Expansion of re-use water at WWTP

Victor Harbor is a popular and rapidly expanding coastal town on the Fleurieu Peninsula, some ninety kilometres south of Adelaide. The Victor Harbor Wastewater Treatment and Reuse Project (VHWTRP) was first commissioned in December 2005. Now with a greater demand for sustainable re-use water in the region, SA Water engaged United Utilities to upgrade the plant and network.

The plant currently consists of an immersed membrane bioreactor (IMB) process using Kubota flat sheet membrane technology. It uses an activated sludge process with carbon dosing (molasses, later replaced by liquid sugar), for biological nutrient removal, and alum for enhanced phosphorous reduction, followed by immersed flat sheet Kubota membranes and ultraviolet disinfection. It also includes three pump stations (PS1, 2 and 3) and some 13 kilometres of pipelines to enable storage of the reclaimed water in the Hindmarsh Valley reservoir.

The expansion project will have to meet the Australian Guidelines for Water Recycling: Managing Health and Environmental Risks Phase 1:2006. Within these guidelines, log reduction values (LRVs) for virus, protozoa and bacteria are required for specific re-use applications (‘fit for purpose’). Each existing process unit at the VHWWTP was assigned an LRV, and the options for the additional disinfection requirement were tabled. The required LRVs for the treatment plant were based on the greatest level of human exposure, that being daytime outdoor irrigation (with hose or similar).

To allow an early expansion of re-use water, an interim chlorination facility was installed under a design and construct (D&C) contract between United Utilities Victor Harbor (UUVH) and SA Water, including three additional LRVs for virus.

Then, an extensive decision-making process was undertaken to ensure that the optimum disinfection solution would be used for the ultimate plant design flow for this contract (5.1 ML/day). Three high-level options were identified:

• Chloramination, requiring CTs of: 712 mg min/L at 15°C; and 534 mg min/L at 20°C;
• UV disinfection (validated); and
• Modification of breakpoint chlorination at 30 mg min/L.

Each of these options had advantages and disadvantages in terms of OPEX, CAPEX, delivery time, compliance, ease of operation and customer supply. A solution that would solve the problem of consistent disinfection and flow while meeting all the needs of stakeholders was required.

Multi-criteria analysis (MCA) encompassing whole-of-life analysis was used to take the decision-making away from the individual, resulting in the selection of the optimum solution with appropriate input from all stakeholders.

Earlier this year representatives from SA Water, United Utilities Australia (UUA) and Arup attended the MCA workshop. Prior to the workshop, ten treatment and re-use options were developed for analysis - outlined in the table. Each option was analysed under a set of criteria and scored out of 10, based on the level of risk.

The MCA workshop resulted in UUVH and SA Water agreeing to proceed with Option 7b which would use breakpoint chlorination and would:

• Improve treatment process upstream of the WWTP by adding balancing storages at PS1 and provide more efficient aeration.
• Enable better disinfection control by moving the dosing point to the discharge at PS2 that continuously achieves the 2006 Reuse Water Guidelines.
• Enable re-use demands to be met via an ‘on-demand’ system by upgrading PS3 and adding a new product water storage.
• Provide the lowest associated risks.
• ]Be delivered in a relatively short timeframe.
• Limit capital costs and present costs.

The chosen system is currently under construction and is scheduled to be commissioned in early 2011.