Work on ponds upgrade going well

Improvements to the oxidation ponds at the Christchurch Wastewater Treatment Plant are progressing steadily, with the first pond being brought back into service and the second one about to be emptied.

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 underway.

The upgrade of the ponds will change how the flow moves through the pond system and provide significant improvement to the wastewater quality before discharge. Currently the flow passes through three ponds in series before it is discharged to the estuary. When the pond upgrade is completed in mid 2004 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. 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,” says Mr Bourke.

The work in Pond 2 is expected to be completed around April next year. 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.

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