Resources, Conservation and Recycling, 1 (1988) 27-37 27 Elsevier Science Publishers B.V./Pergamon Press plc - - Printed in The Netherlands

Influence of Temperature , Buffer, Composit ion and Straw Part ic le Length on the Anaerobic Digest ion of Wheat S t r a w - P i g Manure Mixtures

P. LLABRI~S-LUENGO and J. MATA-ALVAREZ

Department o[ Chemical Engineering, University of Barcelona, Barcelona (Spain)

(Received June 8, 1987; accepted in revised form September 28, 1987)

ABSTRACT

Anaerobic digestion of manure-crop residue mixtures presents several advantages, one of them being the larger biodegradation achieved. In this paper the effect of: (a) the wheat straw-pig manure ratio of the mixture; (b) the straw particle size; (c) the inoculum content; (d) the buffer addition; and (e) the temperature, on the biodegradation achieved in straw-pig manure mixtures is studied in a series of laboratory batch fermenters. Quantitative information is given about the relative effect of these parameters. Composition is the most important one. Inoculum is only relevant in process kinetics. Addition of buffer allows the fermentation of mixtures with higher straw content, while temperature exerts a moderate effect on the final biogas yield.

INTRODUCTION

In some areas the large quantities of manure produced on animal farms rep- resent a major waste disposal problem. Their direct application as organic fer- tilizer may be a great source of pollution for soil and growing plants with pathogenic bacteria and fungi. Crop residues also represent a problem when they are produced in large amounts. Burning or ploughing them in the field presents serious drawbacks, so that alternative solutions have to be applied. Anaerobic digestion of combined organic wastes is an attractive solution that could be applied to many areas where both types of residues occur. Its main advantages are the production of a clean fuel and a residue free of odors that can be used as a fertilizer.

Anaerobic fermentation is a complex process in which several distinct groups of microorganisms are involved. Two main steps can be distinguished. In the first step, complex organic compounds are converted to less complex soluble organic substances by enzymatic hydrolysis. These hydrolysis products are then fermented to simple organic compounds, mainly volatile fatty acids (VFA), by acidogenic bacteria. In the second step, these simple organic compounds are converted to methane and carbon dioxide by the methanogenic bacteria [ 1 ].

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TABLE 1

Initial composition fo the wheat straw, pig manure and inoculum used in the experiments

Wheat straw Pig manure Inoculum

T.S. 91.7% T.S. 3.40% V.S. 83.9% V.S. 2.07% Cellulose and pH 7.55 hemicellulose 78%" VFA 2500ppm (HAc) Lignin 8% a N (NH4 + ) 2320 ppm Ash 4% a N (Kjeldahl) 4740ppm N (Kjeldahl) 2150 ppm Redox pot. - 50 mV

T.S. 2.30% V.S. 1.05% pH 8.40 VFA 310 ppm (HAc) N (NH + ) 2760 ppm N (Kjeldahl) 4530 ppm Redox pot. - 120 mV

aValues expressed as percent of Total Solids.

The anaerobic digestion of manure-crop residues mixtures to biogas has several advantages. One of them is that more methane can be produced when crop residues are mixed with manures. For instance, in a comparison of the combined fermentat ion of pig manure and corn stover at thermophilic and mesophilic conditions, it was reported [2] that the methane yield was sub- stantially higher than that from either the pig manure or corn stover alone. Other authors report similar results [ 3-5] . The C / N ratio has been reported to be responsible for this behavior. At low C / N ratios, carbon addition stimu- lates methane production by reducing ammonia inhibition. At high C / N ratios, carbon addition reduces methane production as nitrogen becomes a limiting nutrient.

In this paper the effect of combining straw and pig manure to form a more nutr ient-balanced substrate for anaerobic digestion is studied so as to optimize the biodegradability of the system. Parameters such as straw particle size, sub- strate ratio, inoculum content, temperature, and buffer addition are analyzed in a series of batch laboratory digesters.

MATERIALS AND M E T H O D S

The manure used in this s tudy was obtained from a fattening pig unit. It was collected from piggeries with a slatted-floor system.

