A novel self-buffering membrane distillation-based thermophilic anaerobic bioreactor

The stability of an anaerobic bioreactor is a function of alkalinity that provides buffer capacity to maintain a suitable pH for anaerobic microorganisms. The greatest advantage of membrane distillation (MD) is the high rejection of most solutes, including alkaline salt, and it has a high potential to retain alkalinity for the anaerobic system. For the first time, a thermophilic anaerobic granular bioreactor integrated with an MD membrane to maintain the buffered pH was studied in an anaerobic bioreactor without alkalinity supplement. This study presented a comparison between a conventional thermophilic anaerobic bioreactor (TAn1) and a thermophilic anaerobic bioreactor integrated with a MD (TAn2-MD). The two TAns (TAn1 and TAn2) were operated in the same conditions except there was no alkalinity feeding for the TAn2-MD. The results showed that the pH in the TAn2-MD unit was always higher than 6.8 with the organic loading rates from 6 to 18 kg COD/m³/day. However, to maintain the pH of the TAn1 unit at the same level as the TAn2-MD unit, the minimum alkalinity requirements for TAn1 were 1.5, 1.56, 1.62, and 1.8 g/L as CaCO₃ at an organic loading rate of 6, 10, 14, and 18 kg COD/m³/day, respectively. Although there was no alkalinity supplement for the TAn2-MD system, two stability indexes, FOS/TAC (volatile organic acids to buffer capacity) and IA/PA (intermediate alkalinity to partial alkalinity), indicated the system was operated at a stable condition. In summary, the designed TAn2-MD can accomplish high-efficiency organic carbon removal (higher than 99.4%) without alkalinity supplement, achieve zero-liquid-discharge, and reclaim high-quality wastewater in one step.