Constructed Wetlands

A vertical flow bed of a constructed wetland
A vertical flow bed of a constructed wetland (Source: Heike Hoffman, Peru | SuSanA Secretariat | Flickr | CC BY 2.0)

Constructed wetlands (see “Constructed Wetlands,” EPA and factsheet from the Center for Clean Water Technology) are engineered systems for wastewater treatment, reuse or disposal that use natural biological treatment technologies that incorporate wetland vegetation, soils and associated microorganisms to remove contaminants. They are shallow water bodies or gravel (or engineered media) filled basins vegetated with plants adapted to continuous or periodic inundation. They range in size from small to large and provide natural treatment by settling solids, degrading organic wastes, and assimilating nitrogen and phosphorus through natural biological transformations. Water quality is improved through system sizing, uniform depth, and flow distribution to maximize contact between water and the wetland sediments and biological communities.

They are typically used as a secondary treatment for blackwater, greywater, and stormwater after primary pretreatment. They are relatively simple to implement on small scales, and can be modified to increase the degree of wastewater treatment, including providing tertiary treatment through enhanced nitrogen and phosphorus removal. Constructed wetlands function as wastewater polishing for cluster systems, satellite systems, or centralized systems, and as such, can serve populations of fewer than 50 all the way up to city-sized populations. They can also be used to address overflow during high flow conditions; they also have the added benefit of being ecologically important and aesthetically beautiful.

Constructed wetlands are found worldwide and are designed to meet a variety of objectives (e.g., mangrove microcosms in Hong Kong) and tertiary treatment through an engineered wetland in New Delhi). In general, there are two basic types of constructed wetlands:

  • Subsurface Flow Systems, also called root-zone systems, rock-weed filters, and vegetated submerged bed systems, that are media-based using soil, sand, gravel and/or crushed rock. Essentially, wastewater flows through natural wetland filters. As designed and operated, these systems provide limited opportunity for benefits other than water quality improvement.
  • Free Water Surface Systems simulate natural wetlands in which water flows over the soil. In these systems, breakdown and decomposition of solids is facilitated by the algae and bacteria that live in the oxygenated water, while larger plants also growing in the system take up the abundance of nutrients that are produced during decomposition. These systems are often designed to maximize wetland habitat value and reuse opportunities, while providing water quality improvement (e.g., Green Cay Treatment Wetlands).

Constructed wetlands can be used to both create and restore wetlands. Many free water surface systems also function as wildlife refuges and/or parks. These systems provide an area for public education and recreation in the form of birding, hiking, camping, hunting, and more. (See the image below for some free water surface systems in the U.S.) It is an elegant eco-friendly solution that converts the problem of wastewater management into a natural benefit for the environment and its inhabitants (see EPA case studies). They are typically decentralized solutions suitable for both small communities and as upgrades to large treatment facilities.

Location of 17 free water surface system success stories. Map of the United States of America is outlined in purple. States Oregon, California, Nevada, Arizonia, Illinois, Michigan, South Carolina, Alabama, Mississippi and Florida is highlighted orange.
Location and characteristics of 17 free water surface system success stories (Source: EPA, 1993)
Characteristics of 17 free water surface system success stories. The state name and its characteristics listed in three columns. 

Hillsboro (Jackson Bottom Wetlands Preserve)
1) Natural Bottomland/15 acres constructed wetlands.2) oikushes/reuses secondary effluent 
3) wildlife enhancement, research, water quality improvement, public recreation and education 

Cannon Beach
1.Natural alder/spruce/sedge wetlands (15 acres)
2.Polishes pond effluent (.68 mgd)
3.June thru Oct operation since 1984

Arcata Marsh and Wildlife Sanctuary 
1.Polishes/reuses secondary effluent 2.3 mgd
2.7.5 acres treatment wetland; 31 acres refuge; plus pond, tidal sloughs and estuary hbitat
3.Managed as wildlife sanctuary for wildlife use, research and extensive public use
4.Ford foundation award for innovation in 1987

Hayward March (union sanitary Distric)
1.Constructed wetland for habitat creation
2.restoration of historical wetlands area
3.Secondary effluent and stormwater reuse
4.172 Acres of fresh & brackish marshes part of a 400 arce marsh restoration effort

Marin Co. (las gallinas valley Sanitary Distric)
1.Constructed wetlands for habitat enhancement
2.polishes/reuses secondary effluent (2.9 mgd)
3.20 acres wildlife marsh; 40 acres ponds; 200 acres pasture (summer irrigation)
4.operational since 1984; no summer discharge

Martinez (Mt. View Sanitary District)
1.85 acres constructed wetlands created for habitat value
2. Restoration of historical wetland area
3. polishes/reuses secondary effluent (1.3 mgd)
4. Staged wetlands construction since 1974

Incline village
1.constructed (total evaporative) wetlands
2. polishing/disposal of secondary effluent (3.0 mgd from late tahoe basin)
3. 390 acres of non-discharging wetlands; 770 acre project site also includes some existing warm water wetlands and 200 acres if uplands
4. Operational since 1984

Show Low, AZ (pintail lake / Redhead Marsh)
1.Effluent (1.42 mgd) currently supports 201 acres of ponds and constructed marshes (total evaporative wetlands)

Pinetop/lakeside, AZ (Jacques Marsh)
1.Effluent (2mdg) currently supports 127 acres of ponds and constructed marshes (total evaporative wetlands)
a.Polishing.disposal of secondary effluent
B. habitat creation on national forest lands
C. initiated in 1970; expanded in 1977, 1978, 1980 and 1985
D. managed as wildlife habitat and for public use

Illinois Des Plaines River
1.Constructed wetlands with 450 acres riparian land
2. Demo of improving river water quality
3. Incorporates 2.8 miles of river drainage
4. Drainage area 80% agricultural, 20% urban
5. Private and government sponsored demo
6. ESA special recognition award 1993

Michigan Houghton Lake
1.Natural peatland wetlands (1,500 acres)
2. Polishes pond effluent (2.6 mgd summer only)
3. 16 years of May-Sept operation
4. ASCE Award of engineering excellence 1977

1.Polishes pond effluent (0.1 mgd)
2. 11.5 Acres wetlands self established
3. Continuous operation for 19+ years

South Carolina
Grand Strand, SC (Carolina Bays)
1.Natural pocosin wetlands (702 acres); mostly previously distributed
2. Polishes/reuses secondary effluent (2.5 mgd)
3. Wetlands managed as Nature park
4. Critical refuge for rare plants and animals
ACEC Grand Conceptor Award 1991

For Deposit
1.Constructed treatment wetlands (15 acres)
2. Polishes pond effluent; 1.6 mgd with additional rainwater input (2.6 mgd total)

Orlando Easterly Wetlands Reclamation Project
1.1,220 Acres of constructed wetlands habitat
2. Restoration of historical wetlands area
3. Polishes/reuses 20 mgd AWT effluent
4. Operational since 1987
5. ASCE Award of engineering excellence 1988

1.Polishes/reuses secondary effluent (14 mgd)
2. Mixed with power plant blow down water
3. Restoration of abandoned phosphate mines
4. 1,400 acres constructed wetlands habitat
5. Operational since 1987