4!!9 War,Res. Vol.32, No. 1,pp. 5-12,1998 01998 ElsevierScienceLtd. Allriehtsreserved Pergamon PII: S0043-1354(97)00201-7 Printedin&eat Britain 00...

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War,Res. Vol.32, No. 1,pp. 5-12,1998 01998 ElsevierScienceLtd. Allriehtsreserved


PII: S0043-1354(97)00201-7

Printedin&eat Britain 0043-1354/98$19.00+ 0,00

ANAEROBIC DIGESTION OF SWINE MANURE: INHIBITION BY AMMONIA KAARE HVID HANSEN, IRINI ANGELIDAKI and BIRGITTE KUER [email protected]* Department of EnvironmentalScienceand Engineering,TechnicalUniversityof Denmark, Building115,DK-2800Lyngby,Denmark (First received September 1996; accepted in revisedform June 1997)

Abstrad-A stable anaerobic degradation of swine manure with ammonia concentration of 6 g-N/litre was obtained in continuouslystirred tank reactors with a hydraulic retention time of 15 days, at four different temperatures. Methane yields of 188, 141,67 and 22ml-CH4/g-VSwere obtained at 37, 45, 55 and 60”C, respectively.The yields were significantlylower than the potential biogas yield of the swine manure used (300ml-CH4/g-VS).A free ammonia concentration of 1.1g-N/litre or more was found to cause inhibition in batch cultures at pH 8.0 (reactor pH), and higher free ammonia concentrations resulted in a decreased apparent specificgrowth rate. Batch experimentswith various mixtures of swine and cattle manure showedthat the biogas process was inhibited when the swine-to-cattlemanure ratio was higher than 25:75,correspondingto a free ammonia concentration of approximately 1.1g-N/litre. Inhibition of the biogas process and, thereby, a reduction of the methane yield followed a four-stage pattern: below a threshold of 1.1g-N/litre free ammonia, the process was uninhibited;over this concentration, inhibition occurred, forming first a phase with an initial inhibition, then a plateau and then an inhibition stage where the apparent specificgrowth rate decreasedwith increasingconcentrations of free ammonia. ~ 1998ElsevierScienceLtd. AIIrights reserved Key wordfiammouia inhibition, anaerobic degradation, swinemanure

INTRODUCTION Anaerobic digestion is widely used for waste treatment. In Denmark a demonstration programme for large scale-biogas plants has resulted in the establishment of more than 20 large collective biogas plants treating a mixture of manure and organic industrial waste (Ahring et al., 1992). The organic content of the waste is reduced with simultaneous production of energy. Both cattle manure and swine manure are used in the large collective biogas

plants, and the importance of the last source is expected to increase due to the abundance of this waste type compared to other wastes. Digestion of stiine manure as sole substrate has previously been shown to be unsuccessful,mainly due to the high content of ammonia in this waste (Van Velsen, 1979; Farina et al., 1988). Ammonia inhibition is especially distinct when digesting swine or poultry manure, which often have total ammonia concentrations higher than 4 g-N/litre. An ammonia concentration of 4 g-N/litre was shown to be inhibitory during digestion of cattle manure (Angelidaki and Ahring, 1993). For unadapted methanogenic cultures, ammonia inhibition has been observed to commence at con*Authorto whomall correspondence shouldbe addressed ~el: +4545251566,Fax: +4545932850,E-mail: [email protected]]. 5

