Wetlands (and aquatic systems)

 

Wetlands and Aquatic systems are one of the most low maintenece wastewater treatment options. The structure and sucess of wetland systems can vary greatly, with the environment in which they are placed playing a large role in their success. Site characteristics that must be considered in a wetland sytem design include topograpghy, soil characteritics, existing land use, flood hazard, and climate. In addition to the benefit of waste treatment, with the large loss of natural wetlands throughout the world, constructed can enhance an ecosystem by providing additional wildlife habitat (see EPA) and a more constant water supply.

Aquatic Systems are used in the same way as wetlands. The main difference between the two systems is the type of vegetation used and the physical requirements from the plants. Design parameters include the hydraulic detention time, water depth, pond configuration, organic/hydraulic loading rates, and the process kinetics.

Types

Free-water-surface (FWS) wetlands - These wetlands look like a marsh or swamp. They are typically flooded with water from 4 to 18 inches in depth. FWS constructed wetlands usually contain cattails, reeds, sedges, and rushes. The retention time of these systems can vary, with some designs only allowing the water to seep into the ground or evaporate. Most FWS's have a liner to prevent seepage.A slope around 5% is desirable. The soil should have a permeability from 0.2 to 2.0 in/hr and the groundwater should be more than 5 feet below the surface (Metcalf & Eddy). Various chemical processes and bacterial activity treat the waste as it flows through the vegitation. See links for more info.

Subsurface-flow systems (SFS) - These wetlands are constructed so that wastewater flows through a porous gravel or sand media. This mediashould be from 1.5 to 3.3 feet deep, with a desirable slope of less than 1% (Crites). The emergent vegitation supplies oxygen and allows bioogical growth to accumulate on its roots. This vegitation is mostly bulrush, reeds, and sometimes cattails. See links for more info.

Floating aquatic plant systems - These are typically hyacinth and duckweed (see link) systems. Hyaciths have long roots that support biological growth that are responsible for organic matter stabilization and nutrient removal. Duckweed have short roots and float on the water surface. As it does in a lagoon systems, the duckweed blocks light and prevents the growth of algae and removes nutrients. Both these plants aren't freeze tolerant, although duckweed can thrive at 7 degrees celcius.

Combinations of wetlands and aquatic systems in series can benefit from one another to produce a higher quality effluent.

Links

HYDRA . - Good pictures of surface and subsurface constructed wetlands

Constructed Wetlands implemented at the Triangle School Wastewater Treatment System - This has nice diagrams along with the Alternative Wastewater treatment Overview

EPA Office of Water - Constructed Wetlands for Wastewater Treatment and Wildlife Habitat. 17 case studies. Great descriptions and Photos

EPA Subsurface Flow Constructed Wetlands For Wastewater treatment: A Technology Assessment

URS Corporation - Constructed Wetlands Offer Flexibility - nice photo

US EPA Technology Status Report - Constructed Wetlands: Passive Systems for Wastewater Treatment

University of Florida (Go Gators!)- Wastewater Treatment Wetlands: Applications and Treatment Efficiency 1

Dr. Wastewater's Duckweed Application Page. Detailed explanation of how duckweed is incorporated into lagoons. See Lagoon section also.

Reference Texts

Crites, R., Tchobanoglous, George, Small and Decentralized Wastewater Management Systems, McGraw-Hill, 1998

Metcalf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal, and Reuse, McGraw-Hill, 1991

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