Sewage treatment contract showcases expertise

Scheduled for completion by May 2004, the project will incorporate the following treatment stages: A new inlet works, activated sludge aeration lanes, final effluent clarification, rapid gravity sand filtration complete with air scouring and backwashing, final effluent pumping station with disinfection and pipe-work, sludge thickening and dewatering, ancillary services and controls.

The phase 6 extension will increase the capability of the treatment works to process an additional 37,400m3/day of sewage from the projected increase in population.

Inlet works

To ensure that loads and flows to the treatment works are proportionally balanced between phases, and to reduce the effect of peaking from any of the pumping stations feeding the existing plant, all incoming streams of sewage will be combined in a new pressure sump at the inlet works. The incoming sewage will be split between the existing phases and the new phase 6 inlet works, which will consist of fine screens, screenings compaction, grit removal and flow monitoring.

Screened sewage will be combined with the return activated sludge in a pre-mixing tank prior to splitting the flow to the aeration lanes. Four 58m x 11.6m x 4.5m working depth aeration lanes will be installed with fine bubble diffused aeration systems to provide biological treatment of the sewage. The aeration lanes will be split into five sections with the first section operating as an anoxic zone to allow de-nitrification and improve sludge settlement. The remaining four sections will have progressively reduced aeration capacity to optimise process control.

Activated sludge

Mixed liquor from the aeration lanes will gravitate to a distribution chamber to be evenly split and fed to four 25m diameter settlement tanks, each fitted with tri-arm scrapers. Return activated sludge from the settlement tanks will gravitate via actuated bellmouths to a return sludge wet well, where four pumps, operating as duty/assist/standby, will control its return to the premixing chamber.

The sludge treatment capacity of the works will be extended by the installation of three belt thickeners and two additional filter belt presses to ensure the additional sludge produced can be dewatered.

To ensure an excellent quality of final effluent, ten rapid gravity sand filters will be installed, together with a screw pump lift station, air scour and backwash facilities to maximise performance. Treated effluent from the plant will be disinfected with ultraviolet radiation and pumped to storage prior to distribution for irrigation.