Abstract
As combined sewer systems and centralized wastewater treatment facilities age, many communities in the world are challenged by management of combined sewer overflow (CSO). Constructed wetlands are considered to be one of the green infrastructure solutions to CSOs in the US. Despite the wide application of constructed wetlands to different types of wastewaters, the stochastic and intermittent nature of CSO presents challenges for design and performance assessment of constructed wetlands. This paper reviews the application status of CSO constructed wetlands in the US, assesses the benefits of CSO constructed wetlands, identifies challenges to designing CSO constructed wetlands, and proposes design considerations. This review finds that constructed wetlands are effective in CSO treatment and relatively less expensive to build than comparable grey infrastructure. Constructed wetlands not only remove pollutants, but also mitigate the event-associated flow regime. The design challenges include incorporating considerations of green infrastructure into permit requirements, determining design capacity for highly variable flows, requiring pretreatment, and needing adaptive design and intensive monitoring. Simultaneous monitoring of flow rate and water quality at both the inflow and outflow of CSO constructed wetlands is required for performance assessment and needed to support design, but is rarely available.
Highlights
More than 700 cities across the US rely on combined sewer systems to collect sanitary sewage, industrial wastewater, and stormwater runoff in the same pipe [1]
Because of the highly varying flow rate of combined sewer overflow (CSO), hydraulic loading rate and retention time could not be reasonably estimated for CSO Constructed wetlands (CWs)
Washington CWs have been designed with a CSO event hydraulic loading depth of 726 mm/event, being much higher than 26–78 mm/event at Arch/Bar and Akron CSO CWs
Summary
More than 700 cities across the US rely on combined sewer systems to collect sanitary sewage, industrial wastewater, and stormwater runoff in the same pipe [1]. Combined sewer systems were designed to convey the combined municipal wastewater for treatment in centralized wastewater treatment facilities. Most of these communities are older cities in the Northeast, the Great Lakes region, and the Pacific Northwest. In the past 50–100 years urban population density and service area of combined sewer systems have been increasing, resulting in higher municipal wastewater flow rates [2]. The old combined sewer systems and wastewater treatment facilities are usually able to collect and treat all the combined wastewater
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