kinetic study of the anaerobic digestion of straw-pig manure mixtures

Biomass 14 (1987) 129-142

Kinetic Study of the Anaerobic Digestion of Straw-Pig Manure Mixtures

R Llabr6s-Luengo and J. Mata-Alvarez

Departament d'Enginyeria Quimica i Metal.hirgia, Universitat de Barcelona, C/Marti i Franqu~s, 1, 6 a, 08028 Barcelona, Spain

(Received 10 August 1987; revised version received 24 September 1987; accepted 27 September 1987)

A B S T R A C T

Anaerobic digestion of manure-crop waste mixtures presents several advantages, one being the larger biodegradation achieved. The kinetic behavior of the anaerobic digestion of wheat straw-pig manure mixtures was studied in batch laboratory fermenters. The digestion period (90 days) is divided into two parts to fit two kinetic models, the first developed under the hypothesis of microbial growth as the limiting step, and the second, based on the availability of substrate as the limiting factor. The results show that the effect of the mixture composition on the two kinetic parameters is not relevant. The effect of temperature (23-37"C) was examined under optimal feed conditions. Specific growth rate showed a significant increase with temperature, whereas increase in the hydrolysis rate was less marked.

Key words." Anaerobic digestion, kinetic model, batch, wheat straw, pig manure, mesophilic temperature.

NOMENCLATURE

B

Bo

K S

k S So

Biomass

methane yield after a given period of fermentation, STP m 3 methane per kg VS added ultimate methane yield (infinite time), STP m 3 methane per kg VS added half velocity constant (mg liter- J) second model kinetic parameter (day- ~) substrate concentration (mg liter- ~) initial substrate concentration (mg liter- ~)

129 0144-4565/87/S03.50 - © Elsevier Applied

England, 1987. Printed in Great Britain Science Publishers Ltd,

130

STP t tm TS VFA VS X x0 Y

tim

P. Llabr~s-Luengo, J. Mata-Alvarez

standard temperature and pressure time (day) time to achieve maximum gas production rate (day) total solids (%, w/w) volatile fatty acids (mg acetic acid per liter) volatile solids (%, w/w) microorganism concentration (mg VSS liter- l) initial microorganism concentration (VSS mg liter- ~) yield constant (dimensionless) microorganism growth rate (day- 1) specific maximum microorganism growth rate (day- 1)

INTRODUCTION

Anaerobic digestion is a complex process widely used to stabilize and reduce the amount of organic wastes. Several steps and several groups of bacteria are involved in this complex processJ Nutrients, primarily nitrogen and phosphorus, are required to optimize the bacterial growth. Bacterial seed is also important in batch systems used at farm level. When a digester is started without inoculum, intermediate compounds can accumulate and inhibit the bacterial activity.

Growth in a batch fermentation system presents three distinct phases: a lag period, an exponential growth phase, and finally a stationary phase. The lag period follows the inoculation of the nutrient medium and is a period of adaptation to the new environment. Once adaptation has occurred, the cells move into the exponential or log growth phase. In a mixed culture, the different microbial species may have different specific growth rates (/a) and, as a consequence, the relative proportions of the different species can vary with time. At the end of the experimental phase, the growth rate begins to decrease, due to the depletion of an essential nutrient or because an inhibitory product accumulates. Stationary phase occurs when the rate of growth equilibrates with the death rate. In a mixed culture fermentation, this phase is not coincident for all species since the nutrient requirements may be different for each species.

Anaerobic digestion of mixtures of manure and crop residues has been investigated because of several reported advantages, one being that more methane can be produced when crop residues are mixed with manures. According to several authors 2-4 this is the result of an improved C/N ratio.

Anaerobic digestion of straw-pig manure mixtures 131

Use of anaerobic digesters on farms demands simple and inexpensive devices. Preliminary pretreatment increases the biodegradability of crop residues, 5-7 but is too complex to be incorporated at a farm level. How- ever, simple mixtures of wastes may provide a satisfactory means to improve farm digester performance.

The purpose of this investigation was to determine the effect of the volatile sohds (VS) ratio on biodegradability and the influence of mixture composition and temperature on the kinetics of the batch fermentation system.

MATERIALS A N D M E T H O D S

The manure used was obtained from a pig fattening unit. The seed material used was sludge collected from an anaerobic digester treating similar pig manure. The wheat straw was ball milled and screened before use. The compositions of the straw, pig manure and inoculum are shown in Table 1.

