the effect of calcium on the anaerobic digestion treating swine wastewater

6
Biochemical Engineering Journal 30 (2006) 33–38 The effect of calcium on the anaerobic digestion treating swine wastewater Johng-Hwa Ahn a , Trong Hoan Do a , Sang D. Kim b , Seokhwan Hwang a,a School of Environmental Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, South Korea b Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-dong, Puk-gu, Gwangju, 500-712, South Korea Received 11 October 2005; received in revised form 30 December 2005; accepted 13 January 2006 Abstract A series of experiments with 0, 1, 3, 5, and 7 g Ca 2+ /l using calcium chloride were performed to evaluate the effect of various calcium concentrations on anaerobic digestion of swine wastewater. The addition of 3g/l gave the best performance. On the other hand, the calcium concentrations of 5–7 g/l had an inhibitory effect on anaerobiosis. The lag phase durations expected in biogas production were 28–31 days with the addition of 0–3 g/l of calcium, and 43–52 days for calcium concentrations of 5 g/l or more. The concentrations of total volatile fatty acids decreased to less than 100 mg/l with calcium concentrations of 3–7 g/l. On the other hand, propionate and i-valerate concentrations remained over 4 and 0.8 g/l, respectively, when 1 g/l or less of calcium was added. © 2006 Elsevier B.V. All rights reserved. Keywords: Anaerobic digestion; Batch; Calcium; Lipid; Swine wastewater; VFAs 1. Introduction The swine industry is growing rapidly along with the world’s human population. The trend is toward more concentrated pig- geries with herd numbers in the thousands. Associated with these increased herds are large quantities of wastewaters which include organic matter, inorganic nutrients, and gaseous emis- sions. The wastewater from these facilities exceeds the capacity for direct land disposal without a severe environmental impact [1]. Its treatment requires overcoming the difficulties of a large amount of residues and a high percentage of solids [2].A commonly used system for removal of organic matter in the wastewater is anaerobic digestion. Anaerobic digestion provides a potentially cost-effective solution for the treatment of high strength organic waste, such as swine wastewater, because it forms methane and produces less sludge [3]. Other advantages of anaerobic treatment of swine wastewater include nutrient conservation and odor reduction. Swine wastewater contains a high concentration of lipids, 30–40% based on chemical oxygen demand (COD) [1,4]. These are potentially attractive for biogas production due to their Corresponding author. Tel.: +82 54 279 2282; fax: +82 54 279 8299. E-mail address: [email protected] (S. Hwang). high theoretical methane yield [5,6]. However, in anaerobic wastewater treatment systems, lipids are readily hydrolyzed to glycerol and long-chain fatty acids (LCFAs), which are inhibitors of anaerobic microorganisms at millimolar concen- trations [5,7–10]. The mechanism of LCFAs toxicity is related to the adsorption of the surface-active acids onto the cell wall, which was reported to exert an acute toxic effect on the microor- ganisms involved in the -oxidation and methanogenic path- ways [6,9,11]. Since the LCFAs precipitate as calcium salt, addition of calcium to lipid-rich wastewater could be an attrac- tive way of preventing the LCFAs from upsetting an anaerobic treatment system [5,12,13]. The effect of calcium addition on anaerobic digestion has been reported [5,12–18]. One study had found that calcium was moderately inhibitory at concentrations of 2.5–4 g/l and strongly inhibitory at 8 g/l [15]. Additionally, the inhibitory concentration 50% (IC 50 ) value of calcium has been reported to be 4.8 g/l [17]. In contrast to these results, another has reported that the addition of calcium concentrations up to 7 g/l had no inhibitory effect on the methanogens treating synthetic wastewater [14]. Thus, the effect of calcium on anaerobic wastewater treatment is still not clearly understood. This paper, therefore, seeks to clarify the situation by examining the positive or negative effect of calcium on the anaerobic digestion treating lipid-rich swine wastewater. 1369-703X/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2006.01.014

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Biochemical Engineering Journal 30 (2006) 33–38

The effect of calcium on the anaerobic digestion treating swine wastewater

Johng-Hwa Ahn a, Trong Hoan Do a, Sang D. Kim b, Seokhwan Hwang a,∗a School of Environmental Science and Engineering, Pohang University of Science and Technology,

San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, South Koreab Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology,

