Aquatic-Based Treatment Systems

• Biological Treatment after appropriate pre- and primary treatment • Plants used to provide substrate for bacterial growth, uptake of nutrients and some oxygen input • Disinfection required upon discharge • Examples include; Subsurface flow (SF) constructed wetlands, Free water surface (FWS) constructed wetlands, Floating Aquatic systems such as hyacinth and duckweed, Living Machines TM

Requirements for Biological Degradation of CBOD and NH

3

• Sufficient oxygen, nitrifiers require > 1.5 mg/L ( Oxygen transfer is often the limiting step in aquatic treatment systems) • Hospitable environment, nitrifiers need pH > 7 • Substrate for attachment • Nutrients

How to provide oxygen in the wastewater?

• Plants provide some oxygen to roots as a mechanism to reduce toxicity of certain compounds (i.e., ferrous iron, reduced manganese and sulfides), depending on the plant some oxygen can be available for bacteria • Oxygen diffuses from atmosphere to wastewater at the air-water interface • Supplemental oxygen can be provided through aeration either to wetland itself or to the WW

Subsurface Flow (SF) Constructed Wetlands

• Wastewater flows through gravel substrate • Typical depths about 2 feet • Plants grow in the gravel substrate • Biofilm grows on rocks and plant roots

SF Constructed Wetlands General Design Considerations

• HLR depends (0.015-0.05 mgal/acre•d) • Detention times (several days to 14 days typical) • OLR (up to 60lb/acre•d) varies with degradation rate constant • Aspect ratio (L:W) > 1:1 • Evapotranspiration rates vary • Plant selection • Nitrification often difficult to achieve due to oxygen limitations (need long Θ) • Depth (1-2.5 ft, 2 ft typical) • Phosphorus adsorption to rocks during first years

SF Constructed Wetlands Other Issues

• Optimizing plant selection for nutrient uptake and oxygen transfer (not well understood).

• Plant harvesting removes nutrients stored in plant bodies (during high growth stages, plants take up more nutrients, mature plants may shade younger plants). Plant aspects not well understood.

• Clogging, especially near inlet, can be a problem. Design with larger gravel at inlet. Regular maintenance required.

Free Water Surface (FWS) Constructed Wetlands

• Wastewater flows through shallow basins where aquatic vegetation grows • Typically much larger systems than SF for same application due to lower bacterial population • Can provide habitat for birds and animals, although mosquitoes can be a problem

FWS Constructed Wetlands General Design Considerations

• Detention times (7-15 days typical) • HLR (0.015 – 0.05 mgal/acre•d) • Organic loading rates (up to 60 lb/acre•d), varies with degradation rate constant • Aspect Ratios >1, but less than 4 • Plant types, cattails, bulrush, sedges • Evapotranspiration depends on climate • Nitrification limits • Phosphorus treatment

Floating Aquatic Treatment Systems Design Considerations

• OLR (150-300 lb/acre•d) • HLR (0.1-0.3mgal/acre•d) • Detention times > 6 d • Depth 3 ft • Aeration required • Warm temperature > 10 o C • Water hyacinths • Aspect ratio > 3 • Regular harvest schedule (2 times per month) • Mosquito control

Water Hyacinths can be a colossal nuisance as shown here at the ferry dock in Kisumu, Kenya.

Living Machine

TM

Systems Treatment

• Objective is to provide a more ecological approach to wastewater treatment, however, advantages not scientifically proven • Design parameters not determined • Still in early stages of development