Two pilot-scale membrane bioreactors were operated alongside a full-sized activated sludge plant in Tunisia in order to compare specific energy demand and treated water quality. Energy consumption rates were measured for the complete membrane bioreactor systems and for their different components. Specific energy demand was measured for the systems and compared with the activated sludge plant, which operated at around 3 kWh m(-3). A model was developed for each membrane bioreactor based on both dynamic and steady-state mass balances, microbial kinetics and stoichiometry, and energy balance. Energy consumption was evaluated as a function of mixed-liquor suspended solids concentration, net permeate fluxes, and the resultant treated water quality. This work demonstrates the potential for using membrane bioreactors in decentralised domestic water treatment in North Africa, at energy consumption levels similar or lower than conventional activated sludge systems, with the added benefit of producing treated water suitable for unrestricted crop irrigation.

A pilot-scale aerobic membrane bioreactor (MBR) was operated for 6.5 months, alongside a full-sized conventional activated sludge (AS) plant, treating high-strength domestic wastewater originating from Sfax in Tunisia. The main target was to investigate whether or not the MBR could produce effluent suitable for unrestricted crop irrigation in Tunisia, a target that the AS plant fails to achieve. Membrane performance analysis and energy benchmarking were also carried out. The MBR did produce irrigation quality water regardless of the mixedliquor suspended solids (MLSS) or feed concentrations. The average chemical oxygen demand (COD) removal efficiency was 88% at an average MLSS concentration of 4.59 g L -1, and 89.7% at an average MLSS concentration of 9.52 g L -1. During membrane performance evaluation, a maximum sustainable membrane permeate flux of 12.81 L m -2 h -1, at an average mixed-liquor temperature of 24 °C and an average MLSS concentration of 9.21 g L -1 was maintained. Finally, energy benchmarking was carried out; the average energy consumption rate was 8.95 kWh d -1, corresponding to an average specific energy demand (SED) of 3.82 kWh m -3. This is a relatively high value compared to the AS plant, whose SED value is always lower than 3 kWh m -3, but further energy reduction is possible for the MBR as well as the AS plant effluent requires further treatment in order to be acceptable for unrestricted human crop irrigation; hence, additional energy input. The work demonstrates the potential impact of MBRs in decentralised domestic wastewater treatment in North Africa. © IWA Publishing 2012