Our sewer network is made up of pipes, pumping stations, storage tanks, a treatment plant and an ocean outfall discharge pipe. In many parts of the city this network is old and leaky and has had little or no renewal over the years. Where the pipes are cracked stormwater can leak into the network (infiltration) and sewage can leak out (exfiltration).

The extra water that enters pipes during infiltration increases the volume of wastewater we have to treat and places heavy loads on our pipes and pumping stations, sometimes causing wastewater to overflow into streams and onto beaches. This usually happens after heavy rain, and wet weather sewer overflow events and urban stormwater runoff are the most common causes of pollution on North Shore City beaches.

The sewer overflows are considered the greatest risk to public health and in order to improve beach water quality we must reduce the number of wet weather overflows. In February 2001 we consulted the residents and ratepayers of North Shore City in order to establish a target for Project CARE and there was widespread public support for a 65 per cent improvement in beach water quality with a programme of works extending over a 20-year period, to 2021. The works will decrease wet weather overflow events in North Shore City from an average 12 per year down to two per year.

Other projects and activities such as stormwater-friendly practices, the installation of Enviropod stormwater filters and the WaiCare stream monitoring programme are starting to address the harmful effects of stormwater on our beaches.

Finding the problems

Before a problem can be fixed the source of the problem has to be found. Over the years, our investigations have told us that the source of a wastewater problem is not always obvious. For example, sewage contamination on Milford Beach can actually result from rain falling in Devonport.

Our wastewater network is made up of pipes, pumping stations, storage tanks, the Rosedale Treatment Plant and an ocean outfall discharge pipe. In many parts of North Shore City the wastewater network is old and leaky. Over the years it has had little or no maintenance.

Where pipes are cracked and broken three problems occur

Infiltration

When stormwater flows into the sewage pipes following heavy rain it increases the volume of sewage to be treated. E.g. through leaky pipes, manholes, leaky private drains, or stormwater flowing directly into the wastewater network from our roofs and gully traps.

Overflow

When infiltration places heavy loads on our pipes and pumping stations and causes sewage to overflow into streams and onto our beaches. Stormwater draining illegally into the sewage network contributes significantly to this problem.

Exfiltration

When sewage leaks out of the network into the surrounding area, it occurs mainly during dry weather. This sewage may eventually find its way into stormwater and ends up once again on our beaches and in our waterways.