Volatile fatty acids concentration in real wastewater by forward osmosis

Abstract

Forward osmosis (FO) was studied as a concentration step for volatile fatty acids (VFA, especially acetic acid) to optimise downstream microbial desalination cell (MDC). First, it was demonstrated that water concentration factor (WCF) of wastewater (WW) above 10 (15–30) is achievable with seawater or brine as draw solution and similar flow for feed and draw solution. It was also observed that VFA rejection by FO membrane is highly connected to pH. At pH = 7.5 rejection rates above 80% are achievable working with domestic WW and therefore concentration of VFA by FO process is realistic. Nevertheless, concentration of VFA present in pre-treated real domestic WW proved to be more challenging with regards to fouling /biofilm formation which favours the biodegradation of VFA. Thanks to fouling mitigation (WW pre-filtration with microfiltration membrane and interbatch osmotic backwashing) and biodegradation strategies implemented (by applying N2 sparging and avoiding air contact with the WW), VFA concentration (and especially acetic acid) from 60–80 up to 300–400 mg L−1 is possible. Maintaining high permeation flux, high VFA recovery and concentration factor of VFA during 20 batches of operation was achieved, being more difficult the stable operation of the FO concentration process at high WCF and VFA concentration.