E F F E C T S O F C A R B O N : N I T R O G E N R A T I O O N A N A E R O B I C D I G E S T I O N O F D A I R Y M A N U R E

DAVID J. HILLS

Agricultural Engineering Department, University o) Cali]ornia, Davis, Cali]ornia, USA

ABSTRACT

This stud), is the initial phase oJ an investigation designed to supply criteria Jor combining various agricultural wastes to produce maximum methane per unit volume o/ an anaerobic digester. The parameter investigated Jot" establishing proper mixtures of manure and carbonaceous waste was the "available-carbon to nitrogen" ratio (C: N). Available-carbon was defined as the total organic carbon minus the lignin carbon. Six Jour-litre laboratory digesters were operated Jor 11 months. Screened dairy cow manure, which had a C: N ratio of 8.0, was combined with glucose and later in the experiment, cellulose, to obtain C.'N ratios varying between 8.0 and 51-7. The loading rate was also varied: 1.0, 1.5 and 2.0 k g Volatile Solids m- 3 tk O, - With an increasingJeed C: N ratio, the concentration oJ methane in the digester gas decreased (e.g. C:N = 8, CH 4 = 67%; C:N = 51.7, CH 4 = 51-7°,o). The greatest methane produetion per unit loading rate occurred when the C: N ratio o[theJeed was 25.

INTRODUCTION

Many intensified livestock operations in the United States provide some treatment to their wastes to avoid air or water pollution. One promising treatment system for animal manures is anaerobic digestion. Although capital costs for such plants are fairly high, the annual operating costs are much lower than for aerobic treatment systems which require a considerable energy input. Several full-size demonstration plants have been constructed in the United States (Coppinger et al., 1978; Pigg, 1977 and Fischer et al., 1978). With future energy price increases, the number of on-farm anaerobic digesters is expected to increase.

267 Agricuhural Wastes 0141-4607/79/0001-0267/$02.25 ~) Applied Science Publishers Ltd, England, 1979 Printed in Great Britain

268 DAVID J. HILLS

Livestock owners are concerned with economics for each unit process on the farm. The more efficient the anerobic digestion process for waste treatment and methane production, the more economically attractive it becomes. One possible means of increasing the methane production efficiency of a farm digester is to optimise the carbon-nitrogen ratio of the digester feed. According to Fraser (1977), the relatively low carbon-nitrogen ratios of most animal manures can be improved for digestion purposes by adding cellulosic wastes, such as grass, cereal straws and other crop residues.

It was estimated by Stephens & Heichel (1975) that 430 x 109 kg of cellulosic wastes were produced in the United States in 1972. At an average cellulose content of 40%, 170 x 1 0 9 kg of cellulose are potentially available. Although much of this material presently rots or is burned in the fields, it would appear that suitable collection procedures might be devised to recover a portion of this resource. Some wastes, such as bagasse, rice and wheat husks and corn cobs, are available at central processing sites.

This research was undertaken to determine guidelines for combining manures and carbonaceous wastes for maximum gas production per volume of digester. The parameter investigated for establishing the proper mixture was the available-carbon to nitrogen ratio. Six four-litre laboratory digesters were constructed and operated for 11 months. Dairy cow manure, having an available-carbon to nitrogen ratio of 8.0, was combined with glucose--and later cellulose--to obtain various carbon nitrogen ratios (8-51.7). Digester performance for the various feeds was assessed by monitoring the following parameters: Total Solids, Volatile Solids, Chemical Oxygen Demand, organic nitrogen, ammoniacal nitrogen, pH, volatile acids, alkalinity, gas production and percentage of gas as methane.

Laura & ldnami (1971) reported on the batch digestion of cow manure combined with other materials, primarily of vegetable origin. Although the carbon-nitrogen ratio was not measured, they concluded that the addition of nitrogenous materials, such as casein and urea, increased the extent of decomposition of dairy manure and resulted in approximately 15 % more gas with slightly richer methane content. Addition of 1% cellulose to the cow manure had a significant stimulating effect on the rate of gas production during the initial period of four weeks, but the total volume of gas obtained over the 80-day batch run was about the same as that from dairy manure alone. No recommendations were made in the study regarding combining wastes for digester feed.

