Westernport Water provides water supply and wastewater collection and treatment services to an area of 300km2 about 100km south east of Melbourne, centred on the holiday resort of Phillip Island. Although the serviced area only has a resident population of about 20,000, it experiences peaks of over 100,000 persons during events and holiday periods.

The Cowes wastewater treatment plant, which was a conventional extended aeration, activated sludge plant with disinfected effluent discharge to Bass Straight, was at full capacity. An analysis of treatment options recommended modifying the existing plant to biological nitrogen removal (BNR) rather than constructing a new plant. The upgrade involved converting the extended aeration process to BNR, while increasing the overall plant capacity to handle projected influent loads up to 2021.

The major changes involved:
• Incorporating an anoxic zone for biological denitrification;
• An internal mixed liquor recycle of up to twice the average daily flow;
• Peak wet weather loading is to be handled by a new storm bypass system to protect the biological process to an enlarged storm storage facility.

To provide increased capacity, the $2.5M upgrades were made by Simon Constructions to:
• the inlet works
• bioreactor
• internal pumping station
• sludge digester, and
• sludge drying systems.

The major challenge was to provide continuous operation of the plant within the licence limits during construction. This was managed by careful scheduling of construction activities to select the best timing and duration of individual work components.

The upgrade was complex and challenging and was completed on time in June 2015 by contractor Simson Constructions and within budget with no breaches of the licence effluent limits. Since commissioning of the anoxic tank effluent quality has improved despite increases in inflow volumes.