1 Oryong-dong, Puk-gu, Gwangju, 500-712, South Korea

Received 11 October 2005; received in revised form 30 December 2005; accepted 13 January 2006

Abstract

A series of experiments with 0, 1, 3, 5, and 7 g Ca2+/l using calcium chloride were performed to evaluate the effect of various calcium concentrationson anaerobic digestion of swine wastewater. The addition of 3 g/l gave the best performance. On the other hand, the calcium concentrations of5–7 g/l had an inhibitory effect on anaerobiosis. The lag phase durations expected in biogas production were 28–31 days with the addition of0–3 g/l of calcium, and 43–52 days for calcium concentrations of 5 g/l or more. The concentrations of total volatile fatty acids decreased to lessthan 100 mg/l with calcium concentrations of 3–7 g/l. On the other hand, propionate and i-valerate concentrations remained over 4 and 0.8 g/l,r©

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espectively, when 1 g/l or less of calcium was added.2006 Elsevier B.V. All rights reserved.

eywords: Anaerobic digestion; Batch; Calcium; Lipid; Swine wastewater; VFAs

. Introduction

The swine industry is growing rapidly along with the world’suman population. The trend is toward more concentrated pig-eries with herd numbers in the thousands. Associated withhese increased herds are large quantities of wastewaters whichnclude organic matter, inorganic nutrients, and gaseous emis-ions. The wastewater from these facilities exceeds the capacityor direct land disposal without a severe environmental impact1]. Its treatment requires overcoming the difficulties of a largemount of residues and a high percentage of solids [2]. Aommonly used system for removal of organic matter in theastewater is anaerobic digestion.Anaerobic digestion provides a potentially cost-effective

olution for the treatment of high strength organic waste, such aswine wastewater, because it forms methane and produces lessludge [3]. Other advantages of anaerobic treatment of swineastewater include nutrient conservation and odor reduction.Swine wastewater contains a high concentration of lipids,

0–40% based on chemical oxygen demand (COD) [1,4]. Thesere potentially attractive for biogas production due to their

high theoretical methane yield [5,6]. However, in anaerobicwastewater treatment systems, lipids are readily hydrolyzedto glycerol and long-chain fatty acids (LCFAs), which areinhibitors of anaerobic microorganisms at millimolar concen-trations [5,7–10]. The mechanism of LCFAs toxicity is relatedto the adsorption of the surface-active acids onto the cell wall,which was reported to exert an acute toxic effect on the microor-ganisms involved in the �-oxidation and methanogenic path-ways [6,9,11]. Since the LCFAs precipitate as calcium salt,addition of calcium to lipid-rich wastewater could be an attrac-tive way of preventing the LCFAs from upsetting an anaerobictreatment system [5,12,13].

The effect of calcium addition on anaerobic digestion hasbeen reported [5,12–18]. One study had found that calcium wasmoderately inhibitory at concentrations of 2.5–4 g/l and stronglyinhibitory at 8 g/l [15]. Additionally, the inhibitory concentration50% (IC50) value of calcium has been reported to be 4.8 g/l[17]. In contrast to these results, another has reported that theaddition of calcium concentrations up to 7 g/l had no inhibitoryeffect on the methanogens treating synthetic wastewater [14].Thus, the effect of calcium on anaerobic wastewater treatment

∗ Corresponding author. Tel.: +82 54 279 2282; fax: +82 54 279 8299.E-mail address: [email protected] (S. Hwang).

is still not clearly understood. This paper, therefore, seeks toclarify the situation by examining the positive or negative effectof calcium on the anaerobic digestion treating lipid-rich swinewastewater.

369-703X/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.bej.2006.01.014

34 J.-H. Ahn et al. / Biochemical Engineering Journal 30 (2006) 33–38

Table 1Physico-chemical characteristics of swine wastewater

Parameter Concentration (g/l)

pH 6.37 ± 0.10Total solid 61.8 ± 0.4Volatile solid 44.5 ± 0.2Chemical oxygen demand 130.8 ± 3.0Soluble chemical oxygen demand 59.7 ± 0.9TKN 7.3 ± 0.1Ammonium nitrogen 4.8 ± 0.1Proteina 15.8 ± 0.9Carbohydrate 9.1 ± 0.2Lipid 20.1 ± 0.1Acetate 11.1 ± 0.2Propionate 4.2 ± 0.1n-Butyrate 6.4 ± 0.1Total VFA as acetate 36.7 ± 0.3Calcium 0.67 ± 0.05Potassium 3.4 ± 0.1

a Protein content = (TKN − ammonium nitrogen) × 6.25.

