The behavior of sulfate-reducing bacteria in acidogenic phase of anaerobic digestion
Introduction
Anaerobic treatment process is widely applied to the treatment of wastewater and sewage sludge. However, it is recognized that a sulfate-rich wastewater such as molasses slop can cause some significant problems resulting from sulfate reduction in the anaerobic treatment process (Hilton and Archer, 1988). Dissimilatory sulfate reduction is associated with the growth of sulfate-reducing bacteria (SRB) which utilize hydrogen or organic matters and sulfate as an electron donors and electron acceptor, respectively (Widdel, 1988; Odom and Singleton, 1992). It is well-known that anaerobic treatment, especially methanogenesis as a terminal step, can be significantly influenced by the sulfide produced from sulfate reduction. In particular, a high concentration of free-hydrogen sulfide can lead to the inhibition of methanogenesis and eventually anaerobic process failure (Parkin et al., 1990). The behavior of SRB in an anaerobic ecosystem is, therefore, of considerable interest not only to the microbiologist but also to environmental engineers for the control of anaerobic treatment systems.
In previous studies, many investigators evaluated the role and behavior of SRB in anaerobic ecosystems including sediments and anaerobic digesters. During methanogenesis as the terminal step of anaerobic degradation, competition between SRB and methane-producing bacteria (MPB) is particularly noted from microbiological and technological aspects (Abram and Nedwell, 1978; Schonheit et al., 1982; Lovley and Tiedje, 1984; Yoda et al., 1987). It is well understood that SRB compete with MPB for acetate and hydrogen which are the most important methane precursors. During the syntrophic oxidation of fatty acids and neutral products, SRB play an important role as a hydrogen consumer in the interspecies hydrogen transfer and their predominance significantly decreases the amount of methane production. It has been reported that SRB could compete with the hydrogen-producing acetogenic bacteria for the substrates of butyrate (Mizuno et al., 1994), propionate (Parkin et al., 1990; Visser et al., 1993; Uberoi and Bhattacharya, 1995) and benzoate (Li et al., 1996). As the result of the SRB participation, the degradation pathways of propionate, butyrate and benzoate are significantly influenced by the COD/S ratio of the substrate. These basic studies focused on the methanogenic phase.
From the engineering point of view, several improved processes for treating sulfate-rich wastewater have been demonstrated (Reis et al., 1988; Buisman and Lettinga, 1990; Buisman et al., 1990). Among them, sulfate reduction in the acidogenic phase has also been reported in a two-phase anaerobic digestion process treating distillery molasses slop effluent (Reis et al., 1988). However, little is known about the role of SRB and sulfide toxicity for acidogens during the acidogenic phase of the two-phase anaerobic digestion.
The objective of this study is to investigate the effect of sulfate reduction on the acidogenesis of organic matter and to clarify the behavior of SRB in the acidogenic phase. For this purpose, four series of continuous experiments using five chemostat-reactors were conducted with different sulfate concentrations using sucrose as the only organic carbon source. The COD balance, sulfate removal and metabolite production at various sulfate concentrations and HRTs, were measured to clarify the interaction among acidogenesis, sulfate reduction and methanogenesis in the acidogenic phase.
Section snippets
Operation of anaerobic chemostat systems
Five anaerobic reactors of 1-liter capacity with 500-ml working volume were used in the present study. A schematic of the experimental apparatus is illustrated in Fig. 1. The contents of the reactor were continuously stirred by biogas recirculated with an air pump. In order to measure the volume of biogas production, the reactor was connected to a biogas collection cylinder placed in an acidic saturated salt solution of NaCl with 2% sulfuric acid. The chemostat-reactors were installed in a
Sulfate removal efficiency
Fig. 2 shows the effects of sulfate concentration and HRT on sulfate removal efficiency under the steady-state condition. Sulfate removal efficiency increased with the increase in HRT at the same influent sulfate concentration but decreased with the increase in sulfate concentration at the same HRT. When the HRT was longer than 8 h, over 90% sulfate removal was obtained at sulfate concentrations of 600 and 1200 mg/l, while only 60% was obtained at 2400 mg/l. On the other hand, only 23 to 33%
Discussion
It has been well known that the SRB in a methanogenic ecosystem could compete with MPB and syntrophic acetogens for intermediates during the anaerobic degradation of organic matters, such as hydrogen (Kristjansson et al., 1982), acetate (Schonheit et al., 1982; Yoda et al., 1987), propionate (Parkin et al., 1990; Visser et al., 1993; Uberoi and Bhattacharya, 1995), butyrate (Mizuno et al., 1994), ethanol (Szewzyk and Pfennig, 1990; Wu et al., 1991) and benzoate (Li et al., 1996). The
Conclusions
Based on the above results and discussion, the following conclusions can be made:
- 1.
Sulfate reduction occurred in the acidogenic phase even at HRTs as short as 2 h. At the sulfate concentration up to 2400 mg/l, the sulfate in the influent wastewater was effectively removed by the acidogenic reactor with an HRT of 8 to 10 h. The SRB in the acidogenic reactor utilize hydrogen as the main electron donor.
- 2.
No inhibition of sucrose degradation was observed under the condition of free-H2S concentration up to
Acknowledgements
The authors wish to thank Mr H. Watari, a graduate student in the Department of Civil Engineering, Faculty of Engineering, Tohoku University, for this excellent assistance. During the time much of this work was performed, Dr Yu-You Li was an associate professor in the Department of Civil Engineering, Faculty of Engineering, Tohoku University.
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