Flux decline control in nanofiltration of detergent wastewater by a shear-enhanced filtration system

Treatment of wastewater containing a cleaning-in-place detergent was investigated by using a rotating disk module equipped with a nanofiltration (NF) membrane. At a rotational speed of 2000 rpm, a pH 7.2 and 25 °C, the permeate flux reached a plateau at 350 Lm−2 h−1 above 35 bar while rejection of conductivity and COD were respectively 93 and 97%. When a pretreatment by ultrafiltration (UF) was carried out before the NF, the NF flux increased linearly with TMP to reach 450 Lm−2 h−1 at 40 bar while COD of NF permeate was a little lower than without pretreatment. Both permeate flux and conductivity rejection increased with increasing pH. High membrane shear rates prevented flux decline with time, as surfactant molecules were dragged away from membrane surface, preventing surfactant aggregates formation. The disk rotational speed necessary for flux stability increased with TMP from 1000 rpm at 10 bar to 2000 rpm at 30 bar. Increasing feed pH had a similar effect by enhancing electrostatic repulsion between surfactant molecules and membrane. Raising the temperature to 45 °C increased the permeate flux to over 500 Lm−2 h−1 at 30 bar and 2000 rpm. This work confirms the high performance of high shear dynamic filtration in detergent wastewater treatment by NF system both in terms of permeate flux and ion and COD rejections due to reduction in concentration polarization, so that a UF pretreatment was not necessary.

► Detergent wastewater was well treated by RDM module with a NF270 membrane.
► Flux decline by surfactant was controlled by high shear rate.
► High pH and high temperature decreased surfactant aggregate formation on membrane.
► Membrane performance was recovered without any chemical cleaning.
► Membrane fouling occurred at acid pH when surfactant existed in system.