The first stage of the upgrade is nearing completion as Pond 1 is refilled following deepening and reshaping. The next stage is to take Pond 2 out of service to allow the construction of an earth baffle to effectively divide the pond into two cells.

Operations and Maintenance Manager Mike Bourke says this will allow similar deepening and reshaping, and also strengthening of the pond banks to reduce the potential for failure in a major earthquake.

Other major works in the ponds involving new piped connections between some of the ponds is already well under way.

The upgrade will change how the flow moves through the pond system and significantly improve the wastewater quality before discharge. At present, the flow passes through three ponds in series. When the pond upgrade is completed the flow will pass through seven ponds in series, and produce an effluent quality that is expected to be some 10 times better than the current discharge in terms of pathogens present.

“This improved, natural disinfection is achieved by reducing the short circuiting of the flow from inlet to outlet of each pond and maximising the time the flow takes to pass through all the ponds,” says Mr Bourke.

Computer modelling of the pond system, coupled with salt tracer testing, has been used to predict the improvements and to determine the optimum shape and depth.”

The work in Pond 2 is expected to be completed around next April. Mr Bourke says that some odour may be evident as the pond initially dries, and any dust potential in the summer will be minimised by restricting the contractors’ work area.