The seed material used was sludge obtained from an anaerobic digester tha t treats the same pig manure used as feed in the experiments. The pig manure was diluted to 2.0% volatile solids (VS) with tap water and stored in a refrig- erator ( - 25 ° C ) until one day before use when it was placed at 4 ° C. The straw was milled in a ball miller and screened at 1.0 cm and 0.5 cm. The composition of the straw, pig manure and inoculum used is shown in Table 1. The total amount of volatile solids (VS) fed to the fermenters was about 20 g each.

Powdered CaCO3, commercially available, was used to buffer the digesters. The fermentat ion vessels used in this study were 1.5-L Pyrex Erlenmeyer

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4

5~ -::: / / / ~ / j -==?-: ~=~: _ _

// ///////// ///////////

Fig. 1. Fermentation of straw-pig manure mixtures. Schematic experimental batch fermentation system: (1) fermenter; (2) gas collection Erlenmeyer; (3) gas measurement Erlenmeyer; (4) bio- gas sample port.

flasks. The digestion units were shaken by hand once a day. The equipment used for the batch experiments is shown schematically in Fig. 1. The experi- mental system was heated by placing it in a controlled temperature room.

Forced nitrogen flow was introduced before the start of the experiment to achieve an anaerobic environment. The amount of biogas that could be made available from the mixed material, was determined by displacement in a gas bottle over salt solution (4 M NaH2PO4, pH = 2 ) to prevent loss of CO2 by absorption. The volume of biogas at standard temperature and pressure (STP) conditions was calculated after correction for the effects of room temperature, water vapour pressure, and for the increase in gas pressure which results from supporting the column of collecting fluid. The biogas production was measured daily after agitation of the sludge mixture.

The biogas composition was analyzed by gas chromatography. Volatile solids (VS), total solids (TS) and total Kjeldahl nitrogen were determined by the methods outlined in Standard Methods [6]. Ammonia nitrogen was deter- mined potentiometrically using a selective-ion ammonia electrode ( Orion 95- 10 ). The concentration of holocellulose ( hemicellulose and cellulose) and lig- nin in a dried straw sample was determined according to ASTM Standards [7,8]. Volatile fatty acids (VFA) were determined directly from the aqueous sample, after centrifugation (7500 g and 10 min) , by a Shimadzu gas chro- matograph GC-9A equipped with a flame ionization detector. The nitrogen carrier gas was saturated with formic acid during the analysis. Gas chromato- graphic separation and analysis was accomplished using a 3-m long, 3.2-mm O.D. steel column packed with 100/120 Chromosorb WAW 6. Other conditions were as follows: carrier gas flow, 30 mL/min; hydrogen flow, 50 mL/min; air flow, 500 mL/min; injector temperature, 240°C; oven temperature, 150 to 190 ° C; heating rate 5 ° C/min.

In order to determine the conditions at which the highest reduction of bio- degradable solid is achieved, the Box-Wilson method was used [9]. Previ-

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TABLE 2

Parameter values of the initial factorial design; experimental results obtained after 60 days of fermentation

Exp. % V.S. Screened % (w/w) % pH % B (60d) Bo No. (straw/total) straw size inoculum reduction methane

m '~ CH4 (STP) /kg VS T . S . V . S . added

1.1 40 1.0 30 12.1 12.5 8.60 76.2 0.327 0.371

1.2 40 1.0 15 18.5 18.0 8.10 74.8 0.338 0.386 1.3 40 0.5 30 19.5 16.8 8.19 75.4 0.343 0.380 1.4 40 0.5 15 27.4 28.2 8.00 77.4 0.366 0.412

1.5 20 1.0 30 9.3 7.7 8.05 79.9 0.309 0.344 1.6 20 1.0 15 8.5 14.5 8.15 81.0 0.344 0.393 1.7 20 0.5 30 5.8 11.9 8.18 79.5 0.356 0.395 1.8 20 0.5 15 11.5 20.4 8.32 80.7 0.334 0.373 1.9 30 0.75 22.5 17.1 10.7 8.80 74.9 0.319 0.362

ously, a 23 experimental factor design was set up in order to evaluate the effect of the parameters and to derive an equation for the optimization [ 10].

