Constructed wetland

Terminology

Many terms are used to denote constructed wetlands, such as reed beds, soil infiltration beds, constructed treatment wetlands, treatment wetlands, etc. Beside "engineered" wetlands, the terms of "man-made" or "artificial" wetlands are often found as well. A biofilter has some similarities with a constructed wetland, but is usually without plants.

However, the term of constructed wetlands can also be used to describe restored and recultivated land that was destroyed in the past through draining and converting into farmland, or mining.

Ponds for wastewater treatment or water purification are not considered as constructed wetlands. They are referred to as stabilization ponds or treatment ponds, respectively.

Overview

A constructed wetland is an engineered sequence of water bodies designed to filter and treat waterborne pollutants found in sewage, industrial effluent or storm water runoff. Constructed wetlands are used for wastewater treatment or for greywater treatment, and can be incorporated into an ecological sanitation approach. They can be used after a septic tank for primary treatment, in order to separate the solids from the liquid effluent. Some CW designs however do not use upfront primary treatment.

Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as they break down organic materials. This community of microorganisms is known as the periphyton. The periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about seven to ten percent of pollutants, and act as a carbon source for the microbes when they decay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment. Constructed wetlands are of two basic types: subsurface flow and surface flow wetlands.

Many regulatory agencies list treatment wetlands as one of their recommended "best management practices" for controlling urban runoff.

Types

The main three constructed wetlands types are:

The former types are placed in a basin with a substrate to provide a surface area upon which large amounts of waste degrading biofilms form, while the latter relies on a flooded treatment basin upon which aquatic plants are held in flotation till they develop a thick mat of roots and rhizomes upon which biofilms form. In most cases, the bottom is lined with either a polymer geomembrane, concrete or clay (when there is appropriate clay type) in order to protect the water table and surrounding grounds. The substrate can be either gravel—generally limestone or pumice/volcanic rock, depending on local availability, sand or a mixture of various sizes of media (for vertical flow constructed wetlands).

Types

Subsurface flow wetlands can be further classified as horizontal flow and vertical flow constructed wetlands. In the vertical flow constructed wetland, the effluent moves vertically from the planted layer down through the substrate and out (requiring air pumps to aerate the bed). In the horizontal flow CW the effluent moves horizontally via gravity, parallel to the surface, with no surface water thus avoiding mosquito breeding. Vertical flow CWs are considered to be more efficient with less area required compared to horizontal flow CWs. However, they need to be interval-loaded and their design requires more know-how while horizontal flow CWs can receive wastewater continuously and are easier to build.

The French System combines primary and secondary treatment of raw wastewater. The effluent passes various filter beds whose grain size is getting smaller (from gravel to sand).

Applications

Subsurface flow wetlands can treat a variety of different wastewaters, such as household wastewater, agricultural, paper mill wastewater, mining runoff, tannery or meat processing wastes, storm water.

The quality of the effluent is determined by the design and should be customized for the intended reuse application (like irrigation or toilet flushing) or the disposal method.

Design considerations

The wastewater passes through a sand medium on which plants are rooted. A gravel medium (generally limestone or volcanic rock lavastone) can be used as well and is mainly deployed in horizontal flow systems though it does not work as efficiently as sand.

Constructed subsurface flow wetlands are meant as secondary treatment systems which means that the effluent needs to first pass a primary treatment which effectively removes solids. Such a primary treatment can consist of sand and grit removal, grease trap, compost filter, septic tank, Imhoff tank, anaerobic baffled reactor or upflow anaerobic sludge blanket (UASB) reactor. The following treatment is based on different biological and physical processes like filtration, adsorption or nitrification. Most important is the biological filtration through a biofilm of aerobic or facultative bacteria. Coarse sand in the filter bed provides a surfaces for microbial growth and supports the adsorption and filtration processes. For those microorganisms the oxygen supply needs to be sufficient.

Especially in warm and dry climates the effects of evapotranspiration and precipitation are significant. In cases of water loss, a vertical flow CW is preferable to a horizontal because of an unsaturated upper layer and a shorter retention time.

The effluent can have a yellowish or brownish colour if domestic wastewater or blackwater is treated. Treated greywater usually does not tend to have a colour. Concerning pathogen levels, treated greywater meets the standards of pathogen levels for safe discharge to surface water. Treated domestic wastewater might need a tertiary treatment, depending on the intended reuse application.

Plantings of reedbeds are popular in European constructed subsurface flow wetlands. Other plants are cattails (Typha spp.) and sedges.

