Plant now treating all household wastewater

All Gisborne’s domestic wastewater is now being treated at the city’s new Banks Street wastewater treatment plant. The eastern side of the city was linked in early this week to join wastewater from the western side of the city, which has been treated in the new plant since late December.

The historic commissioning of the plant on 28 December 2010 – on time and under budget – went under the radar for most of the community in holiday mode. Commissioning the plant before 31 December 2010 was one of the conditions of the 35-year wastewater resource consents for treatment and disposal granted in late 2007.

Engineering and works manager Peter Higgs said the low profile of the historic occasion was more to do with the commissioning having gone according to plan.

“Everything has gone well so far with only some minor, intermittent odour from the plant. Some odour was produced from sludge in the disused inlet channel at the old Stanley Road site, but this should be gone now the channel has been flushed.

“The Banks Street biological trickling filter tank is fully enclosed with a geodesic dome roof and we have an extensive odour control system in place which, in simple terms, sucks out odour and filters it through three large shingle and bark beds.

“We are pleased with the way everything is going to date with biofilm growing well on the plastic media within the tank. The clarity of the wastewater after being treated in the trickling filter tank is noticeably different to that before treatment.”

Mr Higgs said the project had been a great team effort of people working together for the common goal of having this plant operational by the date specified.

Colin Newbold, project manager for HEB Structures – the company responsible for constructing the plant – said just half a dozen people remained on site to complete remaining works. In mid-December, about 70 people were working on site, half of them from Gisborne.

“We probably have another three to four weeks’ work to go with roadworks and general tidying on site. We are working on pipes in Banks Street so wastewater can be diverted into new sewers. Cedenco’s industrial wastewater will be separated from the main flow within the next few weeks and the rest of the industrial wastewater is expected to be diverted to the new plant by the end of the month.”

The plant will be officially opened on 22 March.

How it works

Domestic wastewater is screened by one of two rotating drum screens (one is on duty, one on standby) and then passed through a vortex grit removal chamber. Two more drum screens are used for industrial wastewater.

The screenings and grit removed from these stages will have the water pressed from them before being bagged and trucked to a landfill at Paeroa.

The screened and de-gritted wastewater gravitates into a biological trickling filter pump station before being pumped up through the tank’s central column to a height of 8m, then distributed through six rotating arms to trickle slowly through many different channels in 10 layers of large plastic media blocks.

In so doing, very fine screened and degritted wastewater solids are transformed into plant-like matter. The resulting treated wastewater is pumped via a new outfall pump station to the existing marine outfall, 1.8km out to sea.