To determine effects of temperature in the low mesophylic range, fermenters were operated between 23 and 37°C.

R E S U L T S AND DISCUSSION

The tested variables were: percent of VS coming from the straw in the mix- ture, inoculum percentage and straw particle size. The values of the variables in the experiments, together with the results obtained after 60 days of digestion are presented in Table 2. The ultimate methane yield Bo [ 5 ], expressed as m 3 methane (STP) per kg VS added, was determined by plotting the cumulative methane production vs. the reciprocal of time and extrapolating to infinite time. This system response has been selected because it represents a good mea- surement of the biodegradability of the straw-pig manure mixture. Other re- corded parameters are the VS reduction, pH and the percent of CH4 in the biogas. As can be seen, pH values are rather high due to the buffer effect of the ammonia ions in the manure used (see Table 1). As could be expected, the highest volatile solids reduction corresponds to the highest Bo obtained. The experiments with the same straw-manure ratio (experiments 1.1-1.4 and 1.5-1.8) show similar results: the smaller the straw size and the smaller the percentage of inoculum, the larger the ultimate yield Bo. The methane produc-

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DAYS I 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016

I I i

0.6 i0

B o ~

cu.vE A ~

~ 6 ~

,.~ 4 N 0.2 ~

I i I I I I

IO 20 30 40 50 60

DAYS

Fig. 2. Fermentation of straw-pig manure mixtures. Daily biogas production ( curve A ), cumula- tive biogas production (curve B) and cumulative gas vs. reciprocal time (curve C) for exp. 1.1.

tion profile ( curve A), together with the cumulative methane production ( curve B ) for experiment 1.1 is presented in Fig. 2 as representative of the series. This figure also shows the plot used to estimate B0 (curve C ).

In order to quantify the effects of the variables studied a first order equation was fitted to the data [ 10 ] :

Bo -- 0.382 + 0.0062 X1 - 0.009 X2 - 0.0085 X3 (1)

where

(% VS straw-manure) - 30 X l - ( 2 )

10

(straw size [cm] ) -0 .75 X2 - (3)

0.25

( % inoculum) - 22.5 X3 - (4)

7.5

From the observation of the sign of the coefficient at tached to each variable it follows that the percentage of VS coming from the straw strongly influences the biodegradability of the mixture: addition of straw to manure increases the ult imate methane yield, Bo. On the other hand, it seems that the enzymatic breakdown of straw is not significantly increased by size reduction within the analyzed range. The hydrolysis of straw mainly depends on its degree of lig- nification. In any case, the decrease in straw size seems to have a slight positive effect on Bo. This effect would be similar to that described by Gharpuray et al.

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O.6

(A)

0 . 2

Io I I I I I 1 20 30 40 50 60

DAYS

Fig. 3. Fermentation of straw-pig manure mixtures. Comparison of experimental biogas produc- tion profiles with inoculum contents of 30% (curve A, exp. 1.3) and 15% (curve B, exp. 1.4).

[ 11 ]. Finally, the decrease in the percentage of added inoculum slightly in- creases the ultimate methane yield Bo. However, there is a remarkable kinetic effect: the more microorganisms initially present, the sooner biogas production starts and the sooner the maximum methane production rate is achieved. Fig- ure 3 shows this effect by plotting the biogas production profiles obtained when using inoculum levels of 15% (exp. 1.4, curve B) and 30% (exp. 1.3, curve A) (weight of inoculum/weight of total mixture). Analysis of variance showed that both straw size and inoculum content are statistically insignificant for the response Bo.