centrations of 1.5–2.5g-N/litre (Van Velsen, 1979; Hashimoto, 1986),However, by adaptation of the biogas process to ammonia, tolerance to 4 g-N/litre total ammonia has been demonstrated (Hashimoto, 1986; Angelidaki and Ahring, 1993). The free ammonia concentration has been suggested to be the active component causing ammonia inhibition (Hashimoto, 1986; Angelidaki and Ahring, 1993). Acetate-utilizingbacteria adapted to ammonia were shown to grow with a free ammonia concentration of up to 700mg-N/litre (Angelidaki and Ahring, 1993), while many lower free ammonia concentrations (100-150mg-N/litre) have been reported for initial inhibition of an unadapted process (Braun et al., 1981;De Baere et al., 1984). The free ammonia concentration depends mainly on three parameters: the total ammonia concentration, temperature and pH. Several authors have found that methane fermentation of high ammoniacontaining wastes is more easily inhibited at thermophilic temperatures than at mesophilic temperatures (Braun et al,, 1981;Van Velsen, 1981;Parkin and Miller, 1983; Angelidaki and Ahring, 1994). This is in agreement with the fact that the free ammonia concentration increases with increasing temperature. Furthermore, the biogas process becomesmore sensitivetowards ammonia when the pH value increases (Koster, 1986), which again increases the concentration of free ammonia.

Kaare Hvid Hansen et al.



Methano~enesis from H2/C02 (80:20)during thermophilicdigestionof swinemanurewasstudiedat threedifferentammoniaconcentrations(3.3, 4.8 and 5.8g-N/litre).The experiment wasperformedin 118-mlserum vials, with a 20-ml inocuH2/C02 experiment (experiment 3).

Increase of pH from 7 to 8 will actually lead to an eight-fold increase of the free ammonia concentration. It has been suggested previously that the interaction between free ammonia, volatile fatty acids (VFA) and pH will lead to an “inhibited steady state”, which is a condition where the process is running stable but with a lower methane yield. This has been shown to happen when the substrate has a high ammonia concentration (Angelidaki and Ahring, 1993;Angelidaki et al., 1993). The present study extends the authors’ previous studies of ammonia inhibition to anaerobic digestion at different temperatures of swine manure with a high content of ammonia. Finally, a mathematical description of the influence of the free ammonia concentration on the biogas process is presented. The description is based on the results from this study.

hrm from the same lab-scale reactor as described in experiment 2 and 20ml of BA-medium (Angelidafd et al., 1990).The ammonia concentration was increased by adding N&Cl. The duration of the experimentwas 8 days. The serum vials were closed with butyl rubber stoppers and sealed with ahnninium crimps. The vials were placed in a 55°Cincubator and were shaken vigorouslyby hand once a day. The methane concentration in the head space were measured one time each day. The potential methane yield (Bo) was determined as the total methane production after complete anaerobic degradation dividedby the amount of volatile solids (W) in the samples. Complete anaerobic degradation was assumed when the culture showed no further increase in the methane production for 15days. CSTR experiments

The effect of temperature on digestion of swine manure was tested in four 4.5-litre biogas reactors with a working volume of 3 litres. The continuously stirred tank reactor MATERIALS ANDMETHODS (CSTR) set-up is shown in Fig. 1. The reactors had a slowlymovingstirring blade (60 rpm), whichwas activated Batch experiments for 1min every 3 min. The gas production was collectedin aluminium bags and measured. The reactor temperature Three batch experimentswere performed in duplicate. was kept at the selected temperature by water circulation Experiment with digestion of mixtures of swine and cattle manure (experiment 1). Degradation of various mixtures in the water jacket surrounding the reactors. The reactors of cattle and swine manure was tested in 58-ml serum were fed six times per day. Swine manure was used as sole substrate in all reactor vials. To the serum vials a 12-ml inocuhrm was added from a lab-scale reactor (PH 8.1, ammonia concentration experiments and the HRT was 15 days. Four reactors 3 g-N/litre) operated on fresh cattle manure at a hydraulic were run at temperatures of 37, 45, 55 and 60°C and retention time (HRT) of 15 days and a temperature of designated R37.C,R450C,R55.Cand RsO.c,respectively.The 55”C. In addition, 8 ml of fresh cattle and swine manure reactors were started with 2.8 Iitres of inoculurn and was added in different mixtures (100:0,75:25,50:50,25:75 200ml of swinemanure. The inoculum used for R37,Cand and O:100 of cattle and swine manure, respectively).The RWC was digested cattle manure taken from a lap-scale reactor operated at 37°C with a HRT of 15 days, an duration of the experimentwas 50 days. Ammonia experiment (experiment 2). Degradation of ammonia concentration of 3 g-N/litre and a pH of 7.9. swine manure at six different ammonia concentrations The total VFA concentration was less than 0.5 g-acetate/ (3.1, 4.1, 5.1, 6.1, 7.1 and 8.1g-N/litre) was tested. The litre. R550Cand Rw.c were inoculated with digested cattle tests were performed in 118-mlserum vials, with 6 ml of manure from the lab scale reactor described for ba~chexfresh swine manure, 20ml of BA-medium (Angelidaki periment 1. The reactors were run for three HRTs: ~efore et al., 1990)and a 14-mlinoculumfrom a lab-scalereactor any changeswere made to ensure steady-state condl~ons. Steady state was defined as a situation where methane (pH 8.0, ammonia concentration 6 g-N/litre) operated on swine manure with a 15 days HRT at a temperature of yield, pH, content of VFAS and ammonia concen~ration 55”C.The ammonia concentration was controlled by add- were constant over a period of at least 10 days. Mf.thane yield was measured every day, while pH, VF+S and ing NH4CLThe-duration of the experimentwas 65 days. o




