Reverse osmosis concentrate: An essential link for closing loop of municipal wastewater reclamation towards urban sustainability

Abstract

With rapid global urbanization and increasing population, the water shortage is becoming a pressing issue for urban sustainability due to limited availability of natural water resource and increasing demand on fresh water. In such a situation, reclaimed municipal wastewater has been actively considered as a supplementary water source for sustaining future water supply. As such, various biological processes coupled with reverse osmosis (RO) for municipal wastewater reclamation to high-grade product water have been gaining growing interest worldwide. However, it should be realized that the use of RO membrane units for water treatment is complicated by the buildup of their concentrated waste. So far, the RO concentrate (ROC) has been recognized as an emerging challenge in municipal wastewater reclamation. Thus, this article aims to look into (i) integrated biological treatment-RO processes for municipal wastewater reclamation which is an emerging source of ROC, (ii) challenges associated with ROC, (iii) post-treatment of ROC and its impacts on overall energy balance in municipal wastewater reclamation processes and (iv) an innovative integrated anaerobic nanofiltration membrane bioreactor-RO process coupled with electrodialysis reversal and ozonation lastly proposed for closing water loop which is imperative in more and more countries. The possible engineering solution to tackle ROC is to minimize its volume by improving the productivity of RO unit in the mainstream wastewater reclamation process, with the ultimate goal of crystallization for zero liquid discharge at an affordable energy demand. Moving forward, reclamation of municipal wastewater to high-grade product water offers a promising path towards urban sustainability.