Index Home Join IPCC

Constructed Wetlands and Reed Beds

Féidhlim Harty of FH Wetland Systems Ltd., Gaggin, Bandon, Co. Cork answers some basic questions about constructed wetlands and reed beds in Ireland.

WHAT ARE CONSTRUCTED WETLANDS?
Constructed wetlands are purpose built wetlands which are specially designed for the treatment of wastewater. The design layout and careful choice of plants maximise the treatment mechanisms active in natural wetlands to cleanse and reoxygenate the water. Although constructed wetlands and reed beds are no substitute for conservation and protection of natural wetland habitats, their widespread use in Ireland over the past 15 years has introduced wetland habitat into areas where there may otherwise be none. These wetlands have attracted many species of wildlife, including butterflies, dragonflies and birds such as mallard, moor-hens and snipe.

WHAT ARE THE APPLICATIONS?
In Ireland constructed wetlands are usually used for effluent treatment. The most common applications are for treating septic tank effluent and polishing municipal sewage. Other common uses include treatment of farm yard runoff and parlour wash-water, as well as industrial effluents and industrial yard runoff. In addition, they have been used for runoff from mines, landfill runoff water, stormwater runoff, and as agricultural buffer zones.

HOW DO CONSTRUCTED WETLANDS WORK?
There are many Physical, Chemical and Biological mechanisms that play a part in the treatment of wastewater within a constructed wetland system. The principal mechanisms are bacterial activity and filtration. Wetland plants have evolved to grow in saturated conditions and one adaptation is the ability to draw oxygen from the leaves to the roots. With enough area and enough plants, the need for electric pumps is replaced by this trait, where the plants themselves make the oxygen available for the right bacteria to thrive and treat the wastewater.

Other mechanisms include adsorption, precipitation, decomposition, nutrient uptake and volatilisation. These all add to the pollution removal efficiency of constructed wetlands for many different wastewater types.

WHAT PLANTS ARE USED IN CONSTRUCTED WETLANDS?
The table below lists the variety of plants used in constructed wetlands. Basically a good mix of local wetland plant species is required.

Plant Name Latin	Plant Name English
Typha angustifolia	Bulrush
Typha latifolia	Lesser Bulrush
Iris pseudacorus	Yellow Flag Iris
Lemna minor	Duckweed
Potamogeton species	Pondweeds
Caltha palustris	Marsh Marigold
Nuphar lutea	Yellow Water Lily
Nymphaea alba	White Water Lily
Phragmites australis	Common Reed
Sparganium erectum	Branched Bur Reed

ARE REED BEDS THE SAME AS CONSTRUCTED WETLANDS?
Reed Beds and Constructed Wetlands are two terms often used interchangeably. Although international terminology appears to be blurred, in Ireland the term "Constructed Wetland" is usually used to describe a soil based marsh system in which the wastewater flows over the soil substrate; while a "Reed Bed" is usually a gravel based system in which the wastewater flows vertically or horizontally through the gravel substrate. Ponds are often incorporated into the design of both constructed wetlands and reed beds to increase the retention volume, habitat value and amenity interest.

The choice of system depends principally on site size and soil type; budget; degree of maintenance required; and adjacent landuse. In brief, constructed wetlands are more robust and maintenance free while reed beds make more efficient use of space and function without open water if a pond is omitted from the design.

WHAT ARE THEIR PARTICULAR ADVANTAGES OVER OTHER TREATMENT SYSTEMS?
From a habitat perspective, constructed wetlands have the obvious advantage that they introduce wetland habitat where none previously existed. Most importantly, as with any system, they help to protect the receiving rivers, streams and groundwater from pollution by achieving high effluent quality. However they also make good economic sense for wastewater treatment, since they are relatively inexpensive to install and can have low to zero running costs.

Another distinct advantage of constructed wetlands over other treatment systems is that they are very adaptable. They can function effectively with a variable waste load, for example at schools or caravan sites where the usage is seasonal, and can also fit in around existing or overloaded systems. This means that it is now quite possible for small communities and villages to treat their wastewater discharges to a high quality where only a large septic tank may have existed before. In addition, it provides farmers with a way of dealing with yard and parlour washings, a particular problem where difficult soil or storage conditions exist.

WHEN WOULD I USE ONE?
There are several reasons why a constructed wetland may be used for treating wastewater. For a domestic situation with just a single septic tank, the soil percolation may be too high or too low, or other site conditions may be unfavourable for a percolation area.

If soil percolation is too low then the septic tank effluent will not flow down through the percolation area but may instead pond on the surface of the lawn. If percolation is too high then the effluent will travel rapidly through the sandy or gravelly soil and pollute the groundwater. This is particularly undesirable if you or your neighbours have a well supplying their drinking water.

For larger applications the main reason is to achieve excellent discharge quality prior to discharging the effluent to a nearby stream or river. Developers, councils and businesses are increasingly required to minimise their impact on the receiving aquatic environment, and constructed wetlands are an attractive option for achieving these aims.

In any scenario where a high water quality is required, whether it is a stream, a septic tank overflow, golf course runoff or rainfall runoff from a housing estate, a constructed wetland can help to protect the local environment and provide habitat into the bargain.

FURTHER INFORMATION
In the mid 1990s Féidhlim Harty set out to design, build and plant constructed wetland systems to help address the problem of water pollution in the Irish environment. Ten years later he has a well established company with a diverse set of environmental services on offer. Féidhlim Harty can be contacted at
FH Wetland Systems Ltd., Gaggin, Bandon, Co. Cork, wetland@eircom.net or phone 023-52983. You can get more information by following this link: www.ballymaloe.ie/fhwetlands

© Féidhlim Harty 2007
FH Wetland Systems Ltd., Gaggin, Bandon, Co. Cork,
wetland@eircom.net, phone 023-52983