2. Materials and methods

2.1. Swine wastewater

Swine wastewater (Table 1), pre-screened with a 4 mm sieve,was obtained from a local pig farm. The volatile solid (VS) con-centration, which indicates the amount of potentially biodegrad-able substances in the wastewater, was 44.5 g/l, 72% of the totalsolid (TS). The ammonium nitrogen concentration was 4.8 g/l,which is close to the IC50 [17]. The wastewater characteristicsfacilitate a high level of acetate production from volatile fattyacids (VFAs) and LCFAs. The potassium concentration in thewastewater was not inhibitory to anaerobic digestion [15].

2.2. Experimental set-up

The anaerobic sludge obtained from a local domestic wastew-ater treatment plant in Pohang, Korea was used as a seed inocu-lum. A full-scale egg-shaped digester with a working volume of7000 m3 was operated at 31 ± 4 days hydraulic retention timeusing sludge. The operational temperature and pH of the full-scale digester were 31 ± 1 ◦C and 6.9 ± 0.1, respectively. Theaverage organic loading rate was 0.61 ± 0.10 kg VS/m3/d.

A series of experiments were performed to evaluate the effectof calcium concentrations of 0, 1, 3, 5, and 7 g/l on anaer-obic digestion, where the corresponding trials were, respec-t(ac

TE

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calcium concentration in each reactor. The experiments werecarried out using 500 ml Wheaton bottles, which were purgedwith nitrogen gas to remove oxygen and sealed with rubber stop-pers. The reactors were seeded with 30% sludge and operatedin a shaking incubator at 100 rpm and mesophilic temperature(35 ◦C) under anaerobic conditions until the methane productionrate was less than 0.01 l/l d. Two-liter Tedlar bags were used forcollecting biogas, and the volume of biogas produced was mea-sured manually using a 60 ml syringe.

2.3. Analytical methods

All analyses were duplicated, and the results given are meanvalues. A Hewlett-Packard gas chromatograph (Model 6890plus) equipped with an Innowax capillary column and a flameionization detector was used to determine the concentrations ofVFAs. Helium was the carrier gas, with a flow rate of 2.5 ml/minand a split ratio of 10:1. The same gas chromatograph with anHP-5 capillary column (film thickness, 0.25 �m; length, 30 m;i.d., 0.53 mm; phase ratio, 3) and a thermal conductivity detec-tor was used to quantify methane in the biogas. Helium was thecarrier gas, with a flow rate of 8 ml/min and a split ratio of 70:0.

The COD and solid concentrations were determined accord-ing to the procedures in Standard Methods [19]. The concen-trations of protein and carbohydrate were measured accordingtramww

3

3

C6cdatwriatiefatpo

2

ively, denoted as the control, Ca1, Ca3, Ca5, and Ca7 reactorsTable 2). The maximum calcium concentration of 7 g/l wasbout 40% higher than the reported IC50 value [17]. Calciumhloride (CaCl2·2H2O, Merk) was added to give the desired

able 2xperimental conditions

Reactor

Control Ca1 Ca3 Ca5 Ca7

alcium addition (g/l) 0 1 3 5 7

o the Kjeldahl [19] and the phenol–sulfuric acid [20] methods,espectively. Lipid concentration was measured gravimetricallyfter extraction of lipid by solvent (33.3% chloroform and 66.6%ethanol) [21]. The concentrations of cations in the samplesere measured using an ion chromatograph (Dionex DX-120)ith an IonPac AS14 column.

. Results and discussion

.1. pH

After calcium addition, the initial pHs in the control, Ca1,a3, Ca5, and Ca7 reactors were 6.51, 6.47, 6.34, 6.24, and.15, respectively, which showed that the pH decreased as cal-ium concentration increased. According to Eq. (1) [22], the pHecreases might be because of precipitation of VFAs or LCFAss calcium salt. As the calcium concentration increased in solu-ion containing only acetic acid, pH decreased (data not shown),hich indirectly supports this explanation. The initial pH in each

eactor was adjusted to pH 7 using NaOH (Table 3). The pHncreases from the initial day to the last day in the control, Ca1,nd Ca3 reactors were 1.34, 1.30, and 1.16, respectively, buthose in Ca5 and Ca7 were 0.85 and 0.41. The different pHncreases could be due to the degree of protein degradation inach reactor because the deamination of protein results in theormation of ammonium bicarbonate, ammonium carbonate ormmonium hydroxide. The protein degradation efficiencies inhe control, Ca1, and Ca3 reactors were at least 4% higher com-ared to those in Ca5 and Ca7 because of the inhibitory effectf high concentrations of calcium, 5–7 g/l

