North Shore City residents were unhappy with the increasing beach pollution over the last few decades, which frequently resulted in warnings against swimming in local beaches due to sewage overflows or poor water quality reports from the sampling program. To tackle the problem of beach pollution, North Shore City launched Project CARE in 1998.

Project CARE

Project CARE (Council Action in Respect of the Environment) had a total estimated cost of $NZ280 million ($A246 million) over a 20 year period. Project CARE was designed to undertake a holistic assessment of the issues facing North Shore City Council and to develop strategic options that would provide an acceptable level of service at a cost the community could afford.

At the inception of Project CARE, an extensive data collection survey was carried out, followed by the development of a wastewater network, storm water network and receiving water models. The most important conclusions drawn from the analysis of the performance of the existing systems were that the wet weather overflows, concentrated in a few critical areas, are the largest bacteriological pollution source and the wastewater system is very leaky and under capacity.

Although the initial approach, regarding wastewater, would be to fix the leaky system, it has been proven in North Shore City and elsewhere that there is a limit as to what can be achieved through sewer repairs. In general it is not possible to get the inflow and infiltration to a level below which it will no longer cause any wet weather overflows. Additionally, this is certainly not possible within an acceptable budget. The Pareto effect also applies. However, the number of events and volume of the wet weather overflows can be reduced by following methodologies:

* Sewer rehabilitation * Providing storage for peak flows during heavy rainfall * Transporting and treating excessive flows by increasing the capacity of the wastewater system and the wastewater treatment plant. This includes alternative new routes to transfer flows, relieving existing lines.

Based on above methodologies, a cost optimisation model (SEWCOM) was developed to identify the optimal set of improvement works to meet the different performance targets and to cater for the growth up to the year 2050. After extensive public consultation, North Shore City adopted a standard, which allows no more than two wet weather overflows per annum on average.

Wastewater network investigation

All components of the sewerage system in the identified problem areas, including both the public sewer network and private drainage, are investigated and renewed as necessary. The first stage of the process is to divide the sewerage network into manageable parts or mini catchments. The severity of I/I is then determined in these areas by flow gauging and the use of a computerised modelling program. The leakiest mini catchments are prioritised and grouped into an appropriate package for the physical works contracts that follow, allowing for a systematic, efficient and cost effective approach.

The next phase of the program is the rehabilitation and renewal phase, where identified defects in the public sewerage system are corrected. Various pipe lining techniques such as cured in place pipe (CIPP), form and fold and spiral winding are used to rehabilitate the public sewers. Most of the areas in North Shore City are highly developed and it is advantageous to use trenchless methods as much as possible in order to reduce the adverse effects on the environment and possible resistance from property owners during the pipeline rehabilitation phase. In certain cases, pipelines are replaced by directional drilling or pipe bursting. Pipe bursting is used predominantly when the existing pipes need upsizing, whereas directional drilling is generally used in situations where pipe re-routing is required. The open cut method is generally used only for short and shallow sections.

In the past, public laterals and lateral connections (junctions) were sealed using polyurethane grouting. Presently, public laterals are lined using the CIPP technique and the junctions are sealed by installing lateral junction repairs (LJR) robotically. This has enabled significant progress to be made in achieving a fully sealed wastewater system.

Leaky manholes are generally repaired by the application of epoxy cement and polyurethane grouting. Structurally weak manholes are either replaced by new manholes or converted to dry manholes.

A key difference between the strategy followed by North Shore City Council and most other Australasian local authorities for I/I reduction is the emphasis placed on reducing leakage from private drainage. Studies carried out have indicated that stormwater I/I into private drains can be significant, hence justifying the effort invested by North Shore City into this aspect of the program.

North Shore City defines a private wastewater drain as a drain which services only one property (the property may however, have more than one dwelling). The drain becomes public where it joins with the drains of one or more other properties or where it crosses into the road reserve or a public reserve. If a drain crosses the boundary into a neighbouring property, it remains classified as a private drain.

Since the start of Project Care in 1999, private wastewater drains of approximately 8,500 properties have been inspected. On average, two out of three properties have drainage defects with the majority failing a water test and/or a CCTV inspection.

The cost to remedy drains which fail a hydrostatic test and/or a CCTV inspection is typically between $NZ1,000 and $NZ5,000 depending on the condition, length and accessibility of the drain. In some special cases, North Shore City Council is able to advance the money for the work after an agreement has been reached with the homeowner on the terms of reimbursement.

All private drainage work is inspected by a council inspector before it is signed off and the drainage condition removed from the property. The inspector uses a variety of inspection methods depending on the work done such as visual inspection, water & air test and CCTV inspections.

Results achieved so far

After the public lines are rehabilitated and the private drainage defects are repaired by property owners, the effectiveness of the program is determined by re-gauging the relevant mini catchments. At this stage, only one full catchment (Devonport catchment) has been re-gauged and has returned positive results with pre-set targets of I/I reduction having been achieved. For the Devonport catchment as a whole, the stormwater ingress has been reduced from 16.7 per cent to 8.2 per cent. Post flow gauging has been completed in the Browns Bay and Old Northcote catchments and the modelling results should be available in a few months time.