Willow facility update: EPA research options for low-percolation sites
23 February 2016
Author: Féidhlim Harty is director of FH Wetland Systems environmental consultancy
When the EPA Code of Practice was produced in 2009, it set out certain criteria for soil percolation rates and depth to groundwater or bedrock. From the time that it was adopted as the planning document of choice, the “t > 90” sites have been a stumbling block for those wishing to build one-off houses in unsewered areas.
This refers to the infiltration rate, or t-value, being greater than 90 minutes to drop 25mm in a standardised percolation test. If the infiltration rate is slower than this value, there is no solution offered in the EPA Code, and thus the site is deemed unsuitable for development.
This has to difficulties for counties such as Wexford, Leitrim, Limerick, Kilkenny and elsewhere with similarly heavy clay soils. As much as 39 per cent of the country has soil conditions that are unsuitable for the EPA Code of Practice criteria, so there has been a pressing need for guidance on how to satisfactorily deal with existing systems that are failing, or performing sub-optimally, and also on how best to protect groundwater and surface water on new developments.
In 2007 Wexford County Council initiated a trial process to explore the potential for using Zero Discharge Willow Facilities. Wexford has an abundance of heavy clay subsoils that makes makes surface waters particularly susceptible to septic tank pollution.
Avoided option of licensed surface discharges for domestic dwellings
Like other councils, Wexford has avoided the option of licensed surface discharges for domestic dwellings, so the remaining option was to limit construction on unsuitable sites until an environmentally acceptable alternative was found. Trinity College Dublin (TCD) was contracted to carry out the research on a number of willow systems permitted for one-off developments within the county. These were designed by TCD and FH Wetland Systems, each in collaboration with willow facility designers in Denmark.
Subsequently TCD received further funding from the EPA to continue this research work. As part of the new brief they broadened the scope of the investigation to include other options for dealing with effluent disposal on challenging sites. The results of that process have just been published by the EPA in its ‘Research Report No.161, Assessment of disposal options for treated waste water from single houses in low permeability subsoils’.
The options investigated under the research project were as follows:
- Zero Discharge Willow Facilities.
- Pump-fed Drip Distribution systems.
- Pump-fed Low Pressure Pipe systems.
- Licensing single house discharges to surface water.
- Disposal to ground into bedrock zones.
- Cesspool storage for export for municipal treatment.
- Connection to an existing sewer.
- Introduction of clustered decentralised sewage treatment systems.
Zero discharge willow facilities were first developed in Denmark in the early 1990s, and in Danish EPA trials they were found to be effective at disposing of 100 per cent of the effluent to the air via evapotranspiration. The treatment of septic tank effluent within the facilities was also deemed to be very satisfactory.
Two Danish EPA Guidance documents were produced to guide home owners, builders and local municipalities in their design, construction and maintenance, one for fully plastic lined facilities (Gregersen et al, 2003,a) and unlined systems (Gregersen et al, 2003,b). Both are considered to be zero discharge willow facilities in Denmark, but in the latter, some infiltration is likely to occur in the first year or so as the willows become established.
Thereafter the willows keep the water balance in check and limit vertical and lateral infiltration of liquid. In the TCD trials it was the plastic lined willow facility model that was assessed.
Trials showed that Irish systems overflowed for some of the annual cycle
The TCD trial process, taking the initial work carried out for Wexford County Council and then for the EPA, carried out an assessment of 13 constructed willow facilities in counties Wexford, Leitrim and Limerick. Climate data was collected, as well as water depths within the facilities. The trials showed that despite Danish successes, the Irish systems overflowed for some of the annual cycle. This was the case for all systems, the larger TCD model and the smaller FH Wetland Systems model designed in collaboration with Peder Gregersen in Denmark.
The overflow was attributed to two principle factors. Firstly the relative humidity in Ireland is higher than in Denmark, so the evapotranspiration rate is that bit lower. The other, arguably more important factor, is that the soil type used in the systems varied considerably from the Danish systems. Essentially clay soils here and in Denmark are the same – except for moisture content.
Willow facilities constructed on clay soils in Denmark are built at a time of year when the clay can dry and break up before replacement. Thus there is a large void space between these chunks of clay. Here in Ireland, the systems were installed on sites of heavy clay conditions – those conditions that would have failed a percolation test hands down – but unlike Denmark, the clay was wet and sticky.
When it was excavated and then replaced, the void space was minimal. This lack of void space has led to reduced storage capacity for effluent and rainfall, hence the overflow during times of higher rainfall and lower evapotranspiration.
