Perth Metropolitan Wastewater Treatment Plant Upgrade, Australia

The Metropolitan Wastewater Treatment Plant (WWTP) upgrade is one of the most substantial wastewater programmes being carried out in Western Australia. It involves upgrade of the three largest WWTPs in Perth. It is the first time in the state that three large WWTPs are being upgraded simultaneously.  These three WWTPs are located at Woodman Point, Beenyup and Subiaco.

The programme was ordered in December 2006 to meet the growing population needs of Perth, the capital city of Western Australia.

The work began in February 2007 and is expected to cost A$352m ($270m). The upgrade programme is expected to be completed by 2011.

Key players

The three WWTPs currently being upgraded are owned and operated by Water Corporation. To implement the project, a strategic alliance was formed between Water Corporation and W2W Alliance. The W2W Alliance consists of Thiess, Black & Veatch, Sinclair Knight Merz, and the Water Corporation.

Project

The project includes conceptual design, engineering, procurement, construction and commissioning services to increase capacity of the three metropolitan wastewater treatment plants.

The treatment capacity of the Beenyup WWTP will be increased by 15m litres a day (mld). Woodman's solids treatment capacity will be increased to make-up with the plant's overall hydraulic capacity.

The work at Woodman also involves reduction in odour emissions by 50%. Odour emissions reduction will help Water Corporation save chemical costs of about $660,000 each year.

The project is meant not only to increase the treatment capacity of the plants, but also to reduce environmental impacts by applying sustainable design and operation features.

Upgrades

Woodman Point WWTP provides services to 600,000 people living south of the Swan River in the Perth metropolitan area. Present treatment capacity of the plant is 160mld of wastewater, but on an average, it treats 120mld of wastewater.

The initial capacity of the plant was 125mld. In 2002, the plant was upgraded at an investment of $150m to raise its capacity to 160mld. Another upgrade began in 2007 to increase the solids treatment capacity of the plant to 160mld. The solids treatment capacity will be improved by constructing a third egg-shaped digester tank with a capacity of 8,000 cubic metres. Odour management facilities will also be improved significantly to reduce odour emissions by at least 73%. The project is estimated to cost $137m.

The present capacity of the Beenyup WWTP is 120mld. It serves 650,000 people in the northern suburbs. A $147m upgrade was started in September 2007 by W2W Alliance to increase the treatment capacity of the plant to 135mld. The plant is being upgraded in such a way that if required, it can be further developed to treat up to 200mld, serving 1.1m people.

The Beenyup WWTP upgrade work involves construction of four secondary sedimentation tanks, three sludge thickeners, two 600 cubic metre sludge digesters, an additional grit washer, three primary sedimentation tanks,  an aeration tank and installation of a sludge dewatering centrifuge. Reduction of odour emissions will be achieved by constructing a new odour scrubber system, and installing an enclosed sludge load-out facility with covers and odour duct work.

Treatment

The wastewater treatment process at the Woodman Point WWTP is computer-controlled and monitored by expert managers and technicians. Before removing grit such as sand etc. through settling tanks, the wastewater is screened to remove any larger objects such as paper, rags etc.

The wastewater is then kept in large sedimentation tanks for several hours to get rid of settleable solids. The sludge coming out from these tanks is treated further. Egg-shaped, two 38m high anaerobic sludge digesters are then used to stabilise the sludge to convert it into residue. These residues are produced through bacterial actions at 35 degrees celsius in the digesters.

An underground pipeline of 23km is used to transfer the treated wastewater to a tower at Cape Peron, from where it is discharged via a 24km ocean outfall into the 20m deep Sepia Depression.

The treatment process at the Beenyup WWTP involves the following steps: first the wastewater is made to pass through step screens for the removal of materials such as paper and rags. The water is then aerated through circular grit removal tanks. This process allows the organic material to pass through and inorganic material to settle. After grit removal, the wastewater is passed through large, rectangular primary sedimentation tanks. Nearly 50% of the suspended solids settle down as sludge.

The sludge is transferred to the solids handling area for thickening and digestion. Secondary treatment is then applied to the treated liquid leaving the tanks. It involves generating microbiological life through aeration tanks to consume the organic material present in the wastewater. The wastewater coming out from the aeration tanks passes through circular clarifiers to obtain high quality secondary effluent. The final treated wastewater is suitable for ocean discharge.

At Subiaco WWTP, materials such as wood and rags are collected from the incoming raw wastewater. Flowing wastewater is then collected into grit chambers. The settled grit, together with the screenings, are disposed of as landfill.

After this preliminary treatment process, the wastewater enters the primary sedimentation tanks, which allow the sludge to settle down.

The settled sludge is pumped directly to the sludge blending tanks for further processing, after being collected into a central hopper through scrapers.

The water from primary sedimentation tanks is collected in 11 reinforced concrete aeration tanks, where it is blended with microbiological biomass to breakdown the organic compounds in the presence of oxygen. The blended liquid then passes into one of the 12 circular sedimentation tanks. The settled sludge from these tanks is removed continuously and returned to the aeration tanks to maintain its microbiological population. If the settled sludge is in excess, then it is pumped to one of two dissolved air flotation thickeners (DAFT). The sludge from DAFT is transferred to the sludge blending tanks for further treatment. The water flowing out of the sedimentation tanks is suitable for ocean discharge.