Page 55 - Council Journal Winter 2019

the LPP is an in-ground trench arrangement with indigenous soil between the pipes and is not an excavated bed.
the meteorological conditions at that time of year. No deterioration in treatment quality was found during overloading trials.
distribution system,” the report found. The reported results were excellent
Drip distribution (DD) systems: DD systems are high pressure drip- irrigation systems with exible 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.
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.
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”.
Evapotranspiration (ET) systems: the ET systems tested were sealed- bed willow systems.
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 infilltration systems.
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.
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.
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 these findings into national guidelines as appropriate.”
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
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
Council Journal 55
FEATURE Development in Poor Soils