Comprehensive assessment of a hybrid ultrafiltration-osmotic membrane bioreactor for potable water reuse and nutrient removal from municipal wastewater, A

Abstract

Existing water supplies and mineral reserves could be augmented using municipal wastewater through water reclamation and reuse and through nutrient recovery. Potable water reuse is increasingly being considered as an option to supplement diminishing fresh water reserves in water scarce regions. The technologies developed for potable reuse have to employ multiple treatment barriers (e.g., chemical, biological, and physical) to protect the health of the public and the environmental from contaminants, including pathogenic microorganism, nutrients, and trace organic compounds (TOrCs). Similar to water reuse, there has been increasing interest in nutrient (phosphorus and nitrogen) and mineral (calcium, magnesium, and potassium) recovery from wastewater for use as fertilizers and other marketable commodities. A pilot-scale hybrid ultrafiltration-osmotic membrane bioreactor (UFO-MBR) treatment system was developed and investigated for water reuse and nutrient removal. The UFO-MBR couples biological treatment with semi-permeable forward osmosis (FO) membranes and low-pressure ultrafiltration (UF) membranes in one integrated process. The FO membrane provides nearly complete separation of suspended and dissolved solids between a low-salinity activated sludge feed stream and a high salinity draw solution (DS). The DS is used to provide the driving force for water production in this osmotically driven process. During operation, the DS is diluted by water diffusing from the feed and it must be reconcentrated using a dewatering/ reconcentration system. In this study, a reverse osmosis (RO) system was used to produce high quality product water and to reconcentrate the DS. As a result of using FO membranes for separation of suspended and dissolved solids in a bioreactor, salts, minerals, and nutrients accumulate in the activated sludge, resulting in reduced driving force (difference in salinity between the feed and DS) and inhibition of microbial activity in the activated sludge. Salt accumulation is mitigated in the UFO-MBR using a UF membrane system to extract dissolved constituents from the activated sludge with the UF permeate while retaining the biosolids in the reactor. It has been demonstrated in this study that increased flux, reduced fouling, and improved nutrient removal is achieved using the hybrid UFO-MBR system compared to an osmotic membrane bioreactor (OMBR) incorporating FO and RO only. Furthermore, the UF permeate stream has been shown to be rich with potentially valuable and recoverable nitrogen, phosphorus, and minerals. Thus, the objective of this dissertation was to assess the feasibility, viability, and sustainability of the UFO-MBR as an integrated water reuse and nutrient recovery technology through pilot-scale testing, system modeling, and life-cycle assessment.