Is The Central Interceptor Necessary?

Watercare's Notice of Requirement and Resource Consent applications which appeared in NZ Herald last week, explained the purpose of its proposed $800 million Central Interceptor project like this:

The Central Interceptor main project works are designed to address wastewater network upgrading needs on the Auckland Isthmus. The works have been designed to:

• provide additional wastewater network capacity for growth and development
• duplicate the lower section of the ageing Western Interceptor
• significantly reduce major wastewater overflows to the environment.

You can see the full public notice here. Submissions close Monday 3rd December 2012.

If you are interested in more detail, then I suggest you consider some of the documents that Watercare has prepared to justify and support this application. To get you going I suggest a good start is the overview document entitled Assessment of Effects. This is 100 pages and reasonably straightforward.

One of the major problems with the Auckland Isthmus sewer network is that it is a Combined Sewer Stormwater network. It was designed to carry sewage in dry weather, and also to carry stormwater in wet weather, and when there was too much stormwater, the system was designed to overflow and discharge into the Waitemata Harbour at various places. In particular Meola River and Coxes Creek.

A hundred years ago this might have been acceptable. But today it is not. In past years Auckland City Council and Metrowater did undertake some works, some with Watercare, to separate stormwater from sewage. But it's a large task, and overflows of stormwater and sewage have continued.

According to Watercare's AEE:

Watercare’s two largest network overflows discharge to the head of Meola Creek, adjacent to Mount Albert Grammar School and the Roy Clements Treeway walkway.....
There are some 122 active combined sewer overflow points on the wastewater network in the Central Interceptor catchment area, which currently discharge in the order of 2,200,000 m³ of diluted wastewater to the environment on an average annual basis. These overflows affect the natural values of Motions Creek, Meola Creek, Oakley Creek, Whau Creek, and the coastal waters around Point Chevalier and the Waterview Inlet, creating potential public health risks for recreational users, and reducing the environmental, amenity and cultural values of the waterbodies.....

The AEE goes on to summarise what improvements can be expected when the Central Interceptor is constructed and operating:

Once the Central Interceptor scheme is completed, the scheme will reduce the average annual wastewater overflow volumes discharged from this network by approximately 80%. This equates broadly to network overflow discharges in 6 – 12 storm events in an average year, down from the many hundreds of events that currently occur....

In other words there will still be significant overflow volumes - most likely in reasonably large rainfalls - and that there will be between 6 and 12 overflow events annually. That's once it is built.

The AEE summarises the work done by Watercare to consider alternatives. It has given itself the task of doing several things at once: addressing trunk sewer network capacity; duplication of the Western Interceptor; mitigation of sewer overflows. The AEE mentions the fact that all of this wastewater will be directed to the Mangere Wastewater Treatment Plant, but does not spend much time on the fact that Mangere is at, or close to, the maximum conditions of its discharge consent when it rains.

This is an important issue. Interestingly, the AEE mentions a Northern Interceptor...

In addition to the Central Interceptor scheme, Watercare is also evaluating a future “Northern Interceptor”, which will eventually see flow from the northern most areas currently serviced by the Western Interceptor and treated at the Mangere WWTP being diverted to the Rosedale WWTP.

When I served as Councillor at North Shore City Council, between 1998 and 2004, we had terrible problems with North Shore's sewer network. It was extremely leaky, and while it wasn't built as a combined sewer/stormwater network - it certainly performed that way when it rained. In fact we were advised that wet weather flows were more than 5 times as much in volume as dry weather flows into the Rosedale Wastewater Treatment Plant. But that even in a moderate rainfall event, the network would overflow and discharge dilute sewage onto beaches and into streams and mangrove wetlands.

To cut a long story short, after staff had experimented with pipe repair and rehabilitation and various other augmentation schemes, it was decided that we should try and store overflows close to significant overflow points, and then when the rain had passed, to pump the collected wastewater on to the Rosedale Treatment Plant.

The picture above shows the first of these being dug out. This is the Kahika Storage tank in Beach Haven which was completed in 2002 and can hold 4500 cubic metres. In the four years since it was built the number of overflows into the Kaipatiki inlet has been reduced from five per year to about one every two years. The tank cost $4.5 million to build.

North Shore's worst overflow point - our equivalent to what Watercare does to Meola Creek - was into the Wairau Creek which drains into Milford Beach. There were several overflow points that routinely discharged in moderate rainfall events.

This picture shows the storage tank that was built to contain most of those overflows. The second of the new storage tanks built by the council, called Silverfield, significantly reduces overflows in the Wairau catchment and pollution at nearby Milford Beach. The $7.2 million, 6500 cubic metre capacity tank - equivalent to nearly four Olympic swimming pools - was built beneath the car park at the North Shore Events Centre and was completed in 2004. You can see the scale of it from the picture which shows two or three workmen in their orange jackets.

The point being that 6,500 cubic metres of storage and pumps etc and connection into North Shore's computerised control system to manage the way wastewater flowed in the network, was built at a cost of $7.2 million. So what is Watercare planning to build, and how will it work? The AEE says this:

The tunnel has been sized to provide capacity for storage of some 200,000 m3 of wastewater. Under dry weather conditions very little of the tunnel’s overall capacity is used. However, under wet weather conditions the tunnel is used to capture and store wastewater that would otherwise overflow to the environment and then convey it to the pump station and treatment plant. As a rain storm event occurs the tunnel gradually starts to fill, and depending on the duration and intensity of the storm, the remaining capacity reduces until the tunnel becomes full. As the tunnel approaches its full capacity, inlet control gates at selected connection points will close and excess inflows will overflow to the environment via new and existing engineered overflow structures. As the storm event finishes, the wastewater in the tunnel will slowly drop as the WWTP processes the stored wastewater.

So. Much the same functional behaviour as North Shore's system.

The thing is, Watercare's project is costing $800 million for 200,000 m3, while the North Shore approach (based on Silverfield and not allowing for inflation) would give that storage capacity for about $200 million and deliver some 30 storage systems at crucial overflow locations.

Makes you think doesn't it. Watercare's casual assessment of this option dismisses it by assessing its cost at over a $billion. That seems way over the top.

If Auckland can more economically deal with overflows than Watercare's $800 million Central Interceptor - then wouldn't that be a sensible option? Freeing up around $500 million which could go toward building another sort of tunnel - one for trains instead of dilute sewage.