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 ( mgal/acred) Detention times (several days to 14 days typical) OLR (up to 60lb/acred) 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/acred) Organic loading rates (up to 60 lb/acred), 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 ( lb/acred) HLR ( mgal/acred) 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 Treatment Systems 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