Guidelines for the Design and Operation of Unit-process, Open-water Wetlands

Unit-process, open-water (UPOW) wetlands are natural water treatment systems designed to promote photo- and biologically-mediated water treatment processes. Treatment processes promoted in UPOW wetlands include photolysis and biotransformation of chemical contaminants (including trace organic compounds), photo-inactivation of microbial contaminants, and biological removal of nitrate (e.g., denitrification and anammox). UPOW wetlands have been observed to have improved treatment efficiency compared to vegetated wetlands due to a diversity of pathways available for contaminant removal, and a hydraulic regime that approaches plug flow. Therefore, these wetlands are an attractive option for the polishing of municipal wastewater effluent, treatment of water from effluent-dominated waterways, or use as the final stage of wastewater treatment pond systems that receive municipal wastewater. Ongoing research is exploring the ability of UPOW wetlands to treat saline waters such as fracking water and brine produced during reverse osmosis treatment of wastewater.

This document is organized as follows: A description of UPOW wetlands, including their definitive features is provided in Section II. Section III presents case studies that highlight a pilot-scale UPOW wetland constructed in Discovery Bay, CA and a demonstration-scale UPOW wetland in Orange County, CA. The mechanisms of removal or transformation of nitrate, select trace organic contaminants, and microbial contaminants from the wetland, as well as the framework for estimating first order removal coefficients, are detailed in Section IV. Section V presents hydraulic model equations that are used to calculate wetland size based on the estimated removal rates calculated using the framework described in Section IV. Section VI discusses additional design criteria that apply to shallow UPOW wetlands, including depth, influent water quality, and hydrodynamics. Section VII provides guidance for the operation and maintenance of these systems.