J. Environ. Sci. Eng. (2004), 6, 58-63
Kinetics of Thermophilic Anaerobic Digestion and Effects of Propionate on Thermophilic Anaerobic Digestion Do Hee Kim* and Seung H. Hyun1 Korea Western Power Co., Ltd. Chemistry and Biotechnology Examination Bureau, Korean Intellectual Property Office (KIPO), Government Complex-DaeJeon, DaeJeon City, Republic of Korea
Serial basic tests were conducted to determine fundamental kinetics and for propionate effects on thermophilic anaerobic digestion using precise measurement of methane production under a thermophilic condition. Results of serial test for the determination of fundamental kinetic coefficients showed the value of k (maximum substrate utilization rate coefficient) and KS (half-saturation coefficient) as 0.24 h−1 and 700 mg l−1, respectively, for non-inhibiting organic loading range. The substrate inhibition coefficient (KH) was 1000 mg l−1 for inhibiting organic loading range. VFAs degradation tendency under thermophilic condition showed that first butyrate and acetate were converted simultaneously and later propionate was converted. Keywords : Anserobic, kinetic, thermophilic, VFA
Introduction Anaerobic process involves the degradation of complex compounds by different groups of bacteria to form methane and carbon dioxide as end products. Within the last two decades, anaerobic processes have become attractive alternative to aerobic treatment for treating moderate to highstrength and for the production of methane gas as energy source. It has been reported that anaerobic digestion process has several unique advantages over aerobic biological processes, including the elimination of oxygen supply and a significantly lower amount of sludge production. In addition, these processes have resistance to complex organic chemicals that are classified as being hazardous or toxic (Hyun, 1997; Andrews and Pearson, 1965; Boone, 1985; Boone and Mah, 1987). Temperature is one of the most important parameters in biological waste treatment particularly, anaerobic treatment. Therefore, many studies about temperature effect under anaerobic process have been reported since 1930’s. Most of the researches about temperature effect were mesophilic treatment (around 35oC) and thermophilic treatment (around 55oC). Recently researches also studied phychrophilie treatment for treating low-strength wastewater under anaerobic *Corresponding author Tel. 017-603-1171, Fax. 031-330-4529 E-mail:
[email protected]
condition (Hyun et al., 1999; van Lier et al., 1996; van Lier, et al., 1996; van Lier, et al., 1997). Among the thermophilic, mesophilic and psychrophilic treatment, thermophilic anaerobic treatment has many unique advantages. Because reaction rates increase with temperature, significantly higher loading potential and considerably shorter retention times can be expected if anaerobic treatment is applied under thermophilic condition. Therefore, thermophilic anaerobic process could be an attractive alternative treatment (van Lier, 1992, 1997). Specific objectives of this study are to find kinetic coefficients under thermophilic anaerobic digestion and to find effects of volatile fatty acids (VFAs) especially propionate.
Materials and Methods This study can be divided into phases. The Phase I and II are related to the fundamental kinetic study under thermophilic conditions and the other three phases are the effects of VFAs. High rate batch tests using glucoseenriched anaerobic cultures were conducted to obtain the necessary data. Phase I tests were set for inhibition screening purpose by measurement of anaerobic biogas production only under variable organic loadings. Phase II tests involved intensive liquid sampling and analysis of remaining organic concentration. The purpose of Phase II tests was to find out the concentration of substrate inhibition and to determine the biokinetic coefficients. Tests in phase III were conducted
THERMOPHILIC ANAEROBIC DIGESTION 59 Table 1. Set-up for batch degradation kinetic tests Phase
Materials added
Function
Analysis
I
Seed Seed + HgCl2 + 1.0 g COD l−1 of Glucose Seed + 1.0, 2.0 and 4.0 g COD l−1 of Glucose
Blank Abiotic cont. Test units
Total Gas Methane Gas
II
Seed Seed + HgCl2 + 1.0 g COD l−1 of Glucose Seed + 1.0, 2.0, 4.0 g COD l−1 of Glucose
Blank Abiotic cont. Test units
COD VFA
III
Seed Seed + 200 mg HBu l−1+ 200 mg HPr l−1 + 200 mg Hac l−1
Blank Test units
Gas VFA
IV
Seed Seed + 1.0 g COD l of Glucose + 3 levels of Propionate
Blank Test units
Gas VFA COD
V
Semi-continuos operation by various organic loading rates
Test units
Gas VFA
−1
