Effect of high temperature on bacterial community dynamics in anaerobic acidogenesis using mesophilic sludge inoculum
Introduction
Swine wastewater has high organic matter, animal nitrogen, and also malodorous compounds (Spoelstra, 1978). However, there are certain difficulties in dealing with swine manure because of the high proportion of suspended solids resistant to biological degradation (Beaudet et al., 1990). Consequently, it is necessary to stimulate a high biodegradation rate in treating swine wastewater.
Anaerobic digestion, widely used to treat high-strength organic wastewater such as swine wastewater, consists of three stages in which the complex organic components of the waste are solubilized, broken down, and fermented into intermediate products such as oligosaccharides, amino acids, or glycerol and long-chain fatty acids that are subsequently reduced into methane and carbon dioxide. The stages are called hydrolysis or liquefaction, acidogenesis, and methanogenesis, respectively (Hwang et al., 2001, Ahn et al., 2006). These stages contain considerably varied species of symbiotic microorganisms, which can be broadly classified as two groups: acidogens and methanogens. For designing the optimum anaerobic process yielding a maximum efficiency, a two-phase anaerobic digestion is often preferred because these two major microbial groups in these processes are different in their physiology, biokinetics, and growth environment (Han and Dague, 1997). Since the methanogenic reaction is believed to be the rate-limiting step in the overall anaerobic digestion, proper control of the methanogenic phase has been a key factor in the successful operation of most anaerobic processes (Grady et al., 1999). However, overall process enhancement must be based on an understanding of optimum growth conditions and behavior of acidogens in a two-phase process because they play a primary role in producing major substrates, short-chain organic acids, for methanogens. While organic acids are important substrates in methanogenesis, high concentrations of acids can cause anaerobic digestion systems to fail (Stronach et al., 1986). Furthermore, the acidogens include many analogous microbial species and little information about their characteristics and growth conditions is available relative to the methanogens.
Thermophilic acidogenesis can provide many operational benefits, such as increases in the rates of digestion and hydrolysis, decrease in digester volume, and ease of liquid–solid separation, with the efficient production of volatile fatty acids (VFAs) (Aitken and Mullennix, 1992). In practice, a thermophilic acidogenesis may start-up with mesophilic sludge inoculum because the thermophilic process is less prevalent in field-scale applications (Yilmaz et al., 2008). Thermophilic anaerobic digesters generally harbor less microbial diversity, however, with thermophilic bacteria as dominant species. Since microbial diversity is closely related to functional stability, it is necessary to study the effects of high temperature on the bacterial community dynamics in the anaerobic digestion process during acidogenesis using mesophilic sludge inoculum (Curtis and Sloan, 2004).
Bacterial communities can be investigated with denaturing gradient gel electrophoresis (DGGE) analysis. The DGGE method is one of the most widely used fingerprinting technique, with the benefit of describing microbial communities by the recovery and sequencing of amplification products (Curtis and Craine, 1993). The main objective of the present research was to investigate the microbial community dynamics in thermal acidogenesis using mesophilic sludge inoculum.
Section snippets
Reactor operating conditions
Three identical automated anaerobic continuously stirred tank reactors (CSTR), each with a working volume of 4 L and equipped with temperature controllers, were operated in batch mode and continuous mode sequentially for acidogenesis. Anaerobic seed sludge from a local municipal wastewater treatment plant in Pohang, South Korea, was mixed with the swine wastewater as the substrate for acidogens as 1% of the total suspended solids (w/v), and cultivated in batch mode in the three bioreactors for 36
Response surface methodology
The substrate concentration was maintained at 70 g chemical oxygen demand (COD)/L and steady-state was assumed after ten turn-overs. The operating condition at the center point was 1.5 days HRT and 50 °C. Repeated observations at the center were used to estimate the experimental error.
First, seven trials from trials 1–5 in Table 1 were run initially. A first-order model, ηi = βo + β1x1 + β2x2, was used to fit this data by least squares and the following models were generated for the TVFAs production.
Conclusion
Mathematical modeling of acidogenesis demonstrated that the optimal temperature and HRT were calculated to be 51 °C and 2.0 days, respectively. According to DGGE analysis, B. halodurans and a swine effluent bacterium worked constantly in the overall process of acidogenesis. In contrast, P. mendocina, present in the swine wastewater, disappeared because of its mesophilicity. Clostridium hastiforme, G. thermotolerans, and T. haemolytica were observed in the continuous process of acidogenesis, and
Acknowledgements
This work was financially supported in part by the Korea Ministry of Environment (MOE) as Human Resource Development Project for Waste to Energy, Manpower Development Program for Energy and Resources by the Ministry of Knowledge and Economy (MKE), and by the BK-21 program.
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