FH WETLAND SYSTEMS Ltd.

An evapotranspiration area is an area of ground which is planted with trees or other deep rooted plants, into which water is introduced for the purposes of partial or full evapotranspiration to air. The water in this case is usually treated effluent. An evapotranspiration area is typically used in the event of limited discharge characteristics of the site, or a striving for minimum environmental impact on the part of the home owner. Limiting site conditions include poor soil percolation characteristics for effluent discharge to ground combined with the lack of a suitable receiving stream/river for surface discharge. This information leaflet discusses evapotranspiration systems for discharge of treated effluent to air from single domestic dwellings, however larger applications are also in use in Ireland .

There are two basic types of evapotranspiration area that are discussed in this information leaflet:

The first is a Partial Evapotranspiration System in which water is introduced beneath the soil in a percolation trench layout. This is typically an unlined system used in soil with poor percolation characteristics. Some percolation can occur, and is not discouraged. This system is most useful where winter dilution rates in a nearby stream are acceptable, but where summer flows are low. Thus during the growing season there is greatest evapotranspiration and lowest discharge, while in the dormant season an overflow pipe can channel excess water to the stream.
The second is a Danish 100% Evapotranspiration System in which the water is introduced into a lined, soil-filled, planted “tank” for full evapotranspiration to air in summer and storage in winter. This is typically lined with plastic or indigenous marl clay and backfilled with soil to the required depth. This approach is typically used where a sensitive watercourse is present downstream of the dwelling and a zero discharge is required all year round.

There is some overlap between these systems, but this characterisation is useful for a general description of the systems available.

Where can an evapotranspiration system be used?

An evapotranspiration system can be used in any location where sufficient area exists for its construction. It is ideally suited however to areas where soil percolation is too low to allow for a percolation area alone as a means of disposing of the final effluent from a treatment unit. There is a relatively large land area requirement for the system, so a minimum site size of 1 acre is a useful guideline. The system itself can vary in size from the standard EPA percolation layout size for the Partial Evapotranspiration System to about 800m 2 for the Danish 100% Evapotranspiration System.

What are the benefits?

The principle advantage of either Evapotranspiration system is that treated sewage effluent may be dealt with in areas where inadequate percolation and discharge conditions exist. In addition to this, the evapotranspiration system will assist in the further removal of nitrates and phosphates from the treated effluent. The system can be incorporated into the overall landscaping plan to provide an aesthetic woodland area within the garden. If coppicing is required for biomass production, the system provides an area of ground with a guaranteed supply of water with typically higher nutrient content than background levels.

With the Partial Evapotranspiration System the wastewater can be discharged to percolation, with an overflow to a receiving water in areas where soil conditions are otherwise unsuitable and where the receiving water is subject to low flows in dry conditions, but suitable during time of higher rainfall.

With the Danish 100% Evapotranspiration System the primary advantage is that it is possible to have a flush toilet with a treatment system and yet have a zero discharge into the surrounding environment.

What are the limitations?

The evapotranspiration system requires a significant area of ground to be dedicated to the purpose of effluent disposal. Consequently a sufficient site size is required to be able to incorporate the system into the overall garden layout plan. Systems of 120-1000m 2 are typical for domestic scale applications depending on the system chosen. The Danish 100% Evapotranspiration System requires the greater land area ( c .800-1000 m 2 x 1.5m deep) due to the requirements for winter storage.

Coppicing is not necessary, but is often desirable for controlling tree height, and thus would require some level of maintenance. The system is intended for use with effluent treated to at least 20/30 standard. While it can be used to good effect as part of a polishing system, it is not intended for groundwater or surface water protection from effluent coming directly from septic tanks.

What happens after the evapotranspiration area?

After the Partial Evapotranspiration System the effluent will be either percolated into the ground, evapotranspired into the air or allowed to discharge to a receiving water body, depending on the time of year and the site conditions. The effluent will be reduced in volume, nutrient content, BOD and pathogen load, but it will still not be sterile and this should be taken into account in the overall system design. In certain areas it may be necessary to sterilise the final effluent via a UV filter system.

