Linking microbial community structure and function to process performance and reactor configuration during high-rate low-temperature anaerobic treatment of dairy wastewater

Abstract

Anaerobic digestion is a sustainable and attractive option for the treatment of wastewaters. It allows the recovery of energy from waste - in the form of biogas - and is more cost-efficient than aerobic biological treatments. In temperate climates, much of the wastewater generated from industry is discharged below 18°C. The energy requirement and capital needed to heat large volumes of this wastewater to the mesophilic temperature range (25-45°C) is high and often negates any potential energy gains from the biogas produced. Low-temperature anaerobic treatment (< 25°C) avoids the need to heat this wastewater, thereby reducing the operating costs and the energy-input required. The process is particularly attractive for high-rate, low-strength wastewaters such as those produced by the dairy industry. Full-scale implementation of low-temperature anaerobic digestion has been limited, largely due to the lack of knowledge regarding the identity and functions of the microbial populations that drive the process, but also due to uncertainty regarding the optimal reactor configuration and operating conditions. In this study, a multiphasic approach was applied to assess the effectiveness of different reactor configurations during low-temperature anaerobic digestion of dairy wastewater. The study employed molecular techniques to investigate the microbial community structure and its function, which could then be linked to bioreactor process performance. A structural and functional understanding of the microbial community’s response to different reactor conditions allows for greater insight into stable low-temperature anaerobic treatment. This increased knowledge can then be used to identify potential problems, provide solutions to disturbance issues, forewarn against process failure, and in general, help determine the appropriate conditions for successful low-temperature anaerobic treatment. We found that low-temperature anaerobic digestion of dairy wastewater was eminently feasible; however, the microbial community was extremely sensitive to mixing. The UASB reactor configuration achieved the highest process performance. Lactococcus was extremely important for the successfully functioning of the reactors at low temperature and appears to have a major role in protein degradation. Floatation of biomass was a major issue and we demonstrated that the microbial composition of floating and settled biomass were different. Overall, combining traditional bioprocess monitoring with advanced molecular analyses, provided valuable insights into the structure and function of the complex microbial communities involved and led to a greater understanding of the low-temperature anaerobic digestion process.