. . . . , . . . .“

2 1 l-i



9. 8 h-


. .




6 5






Fig. 1. CSTR set-up, (1) influent flask, (2) pump, (3) controller, (4) gas bag, (5) effluentflask, (6) reac- ~ tor, (7) motor for stirrer, (8) water bath with pump, (9) magneticstirrer.

Ammoniainhibition of anaerobic digestion ammonia concentration were measured three to four times per week.


Definitions of used growth rates

The apparent specificgrowth rate, is p, and the apparent specific growth rate at a free ammonia VS and pH were determined according to standard concentration of 1.1g-N/litre in each experiment is methods (Greenberg et al., 1992).Gas composition (CH4 #ref. The apparent specific growth rates at other and COJ and VFAS were measured by gas chromatography as previously described (Angelidaki et al., 1990). conditions were normalized relative to this p,.f. The Total ammonia content was determined by the Kjeldahl relative p are designated as p, (~,= p/~,cf). Analytical metho~

method without the destruction step, and Kjeldahl nitrogen was determined by the Kjeldahl method (Matissek et al., 1992).The lipid content was determined by a Soxhlet method with dichloromethane as eluent (Matissek et al., 1992).The protein content was calculated by multiplying the organic bound nitrogen by the factor 6.25(Matissek et al., 1992).The organic bound nitrogen was assumed to be the differencebetween ammonia nitrogen and total nitrogen. Carbohydrates were determined as the fraction of the VS which was left after subtraction of the proteins and lipids.

THEORETICAL CALCULATIONS Calculation of the free ammonia concentration

Free ammonia concentration was calculated from the followingformula (Ostergaard, 1985): –1 1o-PH [NH3] (1) ~NH,] = 1 + - ~,w,8+)) ( 10


RESULTS The ammonia nitrogen was 5.3 g-N/litre for the swine manure and 3.3 g-N/litre for the cattle manure. The content of VS was 5.8°/0 for cattle manure and 4.5°/0 for swine manure. The lipid content was found to be higher for swine manure (10.8+ 0.2%) than for cattle manure (5.8 + ().5Yo). However, the

content of carbohydrates and proteins was nearly equal in these two types of manure. The total content of VFAS was 11060and 8860mg/litre as acetate for swine and cattle manure, respectively. B. was determined as 300 and 285ml-CH4/g-VS for swineand cattle manure, respectively. Batch experiments

