Chemical kill-off

Illegal drugs cause harm in many ways, including killing Porirua’s sewage treatment plant two years ago.

In 2011 chemicals from a Tawa P lab were dumped into the sewage system about the time of a police raid, says Peter Bailey, Porirua City Council’s general manager of asset management and operations.

The chemicals ended up in the council’s Titahi Bay sewage treatment plant, where they killed much of the beneficial bacteria that breaks sewage down.

‘‘It’s like using Roundup on your vege garden, in a way.’’

As a result, both the harmful chemical and untreated sewage flowed into the sea, Mr Bailey says.

There was no way to contain the untreated sewage. ‘‘The volume is so great there’s nothing we can do. It would be a massive job to store it in holding tanks.’’

The plant treats all of Porirua’s sewage and much of north Wellington’s, servicing 70,000 people in all.

Its size works in its favour when illegal chemicals are dumped – the huge volume of water flowing through the plant dilutes the chemicals, making them less harmful both to the ocean and to waste plant staff, Mr Bailey says.

Smaller cities are not so lucky and unwanted chemical buildup in small plants can cause explosions.

The P lab incident was the third chemical kill-off in a decade, but usually the culprit is a business illegally dumping industrial chemicals into the system.

Firms can legally dump chemicals in Seaview, Lower Hutt, but there is a steep fee to do so, Mr Bailey says.