Low-temperature anaerobic digestion as a core technology for the sustainable treatment of municipal wastewater
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The currently applied paradigm for municipal wastewater treatment in the European Union does not meet basic sustainability criteria. Indeed, it runs counter to the stated goals of recent European Council policies regarding sustainable development. This disposal based linear system uses aerobic microbiology as the core technology. This results -- in the case of activated sludge plants, for example -- in a requirement for large capital investment, heavy usage of fossil fuels, high technology operational control and the generation of large quantities of sludge requiring treatment before safe reuse/recycle. A direct anaerobic treatment approach is proposed in this study would assist in meeting sustainability criteria and offer significant advantages to the conventional treatment approach. Anaerobic digestion (AD) has long been recognised as a sustainable waste/wastewater treatment strategy. The major application of high-rate AD has been for high-strength industrial wastewaters, e.g. food processing, brewing, etc., where mesophilic high-rate AD has been tremendously successful. There is great potential for expanding the application of anaerobic wastewater treatment. One area of particular interest is AD for the treatment of low-strength, high volume wastestreams which are discharged at sub-mesophilic (<20°C) or low temperatures (e.g. municipal wastewater in countries with temperature climates). The implementation of anaerobic treatment of these low-temperature wastestreams is severely limited by economics, whereby a significant energy input to heat bioreactors would be required for treatment to proceed within a mesophilic temperature range, therefore negating the cost saving associated with direct anaerobic treatment. The objective of this thesis was to investigate the feasibility and applicability of direct AD of municipal wastewater under Irish conditions; low-temperature anaerobic digestion (LtAD). Process technology trials were supported by microbial physiology and 16S rRNA gene community analyses. In the first phase of this study (chapter 2), the feasibility of long-term (>1 year), low-temperature (10-15°C) anaerobic bioreactor operation, for the treatment of synthetic sewage, was investigated. The effect of temperature on the bioprocess was investigated through the use of a mesophilically (37°C) operated control bioreactor. Three hybrid bioreactors (R1-R3) were seeded with a mesophilic inoculum, and used to treat a synthetic sewage wastewater at 37, 15 and 10°C respectively. Organic loading rates (OLRs) of 0.5-6 kg chemical oxygen demand (COD) m-3 d-1 and hydraulic retention times (HRTs) of 1.5-12 h were applied during a 1.5-year trial. Despite transient disimprovements, mean total COD removal efficiency and methane biogas concentrations exceeded 70% and 50%, respectively, for all bioreactors. Specific methanogenic activity (SMA) testing indicated that a psychroactive biomass developed in the low-temperature bioreactors. The data obtained suggest that a mesophilic inoculum can physiologically adapt to sub-optimal temperature, and efficiently treat low-strength wastewater at temperatures as low as 10°C. In the second phase of this study (chapter 3) two hybrid bioreactors (R4 and R5), were each seeded with a mesophilic biomass, and employed for the treatment of synthetic sewage at 12°C and applied OLRs of 0.5-6 kg COD m-3 d-1, and HRTs of 2-24 h. Based on the results obtained in chapter 2 the use of an alternative fixed-film matrix material; granulated pumice stone, to enhance process efficiency and stability was evaluated. In addition the reproducibility of LtAD for municipal wastewater treatment was investigated. Stable bioprocess performance was demonstrated with COD removal efficiencies of >70% obtained by both bioreactors. SMA and biodegradability assays demonstrated the development of a psychrotolerant, sewage-degrading consortium within the biomass of both bioreactors. Bacterial communities, as deduced from clone library analysis at the conclusion of the trial, were phylogenetically diverse, consisting of important fermentative and hydrolytic populations. Archaeal methanogenic dynamics indicated that acetoclastic methanogenic activity directly correlated to bioreactor performance. In the final phase of this study (chapter 4) the feasibility of direct anaerobic treatment of raw and settled sewage under temperate climatic conditions (12°C) was investigated. Two expanded granular sludge bed (EGSB) anaerobic filter (AF) hybrid bioreactors, R6 and R7, were employed to treat raw sewage and settled sewage (primary effluent; the liquid fraction following settlement of primary sludge), respectively, sourced from the city of Galway, Ireland. The bioreactors were operated at HRTs of 3-24 h in a trial of c.140 days. Successful treatment of both influent types was achieved with consistent effluent quality of <125 mg l-1 COD being obtained at OLRs of 0.2-2.4 kg COD m-3 d-1, volumetric loading rates (VLRs) of 1-4 m3 wastewater m-3 bioreactor d-1 and HRTs of 3-6 h. Bacterial clone libraries at the conclusion of the trial demonstrated communities that were phylogenetically diverse, similar to those observed in chapter 3, with the identification of key hydrolytic and fermentative populations being achieved. Archaeal clone library analysis indicated that acetoclastic methanogenesis was dominant in both R6 and R7 by the completion of the trial. In summary, LtAD was demonstrated as an effective and efficient wastewater treatment approach for municipal wastewater. Combining bioprocess monitoring with physiological and molecular analyses, provided valuable insights into the complex microbial communities underpinning the LtAD process.