Direct membrane filtration of municipal wastewater: Assessment of different fouling control strategies to enhance its energy, environmental and economic viability

This study assessed the viability of direct membrane filtration (DMF) to enhance resource recovery from municipal wastewater (MWW). A demonstration scale ultrafiltration plant equipped with industrial commercial membranes was operated for several months to evaluate its most suitable operating conditions. Different strategies were evaluated to mitigate membrane fouling, determining air sparging and classic backwashing as low effective methods when used in the DMF, suggesting they should be reduced to diminish operating expenses. Conversely, chemical-enhanced backwashing proved to be highly effective in mitigating fouling (membrane permeability recoveries of about 0.34 LMH bar-1 per g of NaOCl were obtained in this work), so that its use at low concentrations and high periodicities can be recommended. When employing a proper CEB protocol, filtration was possible for more than 45 days at low fouling rates (about 0.75 mbar day-1 when filtering at a transmembrane flux of 11.9 LMH), while a great permeate quality that meets EU discharge standards for nonsensitive environments was produced. An energy, carbon footprint and economy analysis of the system was performed to determine its viability after the application of the operating conditions obtained in this work. Optimistic results were achieved with net energy productions of about 0.47 kWh per m3 of influent MWW, thanks to the particulated COD recovery. This represented net carbon footprint savings of about 0.19 kg CO2-eq. per m3 of influent MWW by increasing energy savings while avoiding the use of fossil fuels. Optimistic results were also obtained for the operating costs, with an estimated net profit of about (sic)0.064-0.138 per m3 of influent MWW. Unfortunately, membrane costs still represent the main bottleneck of this alternative, with high investment demands despite considering membrane acquisition costs only. However, thanks to the significant economic benefits achieved by this technology, membrane amortization could be greatly reduced, with an estimated payback period of between 2.6 - 5.6 years.