Ultraviolet (UV) light is sometimes used in wastewater treatment because it kills a wide range of bugs in the water without chemicals.

The sun is the cheapest and most easily used source of UV light, which is why Christchurch’s treatment plant has large ponds as its final stage.

However, some places also build artificial UV plants to back up their sunlight-on-pond water treatments and get a better bug kill. According to research given to the City Council last month, new public health rules in New Zealand would mean the city would need to install a UV treatment plant if it wanted to keep putting its treated wastewater into the Estuary. And while this would improve safety it would still not lead to a significantly improved safety rating.

The ECan commissioners’ decision earlier this year in effect told the Council to stop using the Estuary. It also said the Council should quickly build a UV plant and operate it until the wastewater outlet was moved out of the Estuary.

This is one of the main points City Councillors must consider when deciding if the Council should challenge the commissioners’ decisions.

That is because UV plants are not cheap, either to build or to operate. According to City Council staff, it would cost at least $6 million to build and $500,000 a year to run.

The treatment plant in east Christchurch makes electricity from biogas. Until now it has been making more than it uses and earning money for the city by selling the surplus.

A UV plant would change the plant from a power exporter to a big importer. That is because the amount of bug-killing a UV plant does is closely tied to the amount of electricity pumped into it.

Three other things need to be considered. First, Council officers say a plant built to help protect the Estuary would not be in the best location to be hooked up to an ocean-outfall system. Second, scientists who reviewed all the work done on the Estuary say it is not certain artificial UV treatment is crucial.

Over the next two summers the Council plans to re-route the flow through its ponds and this will lead to a big improvement in the bug kill, providing a useful substitute for the stipulated UV plant. And third, the scientists also concluded that it was far from clear that a UV plant was needed for an ocean outfall.