The pH was measured during start-up and when terminating the batch experiments. pH never varied more than 0.2 units between these measurements. where [NH3]is the concentration of free ammonia, Therefore, the pH values used in the calculations [TNH3] is the total ammonia concentration and are the average of the two measurements. T(K) is the temperature (kelvin). The apparent specificgrowth rate (p) in batch experiment 1 was 0.150d-* at a free ammonia concenCalculation of the specljic growth rate from batch extration of 1.1g-N/litre. This was used as the periments reference apparent growth rate @ref)and was given Assuming that the methane production rate is the value of unity. proportional to the growth of the methanogenic The results of batch experiment 1 clearly show bacteria, and assuming that there is no lag phase that both p and p. decreased when the amount of (Powell, 1983),it is valid that swine manure increased (Table 1). It was calculated that an increase in the free ammonia concentration (2) ln(p +po) = v x t + WO) from 1.1 to 1.3g-N/litre lead to a decrease in the where p is the measured methane production, p. k relative apparent specificgrowth rate (,ur)from 1.0 the methane production derived from the inoculum, to 0.67 (Table 1). The p. value decreased at free ~ is the apparent specificgrowth rate and t is the ammonia concentrations exceeding 1.1g-N/litre, which indicates that a smaller fraction of the bactime passed. In mathematical terms, po=Xox Y (where X. is teria in the inoculum were active beyond this value. Increasing concentrations of ammonia (experthe part of the cell quantity in the inoculum which remains active, and Y is a yield constant indicating iment 2) resulted in lower apparent specificgrowth the amount of methane formed per gram of sub- rates, A total ammonia concentration of 5.1g-N/ of strate utilized). Assumingthat Y is constant when a litre resulted in a value of p which was only 64’XO given substrate is added at identical concentrations, that at 4.1 g-N/litre (Table 1). However, the value a change in PO is caused by a change in XO.This of p was the same for ammonia concentrations of means that p. is a parameter proportional to the 3.1 and 4.1 g-N/litre, indicating that a threshold number of bacteria in an inoculum which is active concentration is needed before ammonia will start after transfer to a new environment. The natural to affect the bacteria. The p,efvalue was determined logarithm of PO was calculated by linear regression as 0.0565d–l at a free ammonia concentration of as the intercept with the y-axis. PO was then added 1.1g-N/litre (Table 1).When the free ammonia conto ln(p + PO) and linear regression was performed. centration is increased to 1.9g-N/litre, the apparent This was repeated until PO and, thereby, p was con- specific growth rate is decreased to 210/0of the value estimated at 1.1g-N/litre (Table 1). However, stant.


KaareHvidHansenet al. Table1.Overview overdifferentmeasuredandcalculatedparametersfromthwrnophilic (55”C)batchexperiments

Batch experiment number 1 1 1 1 1 2 2 2 2 2 2 3 3 3

Designation 3.3 3.4 3.8 4.1 4,4 3,1 4.1 5.1 6,1 7.1 8.1 3.3 Hyd 4.7 Hyd 5.5Hyd

p (d-’)

0.150 0.131 0.104 0.102 0.101 0.0565 0.0565 0.0362 0.0153 0.0102 0.0119 0.27 0.27 0.21


p$) (ml-CHJ

1,0 0.87 0.69 0.68 0,67 1.0 1.0 0,64 0,27 0.18 0.21 1.0 1,0 0.77

62 62 47 19 6 19 17 15 15 18 17 1 0.42 0.15

Totalammonia (g-N/litre) 3.3* 3.4* 3.7* 4.1* 4.4* 3.1* 4.1* 5.1* 6.1* 7.1* 8.1* 3.3* 4.7* 5.5*

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0,1 0.1 0.1 0.1 0.1 0.1

Freeanqnonia (g-N/litre)


1.1 l.! 1.2

8.1 8.1 8.1 8.0 8.1 8.0 8.0 8.0 8.0 7,9 7.9 8.0 7.9 7.9

1.2 1.3

o.dg 1.1 1.4 1.6 1.$ 1. o.?9 1.2


‘pO hasan average standarddeviation of t2ml-CHl

the p. values were constant at all the different ammonia concentrations. When the ,u, values found in experiment 1 and 2 are plotted against the free ammonia concentration, a four-stage curve is seen (Fig. 2). Until the free ammonia concentration was approximately 1.1mgN/litre, p, was constant. In the second stage, p, decreased to 0.67 when the free ammonia concentration increased from 1.1 to 1.2g-N/litre. At stage three, pr remained constant at a decreased rate of 0.67, when the free ammonia concentration was between 1.2 and 1.3g-N/litre. In stage four, ~, decreased with a more or less constant rate with increasing free ammonia concentration (Fig. 2). These four stages can mathematically be described as follows: 0< ~H3] <1.10, Stage 1 Stage 2 1.10< [NH3]<1.16, Stage 3 1.16< [NH~]<1.34, Stage 4 1.34< [NHJ],

