8.3 Constructed Wetlands & Natural Treatment Systems
Key Takeaways
- Constructed wetlands and their appurtenances are classified as Grade I biological water pollution control systems under 15A NCAC 08G .0302(a).
- Free water surface wetlands expose open water and are effective for polishing, while subsurface flow wetlands keep water below a gravel bed surface, avoiding mosquito habitat and public contact.
- Treatment occurs through sedimentation, filtration, microbial attached growth on media and root surfaces, plant uptake, and adsorption — not primarily through the plants themselves.
- Wetlands are polishing systems: they require preliminary and primary or secondary treatment upstream and perform poorly when loaded with raw solids that clog the bed.
- Operational control is mostly hydraulic — managing water level, flow distribution, short-circuiting, vegetation health, and mosquito and nuisance control — with routine inspection of berms, inlets, and outlet structures.
8.3 Constructed Wetlands & Natural Treatment Systems
15A NCAC 08G .0302(a)(3) lists "constructed wetlands and associated appurtenances" as Grade I biological systems, and the Biological Grade 1 course dedicates an hour to the topic.
1. Types
| Type | Water surface | Typical use | Notes |
|---|---|---|---|
| Free water surface (FWS) | Open water above a soil bed | Polishing secondary effluent; stormwater; wildlife habitat | Public access and mosquito issues; strong TSS and nutrient polishing |
| Subsurface flow — horizontal (HSSF) | Below the surface of a gravel bed | Small domestic systems, schools, parks | No exposed water; less odor and mosquito risk; clogging is the main failure |
| Subsurface flow — vertical (VF) | Dosed onto the surface, percolates down | Higher oxygen transfer; nitrification | Requires dosing pumps or siphons |
| Floating treatment wetlands | Plants on floating mats over open water | Retrofits into ponds and lagoons | Roots provide attached growth surface |
2. How treatment actually happens
The plants are the visible part, but most of the work is done by physical and microbial processes:
- Sedimentation and filtration of suspended solids in the slow, shallow flow and within the gravel matrix.
- Attached microbial growth on gravel, litter, and root surfaces — the same biofilm chemistry as a trickling filter, oxidizing BOD and, where oxygen is available, nitrifying.
- Denitrification in anoxic zones deeper in the bed, using the carbon available in the wastewater and plant litter.
- Plant uptake of nitrogen and phosphorus — real but modest, and only permanently removed if the biomass is harvested.
- Adsorption and precipitation of phosphorus and metals onto media and soils, a finite capacity that eventually saturates.
Typical vegetation: cattail (Typha), bulrush (Schoenoplectus), soft rush (Juncus), and common reed. Plants stabilize the bed, transfer a small amount of oxygen to the root zone, provide surface area, and shade the bed against algae.
3. Design and loading
| Parameter | Typical range |
|---|---|
| Water depth (FWS) | 0.5–1.5 ft |
| Bed depth (HSSF) | 1.5–2.5 ft of gravel, water level kept 2–4 in below the surface |
| Media (HSSF) | Washed gravel, commonly 0.75–1.5 in |
| Hydraulic retention time | 2–10 days |
| Length-to-width ratio | Commonly 2:1 to 4:1 to limit short-circuiting |
| Pretreatment | Required — septic tank, primary clarifier, or secondary treatment |
Cold weather slows microbial kinetics and can reduce nitrification substantially; North Carolina's climate is favorable, but winter performance should be expected to fall.
4. Operating a wetland
- Manage the water level. In an HSSF wetland the level is kept just below the gravel surface using an adjustable outlet — high enough to keep the bed saturated, low enough to prevent surfacing. Surfacing water in an HSSF bed is the first sign of clogging.
- Distribute the flow. Inspect and clean inlet manifolds and distribution piping; uneven distribution creates short-circuiting and dead zones. Dye testing confirms actual residence time.
- Watch for clogging. Caused by excessive solids from inadequate pretreatment, biomass accumulation, and root growth. Remedies range from resting the bed and adjusting flow to replacing the inlet-zone media — which is why solids removal upstream is non-negotiable.
- Vegetation management. Maintain healthy stands, replant bare areas, control invasive species, and harvest where nutrient removal is credited. Do not let woody vegetation establish on berms.
- Mosquito and nuisance control. FWS wetlands need mosquito management — water-level manipulation, mosquito fish, or larvicide. Subsurface flow designs avoid the problem entirely, which is why they dominate at schools and parks.
- Structures. Inspect berms and liners for erosion and burrowing animals, keep outlet structures free of debris, and confirm that flow measurement devices remain accurate.
5. What wetlands do well — and what they do not
Do well: BOD and TSS polishing, pathogen reduction with adequate retention, nitrogen removal where aerobic and anoxic zones both exist, buffering of flow variations, and very low operating cost and energy use.
Do not do well: handle raw wastewater solids; produce consistently low ammonia in cold weather without a vertical flow or aerated design; remove phosphorus indefinitely (adsorption sites saturate); or tolerate hydraulic overloading, which short-circuits the bed and can wash out accumulated solids.
[!NOTE] Reclaimed water and reuse. Where a wetland or other natural system produces effluent destined for reuse, North Carolina's reclaimed water rules in 15A NCAC 02U govern the treatment reliability, monitoring, storage, and permitted uses. A constructed wetland is not by itself a reclaimed water treatment system.
Water is surfacing across the middle of a horizontal subsurface flow constructed wetland. What does this most likely indicate?
Which statement best describes the role of plants in a constructed wetland?
Why must a constructed wetland be preceded by preliminary and primary or secondary treatment?