SiteAssessor.com Blog

1. Introduction

Following on from the recent Zero Discharge Willow System Seminar I have thrown together a summary of the day. There has been a huge amount of interest in this topic over the last few weeks.

As the community is now aware in May of this year (2011) An Bord Pleanala (ABP) overturned a decision by Kilkenny Co Co and granted full planning permission on a green field site with a T-value >90 for a Zero Discharge Willow system (ZDWS).

As far as I know this is the first time that ABP have granted planning permission for a system that has not fully been ratified/endorsed by the CoP. Purely from a site assessor point of view I was delighted as I now felt I had an option to put forward to clients who had failed percolation. The only issue was I did not know anything about the ZDWS so I decided to organise a seminar on the issue.

The seminar was held on Tue 30th August in Cork and was extremely well attended with people travelling from as far as Monaghan and Roscommon.

Presentations on the day were made by:
Féidhlim Harty of FH Wetland Systems.
Brendan Cooney, Senior Executive Scientist of Wexford Co Co.
Dr. Pamela Bartley of HYDRO-G

2. Féidhlim Harty of FH Wetland Systems

The first speaker on the day was Féidhlim Harty of FH Wetland Systems, one of Irelands leading wetland experts. Over the last number of years Féidhlim has been designing and constructing zero discharge willow facilities under the watchful eye of Peder Gregersen who is one of Denmark's leading authorities in Zero Discharge Willow Systems.

Féidhlim explained the principal differences between willow facilities and the concepts of the Danish ZDWFs.

I have broken down the presentation into a Q&A format .

2.1. What are the main differences between Willow Facilities and Wetlands?

The terminology around constructed wetland systems has led to some confusion over the years. Basically there are three principle types of constructed wetland system. These are soil-based constructed wetland systems, horizontal flow gravel reed beds and vertical flow reed beds.

The terms Free Water Surface (FWS) wetlands, Surface Flow Wetlands and Integrated Constructed Wetlands all fall within the category of soil based constructed wetlands.

The terms Sub-Surface Flow (SSF) wetlands and Horizontal Sub-Surface Flow (HSSF) wetlands or Gravel Reed Beds are used to describe this type of gravel reed bed treatment system.

Vertical Flow (VF) wetlands or vertical flow reed beds are the terms usually used to describe this type of system.

These systems are all characterised by being contained (in plastic or indigenous subsoil) filter systems planted with emergent wetland vegetation designed to for effluent treatment prior to discharge.

The zero discharge willow facilities, sometimes called evapotranspiration systems or 100% evapotranspiration systems, resemble constructed wetlands insofar as they are contained, planted areas used for effluent management. However they are differentiated from treatment wetlands in that they are designed specifically to contain and then evaporate all of the effluent introduced and not have any discharge point. While willows can also be used for filtering water, it is the zero discharge facilities that are dealt with here.

2.2. 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 has teamed together with the original developer of zero discharge willow facilities, Peder Gregersen of the Centre for Recycling in Denmark. 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. 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.

2.3. 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.

2.4. 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.

2.5. What happens after the Zero Discharge Willow Facility?

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”.

2.6. 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.

2.7. 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.

2.8. 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.

2.9. 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 €10-15,000, 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.

2.10. 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. Approximately ten systems have been built or granted planning in Ireland and many of these are undergoing trials at present. Contact FHWS for updates on Irish examples of zero discharge systems.

FH Wetland Systems Ltd. is in partnership with the Centre for Recycling in Denmark for Zero Discharge Willow Facilities in Ireland. In this way we can offer over a decade of Danish experience and expertise with zero discharge willow systems, as well as our own experience with constructed wetland systems and reed beds to meet all your ecological wastewater treatment requirements.

For further information contact FH Wetland Systems Ltd. or visit www.pilerensning.dk to view the Danish willow systems website.

2.11. What is the current position on the use of willow facilities in the Irish context?

Currently the code of practice does not extent to zero discharge facilities. Thus the building regulations are the relevant legal document for their use in Ireland. Although the building regulations do not specifically identify zero discharge willow facilities, they identify the requirements that any sewage systems must meet. Since willow facilities can be designed to meet all of these requirements, their use is at the discretion of each planning office at county council level. Brendan Cooney, Senior Executive Scientist of Wexford Co Co.

3. Brendan Cooney, Senior Executive Scientist of Wexford Co Co.

As most of the community are aware this year Trinity college Dublin have started constructing zero discharge willow systems with a view to trialling and monitoring over the next few years in various parts of the country. However, what many of you may not be aware of is Wexford Co Co are at least 3 years ahead of the curve.

