Wetlands for water treatment

The ability of wetlands, and in particular reedbeds, to filter out solid waste and pollutants has been recognised for many years. With increasingly stringent controls on the quality of water discharged into rivers and the sea, water companies, developers and businesses have begun to invest in constructed wetland systems for treatment of waste. Depending on the type and volume of waste, these systems vary from simple reedbeds, to more complex systems with settling ponds and cascades.Aerators may be included to improve aeration within the pond. Although constructed wetlands tend to be dominated by common reed, a diverse wetland plant community is preferable for effective water treatment, and for wildlife value. On some sites, treated water can be reused within the site, thus lessening the water supply requirements.

There are at least 300 operational constructed wetlands for water treatment in the UK, ranging from small DIY- constructed domestic systems, to large scale industrial systems such as the 5 hectare system at Billingham, Cleveland, constructed by ICI to treat effluent from a chemical plant. Constructed wetlands and ponds can be used for the following:

1. Controlling pollution from diverse sources which feed into a pond or water course (see first example below).
2. Small-scale sewage and waste water treatment for isolated communities, holiday developments, visitor centres and so on. Where conventional sewage disposal is still used, simple systems can be designed to treat greywater (from sinks, baths, washing machines etc) so that the water can be used for wildlife ponds or garden irrigation.
3. Tertiary treatment at sewage works for populations up to about 2,000.
4. Treatment of industrial waste.
5. Treatment of farm waste.
6. Treatment of run-off from roads, runways and other hard-surfaced areas.
7. For balancing ponds, constructed on building developments to trap polluted and sediment-laden run-off produced during the building phase.

Most systems have proved efficient at removing suspended solids, and in greatly reducing BOD (Biological Oxygen Demand) in the discharged water. Reductions in ammonium and phosphate are more variable. Construction and maintenance costs are low, and the benefits for wildlife and landscape are high. However, the amount of land required may be a limiting factor on some sites.

A simple marginal reed bed to improve the quality of water entering a pond or watercourse can be constructed as shown below. A mixture of free surface flow and subsurface flow should be encouraged for maximum efficiency of the reedbed.

1. Dig a trench about a metre in from the edge of the pond or watercourse. The trench should be about 500mm (20″) wide, and 750-1000mm (2’6″-3′) deep to intercept subsurface flow.
2. Use the spoil to build outwards into the pond to create the marginal shelf. Extra spoil is likely to be required. Temporary boarding may be necessary to support the sides of the trench.
3. Fill the trench with coarse gravel.
4. Plant the marginal shelf with a variety of plants including common reed (Phragmites communis), common clubrush (Schoenoplectus lacustris) and reedmace (Typha angustifolia). Reed grass (Phalaris arundinacea) and reed sweet-grass (Glyceria maxima) are also suitable.

Nutrients can also be removed by allowing water to flow through a waterlogged zone 1-5m (3-16′) wide, planted with willows or poplars.

There are two basic types of constructed wetland or reedbed which can be used for treating domestic, agricultural or industrial effluents:

1. Horizontal flow. The effluent passes horizontally through the reedbed. Factors which can be altered include the gradient of the bed, layering of soil or gravel within it, and the inlet and outlet arrangements. A generalised diagram is shown.
   
2. Vertical flow. The effluent is introduced to the system via a perforated pipe, which goes down through graded layers of gravel.
   

A hybrid system, with a vertical flow bed followed by a horizontal flow bed normally gives the most complete treatment.

A further variation is the reverse flow vertical system, which has a solid pipe down to the bottom of the layers, thus forcing the effluent up through the layers to the rooting zone as shown.

For further information on large scale systems, with case studies, see Hammer (1989), Merritt (1994) and Hawke, CJ and Jose, PV (1996). Permaculture Magazine and books on permaculture and alternative technology are a good source of information on systems for treatment of domestic waste. See also Chapman T and Shields J (1994) and Centre for Aquatic Plant Management Information Sheet 36 (1995). Local authorities and water companies are increasingly interested in wetland systems for water treatment and may be able to offer advice.