to see the VFAs degradation efficiency under thermophilic conditions. Phase IV tests were detailed kinetic studies for propionate effects. Phase V tests were semi-continuous tests for the determination of maximum injection concentration of propionate. Table 1 shows the detailed experimental conditions. Thermophilic Master Culture reactor (T-MCR) was operated to provide cultures of identical and repeatable characteristics so that measurements conducted at different times and by different sets of test would have a common microbial and biochemical basis (Young and Tabak, 1989, 1993). TMCR used in this study received only glucose as substrate. Pyrex glass vessel of 13 liter capacity was used and contained 12 liters of culture. Mixing and heating was achieved by magnetic hot plate. The T-MCR was maintained in a constant chemical environment and was operated at fixed environmental conditions. Tests were initiated by transferring cultures from T-MCR under anaerobic conditions to small test reactors. The Table 2 showed the conditions of TMCR. Quality control for T-MCR was based on daily gas production records by respirometer (AER 200, CES, USA) and supplemented with measurements of residual volatile fatty acids (VFAs). Gas production deviation of more than 10% above or below on average of 1 g COD l-1 glucose added, or
breakpoint deviation more than allowable difference from the typical curve indicated the presence of problems with culture environment (Young and Tabak, 1993). Cultures exhibiting gas production curves falling outside these limits were not used for test purposes until the problem was corrected and the reactor had achieved acceptable performance. Fig. 1 shows the schematic diagram of T-MCR and serum bottle. Temperature and pH were measured by portable meter (ISE/pH/mV/ORP/Temp. meter, Orion, model 290 A) and all other experiments in this study were performed by Standard Method 2540 (1995), unless indicated. Kinetic Models Several mathematical relationships have been proposed to describe kinetics of anaerobic degradation. Followings are the kinetic models used in this study. Monod Model Monod kinetics, because of its hyperbolic shape, is an equation that interpolates well data of reaction kinetics
Table 2. The operational condition of T-MCR Items Temp. Carbon source Organic Loading rate SRT MCR Volume
Condition 55oC Glucose 1.0 g COD l−1 day 40 days 12 l
Fig. 1. TSS and VSS results from T-MCR.
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which tend to a saturation value at increasing substrate concentrations, as normally happens for enzymatic conversions. dS kSX ------ = – --------------dt KS + S
(1)
where, k : maximum specific substrate degradation rate, h−1 KS : half-saturation constant, mg l−1 S : substrate concentration, mg l−1 X : biomass concentration, mg l−1 When steady-state condition exits in continuous-culture reactors, Equation (1) can be arranged in the LineweaverBurk linear form from which the kinetic coefficients can be obtained directly. But a batch culture anaerobic reaction is a transient or non-steady state condition. Therefore, the approach generally used for continuous systems does not apply, and Equation (1) must be simultaneously solved for S and X to keep track of the change of substrate and bacterial concentration in the reactor. Substrate Inhibition Model Various models have been devised to describe the relationship between the substrate conversion rate (dS/dt), S as a substrate and an inhibitor (i.e. substrate inhibition). The Haldane equation (1930) is regarded as the first and most often used substrate inhibition model: dS kSX ------ = – -----------------------------------2 dt KS + S + S ⁄ Ki
Uncompetitive Mixed
: k* < 1.0, KS* < 1.0 : k* < 1.0, KS* > 1.0
(7) (8)
Results and Discussions Results from T-MCR Operation Operational results for MCR comprising of VSS, TSS measured over the test period are shown in Fig. 1. An average VSS concentration was at 1341 mg l−1. pH and ORP were maintained consistently at 7.1~7.3 and −335~ −360 mV, respectively. The average gas production volume was 7,300 mg l−1 COD and it represented 93% of theoretical gas production volume. These results were in good agreement with calculated values. This daily gas production data was obtained by a computer data acquisition system connected to an anaerobic respirometer (AER 200, CES, USA). All organic compounds in MCR were biodegraded almost completely within 20 to 24 hours. Results from Fundamental Kinetic Tests (Phase I and II) The results of phase 1 screening tests are presented in Fig. 2. At each organic loading rates was 1, 2 and 4 g COD l−1,
(2)
where, Ki = inhibition coefficient, numerically equal to the highest substrate concentration at which the specific growth rate is one-half the maximum specific growth rate in the absence of inhibition, mg l−1 Inhibition Kinetic Model Han and Levenspiel (1988) proposed the impact of a toxicant by the effect on the value of k and KS. The equation were expressed as following relationship k = k 0 [ k* ]
(3)
K S = K S0 [ K *s ]
(4)
where, k*, KS* : inhibition terms, k0 : maximum specific substrate conversion rate in the absence of inhibitor, h−1, KS0 : half-velocity coefficient in the absence of inhibitor, mg l−1, Inhibitory effects can be classified further as Competitive : k* = 1.0, KS* > 1.0 Non-competitive : k* < 1.0, KS* = 1.0
(5) (6)
Fig. 2. Cumulative biogas production volume from phase I [(a) : total biogas, (b) : methane gas].
THERMOPHILIC ANAEROBIC DIGESTION 61
Fig. 4. The VFAs degradation tendency in various VFAs injection.