The fermentation vessels used were 1.5-liter Pyrex Erlenmeyers flasks, which were shaken by hand once a day. The total VS fed to the fermenters was 20 g each, approximately, in all the experiments. Diges- tions were carried out in a controlled-temperature room.

Biogas production was measured daily by means of a water displace- ment system and corrected for temperature, water vapor pressure and

T A B L E 1 Initial Compos i t ion of the Wheat Straw, Pig Manure and Inoculum used in the Exper i -

ments

Wheat straw Pig manure Inoculum

TS (%, w/w) 91.7 3.40 2"30 VS (%, w/w) 83.9 2.07 1.05 Holocel lu lose (%, w/w)" 78"0 - - - - Lignin (%, w/w)" 8-0 - - - - Ash (%, w/w)" 4-0 - - - - N-Kjeldahi (ppm) 2150 4740 4530 N - N H ] (ppm) - - 2320 2760 V F A (ppm HAc) - - 2500 310 pH - - 7-55 8.40 Redox potential (mV) - - - 50 - 120

"Values expressed as percent of TS.

132 P. Llabr~s-Luengo, J. Mata-Alvarez

for the increase in gas pressure resulting from supporting the column of collecting fluid. The measurement was performed after agitating the sludge mixture and values are given as volumes at STP.

The composition of biogas was analyzed by gas chromatography. VS, total solids (TS) and total Kjeldahl nitrogen were determined by the methods outlined in Standard Methods. 8 Ammonia nitrogen was determined potentiometrically using a selective ion ammonia electrode (Orion 95-10). The concentrations of total cellulose (hemicellulose and cellulose) and lignin in samples of dried straw were determined according to the methods approved by ASTM Standards.9,"~Redox potentials were measured using a Radiometer PHM62 equipped with a Radiometer Platinum P101 electrode. The volatile fatty acids (VFA) were determined directly from the aqueous sample, after centrifugation ( 10 min at 7500 x g), by gas chromatography using a 3 m, ~ in (3.175 mm) OD steel column packed with 100/120 Chromosorb WAW 6.

The Box-Wilson method was used j ~,12 to determine the conditions giving the highest reduction in biodegradable solids. An experimental factorial design was set up to evaluate the effect of straw size, VS proportion in the mixture and inoculum content (inoculum dry matter/total dry matter) on the biodegradability and on the kinetic parameters.

Fermenters were operated at eight temperatures in the range 23 to 37°C to determine effects of temperature in the low mesophilic range.

RESULTS AND DISCUSSION

Figure 1 shows a typical biogas production profile as well as change in several significant parameters. A maximum rate of gas production was reached quite quickly, after which it decreased to become negligible at the end of the fermentation period. The early maximum may be explained in terms of the content of easily-biodegradable material in the pig manure and the level of active biomass introduced as seed. During the first 4-5 days all the easily-digested material is rapidly consumed with the biogas obtained, amounting to about 20% (v/v) of the total. Once the easily-degradable compounds are consumed, biogas is produced from the degradation products of the rather slow hydrolytic step.

The experimental period of fermentation was split into two parts to take account of these facts and to overcome the difficulty in fitting the kinetic models. The first period comprised the time before the maximum biogas production was achieved (time, tin) and the second the time after tm. Two different simple models were fitted to the data.

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First model

The hypothesis for the first model was that microbial growth was the limiting factor for biogas production. This is supported by the observed decrease in the ammonia nitrogen before t,,, associated with the syn- thesis of biomass (see Fig. 1 ). Amounts of pig manure and straw available were very large relative to that consumed and hence they could be con- sidered constant during this first phase.

Over the period of nutrient availability, the microbial growth depends only on the concentration of microorganisms. In this case ~ (specific microorganism growth rate) remains constant and is equal to its maximum value named ~m" Thus, it was assumed that microbial growth rate followed a first order kinetic law during this interval

d g / d t = ~m g (1)

Considering the Monod equation

dX_ /.lmSX dt S+K~

(2)

where K~is the constant representing the concentration of S where the cells reproduce at & their maximum rate (~m), it can be seen that it co- incides with the proposed equation (eqn (1)), if K~ is negligible compared with S.