From eqn. (1) the steepest ascent path was computed [9]. Using a step increase of 3% VS (straw VS/total mixture VS) for variable X,, eleven exper- iments were designed (Table 3 ). They were performed simultaneously in order to keep the same manure concentration and the same level of inoculum activ- ity. Only the VS percentage ( related to X1 ) and the inoculum content ( related to X2) were varied. Straw size was held constant because of its negligible sta- tistical effect on the response. VS ranged from 40% to 70% (straw VS/total mixture VS) and inoculum content from 20% to 15% (inoculum weight/total weight of manure) . Table 3 also shows the results obtained after 90 days of fermentation. As can be seen, Bo reaches a maximum value of 0.420 m 3 CH4 (STP) per kg VS added at 43% VS (straw VS/total VS). A higher efficiency is not possible due to the low moisture values: Bo decreases slowly until a toxic VFA concentration level is soon reached (experiments 2.10 and 2.11 ), leading to a failure of the fermentation. These results corroborate the validity of eqn. (1), which states that addition of straw improves the biodegradability of the mixture as compared with their components alone.

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TABLE 3

Fermentation of straw -pig manure mixtures; results obtained from steepest ascent path experiments

Exp. To V.S. To (w/w) fermenter % B (90 d) B,, No. (straw/total) inoculum load ( TO TS ) methane

( m 3 methane ( STP )/kg VS)

2.1 40 20.7 4.83 77.6 0.373 0.401 2.2 43 20.2 5.10 77.7 0.385 0.420 2.3 46 19.7 5.19 74.0 0.360 0.393 2.4 49 19.1 5.39 74.6 0.361 0.396 2.5 52 18.6 5.70 74.4 0.349 0.381 2.6 55 18.0 6.01 72.0 0.314 0.348 2.7 58 17.5 6.31 71.3 0.306 0.353 2.8 61 16.9 6.66 74.7 0.331 0.326 2.9 64 16.4 7.10 67.2 0.289 0.309 2.10 67 15.9 7.60 37.3 0.087 0.164 2.11 70 15.4 8.20 31.0 0.043 0.047

In order to study in more detail the anaerobic digestion process under the optimal mixture proportions ( experiment 2.2), nine replicated experiments in nine identical fermenters were carried out simultaneously. Every 10 days, one of the digesters was opened, and its contents analyzed. Thus, the last analysis was performed on day 90. The results of the leachate and solid analysis are presented in Fig. 4. Mixed liquors TS and VS values are somewhat high due to the non-degradable suspended matter originating from the straw. Redox po- tential evolves in a similar manner as biogas production, and is closely asso- ciated to the methanogenic activity. Ammonia ion concentration shows a not very sharp minimum around day 30, which is related to the biomass formation. Under nutr ient shortage, some cellular lysis occurs and ammonia ion concen- trat ion begins to rise. Cellulose and hemicellulose content substantially de- crease during the fermentat ion period. The low VFA concentration level exhibited during the whole fermentat ion period illustrates the high stability of the process when these substrate proportions are used.

Effect of buffer addition and temperature

In a following series of experiments the effect of a buffer addition was tested. The experiments on the steepest ascent path were performed at VS propor- tions ranging from 40% to 88% (straw VS/total mixture VS ). The buffer con- tent was held constant in all experiments at a level of 50% (w/w) of the total VS weight ( about 3.2 % based on total weight ). The maximum Bo was obtained at a VS proportion of 52% (straw VS/total VS), a little higher than before. Thus, the addition of CaCO3 allows the use of more straw per unit of manure

34

/ T.O

-100 ,=

-120

o ~ ° ~ o ~ °

0.5O ~ v ~ v

~ ~ °

3000" ,.~ • 1500 '0 ~ ~ O~o-----~_ I m / m

O O -- ~ lOGO

GO 50O

I | I I 0 10 20 30 ~0 50 GO 70 80 90

DAYs

Fig. 4. Fermentation of straw-pig manure mixtures. Evolution of leachate parameters during the anaerobic digestion of straw-pig manue mixture at optimal conditions (exp. 2.2): A, pH; $ , redox potential (mV); Y, TS (%); V, VS (%); [3, total Kjeldahl nitrogen (ppm); l , NH + nitrogen; i , VFA content; O, cellulose and hemicellulose content of the straw.

substrate, resulting in a better utilization of the digester's working volume. In practice this would imply, taking an average composition of substrates similar to those presented in Table 1, that it is possible to add as much as 82% of straw (straw VS/total VS mixture) (experiment 3.8, Table 4), as compared with 64% (experiment 2.9, Table 3 ) when no buffer is added. Comparing the values of Bo obtained from the mixture without buffer addition (Table 3 ) with those of Table 4, for the same mixture proportions, it is seen that the yield is im- proved in the range of 52-70% VS. Moreover, there is a significant increase in the ultimate methane yield in those experiments of the first series which pro-