Fig. 3. COD degradation rate [(a) : non-inhibition loading rate (1 g COD l−1), (b) : inhibition loading rate (4 g COD l−1)].
the total gas production was about 168, 336 and 672 ml, respectively and the methane gas production was about 92, 178 and 360 ml, respectively. This methane volume shows approximate 55% of total gas production. When 1 and 2 g COD l−1 of glucose were injected to serum bottle reactors, the same maximum gas production rate was obtained. But when 4 g COD l−1 was injected, substrate inhibition was observed. Fig. 3 shows the degradation reactions for non-inhibition loading and inhibition loading rate (1.0 and 4.0 g glucose COD l−1). Symbols are measured data and lines represent simulated results. The period of time for glucose degradation was proportional to the initial glucose loading rate. Kinetic coefficients were obtained using a non-linear least square method. When loading rate was not inhibited, the maximum specific substrate utilization coefficient (k) and half-saturation constant (Ks) for conversion were 0.24 h−1 and 700 mg l−1, respectively. When organic loading of 4 g COD l−1 was applied, the reaction showed initial substrate inhibition so that the Haldane model was taken into the consideration. This substrate inhibition was allowed to increase to high levels, so that the growth became slow and ultimately ceased. The value of KH was determined as 1000 mg l−1 for the same condition, which indicated appearance of substrate inhibition at 4 g COD l−1.
Results of VFAs Effects Tests (Phase III) 200 mg l−1 of butyrate, propionate, and acetate, respectively were injected in serum bottle reactor to evaluate VFA degradation efficiency. Fig. 4 shows VFAs concentration from phase III experiments. VFA degradation tendency under thermophilic condition showed that firstly butyrate was converted to acetate and then the acetate was converted to methane gas simultaneously (most of the butyrate was converted within 8 hours). Secondly acetate was converted to methane, then propionate was converted to acetate, and finally residual acetate from propionate was converted to methane. It appeared that the most difficult degradation was degradation of propionate. It may be explained that because butyrate is more favorable substrate than propionate, butyrate degradation is faster than propionate. Results of Propionate Effects Tests (Phase IV) Fig. 5 shows biogas production volume, Fig. 6 shows COD degradation rate, and Fig. 7 shows propionate con-
Fig. 5. Cumulative biogas production volume from propionate effect test.
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Fig. 8. Results from semi-continuous operation of serum bottle reactor.
Fig. 6. COD degradation rate of (a) : 1 g COD l−1 with 100 mg HPr, (b) : 1 g COD l−1 with 200 mg HPr, (c) : 1 g COD l−1 with 400 mg HPr.
centration from Phase IV. When 100 mg HPr l−1 were injected with 1 g COD l−1 as glucose to serum bottle reactors, the same maximum gas production rate was obtained. But when 200 and 400 mg
HPr l−1 were injected, a slight inhibition was observed. Kinetic coefficients were obtained using a non-linear least square method. When an initial concentration of 100 mg HPr l−1 was added into the serum bottle reactors, maximum specific substrate utilization coefficient (k) and half-saturation constant (Ks) for the conversion were obtained as 0.24 h−1 and 700 mg l−1, respectively. In case of 200 mg l−1, the reaction showed inhibition so that the value of k and KS were determined as 0.21 h−1 and 700 mg l−1, respectively. In case of 400 mg l−1, the value of k and KS were determined as 0.17 h−1 and 700 mg l−1, respectively. Every loading rate of propionate showed the same KS value, but k value decreased with increasing concentration of propionate. Therefore, propionate functioned in this reaction as non-competitive inhibitor. Results from Semi-continuous Tests (Phase V) Fig. 8 shows semi-continuos operation of serum bottle reactor. Up to 200 mg l−1 of propionate loading, there was no inhibition in the reactor. However, when 400 mg l−1 of propionate was injected in the reactor, lag phage was observed during the initial 4 days and in case of 800 mg l−1 of propionate loading, gas production was stopped. Therefore, the maximum injection concentration of
Conclusions The average VSS concentration of T-MCR was 1341 mg l . The pH and ORP were maintained consistently between 7.1~7.3 and −335~360 mV, respectively. The average gas production volume was 7300 ml g COD−1, and it represented 93% of theoretical value as production volume. From fundamental batch kinetic test using glucoseenriched steady state culture, the values of k (maximum substrate utilization rate coefficient) and KS (half-saturation coefficient) were 0.24 h−1 and 700 mg l−1, respectively, for −1
Fig. 7. Propionate degradation rate from phase IV.
THERMOPHILIC ANAEROBIC DIGESTION 63
non-inhibiting organic loading range. The value of KH (substrate inhibition coefficient) was 1000 mg l−1 for inhibiting organic loading range when Haldane Model was applied. VFA degradation efficiency under thermophilic condition showed that the first butyrate was converted to acetate, and then the acetate was converted to methane simultaneously. The second conversion started from acetate to methane, and then propionate was converted to acetate and finally residual acetate from propionate was converted to methane. In case of 200 and 400 mg HPr l−1, the reaction shows an inhibition so that the value of k and KS were determined as 0.21 h−1 and 700 mg l−1, and 0.17 h−1 and 700 mg l−1, respectively. The maximum injection concentration of propionate was approximately 400 mg l−1. The reaction failed at 800 mg l−1 of propionate.
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