Operation and maintenance

Overloading peaks should not cause performance problems while continuous overloading lead to a loss of treatment capacity through too much suspended solids, sludge or fats.

Subsurface flow wetlands require the following maintenance tasks: regular checking of the pretreatment process, of pumps when they are used, of influent loads and distribution on the filter bed.

Comparisons with other types

Subsurface wetlands are less hospitable to mosquitoes compared to surface flow wetlands, as there is no water exposed to the surface. Mosquitos can be a problem in surface flow constructed wetlands. Subsurface flow systems have the advantage of requiring less land area for water treatment than surface flow. However, surface flow wetlands can be more suitable for wildlife habitat.

For urban applications the area requirement of a subsurface flow CW might be a limiting factor compared to conventional municipal wastewater treatment plants. High rate aerobic treatment processes like activated sludge plants, trickling filters, rotating discs, submerged aerated filters or membrane bioreactor plants require less space. The advantage of subsurface flow CWs compared to those technologies is their operational robustness which is particularly important in developing countries. The fact that CWs do not produce secondary sludge (sewage sludge) is another advantage as there is no need for sewage sludge treatment. However, primary sludge from primary settling tanks does get produced and needs to be removed and treated.

Costs

The costs of subsurface flow CWs mainly depend on the costs of sand with which the bed has to be filled. Another factor is the cost of land.

Surface flow wetland

Surface flow wetlands, also known as free water surface constructed wetlands, can be used for tertiary treatment or polishing of effluent from wastewater treatment plants. They are also suitable to treat stormwater drainage.

Pathogens are destroyed by natural decay, predation from higher organisms, sedimentation and UV irradiation since the water is exposed to direct sunlight. The soil layer below the water is anaerobic but the roots of the plants release oxygen around them, this allows complex biological and chemical reactions.

Surface flow wetlands can be supported by a wide variety of soil types including bay mud and other silty clays.

Plants such as Water Hyacinth (Eichhornia crassipes) and Pontederia spp. are used worldwide (although Typha and Phragmites are highly invasive).

However, surface flow constructed wetlands may encourage mosquito breeding. They may also have high algae production that lowers the effluent quality and due to open water surface mosquitos and odours, it is more difficult to integrate them in an urban neighbourhood.

Hybrid systems

A combination of different types of constructed wetlands is possible to use the specific advantages of each system.

Overview

Physical, chemical, and biological processes combine in wetlands to remove contaminants from wastewater. An understanding of these processes is fundamental not only to designing wetland systems but to understanding the fate of chemicals once they enter the wetland. Theoretically, wastewater treatment within a constructed wetland occurs as it passes through the wetland medium and the plant rhizosphere. A thin film around each root hair is aerobic due to the leakage of oxygen from the rhizomes, roots, and rootlets. Aerobic and anaerobic micro-organisms facilitate decomposition of organic matter. Microbial nitrification and subsequent denitrification releases nitrogen as gas to the atmosphere. Phosphorus is coprecipitated with iron, aluminium, and calcium compounds located in the root-bed medium. Suspended solids filter out as they settle in the water column in surface flow wetlands or are physically filtered out by the medium within subsurface flow wetlands. Harmful bacteria and viruses are reduced by filtration and adsorption by biofilms on the gravel or sand media in subsurface flow and vertical flow systems.

Nitrogen removal

The dominant forms of nitrogen in wetlands that are of importance to wastewater treatment include organic nitrogen, ammonia, ammonium, nitrate and nitrite. Total nitrogen refers to all nitrogen species. Wastewater nitrogen removal is important because of ammonia’s toxicity to fish if discharged into watercourses. Excessive nitrates in drinking water is thought to cause methemoglobinemia in infants, which decreases the blood's oxygen transport ability.

Ammonia removal occurs in constructed wetlands - if they are designed to achieve biological nutrient removal - in a similar ways as in sewage treatment plants, except that no external, energy-intensive addition of air (oxygen) is needed. It is a two-step process, consisting of nitrification followed by denitrification. The nitrogen cycle is completed as follows: ammonia in the wastewater is converted to ammonium ions; the aerobic bacterium Nitrosomonas sp. oxidizes ammonium to nitrite; the bacterium Nitrobacter sp. then converts nitrite to nitrate. Under anaerobic conditions, nitrate is reduced to relatively harmless nitrogen gas that enters the atmosphere.