The Danish 100% Evapotranspiration System will not have an overflow because of the combination of storage and evapotranspiration dealing with all of the water introduced.

What happens to water not evapotranspired?

Because willow trees do not grow during the dormant season there is a time of the year when the system will not evapotranspire the water entering it.

In the Partial Evapotranspiration System water that is not transpired through the leaves of the trees will percolate into the ground and will also have the potential to overflow to a receiving water body in times of low evapotranspiration, low percolation and/or high rainfall.

In the Danish 100% Evapotranspiration System any water that is not evapotranspired during the winter months is stored within the soil in the lined basin. The basin is designed to cater for the design discharge volume from the treatment system plus the anticipated rainfall volume for the area where the system is located. Thus full storage occurs in winter and this is soaked up by the trees in the growing season.

What trees are used?

The trees used in the evapotranspiration area are chosen for their capacity to absorb water, thrive in moist conditions and tolerate coppicing where required to control tree height and growth rate. For quick initial growth, the area is planted thickly with the selected species. These can be thinned in subsequent years should the need arise. For coppicing purposes as well as maximum uptake of nutrients, chosen species are quick growing varieties.

In the Partial Evapotranspiration System the trees can include species of willow, alder and eucalyptus. While willow, alder and eucalyptus are all fast growing trees, some willow species are specifically bred for coppicing for biomass production or for weaving and wattle production. Any of these cultivars would be suitable for the evapotranspiration area. Contact a local garden centre or FHWS for sourcing these plants.

In the Danish 100% Evapotranspiration System willows alone are used, typically the weaker rooted varieties such as purple willow, Salix purpurea . The presence of the liner restricts the choice of trees that are suitable.

Are there seasonal variations in water quality or volume throughput?

During winter months the evapotranspiration rate in the Partial Evapotranspiration System will be at its lowest. The absence of leaves on the willow and alder species means that no evapotranspiration will occur. For the eucalyptus, where used, the lower air temperatures and reduced sunlight will minimise the evapotranspiration rate.

However, to balance this, the increased rainfall rates in winter will increase the dilution rate in the receiving water, and thus make disposal of excess effluent acceptable via this route. The evapotranspiration area will be designed to allow for such overflow during wet conditions and low evapotranspiration. The final overflow of the partial evapotranspiration system should be constructed c .300mm higher than the rest of the pipe network, to allow water to settle within the system before overflowing. The base of the overflow drain should be filled with sand rather than gravel. This will allow for horizontal filtration through the sand and associated bio-film before discharge.

The Danish 100% Evapotranspiration System will vary in saturation, but all the water will be stored within the system and thus discharge quality is not an issue.

How much will it cost?

The cost of the Partial Evapotranspiration System will be similar to that of a standard percolation area with the additional expense of buying, planting and maintaining the trees. The willows will be planted as cuttings and the eucalyptus and alder, where used, will be planted as potted or bare rooted trees. For the partial evapotranspiration system a discharge licence may be required.

The cost of the Danish 100% Evapotranspiration System will be considerably more. The system will be approximately 1.5 – 2m in depth over an area of c .800m 2 ; fully plastic lined with HDPE or Butyl Rubber; refilled with a soil of suitable percolation characteristics and planted with willow cuttings.

The costs outlined for both systems is additional to the secondary treatment system, pumping facilities that may be necessary, design details for planning and construction, landscaping and other such costs.

Where have these been used before?

FH Wetland Systems have used Partial Evapotranspiration Systems as part of the sewage treatment layout on sites using constructed wetlands. These systems have been used either on soils of suitable percolation characteristics where the homeowner specifically wanted to maximise the treatment of effluent prior to discharge to ground; or on poorly percolating soils where the willows help to keep the soil drier by soaking up effluent in the summer.

Willow Evapotranspiration Systems have also been used by other constructed wetland and reed bed designers around the country.

The Danish 100% Evapotranspiration System has been tried and tested by the Danish EPA and determined to be a successful method for dealing with 100% of the effluent generated on a domestic scale.