Also using batch reactors, Hassan et al. (1975) investigated the effects on methane production of adding potato waste and sawdust to poultry manure. When the concentration of sawdust was 2-4 %, the researchers observed about 20 % increase in methane after 110 days of batch operation. When the sawdust concentration was increased to 8 % the total methane produced was nearly equal to that produced from pure poultry manure. From these observations the researchers concluded that an added carbon source stimulates methane production in a poultry manure digester

C : N RATIOS IN ANAEROBIC" DI(IESTION 269

provided that the carbon source is easily degradable and that the concentration is not too high. The carbon--nitrogen ratios were not measured and no guidelines were presented for combining various digestible wastes.

Sanders & Bloodgood (1965) reported on the effect of the carbon nitrogen ratio on anaerobic decomposition. From research with four-litre laboratory digesters which were fed daily with pig feed and varying amounts of caproic acid, maltose, glucose or leucine, they concluded that the carbon nitrogen ratio was an important factor for good digestion. After varying the carbon nitrogen ratio between 16.5 and 21.3, they felt that a maximum ratio of 16-5 would provide successful decomposition and that values as high as 21.1 would result in digester failure.

In a recent study Sievers & Brune (1978) found that a carbon nitrogen ratio range of 15.5:1 to 19 : 1 was optimum for maximum methane production tbr swine manure. They used two-litre laboratory digesters and adjusted the carbon nitrogen ratio between 1-7:1 and 24.7:1 by addition of either urea or glucose to the swine manure. They concluded that for carbon nitrogen ratios greater than 19 the digesters tended to be ecologically unstable and that the lower ratios were better because of the ammonia ion ammonia gas buffering capacity. They also concluded that digesters operating with a carbon nitrogen ratio less than 15.5 could attain toxic levels of ammonia gas at high loading rates. In their study, however, no distinction was made between readily available carbon and non-available carbon, such as is found in slow- degrading lignin, when determining the carbon nitrogen ratio.

METHODS

Six four-litre laboratory digesters were fabricated from lucite plastic as shown in Fig. 1. Mixing was performed by impeller stirring in each digester, the impellers being gear-driven from one common motor, The gas produced was collected in counterweight collectors filled with saline water. The entire assembly, shown in Fig. 2, was housed in a constant temperature room where the temperature was maintained at 35 + 1 °C.

Digester feed consisted of dairy cow manure and various concentrations of glucose or cellulose CReagent Grade' (Microcrystalline), J. T. Baker Chemical Co., Phillipsburg, New Jersey, USA. Used for TLC). Fresh dairy manure was scraped from the concrete surface of the Dairy Farm at the University of California, Davis. The animals were on a diet oficubed alfalfa and carbohydrate concentrate. Sufficient manure for the entire experiment was gathered at one time, was thoroughly mixed, then passed through a 'US Standard No. 8' sieve using a minimal amount of tap water to facilitate screening. The manure feed was then kept frozen and not thawed until immediately prior to use.

During start-up, each digester was innoculated with two litres of municipal anaerobic digester effluent and two litres of dairy manure slurry. Following a two- week period without disturbance, the reactors received their first feed. Subsequently,

270 DAVID J. HILLS

Gear drive f to motor ,h"

Pockin9 =~/ ~ Feed se°l ~= I

lucite tank I ~

DIGESTER

Fig. 1.

GAS COLLECTOR

Experimental digester (one of six).

~:~Weight

Fig. 2. Photograph of digester assembly.

C : N RATIOS IN A N A E R O B I C DIGESTION 271

over a one-month period the loading rates were increased gradually to those cited in Table 1. Feeding was performed every two days with sufficient water to establish a 25-day detention time in each digester. Complete mixing occurred for ten minutes every hour and was controlled by a cam-timer.