IL,= 1.0 v, = ~ -7.6-## ~, = 0.67

methanogenicbacteria were inhibited at free ammonia concentrations higher than 1.2g-N/litre (Table 1). For the H2-utilizingbacteria, inhibition of free ammonia commenced at a significantly higher free ammonia concentration than for the experiment with digestion of manure. During rdanure digestion an increase in free ammonia from ~.1 to 1.2g-N/litre resulted in a decrease of p, from 1.0 to 0.69; an increase from 1.1 to 1.2g-N/litre had no effecton the value of p of the Hz-utilizingmethanogens. The value of p, was still 1 (Table 2), and v was 0.27d-l. At a free ammonia concentration of 1.3g-N/litre, p, was reduced by 23Y0compared to 1.1g-N/litre free ammonia for the Hz-utilizing methanogens, while a 33% reduction was ob$erved when the overall process was tested (Table ‘l). In the experimentswhere hydrogen was added as substrate it was evident that POdecreased with itwreasing free ammonia concentration and also w~en p~ was constant (Table 1).

W = +


CSTR experiments

The experiment where H2/C02 was used as subWhen swine manure was used as substrate for a strate (experiment 3) showed that the H2-utilizing biogas reactor, increasing temperature led to





Fig. 2. A plot of the relativespeeificgrowthrate as a functionof the free ammoniaconcentration

([NH3]). Thedata plottedare frombatchexperiments1 and2.


Ammoniainhibition of anaerobic digestion


Table2. Overview overdifferent measured andcalculated parameters fromtheCSTRexperiments


Methaneyield Methaneinthe Totalarmnonia (ml-CH4/g-VS) biogas(%) (g-N/litre) 188 141 67 22

R3,.C R4sc RS5C RWc

71 69 51 40

5.9* 0.1 6.0k 0.1 6.0+ 0.1 6.1+ 0.1

Acidification Freeammonia VFAS(g-AC/ yield”(ml-CH4/ g-vs) (g-N/litre) litre)

PH 8.06 8.15 7.97 8.15

0.75 1.4 1.6 2.6

4.8 5.6 11.5 15.8

127I 91k 67+ 73+

16 16 16 16

‘Calculatedas tbe CHAobtained when the uotential CH~ rxoduction from the VFASfound was includedand the rrotentialCH~ . .rwoductiOn frOmtheinhUent VFASWaSSUb(raCted ‘“

taneous increase in the VFA content. After day 26 the VFA content stabilized, while the methane yield slowlyincreased until day 63, whereafter no further increase was observed. The methane yield was 67ml-CHl/g-VS and the VFA concentration was 11.5g-acetate/litre (Fig. 3C and Table 2). The gas yield from R60.cdecreasedfrom day 10 to day 45, when it stabilized, The VFA content continued to increase in the reactor while all other parameters (methane yield, pH, ammonia, methane percent) were constant from day 45. Due to the severe inhibition indicated by the low methane yield and high VFA content, this reactor were stopped at day 54. Measured and calculated parameters from the bio(8.06+ 0.05), ammonia content (5.9g-N/litre) and gas reactors taken prior to termination are shown the methane percent (710/0) (Table 2). The biogas in Table 2. When swine manure was anaerobically digested reactor at 45°C (&5.c) showed a similar development in biogas production and VFA content as at temperatures from 37 to 60”C, the methane yield R37.C.Steady state was reached after approximately decreased from 188 to 22 ml-CHd/g-VS(Table 2). 45 days, with a methane yield of 141ml-CHJg-VS The amount of VFA increased with increasing temand a VFA concentration of 5.6g-acetate/litre perature from 4.8 to 15.8g-acetate/litre (Table 2). (Fig. 3b and Table 2). The gas production from When the efficiencyof hydrolysis and acidification R55.Cdecreased from day 1 to day 26, with a simul- was examined using the amount of methane which