The death of microorganisms during this period has not been con- sidered as significant. Equation (1) is related to substrate removal according to the yield equation

d X / d t = ( - Y ) d S / d t (3)

If B denotes the accumulated methane production for each unit of VS added, and B 0 is the ultimate methane yield, ~3 the concentration of biodegradable VS in the fermenter (S, mg liter- ~) will be directly related to gas production in accordance with

(B 0 - B)/B,, = S/S,, (4)

The integration of eqns (3) and (4) gives

B / B,, = Xo/ YSo[ exp(/~mt ) - 1] (5)

When gas production is significant, exp(/~mt)'> 1 and eqn (5) can be simplified to

In(BIB o) = In(X0/YS0)+ ~m t (6)


This equation can be linearized and B 0 values can be estimated from a representation of B versus inverse time. The plot approaches a straight line with 1/t--" 0 as B ~ B 0. From eqn (6), the plot of ln(B/Bo)versus time (0 < t < tin) yields a straight line with the intercept Xo/YS o and slope/%. This equation was used to determine the specific maximum growth rate constant, ~m"

Second model

The second model is based on the availability of substrate as the limiting factor. When microorganisms are grown on water-insoluble nutrients, growth can become limited by the rate of diffusion or the rate of dissolu- tion of that nutrient. Under the hypothesis described above, in a second period of fermentation (after an exponential growth phase) the concentration of hydrolytic cells reaches a steady level and, as a consequence, the rate of hydrolysis of straw also reaches a steady level. Figure 2 shows, that after reaching a maximum level, gas production and the level of VS both decrease and the VFA content becomes negligible. This suggests that a first order kinetic law could represent adequately the degradation process

- d S / d t = k S (7)

where k is a kinetic constant and S the substrate concentration. This model is valid for the second period of fermentation ( t> trn), when the maximum biogas production rate has been achieved. The rate control- ling step, during this second period, is the hydrolysis of polymers. Hydrolysis has previously been modeled as a first order process by several authors.~4~5

Combining eqns (4) and (7) and integrating

B 0 - B - - - exp( - k t) (8)

B0

From eqn (8) the plot of ln(Bo-B/Bo) versus time (t> tin) gives a straight line, and was used to determine the kinetic constant, k.

Experimental results

In the first set of experiments the tested variables were: %(w/w)of VS coming from the straw in the mixture; inoculum percentage and straw size. The levels of the variables in each experiment and the results obtained after 60 days of digestion are presented in Table 2. The ulti-

136 P. Llabr#s-Luengo, J. Mata-Alvarez

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138 P. LlabrOs-Luengo, J. Mata-Alvarez

mate methane yield (BIj) is expressed as m ~ methane per kg VS added. Other recorded parameters after 60 days of fermentation are pH, the reduction in TS and VS, %(v/v) of methane and the cumulative biogas production (B).

The CH 4 percentage in the biogas was about 77% in all experiments. As might be expected, the highest volatile solids reduction corresponds to the highest B¢~ achieved. This table also includes the fitted kinetic parameters, discussed above. The values were analyzed, taking into account the level of the variables, and only the level of inoculum slightly affected the kinetic constant /~m, a larger inoculum resulting in a faster fermentation. The other constant, k, coming from the second model, was only slightly affected. Neither the effect on/~m nor on k was statistically significant and, additionally, k and /~nl were independent of the other studied variables (straw size and VS mixture composition). Thus, it was concluded that the kinetic constants remain approximately constant and that the observed slight variations come from the inherent variability of the data.

From the Bc~ results obtained, the steepest ascent path was computed r~ to maximize the biodegradability of the mixture. Eleven experiments were performed taking steps of 3% (w/w)VS (straw VS/totai mixture VS) (Table 3) and keeping the manure concentration and inoculum constant. Straw particle size was kept constant, though it had a negligible statistical

Fermentation of Straw-Pig TABLE 3

Manure Mixtures: Results Obtained from the Steepest Ascent Path Experiments

Experiment Straw~total Inocuhtm Methane B (90 d) B~, uH, k number (% VS, (% w/w) (% Uv) (~kO'- i) (duy I)

identification w/w) ($77' m; methane per kg VS)

2-1 40 20"7 77"6 0"373 0"4(I 1 0"95 0.(I 176 2-2 43 20"2 77-7 0-385 0-420 0'88 0'0193 2-3 46 19'7 74"0 0"360 0-393 0'95 0'0174 2-4 49 19'1 74"6 0'361 0'396 ( } ' 9 2 0'0196 2-5 52 18"6 74"4 0'349 0"381 (1'93 0.(t186 2-6 55 18"0 72'0 0"314 0'348 0"97 0-0178 2-7 58 17-5 71'3 0'306 (}'353 0"90 0"0195 2-8 61 16"9 74'7 0'331 0"326 0"95 0'0184 2-9 64 16'4 67'2 0'289 0'309 0"93 0"0174 2-10 67 15"9 37"3 0'087 0'164 /)'98 0'0171 2-11 70 15"4 31"0 0'043 0"047 (}'93 0'0198


effect on B o. VS percentage ranged from 40 to 70% (straw VS/total mixture VS).

