Combined membrane aeration and filtration for energy- and space-efficient COD removal in water reuse

• MA-MBR could treat greywater at high rates with >95 % COD removal efficiency. • Biofilm-floc equilibrium enabled SRT control at low MLSS, regulated by ORP. • Flocs re-attached to aeration membranes after scouring reduced MLSS. • MA-MBR could achieve lower TMP buildup compared to BA-MBR. • Distinct microbial communities between floc and film fractions suggest a loosely bound floc-film assemblage. Due to climate change and increasing droughts, wastewater treatment and water reuse are gaining importance. Yet, the state-of-the-art bubble-aerated membrane bioreactor (BA-MBR) faces competitiveness challenges due to its high energy use and maintenance requirements, especially at small scale. This study investigates a novel membrane-aerated MBR (MA-MBR) that integrates membrane aeration and filtration to reduce energy consumption and system footprint, enabling resource-efficient non-potable reuse. The MA-MBR treated greywater for domestic reuse and achieved stable chemical oxygen demand (COD) removal efficiencies up to 95 % at high loading rates (up to 4 g L⁻¹ d⁻¹) and produced effluent with biological oxygen demand (BOD₅) values below 5 mg L⁻¹, meeting stringent reuse standards. Biomass dynamics revealed two distinct forms: biofilm on aeration membranes and flocs in suspension. Coarse bubble scouring facilitated biofilm detachment, enabling solid retention time (SRT) control. Oxidation-reduction potential (ORP) was linked to the biomass detachment efficiency, with negative ORP reducing mixed liquor suspended solids (MLSS) after scouring 5–10 times compared to operation at positive ORP. Reattachment of flocs reduced MLSS levels by 90 % within 60 min. A 25 % lower transmembrane pressure (TMP) in the MA-MBR compared to the BA-MBR after 72 h indicated lower fouling rates. Microbial communities were distinctly different between biofilm and flocs, especially under negative ORP conditions. These findings suggest the MA-MBR as low-footprint, low-fouling alternative for carbon removal from wastewaters with relatively high COD/N-ratios, and may improve resource efficiency for non-potable water reuse, for instance in decentralized source-separation applications.