The upgrade is part of an ongoing programme to improve harbour water quality following heavy rain events, and follows on from the $4.5m upgrade of the Okara Park pump station and installation of a new rising main completed in June 2010, and the installation of a $5m storage tank and treatment system to prevent sewage discharge from the Hatea pumping station which is due for completion in early January 2012.
The wastewater treatment upgrade approved on 10 August will address the public health issues associated with partially treated wastewater that leaves the treatment plant. Wet weather flows up to 90,000 cubic metres a day will be discharged to the wetlands after passing through existing processes and any flow in excess of this will be fully disinfected with a new disinfection system and discharged into Limburners Creek. Council has also resolved to investigate options to discharge this flow through a wetland as well.
Infrastructure and Services Group Manager Simon Weston says that when it rains, wastewater flow within the sewer network increases as stormwater makes its way into the sewer system.
“The amount of stormwater that gets into the network increases with the duration and intensity of the storm. In some cases the sewer network can’t transport all the wastewater and localised spills occur from the network. During these events sewage pump stations run at their maximum rate, putting high flows into the wastewater treatment plant. The plant has systems within it that divert high flow around various treatment units. This is normal in treatment plants and is done to protect the biological process, recognising that the units that treat wastewater during normal flows are not designed to treat diluted sewage. However this means that a large fraction of the flow that enters the treatment plant gets discharged into the environment after being screened and primary treated. This fraction of water, known at the treatment plant as the ‘extreme bypass’, still contains high concentrations of pathogens.”
The planned upgrade of the treatment plant has come after extensive studies on the effects of sewage spills in the harbour, to make sure that the works address the community’s concerns in relation to public health.
Mr Weston says the impact of sewage spills has been investigated through Northland Regional Council’s state of the environment monitoring, NIWA’s harbour dispersion model, and NIWA’s Quantitative Microbial Risk Assessment (QMRA). This work has shown that the effects of sewage spills primarily affect the upper harbour but they can extend seaward of Onerahi in large storms.
The modeling work indicated that a wastewater treatment process providing a thousand-fold improvement in bacteria levels and a fifteen-fold improvement in virus levels would address the public health risk from the discharge point at the wastewater treatment plant. A number of technologies were found that could provide this level of treatment however the most cost effective option, and the one with the least environmental impact, was to use ultraviolet light disinfection.
The approved upgrade is one of three options considered by Council. The range of options looked at upgrades to the treatment plant, storage, reconfiguration of the treatment plant and disinfection. The options were also assessed for their ability to be upgraded in the future if required.
To help address iwi and hapu concerns in relation to land contact of the wastewater, the option of passing treated effluent through the existing wetland will also be investigated.
“This is technically possible and environmentally favourable,” Mr Weston says, “but it may require changes to the existing consent. Council staff will continue to investigate the option of discharging the treated wastewater to Limeburners Creek via a wetland, with Northland Regional Council and hapu.”
A change to WDC’s resource consent to discharge treated wastewater to Limeburners Creek is currently being heard by independent commissioners acting for Northland Regional Council. The effects of the proposed upgrade will need to be considered by them as part of the consent change. Provided they support the upgrade and provide timely feedback, the draft construction programme indicates that the upgrade of existing ultraviolet disinfection facilities could be completed by March 2012, a capacity upgrade by November 2012 and the new ultraviolet facility finished by April 2013.
More detail can be found in the Council Agenda item here:
Infrastructure and Services Committe agenda [10.6mb]