Brendan Cooney of WX Co Co informed the gathering that following consultation with the EPA in 2007/2008 prior to the publishing of the their Code of Practice, Wastewater Treatment and Disposal Systems Serving Single Houses, it was becoming apparent to Wexford Co Co that while the forth coming CoP would deal with sites which would pass the Site Suitability test, their was going to be a problem with those areas of Wexford where the soils were less than optimal, eg. Macamore soils. This would lead to considerable areas of the county which would be excluded from future planning for single houses due to their failure to pass the percolation tests. There was also the problem of what to do with the existing housing stock built in these areas which had wastewater treatment systems that were failing.

Brendan had previously worked with Cavan Co Co during the time when it brought in its “Septic Tank” Bye laws. It was also obvious that this was an area that needed to be addressed and that this problem would soon be exacerbated with the ongoing court action by the European Commission against Ireland re the registration and control of single house “septic tank” discharges.

To that end Wexford Co Co personnel visited Denmark in 2008 with Dr Bartley and Dr Laurence Gill, Dept of Civil Structural & Environ Eng., TCD and inspected a number of existing Zero Discharge waste water treatment system sites. The inspections were carried out with Mr. Arni Backlund who has considerable experience of designing and operating Zero Discharge Willow systems in Denmark and Germany. Arni also was involved in a number of EU / Danish Environmental Protection Agency research programs involving the use of willow in the disposal of wastewaters.

As part of the inspection WX Co Co met with a number of personnel from the Danish equivalent of our County Council Environment Sections and discussion was has over a number of hours, both in the field and in their offices as to what their experiences were of such systems. One of the first things that WX Co Co noticed and was brought to our attention was that while the systems were accepted, they were being routinely monitored as the technology was considered relatively new.

All of the applications were subject to considerable scrutiny by the local regulatory body before being granted planning permission. In some cased they were refused or met with considerable regulatory resistance. This scrutiny included biological, social, geological, etc and involved the presentation of reports from a large number of experts in the local authority’s environmental sections (geologists, hydrogeologists, engineers, plant specialists).

It was also noted that all of the people applying for such systems and living in the houses are considerably more “green” than the majority of people in this country in that they had substantially eliminated most bleaches form the waste stream and the detergents they used for household use were often of a biological nature. There were only a few sites which were accepting effluent of a type commonly seen in an Irish context. The volumes of water being discharges were in all cases quite low also.

Following the tour Wexford contracted TCD/Dr Gill to carry out a research project on the applicability and suitability of using similar technology in Ireland. Following the awarding of the contract to TCD a total of 12 sites have been selected around Wexford, each of different sizes, willow cultivars, construction, orientation etc.

The trials were to begin in 2009 but had a number of hiccups due to the initial joiners opting out as soon as they had planning permission and the sites were put up for sale.

The trials are ongoing at present and hope to be finalised sometime in 2012 and the results will be made available to the public and to all Local Authorities.

Monitoring results are now being recorded. Preliminary results show that the addition of both primary and secondary treated effluent induces a marked increase in evapotranspiration, compared to the varieties dosed with rain water. The increase varied from 20% to over 100% in a growing season, presumably due to the faster growth rate of the trees under the application of organics and nutrients. Equally, the willows receiving septic tank effluent also showed a slightly higher evapotranspiration rate than those receiving secondary treated effluent.

The growth of the biomass over the first year is also in the process of being analysed and the willows were harvested over the winter. Samples are currently being analysed for nutrient and carbon uptake.

The results also indicate that the size of such systems is going to be bigger that most people expect but less that what we modelled for.

There is also the maintenance of the systems with annual coppicing to prevent roots becoming too strong that they cause preferential pathways etc.

As part of the deliverables it is expected to have design and construction details for Zero Discharge Willow Wastewater Treatment Systems which take cognisance of Irish environmental and effluent conditions and which are based on sound science.

Brendan also pointed out that too many times he has seen evidence of alternative systems which don’t work and could never work and which have caused untold anguish to the person who paid for them and which the local Co Co as regulators have to deal with the consequences of.

4. Dr. Pamela Bartley of HYDRO-G

In the past KK Co Co Environment Section had recommended that this application be refused on three separate occasions in the past. In addition, the Environment Section recommended refusal on a further two applications which were subsequently withdrawn before a decision was made. (Basically the percolation tests were failing)

For this application Pam was the primary consultant who prepared the proposed solution for the treatment of wastewater, ZDWS, for the planning application that landed on the desk of An Bord Pleanala.

Pam has spent years researching the ZWDS and is intrigued by the overall hydraulics of the system. As you have read from above she was one of the personnel who visited Denmark in 2008 with Brendan Cooney of Wx Co Co.

The basis of Pams proposal was that the effluent could be treated and was able to discharge. The willows would treat the effluent and the method of discharge was via evapotranspiration.

