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Direct membrane filtration (DMF) has recently gained attention as an alternative secondary biological wastewater treatment process. This study evaluated direct microfiltration (MF) and ultrafiltration (UF) performance and cleaning protocols during crossflow DMF of primary municipal wastewater effluent. Several types of MF and UF membranes were examined by threshold flux determination, and two types of membranes (MF, 0.08 μm; UF, 100 kDa) were chosen for exploring membrane fouling mechanisms at different feed pressures via both fouling resistance analysis and optical coherence tomography (OCT) observation. The results revealed that both MF and UF displayed three-stage fouling behaviors, i.e., initial intermediate pore blocking followed by two-stage cake filtration. Increasing feed pressure from 8 kPa to 50 kPa could accelerate physically reversible fouling rate (consistent with simulated cake filtration constant). During physical flushing, the cake layer was more readily removed from the UF membrane; while residual porous cake layer was present on the MF membrane, regardless of the feed pressure. With extending filtration-cleaning cycle, shortening filtration duration and elevating cleaning solution temperature to from 25 °C to 50 °C benefited for irreversible fouling alleviation. At 50 °C, the geothermal water performed similar cleaning behaviors as clean water, facilitating lower reversible and irreversible fouling than the geothermal brine. This study shed light on the feasibility of using high temperature geothermal water for periodic physical cleaning during DMF of wastewater under Icelandic scenario. Economic Development Board (EDB) This work was supported by the University of Iceland Research Fund. The Student Innovation Fund from Rannís in Iceland was acknowledged for providing summer research grant to Dagmar Olafsd ´ ottir. The Economic Development Board (EDB) of Singapore is acknowledged for funding the Singapore MembraneTechnology Centre (SMTC) , Nanyang Technological University.