increased inhibition of the biogas process (Table 2). The original inoculum was digested cattle manure, but as this was replaced with swine manure the process deteriorated. The effect was first observed in the thermophilic reactors (55 and 60”C), but eventually an effect was seen at mesophilic temperatures. The biogas reactor at 37°C (R370C) reached steady state after 52 days, with a relatively high content of VFAS (4.8 g-acetate/litre) in the effluent. The methane yield of 188 ml-CH4/g-VS was much lower than the potential yield value of 300 ml-CH4/g-VS (Fig. 3a); gas production was, however, stable as such as pH was the case for parameters








Tm (%ys)










Fig. 3. Time-course overgasproductionand VFASfromthe reactorexperiments(CSTR),withdigestion of swinemanure.Digestionat (a) 37”C.(b) 45”C,(c) 55°Cand (d) 60°C.A hydraulicretention time of 15 days was used~or all tem’~ratures.‘(6 Gas production (ml biogas); (A) ‘TotalVFAS(mg-



Kaare Hvid Hansen et al.

methanogenic bacteria resistant to very high free ammonia concentrations apparently could be absent. The inoculum effect p. was constant at dll free ammonia concentrations when the process was adapted to ammonia concentrations of 1.5g-N/litre. However, p. decreased with increasing free ammonia concentrations when the process was adapted to lower free ammonia concentrations than the threshold value. The p. value began to decrease when the free ammonia concentration expeeded 1.1g-N/litre, corresponding to the value where ammonia inhibition started. This could indicate that the initial ammonia inhibition occurred; when a part of the fastest growing rate-limiting b~cteria DISCUSSION became inactive. The specific apparent growth rate decreaped at In this study it was demonstrated that it is posshigher ammonia concentrations. Correlation ible to degrade swine manure from Danish pigs at between the calculated free ammonia [email protected] thermophilic temperatures. The performance of the and the degree of inhibition was observed. reactor was shown to be stable, but the methane Inhibition of the biogas process by free arqmonia yield was low (inhibited steady state). Similar results followed a four-stage pattern (Fig. 2). Similar patwere found when cattle manure with high ammonia concentrations were digested (Zeeman et al., 1985; terns of inhibition were found by Poggi-Varqldo et Zeeman, 1991;Angelidaki and Ahring, 1993).The al. (1991)for unadapted mesophilic acetate-utilizing combination of low methane yield, low methane methanogens and also for thermophilic acetate-utipercent in the biogas and a high VFA concentration lizing methanogens in adapted manure (Angplidaki clearly indicate that the activity of methanogenic and Ahring, 1993). The inhibition patterns ifound bacteria was rate-limitingduring thermophilicdiges- by Angelidaki and Ahring (1993) and Poggition of swine manure. From the calculated methane Varaldo et al. (1991)were, however, limited to the yield including the methane which can be derived acetate-utilizing methanogens, while the present by wnversion of the VFAS produced (Table 2), it mathematical description was found valid fpr the can be concluded that the activity of hydrolytic/fer- biogas process as a whole. The proposed description further deseribps the mentative bacteria is more or less constant, independent of the fermentation temperature and the relative apparent specific growth rate as canstant free ammonia concentration. Acetate constituted for free ammonia concentrations below a threshold the main part of the VFA concentration, indicating value at 1.10g-N/litre. The idea that a certain that it was the acetate-utilizing methanogenic bac- threshold concentration exists before inhibition occurs is widely accepted (Braun et aI., 1981; teria which were primarily inhibited. The batch experiments with digested swine man- Parkin and Miller, 1983; Koster and Lettinga, 1988; ure demonstrated that a free ammonia concen- Bhattacharya and Parkin, 1989; Angelidaki and tration of 1.1g-N/litre was needed to introduce Ahring, 1993) although it has not been included inhibition of the process. Below this value the previouslyin models by other authors. The present new description of ammonia inhispecific apparent growth rates were found to be constant. This threshold levelwas 7–10 times higher bition of the overall biogas process shows great than earlier reported by Braun et al. (1981)and De similarities with the model previously used to Baere et al. (1984) and was even higher than the describe inhibition patterns for the acetate utilizing 1991; level of 0.70g-N/litre reported for digestion of methanogens (Poggi-Varaldo e~ al., cattle manure (Angelidaki and Ahring, 1993).This Angelidaki and Ahring, 1993). This corresponds high threshold value must be related to an adap- with the importance of the acetate-utilizing methatation of the inoculum used (thermophilic digested nogens for the overall performance of the biogas swine or cattle manure). Severalof the previous stu- process (Robbins et al., 1989; Angelidaki and dies were done with pure cultures of methanogens Ahring, 1993; Koster and Lettinga, 1994]. The (Sprott and Patel, 1986;Jarell and Saulnier, 1987) description is in contradiction to what has preat low pH (6.5–7.0)where it was possible that the viously been described, where the Hz-utilizing NH~ ion or the total ammonia concentration can methanogens were found to be the first bacterial reach toxic levels before the free ammonia concen- group to be inhibited (Wiegant and Zeeman, 1986). A practical implication of the results is that tration becomes toxic. For the previous experiment with digested manure, inoculum came from biogas increased ammonia concentrations in a biogas reacreactors with low ammonia concentrations (Van tor will not always result in an decreased methane Velsen, 1979;Angelidaki and Ahring, 1993)where yield. Ammonia will only be a serious problem after could be obtained from the VFA produced together with the actual methane yield after subtraction of the influent VFA, a potential methane yield of about 90 ~ 16ml-CH.4/g-VScould be calculated at all the temperatures tested (Table 2). This result shows that increasing concentrations of free ammonia within the limits tested had no significantinfluence on hydrolysis and acidification of swine manure. However, the calculated maximum potential gas production was only 730/.of the determined potential methane yield (Bo). This shows that the same inhibition will occur on hydrolysisand acidification at the HRT used.

