MEDIA RELEASE

For immediate release, 15 February 2008

Seeking ‘best for Gisborne’ infrastructure

Design consultants for Gisborne’s wastewater project are into some serious number-crunching.

The CH2M Beca team is assessing the best size and location of some equipment to ensure the wastewater infrastructure runs effectively long term, meets the level of environmental protection people expect and is cost-efficient.

Project manager Heather Smith says part of this involves determining the best-sized pipes and hole size of milliscreens, the number of pumping stations, the best location for pre-treatment milliscreening, the best sludge treatment process and maximum flow rate of wastewater to the treatment plant.

These issues will be discussed at the inaugural Wastewater Management Committee next Wednesday (subs:20 February).

The design team suggests the Aerodrome Road wastewater treatment plant site is better for more pre-treatment screening than the current milliscreening plant site at Stanley Road.

“For economic, hydraulic and construction reasons, the Stanley Road site is not the most practicable solution. The Aerodrome Road option would minimise cost and enable a more innovative layout to be developed. Much of the existing infrastructure at Stanley Road is at the end of its useful life and would have been replaced in the next few years even if a new wastewater treatment plant wasn’t required.”

Other benefits of the Aerodrome Road option include the ability to pump wastewater from the western industrial area and Makaraka, if reticulation goes ahead, straight to the treatment plant for screening and treating rather than being pumped into town. Improved aesthetics of the Adventure Playground-Stanley Road area would also result.

“We can have a dedicated, cost-effective pre-treatment plant with all cleaning and dewatering processes for screenings and grit centralised at Aerodrome Road.”

Beca also favours a more appropriate level of milliscreening to ensure good environmental protection without excessive capital and operating costs.

“We want screen sizes to better suit the three main wastewater flows rather than automatically retaining the current 1mm sized screens,” Heather Smith says. “We propose 1 to 3mm screens (depending on the screens being slotted or perforated) for domestic sewage and separated industrial waste stream, and 4 to 6mm screens for overflows likely to occur in extreme storm events once a year when the flow would be diluted. Very fine screens can actually cause more operational problems, removing solids that would be better converted by treatment through the biological trickling filter plant. The secret is selecting the right balance between performance and cost.”

The team is also focusing on the maximum rate at which domestic sewage and non-separated industrial wastewater flows are pumped to the Aerodrome Road site. This design “flow rate” determines the size of the pipes and pumping stations required.

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