TABLE 4

Fermentation of straw-pig manure mixtures: ultimate methane yield using CaCO3 as a buffer

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Exp. % VS % (w/w) % B (90 d) Bo No. (straw/total) inoculum methane

( m ~ CH4 (STP)/kg VS added)

3.1 40 20.7 71.3 0.322 0.354 3.2 46 19.7 69.4 0.331 0.368 3.3 52 18.6 68.6 0.363 0.395 3.4 58 17.5 66.6 0.299 0.336 3.5 64 16.4 71.7 0.290 0.324 3.6 70 15.6 67.8 0.387 0.351 3.7 76 14.2 65.3 0.324 0.352 3.8 82 13.2 54.5 0.300 0.334 3.9 88 12.1 67.8 0.117 0.132

duced an VFA inhibition (experiments 2.10/2.11, Table 3 and 3.5/3.6, Table 4).

In order to assess the effect of the temperature in the mesophilic range, a final set of experiments was carried out. The VS proportion s t raw-manure was maintained at the optimal level (experiment 2.2) in all the runs. The tested temperatures are shown in Table 5, together with the results obtained. As can be seen, a maximum biodegradability is achieved at temperatures between 31 and 33 ° C. Over the whole range, biogas methane content was about 70%. These results are similar to those reported by Van Velsen [ 12 ]. The operation within this range is not only kinetically more favorable, as has been reported else-

TABLE 5

Fermentation of straw-pig manure mixtures: influence of temperature at the optimal conditions (mixture of 43 % VS straw/total VS mixture )

Exp. Temperature % B (90 d) Bo No. ( ° C ) methane

(m 3 CH4 (STP)/kg VS)

4.1 23 67.8 0.258 0.290 4.2 25 70.5 0.268 0.294 4.3 27 69.2 0.323 0.359 4.4 29 68.2 0.359 0.386 4.5 31 69.8 0.365 0.400 4.6 33 76.1 0.365 0.393 4.7 35 71.3 0.357 0.397 4.8 37 69.9 0.356 0.387

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where [13], but also low increments of biogas could be obtained for a given quantity of VS and at a given time. Nevertheless, these differences are not large and, even at room temperatures, the biodegradation obtained is of practical value and, as mentioned before, can be enhanced if optimal proportions of wastes are employed.

From the values of B (Table 5 ), it is also seen that after 90 days of fermen- tation the yields are not very different from the ultimate yields. As a conse- quence, the effect of temperature on kinetics after such a period is not very strong.

CONCLUSIONS

The addition of small quantities of straw (around 40% w/w based on VS) to wastes with a deficient C :N ratio (such as piggery waste) increases the biodegradability of both substrates. This effect could be of practical interest in small batch digesters even if operated at room temperature (in the range 20-37 °C ). The addition improves the agronomic value of the digested residue because of its more balanced organic content. Thus, from the experimental data, 100 g VS coming from pig manure would be reduced, at the end of a fermentation period of 90 days, to 54 g VS, whereas 100 g VS of the optimal mixture would leave only 44.9 g VS.

As expected, inoculum addition increases the degradation rate, but does not affect the final biodegradation yield. Finally, straw particle length does not have a significant effect on the ultimate yield within the experimental range (screened at 0.5/1.0 cm).

NOTATION

B

Bo

STP TS VFA VS X1

Methane yield after 90/60 days of fermentation, m s methane (STP) per kg VS added Ultimate methane yield ( infinite time ), m 3 methane (STP) per kg VS added Standard temperature and pressure Total solids, % (w/w) Volatile fatty acids, mg HAc/1 Volatile solids, % (w/w) Variable related to the composition of the straw-manure mixture ( eqn. 2 ), dimensionless Variable related to the straw size added to the mixture (eqn. 3), dimensionless Variable related to % inoculum content (w/w) in the mixture ( eqn. 4 ), dimensionless

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