Concrete Protection to Primary Settlement Tanks & Common Collection Chamber Project, Bellozanne STW (Revised Tender - Concrete Protection to Primary Settlement Tanks & Common Collection Chamber Project, Bellozanne STW)

The construction of the new Bellozanne Sewage Treatment Works (STW) was completed in December 2023 following a phased handover and commissioning. The project replaced the primary, secondary and tertiary treatment systems and refurbished elements of the original infrastructure to increase the storm water storage capacity.

The STW is designed to accommodate flows and loads associated with a population equivalent of 118,000, with the provision for further 20% increase in capacity. All process structures (tanks, chambers, channels etc) are designed for an 80-year design life.

The works included the construction of three circular precast, post-tensioned concrete Primary Settlement Tanks (PSTs), each with an internal diameter of 28.76m, and a downstream concrete PST Common Collection Chamber of 2.4m diameter. These structures were completed and commissioned early in 2022 and have been in continuous operational since.

Due to the covered configuration of the PSTs, an enclosed atmosphere is maintained to control odour. The environment promotes the accumulation of hydrogen sulphide (H2S), which oxidises and condenses on exposed concrete surfaces above waterline as biogenic sulfuric acid (BSA).

This has resulted in early-stage chemical corrosion of exposed concrete surfaces within the PSTs and the PST Common Collection Chamber.The corrosion is present to varying degrees across the structures. If left untreated, deterioration will continue, leading to a reduction in asset life, increased risk of structural degradation and a greater likelihood of unplanned capital intervention and process disruption.

Works are therefore required to mitigate the effects of ongoing BSA attack and restore the durability of the affected structures. The rehabilitation strategy comprises the preparation of existing concrete surfaces and the application of a protective coating system by a suitably qualified specialist contractor. This solution has been identified as the preferred approach to protect the assets and extend their operational life.