New milliscreen units fitted at Banks St plant

The first major mechanical componentry for the Banks Street Wastewater Treatment Plant was fitted this week, marking another significant milestone for the $39.5 million project.

Four sparkling new stainless steel milliscreen units and their individual covers, weighing a total of 10 tonnes, arrived on two truck and trailer units on Wednesday, having been loaded in Dunedin on Monday.

Dunedin’s Farra Stainless Ltd was awarded the $1.26 million contract to provide the milliscreens and screening handling equipment. Farra designed and built the milliscreens in Dunedin, and the special Kuhn screenings presses were imported from Germany.

The units were craned into position within the pre-treatment building, the roof of which is yet to be installed. They were lined up with four large pipes that will in time convey untreated domestic and industrial wastewater to them. Two units are specifically for domestic wastewater, the other two for industrial. The milliscreening units currently used at Stanley Road are 20 years old and have been due for replacement for some time.

Gisborne District Council project manager Peter McConnell said only one domestic and one industrial milliscreening unit would operate at a time, with the other unit always on standby. “The units will be operated by a pre-determined programme with each screen operating alternatively, week about. This will enable both to be well-maintained and kept in good working order.”

A stainless steel drum rotates within the milliscreen unit. Solids are sieved out as the screened wastewater passes through very fine 1mm stainless mesh and into a pipe to be conveyed through to a grit chamber, where sand and fine grit less than 1mm in size is removed via a vortex system, dewatered and bagged for landfill disposal.

“The screened and de-gritted wastewater is then pumped up through the biological trickling filter tank’s central column to a height of 8m. From here it will be distributed via six rotating arms to trickle slowly through 10 layers of plastic media, standing 6.1m high, as part of the biotransformation process which transforms the remaining fine solids into plant-like matter.

“The treated wastewater will be collected at the bottom of the trickling filter tank and combined with 1mm milliscreened industrial effluent in the new outfall pumping station, then pumped through to connect into the existing outfall, and discharged 1.8km into the sea.”

The solids removed in the milliscreening process are collected, the water squeezed out of them and the dewatered solids are loaded out into bins before being trucked to the Paeroa landfill.

HEB Structures project manager Colin Newbold said there were now 51 people working on the Banks Street site, 26 of them from Gisborne. Gisborne’s Universal Engineering has a significant subcontract fabricating and installing pipework and fittings in the pre treatment building and the three major pump stations on site. The company fabricated and supplied the stainless steel componentry that fits between the main inlet channel and the four milliscreens.

Sidebar on liquefaction

The new Banks Street wastewater treatment plant has been designed to cope with potential liquefaction of the site from a severe earthquake such as occurred in Canterbury.

Gisborne Wastewater Project director Garry Macdonald, of CH2M Beca, said this week that it had been interesting to see the damage to all sorts of structures in Christchurch caused by the liquefaction of sandy, silty soils in combination with high groundwater levels.

“That’s one of the big seismic risks at the Banks Street site and the reason for the hundreds of stone columns constructed to strengthen the ground, and for the heavily-reinforced concrete structures.”