Effective physical cleaning and adequate membrane flux for direct membrane filtration (DMF) of municipal wastewater: Up-concentration of organic matter for efficient energy recovery

Abstract

Abstract Organic matter in municipal wastewater should be regarded as a resource for energy production in the future. Direct application of anaerobic digestion to municipal wastewater is easy if the concentration of organic matter is increased. Although microfiltration membranes can concentrate organic matter in raw municipal wastewater, severe membrane fouling is likely to occur. In this study, efficient physical cleaning methods for direct membrane filtration (DMF) of municipal wastewater were investigated. Agitation of the tank, vibration of membrane modules, and use of granular materials were examined. Aeration was not tested because it leads to microbial degradation of organic matter (loss of recoverable energy). No single cleaning method worked well for DMF. However, a combination of membrane vibration and agitation of the tank was found to be effective. When the effective physical cleaning was performed with chemically enhanced backwash using citric acid and an adequate membrane flux, DMF of real municipal wastewater could be continued for almost one month without conducting any off-line membrane cleaning. The increase in transmembrane pressure was almost perfectly controlled in the long-term operation, during which 75% of organic matter was successfully recovered and 50-fold concentration (based on volume) of wastewater was continuously achieved with hydraulic retention time of 3.1 h.