RCOOH + Ca2+ = (RCOO)2Ca + 2H+ (1)

J.-H. Ahn et al. / Biochemical Engineering Journal 30 (2006) 33–38 35

Table 3Performances of the reactors at various calcium concentrations

Parameter Reactor

Control Ca1 Ca3 Ca5 Ca7

pHDay 0 7.00 ± 0.01 7.02 ± 0.01 7.01 ± 0.01 7.03 ± 0.01 7.02 ± 0.01Day 164 8.34 ± 0.01 8.32 ± 0.01 8.37 ± 0.01 7.88 ± 0.01 7.43 ± 0.01

Chemical oxygen demand (g/l)Day 0 102.9 ± 9.8 99.9 ± 2.0 97.5 ± 0.6 98.4 ± 4.4 94.1 ± 0.1Day 164 43.3 ± 1.0 39.2 ± 0.7 31.9 ± 0.9 38.4 ± 1.8 39.7 ± 1.5

Soluble chemical oxygen demand (g/l)Day 0 38.2 ± 0.5 40.8 ± 0.1 42.1 ± 3.9 40.2 ± 1.0 43.5 ± 1.9Day 164 16.7 ± 0.2 14.1 ± 0.1 4.5 ± 0.1 5.1 ± 0.1 5.1 ± 0.1

Calcium (g/l)Day 0 0.46 ± 0.04 1.64 ± 0.05 3.41 ± 0.06 5.28 ± 0.05 7.58 ± 0.02Day 164 0.08 ± 0.05 0.13 ± 0.01 0.12 ± 0.03 0.15 ± 0.02 0.38 ± 0.02

Initial ratio of calcium and COD 0.004 0.016 0.035 0.054 0.081Maximum biogas production (l/l d) 1.08 1.28 1.61 1.03 0.78Methane content (%) 77 ± 2 78 ± 3 76 ± 2 71 ± 1 66 ± 1Methane yield (l CH4/g CODrm) 0.35 0.35 0.38 0.36 0.36Lag phase duration (day) 29.2 31.2 28.2 43.1 52.4

3.2. Organic removal

The relationship between various calcium concentrations andCOD removal efficiency is shown in Fig. 1. The Ca3 reactor hadthe maximum COD removal efficiency, 67%, which is higherthan those in 16 cases, 29–62%, treating swine wastewater inother reports [1]. The other reactors had similar COD removalefficiencies, 58–61%, because control and Ca1 reactors hadhigher carbohydrate and protein removal efficiencies than Ca5and Ca7 but the latter had higher lipid removal efficiencies thanthe former (Fig. 2). The optimum value of calcium concentrationobtained by regression analysis was 3.4 g/l (Fig. 1). The initialratios of calcium and COD in all reactors were 0.004–0.081(Table 3). The COD removal efficiency increased with the ratioof calcium and COD and reached a maximum at the ratio of0.035, and then decreased at higher ratios. In contrast, it has

been reported that the ratios of calcium and COD up to 2.62 hadno inhibitory effect on the methanogens in anaerobic digestion[14]. The final soluble CODs (SCOD) in the Ca3, Ca5, and Ca7reactors was 4.5–5.1 g/l but those in control and Ca1 were 16.7and 14.1 g/l (Table 3), respectively, because the propionate andi-valerate concentrations were not degraded (Fig. 4B and E).

The percentages of carbohydrate removal in the control, Ca1,and Ca3 reactors were 62%, 61%, and 67%, respectively, whilethose in Ca5 and Ca7 were 50% and 43% (Fig. 2). The car-bohydrate removal efficiency was also maximum in the Ca3reactor. The addition of more than 5 g/l calcium caused worsecarbohydrate degradation than no calcium addition, possibly dueto calcium inhibition. The percentages of protein removal inthe control, Ca1, and Ca3 reactors were 29%, 28%, and 30%,respectively, while those in Ca5 and Ca7 were 24% and 21 %(Fig. 2). The protein removal efficiencies in the Ca5 and Ca7

Fc

Fig. 1. COD removal efficiencies with various calcium concentrations.

ig. 2. Removal efficiencies of carbohydrate, protein, and lipid with variousalcium concentrations.