Ammoniainhibition of anaerobic digestion

the free ammonia concentration has exceededa certain threshold value (1.10g-N/litre). Inhibition of the HJ-utilizing methanogens occurred at higher free ammonia concentrations (> 1.2g-N/litre) than found for the process as a whole. Similar results were found by Koster and Lettinga (1988); Robbins et al. (1989) and Angelidaki and Ahring (1993). The H2-utilizingmethanogens had a much higher apparent specific growth rate (0.27d-l) than the overall biogas process (0.150 and 0.0565d-l). At the same time the maximum specificgrowth rate for the Hz-utilizingmethanogens have been reported to be in the range of 0.198– 0.433h-l (Schonheit et aZ., 1980;Ferguson and Mah, 1983),while acetateutilizing methanogens had maximum specific growth rates in the range 0.0115–0.0578h-l (Zinder and Mah, 1979;Ahring and Westerrnann, 1985).As the acetate-utilizing methanogens are responsible for approximately 70~o of the methane produced in a biogas reactor (Ahring et al., 1995),the assumption that acetate-utilizing methanogens constitute the rate-limiting step during anaerobic digestion of swine manure with a high content of ammonia is evident from this study. CONCLUSIONS 1. A threshold value was found for inhibition of the biogas process of 1.1 g-N/litre free ammonia. 2. A plot of the relative apparent specific growth rate (j,) as a function of the free ammonia concentration showed a four-stage inhibition pattern of the biogas process. 3. The methane yield of swine manure was found to decrease with increasing temperature. 4. Degradation of swine manure in a biogas reactor was found to be possible at thermophilic temperatures (55°C) even with ammonia concentrations of up to 6 g-N/litre. However, the process was severely inhibited and the methane yield was constantly low. 5. The acetate-utilizing methanogens can be regarded as rate-limiting during anaerobic digestion of swine manure with a high content of ammonia.


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