Aquatic Based Treatment Systems

Report
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
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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
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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

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