36 J.-H. Ahn et al. / Biochemical Engineering Journal 30 (2006) 33–38

Fig. 3. Cumulative biogas production with various calcium concentrations: (©)control, (�) Ca1, (�) Ca3, (♦) Ca5, (�) Ca7 reactors. Note that the tangentdetermining the lag period for the Ca7 reactor is shown.

reactors were lower than those in the others, which could alsobe because of calcium inhibition. The lipid removal efficiencyincreased with increasing calcium concentration and reached amaximum at the concentration of 3 g/l, and then decreased athigh concentrations of 5–7 g/l (Fig. 2). The calcium concentra-tions on the last day were reduced to less than 400 mg/l regardlessof the initial concentration (Table 3), which suggests that VFAs,LCFAs, or others precipitated as calcium salt [5,13].

Overall, the Ca3 reactor was the most efficient, while Ca5and Ca7, with calcium concentrations of 5–7 g/l, were not betterthan control and Ca1, with 0–1 g/l. Therefore, it can be said thata calcium addition could improve the efficiency of anaerobicdigestion, but an overdose of calcium should be avoided.

3.3. Biogas production

Fig. 3 shows the cumulative biogas productions with timefor each reactor. The cumulative biogas productions increasedas a combination of sigmoid-like curves. Therefore, sigmoidequations (SigmaPlot 2000 for Windows Version 6.00) wereused for fitting them. There was dual lag phase (Fig. 3).

The effect of calcium addition on anaerobic digestion wasevaluated by the duration of first lag phase before the initiationof active biogas production, which was quantified as the timeobtained by extrapolating the tangent at the exponential part oftstc

ct(btat

reactors were higher than those in control and Ca1 partly becausethe propionate and i-valerate concentrations both decreased toless than 10 mg/l in Ca5 and Ca7, while they remained over 4 and0.8 g/l, respectively, in control and Ca1 (Fig. 4B and E). How-ever, the maximum daily biogas productions in the Ca5 and Ca7reactors were lower than those in control and Ca1 (Table 3).

The methane contents in the control, Ca1, and Ca3 reac-tors were 76–78%. However, the methane contents in the Ca5and Ca7 reactors were 71% and 66%, respectively, significantlylower than those in the others (Table 3). Calcium concentra-tions of over 5 g/l thus seems inhibitory to anaerobic digestionof swine wastewater, and the degree of inhibition increasesas the concentration increases over 5 g/l. Methane yields were0.35–0.38 l CH4/g CODremoval (rm) in all reactors, which showsthat methane yield was not affected by the calcium doses. Thisindicates that the metabolisms of hydrolytic bacteria or H2 pro-ducers may be more affected than those of methanogens. Themethane yields in all reactors were below the theoretical value of0.38 l CH4/g CODrm at room temperature (25 ◦C) and standardpressure (760 mmHg) because some of the initial COD couldhave been used for biomass increase.

3.4. Volatile fatty acids removal

At all conditions, acetate was the first VFA component to startto decrease (Fig. 4). This can be explained thermodynamicallyoflAwVVa

csfctCam

aopabyocbtn(t

he curve as shown in Fig. 3 [5,23,24]. The Ca3 reactor had thehortest lag phase duration. However, the calcium concentra-ions of 5–7 g/l increased the lag phase duration by 15–25 daysompared to the Ca3 reactor.

The first step of the cumulative biogas productions (Fig. 3)ould represent the digestion of carbohydrate and protein, andhe second step could arise from the degradation of lipidsLCFAs) because of different substrate utilization rates for car-ohydrate, protein, and lipid [5,17,23]. The total biogas produc-ion was maximum in the Ca3 reactor. The lag phases in the Ca5nd Ca7 reactors were longer than those in control and Ca1. Onhe other hand, the total biogas productions in the Ca5 and Ca7

