Startup and stabilization of anaerobic membrane bioreactors at ambient temperature

There has been an increasing interest in wastewater treatment in last decades to reduce human footprint. Primarily, anaerobic technology focused on treatment and stabilization of sludge, but now the tendency is to give it a major role in low cost treatment of high/low strength wastewaters, since anaerobic digestion offers energy generation through gas production. Anaerobic membrane bioreactors (AnMBR) combine anaerobic digestion with membrane fil-tration. They are becoming a feasible option for treating previous unsuitable low-strength wastewaters, decoupling hydraulic and solid retention times, and providing successful treat-ment with the benefits of biogas production. However, the digestion process is optimal at mesophilic or termophilic (35-37 ºC), requiring heating of reactors. The more inexpensive option to treat the wastewater at its ambient tem-perature is feasible using AnMBR since this type of reactor can offer long sludge retention times. On the other hand, the digestion equilibrium turns out more sensible and delicate, and per-forming a proper and robust start-up of AnMBR in ambient temperatures is still challenging. The aim of the thesis was the successful startup and stabilization of AnMBR systems at ambi-ent temperature (25 ºC) and low organic loading rate (OLR). Reactor operation was moni-tored, and the most relevant process parameters were considered for the aim. Two pilot-scale AnMBR’s (120L) were used with an external membrane configuration. The experiment was carried out in 100 days. Substrate feeding consisted of synthetic dairy wastewater with added nutrients solution. The inoculum was provided from a full-scale an-aerobic plant at a digester of BV dairy (UK) treating dairy wastewater at 30ºC. Main operation parameters were monitored every day, along with gas production and methane yield. Laboratory tests were performed twice a week with samples of the reactors and efflu-ent. A number of parameters were analyzed, the most important of which were total solids content (TSS), alkalinity, fatty acids, biogas content and chemical oxygen demand (COD). The startup of the two AnMBR’s differed greatly. In System 1, stable conditions were ac-quired in one month of operation. System 2 failed after 20 days of function, and did not achieve successful startup. It was not possible to fully recover it during the days of study due to dramatically slow growth of microorganisms and low stability of the process. Thus, satisfactory system performance could be achieved but the ambient anaerobic process was vulnerable to inhibitory conditions. Both systems showed that the delicate process opera-tion required fast corrective measures to prevent digestion failure. Causes of instability and failure were: washout of biomass, high content of VFA, low buffering capacity and poor per-formance of technical equipment and low pH. However, the digestion could stand a lower pH range than found in literature. In conclusion, the best parameters to control the startup were pH, alkalinity, methane content, biomass content and organic removal. In this sense, low buffering capacity of a reactor makes it vulnerable to inhibition by sudden pH changes, easily solved by systematic addition of a buffering compound. Finally, the use of simple and fast alkalimetric methodologies can give satisfactory process overview compared to complex and more precise techniques for alkalinity measurement.Why should we keep polluting rivers and lakes when we have AnMBR? Those fantastic sys-tems with microbes that eat waste, produce biogas and leave a cleaned water. They don’t even use air to live! It is a good idea, if starting up one of these engineering wonders was as easy as riding a bicycle. Actually, it is like riding a bicycle, but on ice