Two-stage nanofiltration for purification of membrane bioreactor treated municipal wastewater - minimization of concentrate volume and simultaneous recovery of phosphorus

Abstract This study evaluates the usability of a novel two-stage tertiary nanofiltration process concept for further purification of membrane bioreactor (MBR) permeate from municipal wastewater treatment and minimization of nanofiltration concentrate volume. In addition to water purification, the studied process aims to recover phosphorus from the second stage. The process concept for tertiary wastewater treatment includes two nanofiltration modules in series, where first stage is a nanofiltration of MBR permeate with a spiral-wound element and in the second stage a minimization of the NF concentrate is done by shear-enhanced nanofiltration combined with spontaneous precipitation and removal of precipitates from the concentration process. A volume reduction factor (VRF) of 300 was achieved with the studied process concept. The average permeabilities of the NF270 membranes used in both stages were 4.3 L/(m2hbar) (first stage, spiral wound module) and 8.9 L/(m2hbar) (second stage, high shear rate module). The two-stage NF process efficiently decreased the chemical oxygen demand (COD) and total organic carbon (TOC) content of the treated water, giving retentions higher than 99% for COD and 97% for TOC. About 52% of phosphorus was spontaneously precipitated as calcium phosphate (presumably as hydroxyapatite, purity 80%) at the second stage of the process. High turbulence (rotor tip speed 13.7 m/s) at the membrane surface during the second stage tertiary filtration prevented the formation of inorganic scale and membrane fouling. Precipitation occurred in the settling tank after the filtration when the turbulent flow conditions subsided. This enabled the continuation of the filtration to a high VRF value of 300 despite the significant calcium and phosphorus concentrations. The results presented here confirm that shear-enhanced nanofiltration is a promising approach for efficient minimization of the NF concentrate volume, as well as for simultaneous phosphorus recovery without the need for precipitation chemicals.