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Aquatic-Based Treatment Systems

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

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Aquatic-Based Treatment Systems

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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 General Design Considerations

  6. 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.

  7. 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

  8. 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

  9. 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

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

  11. 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

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