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Ewalts Constructive Wetlands

A constructed wetland (CW) is an artificial wetland to treat municipal or industrial wastewater, greywater or stormwater runoff. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development.

Constructed wetlands are engineered systems that use natural functions vegetation, soil, and organisms to treat wastewater. Depending on the type of wastewater the design of the constructed wetland has to be adjusted accordingly. Constructed wetlands have been used to treat both centralized and on-site wastewater. Primary treatment is recommended when there is a large amount of suspended solids or soluble organic matter (measured as BOD and COD).

There are two main types of constructed wetlands: subsurface flow and surface flow constructed wetlands. The planted vegetation plays an important role in contaminant removal. The filter bed, consisting usually of sand and gravel, has an equally important role to play. Some constructed wetlands may also serve as a habitat for native and migratory wildlife, although that is not their main purpose. Subsurface flow constructed wetlands are designed to have either horizontal flow or vertical flow of water through the gravel and sand bed. Vertical flow systems have a smaller space requirement than horizontal flow systems.

The following six major biological process takes place simultaneously in CWs-

  • Photosynthesis
  • Respiration/oxidation of organic carbon
  • Fermentation
  • Nitrification
  • Denitrification
  • Microbial phosphorus removal

The six main pollutant removal process taking place in CWs-

  • Adsorption
  • Sedimentation
  • Filtration
  • Microbial uptake (sediments & biofilm)
  • Plant uptake
  • UV degradation

Types of Constructive wetlands

At the current stage of technology development, three types of wetlands are in widespread use:

  • Free water surface (FWS) wetlands have areas of open water and are similar in appearance to natural marshes.
  • Horizontal subsurface flow (HSSF) wetlands, which typically employ a gravel bed planted with wetland vegetation. The water, kept below the surface of the bed, “flows horizontally from the inlet to the outlet.
  • Vertical flow (VF) wetlands distribute water across the surface of a sand or gravel bed planted with wetland vegetation. The water is treated as it percolates through the plant root zone. Biosolids dewatering wetlands can be thought of as a type of VF wetland system.

The larger aquatic plants growing in wetlands are usually called macrophytes. The term includes aquatic vascular plants (angiosperms and ferns), aquatic mosses, and some larger algae that have tissues that are easily visible. Although ferns like Salvinia and Azolla and large algae like Cladophora are widespread in wetlands, it is usually the flowering plants (i.e. angiosperms) that dominate. Macrophytes, like all other photoautotrophic organisms, use the solar energy to assimilate inorganic carbon from the atmosphere to produce organic matter, which subsequently provides the energy source for heterotrophs (animals, bacteria and fungi). As a result of the ample light, water and nutrient supply in wetlands, the primary productivity of ecosystems dominated by wetland plants are among the highest recorded in the world. Associated with this high productivity is usually a high heterotrophic activity, i.e. a high capacity to decompose and transform organic matters and other substances.

Constructed wetlands Advantages:

  • Alternative to conventional systems:
  • Lower costs (capital and significantly less O&M costs)
  • Reasonably simple installation
  • Low maintenance
  • Reduced need for electrical power (can be nil)
  • Good removal rates for many pollutants
  • Favorable aesthetics, acceptance by the general public and can provide ecological habitat.

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