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 MachinesTM

Requirements for Biological Degradation of CBOD and NH3

• 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 > 10oC• 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 MachineTM 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