AnMBR digestion of mixed WRRF sludges: Impact of digester loading and temperature

This study compared the performance of a pilot-scale anaerobic membrane bioreactor (AnMBR) and a conventional anaerobic digester (AD) when treating raw mixed sludges from two water resource recovery facilities (WRRF). The study was carried out over a 2-year period and examined ambient (25 °C), mesophilic (35 °C) and thermophilic (55 °C) operations. Four operating conditions that spanned these temperature regimes and a range of loading conditions (0.5-2.5 kg VS/m3 d) were assessed. The AnMBR consistently had greater volatile solids reduction (VSr) and increased methane yields (0.19-0.34 Nm3/kg VS fed) when compared to the AD. The VSr in the AnMBR was maintained at high values (49-64%) at an HRT of 7 days. An energy balance showed improved net energy production was realized using AnMBR over AD due to the higher methane yield (38% increase on average) and higher VSLR employed. The quality of the produced biosolids was assessed and it was found that the TS in the waste stream from the AnMBR were 2-3.4 times more concentrated than those of the AD because of the extended SRTs. The nutrient contents (N and P) of the digested sludges fell within the ranges typically reported. The results of this study extend previous work and are considered applicable to full-scale applications. Further the results suggest the potential benefit of a positive energy balance towards a sustainable practice when treating mixed municipal sludge using AnMBR at ambient and mesophilic operating conditions.