From a construction perspective, newer willow facilities built with a greater void space should be almost as effective as Danish equivalents. Practically speaking, import of soil to the site raises the cost and carbon footprint of the project, unless suitable material is available nearby.
Nonetheless, it is possible that compacted rushes or willow brash (bundles of twiggy branches) could be used to create low-cost, low-resource void space if incorporated into the willow backfill process. It will also be interesting to observe the existing systems as the willow roots develop and create increasing void spaces within the basins and to see how mature systems perform.
Water quality leaving facilities was the same for systems with sewage effluent as for rainfall runoff from land with no effluent
Despite the observation of overflows, the EPA Research Report was positive about the willow facility technology. The water quality leaving the facilities was the same for systems with sewage effluent as for rainfall runoff from land with no effluent. In other words, the impact on the receiving environment from the willow facilities was essentially nil.
The report also noted that the overall volume leaving the willow facility was reduced by the evapotranspiration process, further protecting adjacent watercourses. The report raised the suggestion that in light of these factors, it is possible that the overflow from willow facilities in Ireland may potentially be classed as exempt from discharge licences.
Other disposal options explored in the report included the pump-fed distribution systems for effluent disposal; licensing of domestic effluent discharges to surface waters; discharge to bedrock in areas with shallow impermeable subsoils; and export of sewage from the site via cesspools, existing sewers or clustered treatment systems.
Two different pump-fed distribution options were investigated under the research programme; namely low-pressure pipe (LPP) and drip dispersal (DD) systems. These were installed on sites with t values in the upper range of acceptable values (t = 73 minutes and 75 minutes on two different sites). Although these were both below the t ≤ 90 limit in the EPA Code of Practice, on one site effluent was found to be migrating laterally within the upper soil horizon towards an adjacent ditch, and on the other site effluent was ponding at surface level.
Each of the two different pump-fed systems was installed on either site to compare the systems side by side. The conclusion was that the effluent was dispersed satisfactorily through the soil, but that in the case of the drip dispersal system significant maintenance was needed to clear blockages until the system was switched to secondary treated effluent rather than septic tank effluent.
The systems were calculated to be suitable for use on sites with higher t values than is currently permitted in the EPA Code. LPP systems could be a solution for septic tank effluent for t < 90 soils; and DD systems suitable for secondary treated effluent for t < 120 soils. The report further identified that a more sensitive percolation test procedure should be used for such sites.
Introduction of licences for domestic discharges to surface waters recommended as solution for low permeability sites
The introduction of licences for domestic discharges to surface waters was recommended as a solution for low permeability sites. In this context, technologies that were passive and low maintenance were specifically identified as being recommended, given the risk of poor performance of unmaintained systems.
Where shallow subsoils are present over porous bedrock it is possible to discharge effluent directly to bedrock after treatment via suitable soil or sand filtration. This would only be viable where suitable bedrock was present within c.3m of ground surface. Between one and 4.7 per cent of potential legacy sites were deemed to be suitable for this approach in the counties examined.
Connections to existing sewers or new clustered treatment systems were explored as a possibility for removing sewage from domestic sites, as was the possibility of using cesspools and tankering effluent offsite. Between four per cent and eight per cent of all potential legacy sites in low permeability areas are within 100m of an existing sewer. This figure increases considerably where clustered treatment systems are added as a possible option.
Clustering has the advantage over one-off systems in that the costs is shared between home owners and the regulatory involvement is reduced to a single discharge rather than many dispersed throughout an area. Due to the high annual cost of effluent collection, cesspools were deemed to be uneconomical except for infrequently used holiday homes. It is possible that by employing water saving measures such as shower head reducers or dry toilet systems etc. (Dubber and Gill, 2013) that cesspool costs could be minimised where other options are unsuitable.
Economically, the options vary widely for both capital and running costs. However, when viewed in the longer term, the difference between the different systems generally become much less pronounced. Following is a table of 20-year costs based on the figures given in the report.
Table 1: Construction and operational costs for a 4-person system over 20 years.
|Cost over 20 years (€)||Cost with
|Cost with urine-diverting toilets|
|Connection to existing sewer||17,680||n/a||n/a|
|Clustered decentralised system||11,920||n/a||n/a|
|Imported filter media onto bedrock||17,656||9,756 – 13,036||9,520 – 12,800|
|LPP system||17,160||9,560 – 12,840||9,360 – 12,640|
|DD system||19,840||11,760 – 15,040||11,520 – 14,800|
|Willow system||25,200||13,600 – 13,680||12,120|
|Storage tank/cesspool (in use 119d/yr)||61,920||35,440 – 36,480||31,920 – 33,040|
Refer to the EPA Research Report for the calculation basis underlying these figures. Note that the connection to existing sewer and clustered decentralised system options above do not account for Irish Water charges of €1.85/m3 for effluent disposal. Taking this into account would raise the cost of a sewer disposal option considerably, and would also introduce savings for dual-flush or urine-diverting toilets that are not currently accounted for in the report.
