Sustainability Report 2006/07Western Treatment Plant
The Western Treatment Plant treats about 52% of Melbourne’s sewage and services about 1.6 million people in the central, northern and western suburbs.
Internationally recognised wetlands at the 11,000-hectare site are listed under the Ramsar Convention and protected by detailed conservation plans.
In recent years, Melbourne Water completed environmental upgrades that complement the natural processes in the plant’s lagoon systems with new technology. Nitrogen discharges to Port Phillip Bay fell to 1239 tonnes this year, a reduction of 67% since 2000/01, exceeding targets set in the Port Phillip Bay Environmental Study.
Protecting the environment
Mixing zone study
Melbourne Water continued work to improve understanding of the way the discharge from the Western Treatment Plant disperses into Port Phillip Bay, and how to retain the benefits of the discharge while reducing environmental impacts in the mixing zone.
In 2007/08, Melbourne Water worked with research partners in two areas, identifying target areas for detailed examination by assessing likely parameters of concern in the mixing zone, including a substantial effluent toxicity testing program, and undertaking trials to improve delivery of effluent to the valuable migratory shorebird habitat on the shoreline.
As a result of this work, we will be focusing our efforts in 2008/09 on improving our understanding of the role played by particulate organic carbon in helping to sustain the high shorebird populations, and on the fate of nitrogen from the discharge.
Using this work, Melbourne Water will develop an action plan for management of the mixing zone. The plan will aim to reduce the impact of the discharge on the marine environment without compromising the benefit of the discharge to the shorebird habitat and to other conservation values.
Improving performance
Undercover desludging
Sludge that builds up in the lagoons can affect the efficiency of the anaerobic reactors in processing sewage and reduce the amount of biogas produced that is used to help power the Western Treatment Plant. Odour and safety concerns do not allow removal of the covers to desludge the lagoons.
Undercover sludge removal trials have been undertaken and two contractors prepared proposals for a full-scale undercover sludge removal program for the 25 West anaerobic reactor. It is anticipated that a contract for the construction and commissioning of the undercover desludging system will be awarded in late 2008 with full-scale desludging of the reactor to begin in July 2009.
Managing odour
A risk assessment completed in June 2007 recommended a staged program of further works to prevent odour reaching the plant boundary.
Major improvement works include chemical dosing at the Hoppers Crossing pumping station, decommissioning of the main eastern carrier, and replacement and extension of the 55 East lagoon cover.
The new cover will have an innovative design, which will allow removal of small segments to enable open areas of the lagoon to be desludged using conventional methods such as dredging. Previously desludging could be carried out only when large sections of the cover were removed, which released odorous gases. The cover will be replaced by the end of 2010.
Upgrading drying pans
Construction of 24 sludge drying pans, covering about 50 hectares, was completed during the year and the pans were brought into operation. The pans enable sludge to be dried and harvested efficiently, reducing odour risk from the treatment processes and creating opportunities for beneficial use of biosolids.
The pans, which have an impermeable clay liner and a lime stabilised earth floor, are designed to dry 20,000 tonnes a year of sludge.
Fire flares in sludge
Sparks from a fault in a powerline started a grass fire at a remote decommissioned sludge drying pan at the Western Treatment Plant in January.
The fire occurred on a day of total fire ban, with a more severe fire risk forecast the following day. It ignited the sludge, which flared and smouldered, producing thick smoke visible for many kilometres.
Melbourne Water employees and contractors and Country Fire Authority firefighters worked throughout the day to contain the blaze, which took almost a full day to extinguish.
An abandoned pipeline was recommissioned, enabling a significant amount of recycled water to be pumped onto the burning sludge during the evening and overnight.