Constructed Wetland for Onsite and Decentralized Wastewater Treatment. CEE484 April 30, 2007

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1 Constructed Wetland for Onsite and Decentralized Wastewater Treatment CEE484 April 30, 2007

2 Types of Constructed Wetlands Free Water Surface (FWS) Submerged flow (SF), also called Vegetated Submerged Bed (VSB) Vertical Flow (VF) Estimated # installations in U.S. ~600

3 Free Water Surface Wetland FWS wetlands have exposed water bodies similar to natural marshes. Water depth= 0.50 to 2.0 ft May have open zone followed by Vegetated zone Impermeable liner Cattails, reeds, sedges or rushes Follows pretreatment primary treatment, lagoons, or secondary treatment

4 Free Water Surface Wetland FWS Wetland operating near Pensacola, Florida Photo courtesy S. Wallace (EPA onsite curriculum)

5 Vertical Flow Wetlands VF wetlands have much higher oxygen transfer rates, allowing for nitrification. 2 design types: Recirculating (more common in US) Single-pass (more common in Europe) Recirculating VF wetland schematic courtesy Reactor Dynamics Inc.

6 SF or VSB Wetland VSB wetlands employ a gravel bed planted with wetland vegetation. The water is kept below the surface of the gravel.

7 Performance affected by temperature insulation needed in very cold climates

8 horinated effluent Would not require fence Why are kidney shapes o common?

9 Design features of VSB wetland Gravel bed 0.70 to 1.2 inch gravel (d 10 ) Inlet/outlet zones stone- 1.6 to 3.0 inch Bed depth ~1.5 to 2.0 ft Depth to water surface ~1-3 3 inches Plants bulrushes, reeds Bed liner native clay, bentonite,, synthetic liners Slope less than 1% Length to width ~3:1 Approximate detention time 2-33 days Partial nitrification at days Cold temperature insulate sides Inlet pipes- space for distribution Outlet perforated pipe at bottom of bed Connected to water depth control method, i.e.effl.. Pipe elevation)

10 Vegetated Submerged Bed VSB Wetland operating near Lindstrom, Minnesota Photo courtesy North American Wetland Engineering

11 Septic tank and Wetland system For school in Jamaica

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15 Why is the VSB wetland preferred for onsite systems versus a FWS wetland?

16 Why is the VSB wetland preferred for onsite systems? Little chance of human contact with wastewater Mosquito control.

17 Role of Plants in VSB Wetlands Oxygen transfer from plants is minimal (about 0.02 g/m 2 d) Plant roots generally do not penetrate to bottom of gravel bed Plant roots support symbiotic bacteria and fungi, resulting in a more diverse microbial environment Net effect of plants on treatment is minimal Biological anaerobic degradation and aerobic degradation at top layer of media Nitrification is minimal due to lack of oxygen unless most of BOD is removed thus longer detention time systems can nitrify

18 Suspended Solids Removal VSB s are extremely effective in trapping and removing TSS Accumulation of sediments affects hydraulic conductivity of bed media: USEPA, 2000

19 Nitrogen Cycling in VSB Wetlands Insufficient oxygen transfer for nitrification Reducing conditions suitable for denitrification Significant nitrogen reduction only occurs when the influent nitrogen has already been converted to nitrate Plant Harvesting Removes less than 10% of applied nitrogen Not a cost-effective nutrient management option

20 Phosphorus Removal in VSB Wetlands Adsorption onto media is only a short-term term removal mechanism with standard medias Expanded shale and clay aggregates with very high phosphorus sorption capacities have been used to increase phosphorus retention in VSBs Sacrificial bed media Plant Harvesting Removes less than 10% of applied phosphorus Not cost-effective

21 Sulfur Cycling in VSBs VSB s will reduce sulfate to sulfide Sulfide can be an odor source (H 2 S) Influent sulfides represent an additional oxygen demand Some VSB s designed to remove heavy metals through sulfide precipitation

22 Typical treatment performance in constructed wetlands Parameter BOD, mg/l TSS, mg/l Fecal coliform removal NH4-N, N, mg/l TN, mg/l TP, % removal FWS <20 < to 99.9% <5.0* < <20 VSB <20 < to 99.9% >10* >10** <20 - Affected by detention time, temperature: VSB has little nitrification at lower detention time. **poor N removal unless nitrified flow is fed to VSB

23 So how can we get nitrogen removal with a VSB wetland?

24 Nitrogen removal scheme with VSB wetland

25 Cost of Nitrogen Removal System in City of Austin, TX Fact Sheet Estimated costs for trickling filter, installed, and including septic tank for pretreatment, pump, control and alarm, valves, and piping, Wetland unit, installed Equipment repair/replacement costs, estimated at $80/year (estimated pump, fan and controls repairs/replacement), O&M, with a maintenance contract of 10 $15/hour * 2.0, including taxes, overhead, profit, and including pump/controls and servicing, Septage and sludge pumping estimated at once every year, Energy costs (using 4 KWH/day energy use), 20-year NPW (not incl. design & permitting costs), $9,000 $6,000 $6.67/month $25/month $14.58/month $9.73/month $21,692.65

26 Another treatment scheme for N removal with a VSB wetland Septic Tank Intermittent Sand Filter Constructed VSB Wetland

27 Summary Constructed VSB wetland is an alternative for onsite or decentralized treatment More aesthetic Poor site for normal drainage field Consider climate Potential for odors Design approach is empirical based on prior observed performance Combined systems needed for significant N removal Process is complex and performance affected by temperature and plant growth and maintenance

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29 Wetland Vegetation The plant species must be matched to the hydrology of the wetland. The plant material (seed, tuber, rhizome, pot, etc.) must be viable at the time of planting. Water level management during the startup phase must be compatible with the needs of the newly-establishing establishing plants.

30 Wetland Vegetation Plant species are grouped by hydrology: Indicator Category Obligate Wetland Facultative Wetland Facultative Facultative Upland Obligate Upland Symbol OBL FACW FAC FACU UPL Description Plants that occur almost always (>99%) in wetlands under natural conditions Plants that occur usually (67-99%) in wetlands but can also occur (1-33%) in upland areas Plants with a similar likelihood (33-67%) of occurring either in wetlands and nonwetlands (uplands) Plants that occur sometimes (1-33%) in wetlands but occur more often (67-99%) in uplands Plants that occur almost always (>99%) in uplands

31 Wetland Vegetation Plants need access to sunlight and air to survive. The water level must be gradually raised during the plant establishment phase Photo courtesy North American Wetland Engineering

32 Commonly Used Wetland Plant Species Scientific Name Common Name Status Region Carex nebrascensis Nebraska sedge OBL Southwest Carex stricta Uptight sedge OBL Southwest Iris missouriensis Rock Iris Mountain FACW, OBL West Iris pseduocorus Yellow Iris OBL Midwest, Northeast Iris versicolor Blueflag Iris OBL Midwest, Northeast Juncus balticus Baltic Rush FACW, OBL Southwest

33 Commonly Used Wetland Plant Species (continued) Scientific Name Common Name Status Region Scirpus acutus Hardstem Bulrush OBL across US Scirpus atrovirens Green Bulrush OBL Midwest, East Scirpus californicus Bulrush (Restorer) OBL West Scirpus fluviatilis River Bulrush OBL Midwest, East Scirpus validus Softstem Bulrush OBL across US Typha latifolia Cattail, Broadleaf OBL across US Typha angustfolia Cattail, Narrowleaf OBL northern US