Nitrification

Nitrification is the biological conversion of organic and inorganic nitrogenous compounds from a reduced state to a more oxidized state, based on the action of two different bacteria types. Nitrification is strictly an aerobic process in which the end product is nitrate (NO−
3). The process of nitrification oxidizes ammonium (from the wastewater) to nitrite (NO−
2), and then nitrite is oxidized to nitrate (NO−
3).

Denitrification

Denitrification is the biochemical reduction of oxidized nitrogen anions, nitrate and nitrite to produce the gaseous products nitric oxide (NO), nitrous oxide (N
2O) and nitrogen gas (N
2), with concomitant oxidation of organic matter. The end products, N
2O and N
2 are gases that re-enter the atmosphere.

Ammonia removal from mine water

Constructed wetlands have been used to remove ammonia and other nitrogenous compounds from contaminated mine water, including cyanide and nitrate.

Phosphorus removal

Phosphorus occurs naturally in both organic and inorganic forms. The analytical measure of biologically available orthophosphates is referred to as soluble reactive phosphorus (SR-P). Dissolved organic phosphorus and insoluble forms of organic and inorganic phosphorus are generally not biologically available until transformed into soluble inorganic forms.

In freshwater aquatic ecosystems phosphorus is typically the major limiting nutrient. Under undisturbed natural conditions, phosphorus is in short supply. The natural scarcity of phosphorus is demonstrated by the explosive growth of algae in water receiving heavy discharges of phosphorus-rich wastes. Because phosphorus does not have an atmospheric component, unlike nitrogen, the phosphorus cycle can be characterized as closed. The removal and storage of phosphorus from wastewater can only occur within the constructed wetland itself. Phosphorus may be sequestered within a wetland system by:

1. The binding of phosphorus in organic matter as a result of incorporation into living biomass,
2. Precipitation of insoluble phosphates with ferric iron, calcium, and aluminium found in wetland soils.

Biomass plants incorporation

Aquatic vegetation may play an important role in phosphorus removal and, if harvested, extend the life of a system by postponing phosphorus saturation of the sediments. Plants create a unique environment at the biofilm's attachment surface. Certain plants transport oxygen which is released at the biofilm/root interface, adding oxygen to the wetland system. Plants also increase soil or other root-bed medium hydraulic conductivity. As roots and rhizomes grow they are thought to disturb and loosen the medium, increasing its porosity, which may allow more effective fluid movement in the rhizosphere. When roots decay they leave behind ports and channels known as macropores which are effective in channeling water through the soil.

Metals removal

Constructed wetlands have been used extensively for the removal of dissolved metals and metalloids. Although these contaminants are prevalent in mine drainage, they are also found in stormwater, landfill leachate and other sources (e.g., leachate or FDG washwater at coal-fired power plants), for which treatment wetlands have been constructed for mines.

Mine water—Acid drainage removal

Constructed wetlands can also be used for treatment of acid mine drainage from coal mines.

Design characteristics

Plants

Typhas and Phragmites are the main species used in constructed wetland due to their effectiveness, even though they can be invasive outside their native range.

In North America, cattails (Typha latifolia) are common in constructed wetlands because of their widespread abundance, ability to grow at different water depths, ease of transport and transplantation, and broad tolerance of water composition (including pH, salinity, dissolved oxygen and contaminant concentrations). Elsewhere, Common Reed (Phragmites australis) are common (both in blackwater treatment but also in greywater treatment systems to purify wastewater).

Plants are usually indigenous in that location for ecological reasons and optimum workings.

Fish and bacteria

Locally grown bacteria and non-predatory fish can be added to surface flow constructed wetlands to eliminate or reduce pests, such as mosquitos. The bacteria are usually grown locally by submerging straw to support bacteria arriving from the surroundings.

Costs

Since constructed wetlands are self-sustaining their lifetime costs are significantly lower than those of conventional treatment systems. Often their capital costs are also lower compared to conventional treatment systems. They do take up significant space, and are therefore not preferred where real estate costs are high.

History

Subsurface flow CWs with sand filter bed have their origin in Europe and are now used all over the world. Subsurface flow CWs with a gravel bed are mainly found in North Africa, South Africa, Asia, Australia and New Zealand.

USA

The Arcata Marsh in Arcata, California is a sewage treatment and wildlife protection marsh.

Australia

The Urrbrae Wetland in Australia was constructed for urban flood control and environmental education.

At the Ranger Uranium Mine, in Australia, ammonia is removed in "enhanced" natural wetlands (rather than fully engineered constructed wetlands), along with manganese, uranium and other metals.