Why we are doing this project
The sewers under the Opotiki township were first installed in 1956 and most of the pipework is still in service today. Pipes are reaching the end of their life and the system is not working for the town as it should be. Many in town suffer from loss of service, particularly during heavy rain. In really large storm events the system can suffer from overflows from manholes and gully traps. These issues are a concern for public health and the ability to accommodate future development.
Council aims to provide reliable and safe disposal of wastewater and to support future development within the district.
At the March 2017 Council meeting, Council resolved to rehabilitate the existing sewers. You can read the media release about the decision or read more about the background to the decision below.
History behind the project
In 2010 Council began a process of looking at potential replacement options for the Opotiki wastewater scheme. Initial research highlighted several different potential systems that could be utilised, unfortunately all of these were very expensive. In 2013 Council decided to form a steering committee with the directive to investigate the severity and locality of issues through the existing network and determine whether repairs could be undertaken rather than full replacement. The investigation work had several phases.
- Phase 1 – CCTV and Smoke Testing. CCTV and smoke testing was carried out through 2013-14 to determine the general degradation of pipes and identify the extent and scale of stormwater inflow.
- Phase 2 – Ground water, river level, rainfall and pump station monitoring. Through 2014 Council installed monitors on both rivers, in 9 bores around the township and within the wastewater pump stations and manholes. Data was collated and assessed to determine the correlation between rainfall volumes, river levels and ground water levels against the increased flows and volumes running through the wastewater system. The assessment determined that the wastewater system was being overburdened by both the direct inflow of stormwater via gully traps and illegal stormwater connections as well as the infiltration of ground water via old broken pipes under ground.
- Phase 3 – Pilot area repairs. The 2015-16 'Find and Fix' project took the smallest wastewater catchment in the Opotiki area and carried out a work programme of finding points of inflow and infiltration and carrying out fixes. Once fixes were completed the change in pump station and manhole volumes were assessed again to determine the effectiveness of repairs. Results were good.
- Phase 4 – Options Analysis. The final phase of the investigation was an options analysis undertaken in 2016-17 which compared full replacement options against repair/rehabilitation options. The options analysis found that rehabilitation of the network by repairing private property pipe lines and relining main pipe lines was the most cost effective solution.
With the investigation having been completed, providing a clear direction for remediation of the wastewater scheme, Council resolved to undertake the town wide wastewater rehabilitation programme starting 2017/18 with a target end date of 2020.
What was the ‘find and fix’ programme?
Since we know that direct inflow (stormwater getting into the sewerage system) is one of the main contributors to the overflow problem, we decided to select a small suburb, fix the 'easy' defects to prevent that water entering the sewers and see how well that solved the issues in that suburb.
We chose the area north of Elliot Street for the pilot area. Council engineers visited properties in the pilot area and talked to residents about the pipes that lead from their property into the sewer mains. They investigated the pipes on the property to see if there were any quick and easy repair jobs. These included things like replacing broken gully traps, fixing broken pipes where they had been damaged by driveways or tree roots. Council carried these repairs out at their own cost. Larger repairs, such as full pipe replacement, was noted but not carried out at that time.
Generally, the pipes within a property are the responsibility of the property owner. However, we believed that doing these minor repairs would reduce the amount of stormwater that is accidentally getting in to our sewer system and reduce the pressure on the pipes during rain events. The pilot gave us a better understanding of the extent of the improvement we can achieve if these repairs are carried out throughout the township.
Ōpōtiki District Council currently has three contracts underway to directly combat the inflow and infiltration issues.
Waiotahi Contractors were awarded the contract to locate and repair our sewer manholes. This involves locating and raising buried manholes, repairing points of groundwater infiltration and sealing manholes against stormwater inflow.
Image: King Street manhole grouting
Interflow Pty Ltd were awarded the contract to reline our sewer mainlines to prevent infiltration and extend their lifespan. Interflow employ an interesting method that involves winding a reinforced PVC profile through the lines, forming a robust internal pipe with no need to dig up the damaged sewer line. It is has proven to be a practical and effective method. After the costs for repairing the first batch of lines came well under Council's anticipated budget, Interflow were offered a second contract to repair more lines.
These projects are directly impacting the amounts of stormwater and groundwater that is making its way into our sewer reticulation, even though they are not complete yet, analysis of volumes passing through the pump stations in heavy weather events shows a definite reduction in infiltration and inflow has been achieved.
Where to get more information
If you would like to know more about the project, issues with inflow and infiltration and ideas for dealing with stormwater on your property, please check out the details on the FAQs page. Feel free to contact us with any other questions about the project.
Image: Interflow on Wellington Street