UC Berkeley

Potable reuse is increasingly incorporated into water resource planning in regions facing water scarcity. The widespread reuse of municipal wastewater has been enabled by reverse osmosis (RO), which is a reliable treatment process that removes many contaminants present in wastewater effluent. However, this treatment technology also produces a concentrated waste stream that presents a key barrier to the adoption of potable water reuse. The wastewater rejected by the RO membrane, termed reverse osmosis concentrate (ROC) contains contaminants at levels known to cause adverse effects on aquatic organisms, and therefore could negatively impact receiving ecosystems if discharged without treatment. Cost-effective ROC treatment technologies, particularly for inland communities where dilution may be limited near the point of discharge, are needed. This research investigates a low-cost natural treatment system for ROC: open-water unit process wetlands. These shallow wetlands remove trace organic contaminants through a combination of photo- and bio-transformation processes and remove nitrate via biological denitrification. This dissertation describes the effectiveness of open-water wetlands for ROC treatment. Furthermore, this research assessed the potential advantages of pairing open-water wetlands with engineered pre-treatment to enhance contaminant removal.

To assess the effectiveness of open-water wetlands for ROC, two parallel pilot-scale (200-m2) wetland cells were installed at a water reuse facility in San Jose, California. The cells received ROC from the adjacent facility, and the ROC entering one of the cells received an ozone pre-treatment. The performance of the pilot-scale treatment systems was assessed over the course of two years. The effect of salts and organic matter on treatment efficacy in ROC was evaluated using photo- and bio-transformation kinetic models for contaminant removal. Phototransformation rate models were modified to account for differences in organic matter reactivity under the higher salinity conditions observed in ROC relative to municipal wastewater (Chapter 2). The effects of seasonality and ozone pre-treatment were also assessed using monitoring data from the pilot-scale system. A risk quotient framework was used to evaluate the ecological risks from trace organic contaminants of concern during discharge of ROC with and without treatment.

Open-water wetland systems can remove nutrients in addition to trace organic contaminants. Monitoring of the pilot-scale treatment systems was paired with laboratory microcosm experiments to assess nitrate removal in open-water wetlands treating ROC (Chapter 3). The development of a wetland biomat capable of denitrification was observed in the pilot-scale system. The role of carbon availability in controlling nitrate removal rates was also assessed. Carbon sources in the open-water wetlands were characterized with measurements of biodegradable organic carbon and estimates of carbon fixation by photosynthetic diatoms. The use of carbon amendments to increase nitrate removal rates was tested in microcosm experiments using wetland biomat. These experiments demonstrated the ability to accelerate nitrate removal using readily available carbon substrates, such as wood chips, which could reduce the area required for open-water wetlands.

To understand the effect of the ROC matrix on phototransformation reactions, experiments were performed with a solar simulator and wetland microcosms. The occurrence of partial denitrification in the pilot-scale system resulted in elevated concentrations of nitrite, which contributed to phototransformation of trace organic contaminants. Transformation product analysis and experiments with radical quenchers were conducted to assess the mechanisms of nitrite-sensitized phototransformation (Chapter 4). Results indicated the importance of reactive nitrogen species during nitrite- and nitrate-sensitized contaminant transformation. The contribution of these reactions to contaminant transformation was integrated into the photolysis kinetic model used to evaluate performance of the pilot-scale treatment systems.

Collectively, these results indicate that open-water wetlands hold promise for removal of nutrients and trace organic contaminants from ROC produced during potable water reuse. Under laboratory and field conditions, the open-water wetland performance was consistent with kinetic models developed for the conditions of ROC. Overall, open-water wetland treatment was capable of reducing the concentrations of nutrients and trace organic contaminants that pose potential ecological risk during ROC discharge.