ACWa wins Sharjah sewage contract

By Peter Ripley

UK-based ACWa Services and its sister company Consolidated Contractors International Company (CCIC) SAL won a contract to design, install and commission the Phase 6 extension to Sharjah Sewage Treatment works in the United Arab Emirates.

The contract forms part of a US$ 28-million project being carried out by CCIC for the Sharjah Municipality, with the Halcrow Inter-national Partnership acting as the client's engineer.

Scheduled for completion by May 2004, the project includes the following treatment stages: 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 37,400 m3/day of sewage from the projected increase in population. All incoming streams of sewage will be combined in a new pressure sump at the 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. 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.

Activated sludge aeration

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 58-m by 11.6-m by 4.5-m working depth aeration lanes will be installed with fine bubble diffused aeration systems to provide biological sewage treatment. 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.

Mixed liquor from the aeration lanes will gravitate to a distribution chamber to be evenly split and fed to four 25-m-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 pre-mixing chamber. Surplus sludge and scum removal will be included in the wet well design.

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 that additional sludge produced can be dewatered. This will ensure that ideal operating conditions can be maintained for the aeration system.

Tertiary treatment

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

A new SCADA system will be installed to incorporate the whole of Sharjah sewage treatment works, connecting various programmable logic controllers (PLCs) from phases 11-V and the new Phase 6, via Ethernet links, to the main control room. The SCADA system will provide operators with monitoring and set point control for each area of the plant.