig slurry was resumed.

It is not clear from this study whether undiluted silage effluent can be efficiently digested in a fixed-bed reactor. The effluent produced during the first seven days after ensiling has an extremely high COD content (Spillane & O'Shea, 1973) and the high VFA levels, in particular, may prove to be inhibitory. It is intended to investigate the feasibility of digestion of undiluted, high-strength silage effluent during the 1981 silage season. Since silage effluent may be channelled to slurry collecting tanks at farm level, the digestion of slurry silage mixtures will also be investigated. Efficient digestion of silage when mixed at 10 or 30 Tov/v with a pig slurry has previously been demonstrated in a high-rate, pilot-plant digester by Bousfield et al. (1974).

The data presented in Figs. 2, 3 and 4 reveal that recycling of digested silage effluent through the fixed-bed reactor resulted in a marked reduction in digestion efficiency. COD removal rates decreased to approximately 2 0 ~ and both the methane content and the gas yield decreased significantly. It would appear that the bulk of the rapidly digestible material in silage effluent is converted into biogas by one passage through the reactor and that the organic material in the digester effluent is either non-amenable to anaerobic digestion or is more slowly decomposed, thus requiring a longer retention time within the digester. A similar decrease in digestion efficiency is observed when digested pig slurry is recycled through a fixed-bed reactor.

ACKNOWLEDGEMENTS

This study was supported by an EEC research grant under Project E of the Solar Energy Programme. It is a pleasure to acknowledge the consistent technical help of Matthew Duane and Frank Concannon and the engineering expertise of Dr P.J. Newell.

REFERENCES

ALLRED, K. R. (1955). The effect of preservatives on dry matter losses that occur when immature unwilted forage is ensiled. Diss. Abstr., 15(6), 924-5.

BASTIMAN, B. (1975). Silage effluent. Ann. Rev. Drayton Exp. Husb. Farm., 20-3. BASTIMAN, B. (1976). Factors affecting silage effluent production. Expl. Husb., 31, 40-6.

DIGESTION OF SILAGE EFFLUENT 239

BOUSFIELD, S., HOBSON, P. N. & SUMMERS, R. (1974). Pilot-plant high-rate digestion of piggery and silage wastes. J. Appl. Bact., 37, xi-xii.

CASTLE, M. E. • WATSON, J. N. (1973). The relationship between the dry matter content of herbage for silage making and effluent production. J. Br. Grassld. Soc., 28, 135 8.

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GROSS, F. (1972). The formation of fermentation liquor during ensilage and its elimination. Bayer Landw. Jahrb., 49, 964 70.

HAM~I_TON. W. D. B. (1960). Silos and silage effluent. Scott. Agric., 40, 80 2. JONES, E. E. & MURDOCH, J. C. (1954). Polluting character of silage effluent. Wtater San. Engr..

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PATTERSON, D. C. & WALKER, N. (1979). Silage liquor in growing pig diets. FeedstuJJs, 51, 40 1. SPIH.ANE, T. A. & O'SHEA, J. (1973). A simple way to dispose of silage effluents. Farm and Food, July

Aug., 80-1. Standard MethodsJor the Examination oJ Water and Wastewater (1975). (14th ed.), American Public

Health Association, Washington, DC. SUTTER, A. (1957). Problem of waste effluent from silage making and feeding of silage. OEEC Paris,

Project 307, 74-82. WATSON, S. J. & SMITH, A. M. (1956). Silage. Crosby Lockwood, London, 144pp. WOOLEORD, M. K. (1978). The problem of silage effluent. Herbage Abstr., 48(10), 397-403. YOUNG, J. C. & MCCARTHY, P. L. (1969). The anaerobic filter for waste treatment. J. Water Pollut.

Control Fed., 41(5), R160-RI73. ZI~MER, E. (1964). Investigations on the seepage fluid from silage. Res. Bull. Hokkaido Nat. Exp. Sta..

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