Winter stormwater flows affect sewer system

Every winter the peak flow of water through our sewer system is up to 15 times higher than in the summer. By national standards, it should only be about six times greater in winter. Why is Nelson's so much more? Too much rain is getting into the sewer pipes instead of draining into the ground or stormwater channels.

Why is this a problem?

Our wastewater system has to cope with a much larger flow than necessary - more pipes, pumps, and storage, which cost money to build and maintain. Wastewater needs treatment before being discharged; treating stormwater through the same system is a costly, unnecessary practice. Our wastewater system will last longer, with fewer repairs, if we can reduce the amount of stormwater going through. During high rainfall, the system is prone to overflow. This can be a serious health hazard if overflows spill into a waterway or coastal area.

How does stormwater get into the sewer system?
While some rain enters through gully traps, most stormwater gets into the wastewater system through broken sewer pipes - the 'mains' and the 'laterals'. We keep the main sewer pipes working, but private property owners are responsible for the laterals - the pipes which connect their property's wastewater drains to the main.

What are we doing about it?

Council adopted the Wastewater Asset Management Plan in April 2005. It sets a long-term goal of reducing the peak wet weather flow to six times the average dry weather flow (it's currently 15 times higher). How are we going to do this? We'll be carrying on with the renewal of our sewer mains. As part of the renewal process we pressure test the laterals connected to the main to check for leaks. If we find a leaky lateral, the property owner will be served with a notice to repair or replace it. To make the repair more affordable, the owner can have it done by our contractor while they are repairing the main.

We'll also be conducting inspections of laterals in areas where we know excess stormwater is getting into the system even though the mains are in good condition. In these cases, we will notify the owner. Repair or replacement will be their responsibility. Before we carry out any sewer main renewals, we'll be getting in touch with the area's property owners and residents to let them know what the plans are and discuss the available options.