Shotover Delta Dose and Drain Effluent Disposal System

Queenstown Lakes District Council (QLDC) Shotover Wastewater Treatment Plant (WWTP) receives municipal sewage from Queenstown in addition to the Arrowtown, Lake Hayes, and Arthur’s Point communities and in the past consisted of a facultative treatment pond system located on the Shotover River Delta. This WWTP has recently been upgraded by the installation of a Modified Ludzack-Ettinger (MLE) system that treats two thirds of the flow, then blends with the oxidation ponds effluent prior to UV sterilisation.

Phase 2 of this upgrade project was to Install a Land Dispersal system between 2016 and 2022 and have all dry weather flows discharge into land. The original consent

(2008) had a reasonably standard Low Pressure Effluent Disposal (LPED) field located inside the Otago Regional Council (ORC) revetment. Groundwater mounding modelling showed that due to the relatively low permeability material on the land side of the revetment, 2.5 m depth would need to be excavated and replaced, resulting in more than 500,000 m3 of material needed to be imported. The cost to import this gravel to construct the LPED platform was estimated to have increased at least 3-fold from the time the original concept was proposed in 2008 to time of design in 2017, resulting in the LPED construction costs going from $7.7 M (2010) to estimated $21 M in 2016.

To reduce cost, a system was investigated on the river-side of the revetment protection works. To meet iwi and other stakeholder’s concerns, the new land dispersal system had to drain naturally through the underlying silts, rather than being forced through under pressure. To allow for this, high void space plastic stormwater storage cells have been used into a gravity flow system instead of a LPED system. A large diameter feeder pipe was designed to allow for large volume doses to be applied to the field with minimal pressure that would rapidly fills the voids and then allows the effluent to drain naturally through the underlying silts and sands. This innovative design reduced the estimated capital cost to $5.3 million.

To prove the operation, a number of piezometers have been installed that will be monitored to confirm groundwater mounding modelling predictions undertaken during consenting. This overall saving has resulted in the direct discharge of effluent to the Shotover River stopping four years earlier than planned.