As indicated in Table 1, the carbon-nitrogen ratio of the digester feeds varied between 8.0 and 51.7. Carbon content was defined in this study as the total organic

T A B L E 1 VARIABLES STUDIED

Diges ter W e e k s I - 15 W e e k s 16--28 Weeks 2 9 ~ , 4 No. C¢. • N Load ing ~ C ! N Load ing ~ C/: N Loading ~

1 8.0 1 "0 8'0 1.0 30-0 2,0 2 11 "2 1 '0 11.2 1 "0 38"3 2,0 3 8"0 1 "5 8"0 1.5 44.0 2.0 4 18-3 1.5 18.3 1"5 51-7 2.0 5 8.0 2.0 8.0 2.0 8.0 2,0 6 25.1 2-0 25.1 2.0 25.1 2~0

"Loading: kilogrammes of Volatile Solids m -3 day ~. Note: Weeks I 15 C: N ratio modified by glucose addition. Weeks 16-44 C: N ratio modified by cellulose addition. See text.

carbon minus the lignin carbon, since lignin is essentially non-degradable within an anaerobic digester (Boreff& Buswell, 1934 and Han et al., 1975). Three loading rates were employed: 1.0, 1.5 and 2-0 kilogrammes Volatile Solids m -3 day 1

Gas production rates were monitored daily and the gas was analysed for methane content every two weeks. Samples of the effluents were obtained every two weeks and chemically analysed for Total Solids, Volatile Solids, Kjeldahl nitrogen, ammonia nitrogen, Chemical Oxygen Demand, pH, alkalinity and Volatile Acids. All analytical testing was performed according to the procedures in Standard Methods ./or the Examination oj Water and Wastewater (Anon., 1975). Ammonia nitrogen was determined with the Orion Ammonia Electrode, Model 95-10. Alkalinity was obtained by titration to pH 3.7 and Volatile Acids were determined by the Chromatograph Separation Method 504A. Gas samples were analysed on an Envirotech Organic Analyzer Model DC-50/52 for methane percentage. All gas measurements are expressed at 20°C and one atmosphere pressure.

RESULTS A N D DISCUSSION

Digester conditions The eleven-month experiment went smoothly and all digesters appeared stable.

The chemical composition of the dairy-manure feed is indicated in Table 2. As noted earlier, the manure had been passed through a No. 8 sieve with screen openings of

272 DAVID J. HILLS

TABLE 2 DAIRY MANURE DIGESTER FEED BEFORE MODIFICATION, BUT SCREENED THROUGH A NO.

8 SIEVE*

COD 49600 mg litre - l Total Solids 6.30% Volatile Solids 81.8 % TS Total - -N 3.91% NH3--N 0.78%

Available-carbon/Nitrogen = 8-0t

*United States Standard Sieve No, 8--Openings 2.4 mm. t See Table 3.

2.4 mm. The larger particles, mainly undigested straw, were screened out to facilitate feeding the model digesters. If not screened out, this straw would have formed a layer of scum within the digester and would not have been readily available for microbial decomposition. Therefore, the Volatile Solids loading rates of this study may not be directly comparable with those of other investigations. The loading rates of 1.0, 1.5 and 2.0 kilogrammes Volatile Solids m- 3 day i would correspond to slightly higher rates for those feeds where the scum-floating materials are included in the Volatile Solids loading rate.

A breakdown of carbon constituents for both the screened and unscreened manure is indicated in Table 3. Of importance is the fact that screening reduced the non-lignin carbon percentage but increased the nitrogen content and therefore lowered the carbon to nitrogen ratio from 18.9 to 8.0. The hemicellulose, cellulose and lignin analyses were performed according to the methods outlined by Goering & Van Soest (1975). Organic carbon was determined by combustion at I I00°C according to the test procedures outlined by the US Department of Agriculture (1972), The percentage of lignin carbon in the feed was calculated using an assumed lignin carbon content of 57.2"J~,o (Rubins & Bear, 1942 and Sarkanen & Lugwig, 1971).

