New wastewater technologies to allow development in poor soils
09 August 2016
Many areas of rural Ireland are classified as unsuitable for new development because of the health risks associated with sewage-related pollution. As a result, there is a pent-up demand for construction on farm homesteads and other unsewered sites, due to the absence of suitable wastewater treatment solutions in those areas.
Additionally, there are many existing houses on sites that would not meet current standards. Many of these will require upgrading in the future, particularly when there is a change of ownership, when the house is being extended or they fail an inspection under the National Inspection Plan for Domestic Wastewater Treatment Systems. This article focuses on two technologies that can provide solutions for many sites previously considered unsuitable for development.
The domestic wastewater of approximately one-third of the Irish population, or approximately 500,000 dwellings, is treated on site by domestic wastewater treatment systems according to the Environmental Protection Agency (EPA). Most of this is discharging to the subsurface soil, while the EPA also estimates that 39 per cent of the country has inadequate conditions for year-round soil treatment with the potential to pollute groundwater or surface water.
The EPA states that the areas presenting the greatest challenge are areas with:
- Inadequate percolation because of low-permeability subsoils; and/or
- Insufficient attenuation because of high water tables and shallow subsoils.
An evaluation project to assess innovative technologies and provide solutions specifically for areas of low permeability was funded by the EPA in 2011. The object was “to address these problematic areas and allow development, while protecting water resources from the risk of pollution by existing septic tanks in these areas”. This assessment of three technologies was completed by Trinity College in 2014 and the report was released by the EPA in January 2016.
Technologies assessed and positive recommendations
The project focused on three technologies:
- Low pressure pipe (LPP) pressure distribution systems: LPP systems use PVC pipe networks with a larger diameter supply manifold and perforated lateral pipes which are similar to existing pressure pipe systems in the EPA Code of Practice 2009 (CoP) for sand and soil filter beds and raised mound polishing filters. The main system difference is the LPP is an in-ground trench arrangement with indigenous soil between the pipes and is not an excavated bed.
- Drip distribution (DD) systems: DD systems are high pressure drip-irrigation systems with flexible polyethylene (PE) drip tubing inserted in the topsoil 6-9 inches below the ground surface. The filtered wastewater drips from emitters moulded inline into the drip tubing to form a densely spaced number of discharge points throughout the percolation area. The drip tubing used was Geoflow-manufactured Wasteflow PC tubing with root and bioslime treatment and emitters at 600mm spacing.
- Evapotranspiration (ET) systems: the ET systems tested were sealed-bed willow systems. The ET systems were reported previously on EngineersJournal.ie.
Both the LPP and the DD systems assessed in the EPA report were designed by this writer and were supplied at each test site by Ash Environmental Technologies.
The report stated that no surface water ponding was found on either low permeability site throughout the period. The soil moisture results did show that the subsoil below the pipe networks became saturated at times in winter after heavy rainfall events as would be expected. Overloading trials were carried out with a doubling of the hydraulic loading. This showed only a mild effect in the soil moisture monitoring and was more related to the meteorological conditions at that time of year. No deterioration in treatment quality was found during overloading trials.
More uniform and more unsaturated soil moisture conditions were found beneath the drip systems throughout. This would indicate more air in the subsoil below the drip system due to the controlled dripping of water in dose and rest cycles from emitters spaced every 600mm along the length and width of the percolation area. There were approximately 215 emitter discharge points in the drip and 44 orifices in the LPP systems in each 80 square metre area.
The almost-five-times as many discharge points in the drip system explain the differences in saturation levels below each system. This also allows more intensive use of the subsoil below drip systems by pollution attenuation thereby maximising the treatment of the wastewater compared to other soil infiltration systems.
Nitrate concentration analysis below the systems suggested that the drip system provided more favourable environmental conditions for denitrification to occur in the soil, which would reduce nitrate losses to groundwater more effectively than the LPP system. “Hence, in areas of particular nutrient concern, the DD system would be a preferable treatment solution than the LPP distribution system,” the report found.
The reported results were excellent for both the LPP and drip systems on both low permeability sites evaluated. The report stated that the systems under test “resulted in a decrease in the faecal contamination of groundwater, as well as the prevention of surface ponding of effluent, at both sites”.
It continued: “Furthermore, the field results and calibrated models of the unsaturated zone show that the LPP system could be a solution for sites with T-values of less than 90, and the Drip system could be a solution for sites with T-values of less than 120 after secondary treatment.” The drip systems are recommended for use on sites with a soil depth of 600mm to limiting horizon and the LPP 900mm soil depth.