Table 3 also presents the fitted regression constants (/a m and k). As can be seen, the experimental data agree satisfactorily with the models proposed, as in the previous set of experiments. Besides, the values of the parameters are approximately the same as before (Table 2).

Table 3 finally shows the obtained results of VS reduction and B after 90 days of fermentation. As can be seen, B 0 reaches a maximum value of 0.420 m 3 CH4 per kg VS added, at 43% (w/w) VS (straw VS/total VS). Further progress was not possible due to the low moisture values: B0 decreases slowly until a toxic VFA concentration level is soon reached (Experiments 2-10 and 2-11 ), leading to a collapse of the fermentation. Thus, mixture proportions corresponding to experiment 2-2 are con- sidered optimal from the biodegradability point of view. Looking at the values of tim and k in Table 3, it appears that kinetics of the fermentation are not affected, even when the process has nearly ceased. This can be explained by taking into account the assumptions of eqns (1) and (7). If the model is true, the rate of degradation depends directly upon the concentration of viable microorganisms (model 1) or upon the available substrate, S (model 2). When their levels are low, the biodegradation proceeds slowly, resulting in poor VS reduction.

Effect of temperature

A final set of experiments was carried out to assess the effect of the temperature in the mesophilic range. The VS proportion in mixtures straw-manure was maintained at the optimal level (that of experiment 2-2) in all the runs. The range of temperatures used is shown in Fig. 2, together with the biodegradation yields and the fitted kinetic constants. As can be seen, a maximum biodegradability is achieved at temperatures between 31 and 35°C. Operating inside this range a small surplus of biogas could be obtained from a given quantity of VS, at any given time. However, from the values of B0 (Fig. 2), it appears that after 90 days of fermentation the yield differences are not very large. As a consequence the effect of temperature on the total yield is slight.

On the other hand, the rate kinetics of the process increased steadily within the experimental range. The most significant effect was on the specific growth rate, characteristic of the microbial behavior, whereas the much smaller change in hydrolysis rate is characteristic of a diffusion process. The Pm data reported here (0.80 to 0.99 day-~ at 35°C), agree with the ranges given by other authors. ~'-~')

140 P. LlabrOs-Luengo, J. Mata-Alvarez

Temperature data fitting

The data plotted in Fig. 2 were used to determine the apparent activation energy (E) and the cOefficient (A), using a simple Arrhenius approach:

~m = ~, exp( - E/RT) (9)

The corresponding equation is:

In/~m = 81"5 -- 25 200/T (10)

where T is expressed in K and ~m in day -~. The regression coefficient for the line is 0.991. Values for/,t 0 and E are, respectively, 2.57 x 1035 day-~ and 50.1 kcal mol -j. This latter value is within the range 6.8-79.2 kcal mol- ~ reported for the anaerobic digestion of several substrates and within the range reported for enzymatic reactions, cellular and life related reactions occurring at these temperatures. 2°-22

CONCLUSIONS

Biodegradability of both straw and pig manure increases when they are mixed together. An optimal proportion exists that gives a maximum VS reduction.

The biodegradation process can be adequately represented by two first order models, the first assuming that microbial growth is the limiting factor and the second that substrate availability becomes limiting. From the obtained results (Tables 2 and 3) it can be deduced that both specific growth rate (~m) and kinetic constant (k) do not have any link with the tested variables (straw-manure proportion, inoculum content and straw size). These results agree with the kinetic hypothesis proposed and con- firm the validity of both models. During the first period of time (t < tin), when the first kinetic model applies, biogas production comes from the digestion of soluble substances from pig manure. In the second interval (t > tin), biogas production is provided by products from wheat straw and this is the limiting step.

Temperature affects the kinetics, increasing the biodegradation rate over the range (23-35°C). The apparent activation energy lies within the expected values for this kind of process. Biodegradability is also affected by temperature. A maximum VS reduction is observed at temperatures between 31 and 35°C.


A C K N O W L E D G E M E N T

Financial support from the Spanish 'Comisi6n Asesora de Investigaci6n Cientifica y T6cnica' Project No. AG-39-82 is gratefully acknowledged.

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kinetic study of the anaerobic digestion of straw-pig manure mixtures

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