Chemical stability in all digesters is reflected by the effluent characteristics listed in

TABLE 3 AVAILABLE CARBON CONTENT OF DAIRY MANURE

Ran' manure Screened Jeed (%) (%)

Hemicellulose 57,30 67-00 Cellulose 26,70 15.71 Lignin 14.05 13.95 Silica ash 1,95 3-34 Kjeldahl nitrogen 1-86 3.91 Total organic carbon 45-3 39.0 Available-carbon 35.1 31.3 Available-carbon/nitrogen 18.9 8.0

C;N RATIOS IN ANAEROBIC DIGESTION

TABLE 4 SUMMARY OF EFFLUENT DATA AT STEADY STATE

273

Digester VS K-N" NH3- N" COD ~ Alkalinity a Volatile pH (%) ( °/o TS) acids ~

Phase l--Glucose Additive 1 2.53 77.7 1328 395 21250 5480 312 7"07 2 2.38 79'0 1096 257 20850 4250 352 7"02 3 3.71 79.3 1737 480 29300 7470 420 7.18 4 2.47 79"8 1105 169 21100 4316 480 7.01 5 4-45 79.3 2156 612 37100 9550 432 7.27 6 2-58 80-2 1106 122 21310 4130 424 7"00

Phase 2--Cellulose Additive 1 2-71 78-0 1276 420 21050 5800 372 7'15 2 2-40 78-0 1060 330 20150 4300 360 7.01 3 3"83 79.6 1536 430 32100 8300 480 7.23 4 2.70 78"5 1102 180 20500 4250 435 6.97 5 4.65 76.3 2258 640 36700 11000 445 7.44 6 2.67 81"1 1098 135 22540 4100 480 6"95

Phase 3--Cellulose Additive 1 2.58 80"0 784 107 25280 3100 530 6"85 2 2.24 80"4 750 86 24270 2800 720 6.54 3 2.21 81'1 655 81 24500 2105 680 6-46 4 1.89 81"0 563 75 20740 1940 710 6"41 5 4.72 78.3 2315 670 36760 10150 425 7.34 6 2-60 79"9 988 125 24640 3960 472 6.93

"Units in mg li tret . COD = Chemical K N, Kjeldahl nitrogen. See Table 1 for loading rates. VS = Volatile Solids. TS = Total Solids.

Oxygen Demand.

Table 4. For conciseness, only a summary of the average values dur ing steady-state operat ion for each digester is presented. Note that the control digester, No. 5, varied very little through the l 1-month period. In general, the digester parameters of

Volatile Acids, alkalinity, p H and a m m o n i a - n i t r o g e n levels are within the acceptable guidelines for municipal digesters according to Metcalf & Eddy (1972).

Characteristics of digester 4 dur ing phase 3, while operat ing with a ca rbon ni trogen ratio of 51.7, did indicate that extreme condi t ions prevailed which could have possibly terminated in shutdown due to low pH if there had been slight envi ronmenta l upsets.

Gas p r o d u c t i o n

Total gas volume produced, corrected to 20oC volume, is shown in Fig. 3. Changes in feed composi t ion, cor responding to phases 2 and 3 of the experiment, began dur ing weeks 15 and 28. The t ransi t ion periods lasted two weeks into phase 2 and three weeks into phase 3. Fair ly steady-state gas product ion was achieved in each digester for each phase.

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F i g . 3 . T o t a l g a s p r o d u c t i o n .

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Rate of gas production between feedings at 0 and 48 h.

C:N RATIOS 1N ANAEROBIC DIGESTION 275

The chief difference between phase 1 and phase 2 is in the na ture of the add i t iona l ca rbonaceous mate r ia l (phase 1 - -g lucose , phase 2---cellulose). Digesters 1, 3 and 5 used s t ra ight da i ry m a n u r e at different load ing rates whereas digesters 2, 4 and 6 used manure combined with vary ing amoun t s of c a rbona c e ous mate r ia l at load ing rates co r r e spond ing to the cont ro ls . As expected, increasing the load ing rate resulted in greater gas p roduc t i on ; however, o f more significance, increasing the ca rbon ni t rogen ra t io also resulted in greater gas p roduc t ion . The source of ca rbon appea r s to be impor t an t . Despi te the fact that the ca rbon is fairly readi ly avai lable t¥om ei ther glucose or cellulose, more gas was p roduced with glucose than with cellulose at equal load ing rates for the condi t ions under which the digesters were opera ted . An exp lana t ion of this fact can perhaps be der ived f rom Fig. 4 which indicates typical rates of gas p roduc t ion for digester 6 dur ing s teady-s ta te condi t ions in phases 1 and 2. Dur ing the 48-h feeding cycles the mean to ta l gas p roduc t ion was ra ther less for cellulose- than for g lucose-conta in ing feeds. Since the rate of gas p roduc t ion was also less for the ce l lu lose-conta in ing feed, the measured gas p roduc t ion could be, perhaps , fur ther decreased by slight and unavoidab le var ia t ions in digester ope ra t ion which resulted in a decreased de ten t ion time or decreased interval between feeds.

