What is the stink all about at Seaview treatment plant?

Wednesday, 25 September 2024

The Post senior journalist Nicholas Boyack recently did a tour of the Seaview Wastewater Treatment Plant.

It is hard to ignore the Seaview Wastewater Treatment Plant. Situated in an industrial area of Lower Hutt, it features in The Post for all the wrong reasons.

The plant services 160,000 Hutt Valley residents and is approaching the end of its life. Locals ‒ and those who live as far away as Waiwhetū ‒ regularly complain about the foul odour.

When there is heavy rain, the operators are forced to pump treated effluent into the nearby Waiwhetū Stream, but faecal levels in the discharges are too high and are having a negative impact on the stream.

Wellington Water is currently spending $13 million to address the odour problem and in the long run a further $205m is going into an upgrade. The issue of the overflow into the stream is not part of the upgrade and a solution ‒ a new outflow pipe ‒ would cost at least $1 billion.

Wellington Water communications advisor John Donnachie says the issue of odour is not straight forward. Some people complain they can smell it miles away, while others much closer can not smell anything.

So what does the plant look and smell like from the inside?

Even up close to the milliscreens where the large bits are separated from the rest, there is remarkably little smell.

Descriptions like a “sheep farm” or a “shearing shed” are used to describe the odour, which overall is not too strong.

The strongest whiff comes out in the open, where the sludge dryer unloads the dried-out material to be taken to the Silverstream Landfill. The dryer is contained in a large building, and inside the building there is next to no smell.

The dryer is the final stage in the treatment of sludge. The de-watered sludge is dried, resulting in small pellets which look a lot like the sheep pellets used for gardening, and are sent to the Silverstream Landfill.

Getting the drying process right is important, said Wellington Water wastewater advisor Steve Hutchison.

“We do not want it to become like sticky mud.”

Work is planned to replace the dryer and to better contain the odour.

The biggest piece of work is upgrading the biofilters, which contain bark and micro-organisms that break down the odour.

Standing right next to the filters there is very little pong and Hutchison happily plays with the bark as if it is a sand pit.

The large aeration basin is a bubbling mass of water, where micro-organisms chomp through the greyish water.

“Anything that is left over goes to the landfill,” says Hutchison.

Like the sludge dryer, the main pumping station ia remarkably free from odour.

Three large pumps send the treated effluent on an 18km journey to Pencarrow where a large outflow pipe deposits it in the Cook Strait.

The current pipe runs under the road to Eastbourne and will one day need to be replaced.

Hutchison notes the joints in the pipe are “prone to failure” and the pressure in the system has to be managed carefully.

“We have to be careful how much pressure we put on the pipe in order to minimise the joint leaks.”

Overall, the plant is noisy in places, a bit smelly in others and the buildings are showing their age.

Hutchison said much of the planned work is to better contain and capture odours. He warns, however, that there will always be some odour.

The aim of the work is to make sure there is “no objectionable or offensive” odour beyond the boundary of the plant.