Zero Discharge Willow Facilities
Zero Discharge Willow Facilities
Zero Discharge Willow Facilities are fully lined, soil-filled, planted basins for 100% evapotranspiration to air in summer and full storage in winter. This approach is typically used in areas of impermeable subsoil (t > 90) or where a sensitive watercourse is present downstream of the dwelling and a zero discharge is required all year round.
FH Wetland Systems have been working with the original developer of zero discharge willow facilities, Peder Gregersen of the Centre for Recycling in Denmark since 2008. In this way over a decade and a half of Danish design experience is available in Ireland.
They are an excellent system for sites that already have problems with percolation or discharge.
What are the benefits?
With the Zero Discharge Willow Facility the primary advantage is that it is possible to have a flush toilet with a treatment system and yet have no discharge into the surrounding environment. This can allow development where discharges are unacceptable, while still protecting the local aquatic environment.
In addition to taking up all the liquid for the year, the nitrates and phosphates are also used by the trees and converted to biomass, which may be used for fuel or chipped for landscaping. If the wood is used instead of fossil energy, it helps to reduce the overall carbon footprint of the development over time.
How much land area do I need?
A minimum site size of 1 acre is a useful guideline for a willow system, although they will work on smaller sites if necessary. The system itself is usually about 250m2 in size for a domestic situation, but each system needs to be carefully sized based on actual water usage, local climatic conditions and other factors.
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.
Coppicing is necessary on a three-year rotation basis. While this ongoing maintenance programme takes time, the resulting biomass can be used for fuel or landscaping.
The design and construction elements require careful attention to detail in order to ensure that the system functions effectively. This means that until landscaping contractors and builders are familiar with the technology, construction costs are likely to be relatively high.
What happens after the evapotranspiration area?
The Zero Discharge Willow Facility will not have an overflow because of the combination of storage and evapotranspiration dealing with 100% of the water introduced, so in that sense, there is no after.
What happens to the effluent during the dormant season?
Because willow trees do not grow during the dormant season there is a time of the year when the system will not evapotranspire all of the water entering it. Some evapotranspiration will occur, but it is significantly reduced from summer rates.
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 volume from the septic tank plus the anticipated rainfall volume falling on the system. Thus full storage occurs in winter and this is soaked up by the trees during the growing season.
What trees are used?
The main tree species used is the willow Salix viminalis, but alders are sometimes interplanted at intervals through the perimeter willows to break the monocultural appearance of the system and to provide additional wildlife interest. Salix viminalis has the advantage of thriving in moist conditions and tolerating regular coppicing. Specific biomass cultivars are selected which are quick growing, and produce a high biomass volume each year, hence maximising evapotranspiration.
Are there seasonal variations in water quality or volume throughput?
The Zero Discharge Willow Facility will vary in saturation, but all the water will be stored within the system and thus discharge quality is not an issue. In the first year, some effluent removal may be necessary to keep water levels below soil surface level. Alternatively, the design can be adapted to eliminate this removal possibility if needed.
The Zero Discharge Willow Facility will vary in saturation, but all the water will be stored within the system and thus discharge quality is not an issue. In the first couple of years, some effluent removal may be necessary to keep water levels below soil surface level.
How much will it cost?
The cost of the Zero Discharge Willow Facility is considerably more than a septic tank and percolation area. The system is fully plastic lined with LDPE and geotextile liners then refilled with soil before planting with willow cuttings. There is a lot of attention to detail required in years 1 and 2 to get the system up and running correctly. Some effluent may need to be removed from the system in the first year while the willows become established. Thereafter however, it has the potential to provide a biomass willow crop and to save money on discharge licence maintenance for every single year following its installation. A ballpark cost for a standard domestic facility is about €15-20,000, including liners, inspection wells, drain pipes, excavation, installation of piping and liners and replacement of soil, sand and cover plastic. This work can be carried out by a local contractor if desired, but very close attention to detail is crucial to the success of the facility.
This cost is additional to the septic tank, pumping facilities that may be necessary, design details for planning and construction, landscaping and other such costs. The Zero Discharge Willow Facility specifically excludes prior treatment systems other than a three chamber settling tank and pump sump. Septic tank maintenance is necessary, to keep your system working effectively and avoid costly system failure.
Where have these been used before?
Zero Discharge Willow Facilities have been tried and tested for domestic applications by the Danish EPA and determined to be a successful method for dealing with 100% of the effluent generated. Several different willow system designs have been built in Ireland and a good proportion of these have been involved in trials by local authorities and the EPA. The results have been somewhat mixed insofar as not all of them achieved a zero discharge at all times, but they all provided greater protection of adjacent waterways than systems with year round discharges. Also, the trials have pointed to amendments that need to be made for the Irish context, and generally the Danish model appears to be a very successful one here. Contact FHWS for updates on Irish examples of zero discharge systems.
For further information contact FH Wetland Systems Ltd. or visit www.pilerensning.dk to view the Danish willow systems website.