A s u m m a r y o f the s teady-s ta te gas p roduc t ion is listed in Table 5. As indicated,

TABLE 5 SU MMA RY OF GAS DATA AT STEADY STATE

Digester Loading C. N Total gas % Methane Methane gas rate ~ production ~ production'

Phase l--Glucose Additive 1 1"0 8"0 355 68"2 242 2 1.0 I 1.2 495 61.4 304 3 1.5 8.0 525 68-6 240 4 1"5 18"3 995 59"7 396 5 2.0 8'0 710 67.7 240 6 2.0 25.1 1510 56.4 426

Phase 2--Cellulose Additive 1 1.0 8"0 378 66" 1 250 2 1.0 11-2 454 59"0 268 3 1.5 8-0 550 67.3 247 4 1.5 18"3 821 60'0 328 5 2.0 8"0 730 67.1 245 6 2'0 25.1 1360 56"9 387

Phase 3--Cellulose Additive 1 2"0 30-0 1200 55"6 333 2 2.0 38.3 1033 53.7 278 3 2'0 44-0 903 53.7 242 4 2"0 51.7 592 51-7 153 5 2.0 8.0 735 66"8 246 6 2.0 25.1 1400 56-5 396

° Grammes of Volatile Solids litre- ~ day- ~. b Litre gas litre- ~ digester day- ~. c Litre methane kilogramme- ~ Volatile Solids loading day ~.

2 7 6 D A V I D J. H I L L S

the percentage of methane decreases with increasing carbon-nitrogen ratio (i.e. 67 ~o methane for C:N = 8.0 and 51.7 ~o methane for C:N = 51.7). Of prime interest is the methane produced per kilogramme of Volatile Solids loading which is shown in the last column. During phases 1 and 2, digesters 1, 3 and 5, which were fed straight dairy manure, produced equal volumes of methane per unit loading rate. The values varied, however, for the other digesters. For phases 2 and 3, values of methane

~4oo

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150 l"-- hi

I 0 0 - -

I °-c0 5

I I I I I ~ I I [

I I I I I I I I I IO 15 20 25 30 35 40 45 50 55

Fig. 5.

A V A I L A B L E CARBON TO NITROGEN RATIO

M e t h a n e p r o d u c t i o n a s a f fec ted b y C : N ra t ios .

produced per kilogramme of Volatile Solids loading are plotted as a function of carbon-nitrogen ratio in Fig. 5. This curve suggests that the optimum carbon-nitrogen ratio for the dairy manure and cellulose used in this study is 25.

Application of results This study is the initial phase of an investigation designed to supply criteria for

combining various agricultural wastes to produce maximum methane per digester volume. Now that experiments have established an optimum carbon nitrogen ratio for a manure with a definable addition (i.e. cellulose), further research is indicated to determine if unmodified or slightly modified complex carbonaceous wastes can be effectively utilised and, if so, if there is a simple test to assay their available carbon so that the optimum amounts may be added to the digester. To implement these

C:N RATIOS IN ANAEROBIC DIGESTION 277

objectives, the author is continuing work with the above digesters using dairy manure and barley straw.

Several laboratory investigations (Han et al. , 1975; Cowling, 1963; Carroad & Wilke, 1978; Klein, 1972 and Prasad et al. , 1970) have been performed to assess the digestibility of organic solid wastes such as vegetable matter and wood products. These studies support the hypothesis that the ligneous structure within an organic complex tends to shield the cellulose materials from enzymatic hydrolysis. These studies did not provide quantitative data, however. Two preliminary treatments used in these studies were found to be effective in releasing much of the cellulose. Both mechanical fine grinding and chemical treatment with a strong base produced substrates that allowed 80 90 ~',i, removal of the cellulose by subsequent digestion.

