OVERVIEW

Melbourne Water manage the water resources in Melbourne, Australia. The Western Treatment Plant treats over 50% of the city’s sewage. Melbourne Water delivers over 430 billion liters (114 billion gallons) of safe drinking water and treats over 300 billion liters (79 billion gallons) of sewage from across Melbourne.

The existing floating cover of the Western Treatment Plant (WTP) had been in service since 1998 collects biogas for electricity generation. Evoqua Water Technologies first provided biogas collection covers to Melbourne Water at the WTP site in 1998. Since then, a variety of maintenance and upgrade projects have been completed until the replacement project for the 25W lagoon in 2016 and this project for the partial replacement of the 55E cover in 2021.

The 55E Biogas Collection Cover project required the removal and replacement of nearly half of the existing 95,000 m2 (1 million ft2) cover. The existing cover was under excessive stress and badly distorted due to floating sludge and materials in the raw wastewater lagoon. This wear had made the cover unsafe and unserviceable.

Melbourne Water’s construction contractor was engaged to remove the damaged 43,000 m2 (463,000 ft2) section of the existing cover. On site technicians from Evoqua Water Technologies replaced the cover with a revised design to minimize future issues with sludge and material accumulation and allow for periodic opening of the cover for solids removal.

CHALLENGE

The design for this project was challenging. The pond needed a cover that would provide reliable biogas collection, simple rainwater disposal, be able to withstand a 1:100-year wind event and have the means to open and close to provide access for solids removal and maintenance.

Relying on the experience from the previous work on the 25W biogas cover, an innovative design was developed. The components of the replacement cover segment are oriented in the direction of flow with no undercover protrusions which could conflict with the accumulation and movement of the floating solids. The replacement segment comprised of eight panels which align with the influent pipework. Embedded foam floatation allows the panels to be separated and folded open for future biosolids removal.

To ensure the cover could withstand the environment, it was made with:

- High UV resistant HDPE   

- GSE HD in white reflective to limit thermal effects of solar heating

- Layout and orientation of the cover promoted drainage and collection of rainwater in consideration of the floating solids accumulation

- Maintenance friendly rainwater removal components

- Computerized wind analysis results used to calculate anticipated forces from high winds

- Control system upgrades for biogas withdrawal

The lagoon remained in service throughout the project, presenting further design challenges. Temporary biogas management systems, intermittent biosolids removal and high-level health and safety protocols were required.

During installation, geomembrane sheets and floats were laid out according to the design and launched across the open water pond with rigorous quality control procedures in place. The cover section was attached to a perimeter concrete ring beam and then attached to the existing portion of the geomembrane cover.

Extensive commissioning activities were undertaken including oxygen testing for multiple 24-hour periods. The new cover system tested exceptionally and was put into service and the asset was handed over to Melbourne Water.

SOLUTION

GSE HD 2.0 mm smooth and single-sided textured geomembranes were used to construct the cover because the material needed to stand up against a harsh environment from chemicals in the lagoon and exposed conditions.

The material was chosen based on the following advantages:

- It is resistant to the effects of floating biosolids accumulation and movement.

- It is retractable for future biosolids removal.

- It is chemically and physically resistant to wastewater and biogas.

- It is resistant to environmental conditions (UV, precipitation, and wind).

- It is simple and safe to maintain.

The replacement cover has economic, environmental, and social benefits. According to Vanessa MacLean Dunphy, Business Development Specialist at Evoqua, the cover collects over 60,000 m³ (2.1 million ft3) of biogas per day worth over USD 2.3 million per year and contributes to generating over 40% of the site’s electricity requirements for treatment processes.

The green energy collected reduces the overall energy demand and reduce emission of greenhouse gas. It also reduces the odor from lagoon, providing a better environment for the surrounding community.

The customer was extremely satisfied with the cover installation, not least because they were able to keep the lagoon in constant operation during the project and utilize it for biogas collection.