Referencing the report of the trials, the EPA website states: “This research has demonstrated with field trials a range of systems that may be a solution for on-site wastewater treatment and disposal in low permeability soils, although changes in current policy and legislation would be required to facilitate their use. EPA staff will review the report and liaise with the Department of Environment, Community and Local Government in relation to the relevant recommendations/findings highlighted by the authors and incorporate the findings into national guidelines as appropriate.”
A revision to the 2009 Code of Practice is planned by the EPA following the review of the report with an estimated timescale of December 2016 to have a document available for public comment. Based on previous experience, it is reasonable to expect that the recommendations of the report will largely be incorporated into the new guidelines and then into Part H of the Building Regulations. For the full report link, please click here.
Low pressure pipe networks
Conventional percolation trench systems rely on gravity to spread the wastewater through the percolation area. Even using a distribution box or other flow splitting devices gravity systems can only partially distribute water over the percolation area. Some segments will consistently receive a larger volume of water.
On sites with suitable soil conditions, this is satisfactory and can provide long term passive soil treatment. But the percolation area is only being partially used when compared to a pressure system.
The advantage of a pressure distribution system is that the water can be pumped throughout the pipe network and spread over the percolation area more effectively than by a gravity system. This means that pressure must exist to the furthermost part of the network; otherwise the percolation area is only being partially dosed which can quickly result in leakage and overloading.
Pressure systems by their nature function by maintaining sufficient pressure throughout the network while taking account of the pressure release points and length and volume of pipes in the network. Similarly, in a pipe network that supplies wastewater to a polishing filter or any pumped system, a balance must be maintained between the pump capacity, the volumes of pipes, the number of orifices and the diameter of the orifices.
This requires the system designer to control the number and size of holes or orifices in the pipe network; otherwise the required design pressure will be released before the ends of the perorated pipe laterals. Similarly, by specifying the number and diameter of pipes the volume of water to be pumped through the pipe network must be part of the overall system design. For this reason, site-specific design proposals are necessary for wastewater pressure systems, and they should be installed and operated by people with knowledge of the importance of the design principles.
This is required by the footnote to tables 8.1 and 8.2 of the EPA Code of Practice 2009 for single houses: “Due to variations in the discharge rating of pumps available on the market, it is important to correctly match the orifice diameter and the lateral diameter in the distribution system to the pump, thus ensuring even and effective distribution of the hydraulic load across the filter area.”
This is also essential for LPP system pipe networks as they are calculated on a similar basis. For larger non-domestic systems, calculated network designs with pump and network dose volume specifications are even more important.
To maintain the design pressure in the pipe network, the number and diameter of orifices must be matched by the capacity of the pump and the volume of water pumped each time the pump runs. Site variables also affect performance such as site elevation, distance to be pumped and diameter and length of pump line or rising main. As a result, the choice of pump and the volume of the pump tank or pump compartment are critical design elements.
So to be effective, designs for pressure pipe networks require these calculated design values and should be provided with the pipe network:
- The minimum operating pump capacity for the specific pipe network at the site TDH head in litres/minute;
- The minimum recommended network dose volume in litres per dose i.e. per pumping event.
Drip distribution systems
Drip distribution of wastewater to soil percolation areas evolved from the development of drip irrigation in Israel in the 1960’s. Later technology improvements and root inhibiting modifications allowed the drip irrigation to be subsurface. The demands of water shortages meant the reuse of wastewater via drip irrigation soon followed.
Drip distribution involves the controlled dripping of minute quantities of water about 6-9 inches below ground at the biologically active root zone of the ground surface vegetation. It is increasingly recognised as the most effective method of distributing wastewater to subsurface soils. The suitability of soil types ranges from fast free draining soils to the low permeability fine textured clay soils. It has now been used in more than one third of the counties in Ireland.
There are many strong endorsements of drip distribution from the US two of which come from the US EPA and the US Electric Power Research Institute (EPRI)’s peer reviewed guidelines. The US EPA wastewater design manual 2002 describes drip as “the most efficient of all distribution methods and is well suited for all types of subsurface wastewater infiltration systems”.
The US Electric Power Research Institute (EPRI) and Tennessee Valley Authority Peer reviewed Drip guidelines 2004 state: “Subsurface drip irrigation, or more appropriately for wastewater applications, subsurface drip distribution (SDD) is the most efficient method currently available for application and subsurface dispersal of wastewater to soil. Because it is so effective, drip distribution represents a viable option for wastewater disposal for all soil types.