Biological pretreatment has been assessed by other investigators (Wise et al. , 1978 and Johnson, 1977) as a means for breaking down complex organic materials. Multistage units are designed to act in series, the first of which is a solubiliser or leacher. The effluent from the leacher acts as the feed into a high-rate digester. The leacher requires fairly long retention times (30 60 days) but is effective at ambient temperatures. These studies indicate the qualitative feasibility of the process but quantitative data on cellulose breakdown were not presented.

Use of anaerobic digestersi on farms, however, demands simple and inexpensive techniques. The preliminary treatments mentioned above may be too complex to incorporate in farm systems so it may not be feasible to aim at maximising the available carbon in a waste additive. If a farmer does desire to combine other agricultural wastes with his manure, he may still achieve maximum methane production within a simple, single-stage digester by combining the manure and chopped wastes to form a carbon nitrogen ratio of 25 as suggested in this study. The available carbon content of the additional carbonaceous wastes, however, will probably be lower than the "total organic carbon minus the lignin carbon" value. Research is continuing to determine how to quantitate the available-carbon.

ACKNOWLEDGEMENTS

This investigation was supported by "Miller Funds' from the College of Agricultural and Environmental Sciences, University of California, Davis.

REFERENCES

ANON. (1975). Standard Methods jo t the Examination of Water and Wastewater (1975). (14th ed.), American Public Health Association, Washington, DC.

BOREFF, C. S. & BUSWELL, A. M. (1934). The anaerobic fermentation of lignin. American Chemical Society Journal, 56, 886.

278 DAVID J. HILLS

CARROAD, P. A. & WILKE, C. R. (1978). Enzymes and microorganisms in food industry waste processing and conversion to useful products: A review of the literature. Resource Recovery and Conservation, 3, 165.

COPPINGER, E. et al. (1978). Report on the Design and First Year Operation o f a 50,000 Gallon Anaerobic Digester. Ecotope Group, Seattle. July.

COWLING, E. B. (1963). Structural features of cellulose that influence its susceptibility to enzymatic hydrolysis. In: Advances in enzymatic hydrolysis of cellulose and related materials, Pergamon Press, London.

FISCHER, J. R. et al. (1978). A control system for an automated anaerobic digester. American Society of Agricultural Engineers Paper No. 78-5017.

FRASER, M. D. (1977). The economics of SNG production by anaerobic digester of specially grown plant matter. Proc. Symposium Fuels/tom Biogas and Wastes, Institute of Gas Technology, Chicago.

GOERING, H. K. & VAN SOmT, P. J. (1975). Forage fiber analysis. Agricultural Handbook No. 379, US Department of Agriculture, Agricultural Research Service.

HAN, Y. W. et al. (1975). Chemical composition and digestibility of ryegrass straw. Journal oJ Agricultural Food Chemistry, 23, 928.

HAS.SAN, H. M. et al. (1975). Characterization of methane production from poultry manure. Proc. International Symposium on Managing Livestock Wastes, American Society of Agricultural Engineers Publication PROC-275, 244.

JOHNSON, A. L. (1977). Final Report on Research in Methane Generation. Aerospace Report No. ATR- 77(9990)-4, The Aerospace Corporation, El Segundo, California.

KLEIN, S. A. (1972). Anaerobic digestion of solid wastes. Compost Science, 13, 6. LAURA, R. D. & IDNAMI, M. A. (1971). Increased production of biogas from cowdung by adding other

agricultural waste materials. Journal Sci. Fd. Agric., 22, 164. METCALF AND EDDY, INC. (1972). Wastewater engineering. McGraw-Hill, New York. PIGG, D. L. (1977). Commercial size anaerobic digester performance with dairy manure. American

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reactions, John Wiley, New York. SIEVERS, D. M. & BRUNE, D. E. (1978). Carbon/nitrogen ratio and anaerobic digestion of swine waste.

Transactions oJ American Society o f Agricultural Engineers, 21, 537. SXEP.ENS, G. R. & HEIC.EL, G. H. (1975). Agricultural and forest products as sources of cellulose. In:

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collecting soil samples. Soil Surveys and Investigation Manual No. 1, Soil Conservation Service. WISE, D. L. et al. (1978). Multi-stage digestion of municipal solid waste to fuel gas. Resource Recovery and

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