How to Rewild

Treatment Wetland

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Treatment Wetland
Management Guide

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Treatment wetlands are shallow pools with marginal plants – inflowing pollutants and nutrients are trapped by the vegetation and treated by bacteria or sink into the sediment. This improves the water quality downstream.

The Problem of Water Pollution

Water pollution isn’t just limited to the typical chemicals and oil that most people picture when they think about pollutants. There are a huge range of compounds which can damage sensitive aquatic habitats, and also impact terrestrial ecosystems when the water washes out across the land during a flood or storm. These include:

  • Pesticides and herbicides
  • Pharmaceutical compounds
  • Sediment (mud, silt, sand, gravel)
  • Microplastics
  • Nutrients (fertilisers)
  • Pathogens (viruses, bacteria)
 
Many of the above pollutants are released into our rivers when untreated sewage flows into the system. This happens frequently during legal ‘sewage overspill’ events, when stormwater floods into the sewer system, and water companies, lacking the capacity to treat this excess, release a mix of sewage effluent and rain into streams and rivers.


This isn’t the only way that our water becomes contaminated, though, as microplastics and fertilisers also wash off fields and urban areas into ditches and streams. The microplastics may be used to encapsulate slow-release fertiliser pellets, or come from artificial grass, tyres etc.

Wetland formation
Planted treatment wetlands can be a mix of marginal plants (as shown here), although Common Reed tends to be used in most situations

Situating the Treatment Wetland

Treatment wetlands are a nature-based solution which directly addresses the issue of multiple pollutants. 

These artificial habitats are effectively a miniature delta, planted out with Common Reed (Phragmites australis) – a native species of plant. The constructed wetland may be placed in a collection basin, like a swale. Or it can intercept an existing watercourse, like a drainage ditch; it may also be planted along the edge of a watercourse/pool at the base of a slope, where it is catches sediment and pollutant runoff.

As water flows into the shallow system, it slows down, and both sediment and pollutants drop out of suspension, becoming trapped between the roots of the reeds. Here, the actions of microorganisms and the plants themselves will treat some pollutants, or these contaminants may be sequestered in the sediment over time.

Long Term Maintenance

While a treatment wetland is effective at reducing nutrient and pollutant inputs, it does require ‘cleaning’. Once the sediment reaches a certain depth, it begins to slow the flow of water, so it has to be mechanically excavated and treated or disposed offsite. The sediment will contain unnaturally-high concentrations of pollutants, typically making it unsuitable for use as fertiliser.

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