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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 - PowerPoint PPT Presentation
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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