Pam outlined the fact that there is a reference to willow evaporation systems in the new Code of Practice for on-site wastewater treatment system (EPA, 2009). The Code does not explicitly exclude their use as a design option. In fact, the Code suggests they can be used as an additional treatment component. On page 36, the following is stated: - … ‘Other treatment equipment, e.g. storage ponds, maturation ponds, willows, etc. may be added to the system to enhance further treatment. The landscape setting may influence the design of these systems to provide secondary or tertiary treatment of wastewater’…”,

Mr. Gill (Pams colleague) accepts that there are no national design guidelines for the willow bed aspect of the proposed wastewater systems, but were confident that the design proposals put forward are based on progressive, innovative and proven environmental technologies that are acceptable to other planning authorities and are also widely used in many continental and Scandinavian countries i.e. Denmark”.

Furthermore Pam also made the point that the planning authority did not determine that the proposed ecological effluent treatment system would not work but merely decided to adopt the precautionary principle and refuse planning permission because the authority does not have experience of such systems in operation”.

The result was that the inspector for ABP Mr Kelly made a recommendation for refusal but the Boad overturned this recommendation and granted full planning with conditions.

Installation: Nigel Quaid of SGW percolations has supplied materials and constructed a number of Zero Discharge Willow Systems throughout the country for both the larger 2 bed (Trinity College Dublin) system and the smaller single bed (FH Wetland Systems / Peder Gregersen) system.

Points of interest from a construction point of view include the area required , size of machine, water table levels, protection of linings, soil compaction and health and safety. However lots of issues can arise during construction and require experience and fundamental knowledge by of how Willow systems work.

As was pointed out site sizing should be a consideration in design as not every site can accommodate the required area. Separation distances from boundary ditches should be able to accommodate at least a 12tonne 360deg excavator ie. 5m around the willow bed excavation facility and even more for a 20 tonne machine around the Trinity model. Construction area should be at least 4 times the size of the beds with good access to public road for delivery of materials. Planting of willows normally takes place in spring so construction may be in winter or times of heavy rainfall which leads to high water table issues during construction. The preferable option is to dig a temporary trench from the base of the bed if elevations allow and if not use of pumping equipment must be considered to prevent the base getting mucky and unmanageable. Integrity of the linings is of utmost importance. Prevention of any holes or punctures can only be prevented if operatives are conscientious of Sharpe stones sticks objects etc. Any breach of the linings causes direct discharge of untreated effluent to groundwater or over loading of the facility by unwanted groundwater (obvious but worth stating). Labour fitting the linings can take up to 6 people . As linings are made up off-site the dimensions of excavations must be exact so they fit correctly and have enough anchorage on top. A final point I would like to mention is compaction while back filling the soil. This really depends on the type of soil being used. If it is of high clay content the right amount of compaction should ensure it doesn't subside too much whilst at the same time should not be over compacted preventing movement of nutrients to roots. Topsoil and sods should go in first and be covered with subsoil to prevent over population of weeds competing with willow plants. If and when ZDWS become recognised as a viable option for on-site treatment of wastewater in this country which incidentally I believe it should, training of supervisory engineers and contractors will be a must along with training of householders in maintenance as some claim it is almost a lifestyle choice.

5. Summary

I did learn during the talk that the area required to construct a ZDWS can vary from 250m²-500m². The areas will vary throughout the country as the amount of rainfall will have a direct effect on the sizing of the system.

These systems are not cheap to install. A budget figure of 20k should be put aside for the installation of a system to cater for 8PE. Also the system needs to be constructed at least 12months prior to occupancy. The system needs to be maintained on an annual basis i.e. branches prunes etc.

The conditions for this planning application listed that the Reed Beds and the Willow beds proposed shall be established prior to the occupation of the house. It shall be certified to the PA by the system installer that the system has been properly installed prior to the occupation of the house.

Details of the maintenance proposals for the system shall be agreed in writing with the planning authority prior to occupation of the house. A monitoring programme, prepared by a suitably qualified hydrogeologist, to regularly check on the scheme efficiency (including water levels in the willow beds) shall be agreed in writing to the planning authority prior to occupation of the house. A brief annual monitoring report, prepared by a suitably qualified hydrogeologist, shall be submitted to the planning authority.

A Final Report on the wastewater treatment plant operation, prepared by a suitably qualified hydrogeologist, shall be prepared following three years operation of the system (i.e. post occupation of the house). The report shall include an appraisal of the scheme operation, maintenance and monitoring. It shall also specify the ongoing maintenance and monitoring required in the longer term.

If we are to talk honestly the site assessor community is delighted that ABP have overturned a local authority refusal and granted planning for a ZDWS. If for no other reason every time a site assessor is faced with a site that has failed percolation then there is now the possibility of placing the option of applying for a ZDWS on the table.

There is a train of thought that ABP have set a precedent but as we all know ABP look at each application on a site by site basis.

Currently the code of practice does not extend to zero discharge facilities. Thus the building regulations are the relevant legal document for their use in Ireland. Although the building regulations do not specifically identify zero discharge willow facilities, they identify the requirements that any sewage systems must meet. Since willow facilities can be designed to meet all of these requirements, their use is at the discretion of each planning office at county council level.

6. Download documents

To download all documents used in this seminar please click here