Feasibility of combined UASB and MBR system in dairy wastewater treatment at ambient temperatures

• Combined UASB–MBR is feasible for dairy wastewater treatment at ambient temperatures.
• The system presented a high tolerance to loading changes and temperature fluctuations.
• High biogas production with more than 70% methane content was obtained.
• Overall biomass yield similar to anaerobic systems was obtained.
• Permeability values similar to those reported for aerobic MBR systems were obtained.

The aim of this study was to investigate the feasibility of a combined UASB and MBR system for the treatment of dairy wastewater at ambient temperatures. The system consisted of a methanogenic UASB stage and a two-compartment post-treatment aerobic MBR stage, with a membrane ultrafiltration module. The objectives of the system were to decrease the COD of dairy wastewater whilst producing biogas rich in methane, diminish overall sludge production, and to obtain a high quality effluent due to the implementation of membrane filtration. The system presented a high tolerance to loading changes (up to 4.85 kg COD m−3 d−1) and temperature fluctuations (17–25 °C). The average total and soluble COD removals were above 95%, reaching 99% during stable operation periods. The observed overall biomass yield was low, from 0.13 to 0.07 g MLVSS g COD−1. Biogas production yield reached 150 L kg−1 of t-COD, with an average methane content of 73%. With respect to membrane performance, permeability values between 140 and 225 L m−2 h−1 bar−1 were obtained, similar to those reported for aerobic MBR systems. The average flux obtained was 13 L m−2 h−1, reaching 19 L m−2 h−1 during stable operation periods depending on conditions. These values were lower than those observed in aerobic MBR systems, but much higher than those found in methanogenic AnMBRs.