Operation of a submerged aerobic membrane bioreactor for decentralised municipal wastewater treatment in North Africa

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 mixed-liquor 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.