The environmental sustainability of the different systems was assessed in the report and greenhouse gas emissions for both electricity and sludge transportation was presented. In line with the financial costs in the previous table, I have calculated the carbon footprint costs over a 20-year period.
Table 2: Estimated energy-related GHG emissions as tonnes CO2 for a 4-person household over 20 years.
|Greenhouse Gas Emissions||t CO2 with
|t CO2 with
|t CO2 with urine-diverting toilets|
|Connection to existing sewer||1.80||1.04||0.93|
|Clustered decentralised system||2.93||n/a||n/a|
|Imported filter media onto bedrock||5.32||4.87||4.81|
|LPP and DD systems||5.32||4.87||4.81|
|Storage tank/cesspool (119 d/yr)||3.98||2.30||2.06|
The willow facilities have considerably lower carbon dioxide emissions from energy use than most of the other options presented. Given that willows also sequester carbon as they grow, the figure here could would be further reduced if this were taken into account. Where used as a home heating fuel in the place of fossil energy sources, willows actively reduce the household carbon footprint over their lifetime.
In addition to the zero discharge willow facilities investigated in the research, the diversity of willows means that they can work well in conjunction with other systems to achieve both nutrient uptake and carbon sequestration. An obvious hybrid is using pumped distribution of effluent in conjunction with a willow planted percolation area, much like the unlined willow facilities detailed in Danish EPA guidance (Gregersen et al., 2003,b).
Overall the publication of the new EPA research report is very welcome, and enables practical solutions to be adopted on sites with low permeability soils – whether they be legacy sites with existing pollution issues, or new developments. In light of the recent climate talks in Paris, the inclusion of sustainability performance for the different systems is invaluable. In order to achieve the goals agreed to in Paris, Ireland will need to examine and reduce greenhouse gas emissions across every sector of the economy, including sewage treatment.
Dubber, D and L Gill (2013) ‘EPA STRIVE programme 2007-2013. Water Saving technologies to reduce water consumption and wastewater production in Irish households’. EPA, Wexford
EPA (2009) ‘Code of Practice: Wastewater Treatment and Disposal Systems Serving Single Houses’ (p.e. <10). Environmental Protection Agency, Wexford.
Gill LW., D Dubber, V O’Flaherty, M Keegan, K Kilroy, S Curneen, B Misstear, P Johnston, F Pilla, T McCarthy, N Qazi and D Smyth (2015) ‘EPA Research Report – Assessment of disposal options for treated wastewater from single houses in low-permeability subsoils’. EPA, Wexford.
Gregersen, P., S. Gabriel, H. Brix, I. Faldager (2003,a) ‘Retningslinier for etablering af pileanlaeg op til 30 PE (Guidelines for willow systems up to 30 PE [in Danish]). Økologisk Byfornyelse og Spildevandsrensning No 25’ Miljostyrelsen, Miljoministeriet. (Danish EPA, Ministry of Environment and Energy).
Gregersen, P., S. Gabriel, H. Brix, I. Faldager (2003,b) Retningslinier for etablering af pileanlaeg op til 30 PE (Guidelines for willow systems with soil infiltration up to 30 PE [in Danish]) Økologisk Byfornyelse og Spildevandsrensning No. 26 Miljostyrelsen, Miljoministeriet. (Danish EPA, Ministry of Environment and Energy).
Féidhlim Harty is director of FH Wetland Systems environmental consultancy and author of ‘Septic Tank Options and Alternatives – Your Guide to Conventional, Natural and Eco-friendly Methods and Technologies’. See http://www.wetlandsystems.ie/watertips.html for additional online resources related to the above article. See also http://www.engineersjournal.ie/2015/02/10/septic-tank-inspection-failed-sites/ for additional options for failed siteshttps://www.engineersjournal.ie/2016/02/23/epa-research-report-on-solutions-for-low-permeability-subsoils/https://www.engineersjournal.ie/wp-content/uploads/2016/02/aaasoil2-1024x633.pnghttps://www.engineersjournal.ie/wp-content/uploads/2016/02/aaasoil2-300x300.pngChemenergy,environment,Environmental Protection Agency