“The technology is commonly used at sites where point source discharges and National Pollutant Discharge Elimination System (NPDES) permits are not appropriate due to the environmental sensitivity of receiving streams. It is also commonly used at sites where conventional drainfields are not appropriate due to site constraints such as shallow soils above a restrictive layer (e.g. rock, groundwater, hardpan, etc.), steep slopes, or clay soils with low permeability.”
The drip system disperses the filtered effluent uniformly over the percolation area using pressure compensating drip tubing. High head submersible pumps pressurise the dripline network while taking account of the site total dynamic head including network friction losses. The drip emitters which are moulded into the drip tubing are designed to restrict the flow typically to ½ US gallons per hour (0.03 Litres/min) during operation.
With approximately 2.4 emitters per square metre throughout the percolation area, each square metre of soil consistently receives the same volume of water. Low permeability soils are dosed less frequently than more permeable soil types.
The dripline is usually inserted into the existing soil at a depth of 6-9 inches (150- 200 mm) below the existing ground level using sub-surface pipe insertion machinery such as a mole plough attached to a farm tractor. Site disturbance is minimal and no aggregate or stone is required. Existing trees and shrubs can be retained with the dripline diverted around them or any obstacles. The dripline is pre-treated to prevent bacterial deposits building up on the tubing walls and also to prevent root intrusion from trees and shrubs.
Dosing is time controlled from a large pump tank providing flow equalisation over a 24 hour period typically less than 10 minutes per dose cycle. The soil is rested between pumping events to optimise water infiltration and dispersal into the subsoil and allow the re-aeration of the subsoil.
The uniform and time controlled dosing and resting cycles combined with the shallow placement of the dripline in the biologically active topsoil is designed to:
- Facilitate horizontal water movement in the unsaturated topsoil;
- Retain air-filled soil pores to allow aerobic wastewater treatment in the topsoil;
- Encourage evapotranspiration during dry weather periods;
- Facilitate plant nutrient uptake and prevent nitrate and phosphorous runoff to waterbodies;
- Enhance the infiltration into subsoils;
- Minimise the impact of water mounding during periods of seasonal high water table;
- Allow the reuse of wastewater with its beneficial nutrients for irrigation purposes.
The dripline has a series of emitters spaced at 600 mm intervals in the drip tubing which are moulded inline in the dripline. Each emitter releases very small quantities (0.03 litres/min) of water which commences to drip into the soil when the design pressure is reached. The PC tubing allows drip laterals of 90 -100 metre lengths to be run along site contours to encourage dispersal of the water along a long and narrow footprint.
Tertiary polishing by drip distribution
The aerobic soil conditions below drip provide the best conditions for attenuation of most pollutants including pathogenic organisms. Additionally, the EPRI peer reviewed drip design guidelines referring to drip dose and rest water balance procedures state “aerobic conditions optimise microbial breakdown of organic pollutants”. Clearly, this provides an excellent opportunity for drip to be used as a tertiary polishing solution. This is not only true in low permeability soils but even more so in free draining and shallow soils where groundwater and receiving waters are at risk due to poor pollutant attenuation typical of free draining soils. Drip technology can optimise soil treatment in all site conditions.
Drip distribution systems are already being used successfully on a range of domestic and non-domestic applications in this country. Once the EPA Code of Practice is revised to give guidance on the use of drip systems, it is likely that their use will provide an option for the development of sites with slowly permeable soils that were deemed unsuitable heretofore.
To read about another of the technologies involved in the EPA research options for wastewater options in low permeability soils in unsewered areas (the willow systems), please click here.
o EPA Research Report No. 161. Assessment of disposal options for treated waste water from single houses in low permeability soils. Laurence Gill et al.
o EPA Code of Practice Wastewater Treatment and Disposal Systems serving single houses (p.e. <10) 2009
o US EPA Onsite Wastewater Treatment Systems Manual 2002
o Wastewater Subsurface Drip Distribution: Peer Reviewed Guidelines for Design, Operation, and Maintenance, EPRI, Palo Alto, CA, and Tennessee Valley Authority, Chattanooga, TN: 2004. J Watson et al.
Joe Walsh, Ash Environmental, Unit W2 Wicklow Enterprise Centre,
The Murrough, Wicklow A67 HD86
Enterprise centre tel: +353 404 66433; Mobile: +353 86 241 2421
Wastewater Percolation Specialists: www.ashtecs.com