r by using Le Chatelier’s principle. Acetate degradation is mostavorable thermodynamically because its Gibbs free energy isowest among the VFAs, −31 kJ/mol at 25 ◦C and pH 7 [25].ccording to Le Chatelier’s principle, the decrease of acetateould accelerate the degradation of other VFAs because otherFAs produce acetate in anaerobic digestion. The decrease ofFAs resulted in the increase of pH and daily biogas production

t all conditions (Table 3, Figs. 3 and 4).Propionate was the last VFA to degrade regardless of calcium

oncentrations indicating that its concentration and degradationhould be considered carefully, since it would be the limitingactor during start-up and reinoculations [26]. The propionateoncentration in the Ca3 reactor started to decrease earliest, buthe propionate was not significantly degraded in the control anda1 within 164 days (Fig. 4B), which indicates that calciumddition is essential to propionate degraders for better perfor-ance of anaerobic reactors.Acetate is the sole product of degradation of normal (n)-

nd iso (i)-butyrate [26]. Some of the propionate increase couldriginate from degradation of n-valerate, but the increases inropionate are about twice as high as the decreases in n-valeratet all conditions (Fig. 4B and F). n-Valerate is degraded intooth propionate and acetate, whereas i-valerate is known toield only acetate [25,26]. Therefore, the almost simultane-us increase in propionate and reductions of n- and i-butyrateould indicate a substrate competition between propionate andutyrate degraders (Fig. 4B–D). n-Valerate was removed earlierhan i-valerate at all conditions. Like propionate i-valerate wasot degraded in the control and Ca1 reactors within 164 daysFig. 4E), which also suggests that calcium addition is essen-ial to i-valerate degraders for better performance of anaerobic

J.-H. Ahn et al. / Biochemical Engineering Journal 30 (2006) 33–38 37

Fig. 4. Acetate (A), propionate (B), i-butyrate (C), n-butyrate (D), i-valerate (E), and n-valerate (F) concentrations with various calcium concentrations: (—) control,(· · ·) Ca1, (– · –) Ca3, (– ·· –) Ca5, (– – ·) Ca7 reactors.

reactors. In this study, propionate and i-valerate were the lastVFAs to be degraded in all reactors. All VFAs except propi-onate and i-valerate decreased earlier in the control, Ca1, andCa3 reactors than those in Ca5 and Ca7, which suggests that thehigh concentrations of calcium were inhibitory to anaerobes.

The maximum i-butyrate and i-valerate concentrations were78–105% and 61–81% higher than those on day 0, respec-tively, while those of n-butyrate and n-valerate were 13–32%and 8–12% higher (Fig. 4C–F). This indicates that iso forms ofVFAs accumulated more easily in anaerobic digestion [25].

The swine wastewater contained a high concentration oflipids, 20.1 ± 0.09 g/l (Table 1). In the Ca3 reactor, the lag phasewas shorter and the performance criteria, such as VFA removal,were better than those in the others. Also, the lag phases of theCa5 and Ca7 reactors were longer than those of the control andCa1, as mentioned previously. The inhibitory effect of LCFAsfrom lipid could be reduced by adding calcium because calciumcan precipitate LCFAs as calcium salt. However, an overdose ofcalcium could be more inhibitory than the LCFAs themselves.Therefore, choosing the optimum calcium dose is important to

improve the performance of anaerobic digestion treating lipid-rich wastewater such as swine wastewater.

4. Conclusions

The performance of anaerobic digestion improved withincreasing concentration of calcium and reached a maximum atthe concentration of 3 g/l, and then decreased at higher concen-trations, 5–7 g/l. The calcium concentration of 3 g/l decreasedthe lag phase and increased biogas production rate and totalbiogas production. The calcium concentration of 3 g/l was thusthe best concentration among the assayed concentrations. Pro-pionate and i-valerate were not degraded at low calcium con-centrations of 1 g/l or less, suggesting that special focus shouldbe given to the concentrations and degradations of propionateand i-valerate. Therefore, it could be said that calcium additionto swine wastewater is essential to improve the performance ofanaerobic digestion. However, an overdose of calcium was foundto inhibit anaerobic digestion.

38 J.-H. Ahn et al. / Biochemical Engineering Journal 30 (2006) 33–38

Acknowledgements

This research was supported in part by the Advanced Envi-ronmental Biotechnology Research Center (AEBRC; Grant No.R11-2003-006-02002-0) and Korea Institute of EnvironmentalScience and Technology (KIEST; Grant No. 071-051-081) pro-grams.

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