Nutrient removal by NF and RO membranes in a decentralized sanitation system

Abstract

Decentralized treatment of domestic wastewater offers the possibility of water and nutrient reuse. In a decentralized sanitation system the household wastewater streams are separated in a large diluted stream (gray water) and a small and concentrated stream (black water) containing important nutrients like ammonium and phosphate. Nanofiltration (NF) and reverse osmosis (RO) membranes might be used to recover the nutrients from anaerobically treated black water. The permeate might be used in a water reuse scheme. In case of water reuse the produced permeate should meet guidelines for potable water or meet new guidelines which might be applied in the future for intermediate quality of water, for example toilet flushwater; when this is not possible the permeate should meet guidelines for discharge. The most stringent guidelines apply for ammonium and phosphate. The focus of this paper is to test commercially available NF and RO membranes to remove nutrients from anaerobically treated black water in order to meet the Dutch guidelines. A large number of commercial tubular, capillary and flat sheet NF and RO membranes was tested on laboratory scale on their performance to meet the Dutch guidelines for ammonium and phosphate. The ammonium and phosphate concentrations used were based on the effluent composition of anaerobically treated black water. Ammonium and phosphate rejection were both measured in synthetic single salt and multi-ion mixtures and in anaerobic effluent. The rejection for ammonium (30–95%) is neither sufficient for discharge nor potable water use. The rejection of phosphate (74–99%) is in most cases almost sufficient to meet the standards for potable water.