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Significant improvement in treatment

17 Feb 2011

Significant improvement in treatment

The city’s Banks Street wastewater treatment plant project is all but complete and already operating above expectations. The plant is now operated by Gisborne District Council staff and the utilities maintenance contractor.

Project manager Peter McConnell told February's Wastewater Management Committee that even after just a month of full operation, with all the city’s wastewater being treated through the plant, the improvement in the quality of the discharge compared with the old Stanley Road plant had been significant. The reduction in total oil and grease, total suspended solids, biological oxygen demand and enterococci levels were better than thought possible at the design stage. This could be attributed to the greater surface area of the structured (rather than random) plastic media with which the biological trickling filter tank was filled.

The lack of a visible plume in the bay since the plant was commissioned was also hailed as a positive sign of improvement.

Councillor Roger Haisman said the results were encouraging considering the plant had been going for such a short time.

Mr McConnell said the data was still preliminary.

“One would need a data set of at least six months to definitely quantify plant performance. But so far so good.”

Enterococci levels were being greatly reduced to date which meant that when UV disinfection is commissioned in mid-2012, the UV plant may be smaller than originally thought with potential savings in energy costs. Installing a UV plant is a condition of the wastewater discharge resource consent.

Mr McConnell said the Banks Street plant was commissioned on 28 December 2010, days before the 31 December 2010 deadline set out as one of the conditions of the 35-year wastewater resource consents for treatment and disposal granted in late 2007.

“The decision to put 30 percent of the city’s wastewater flow through the plant to begin with was good because of the little issues that arose during commissioning. It was much easier to deal with a smaller volume than the total city’s flow. Some changes were made to the process control procedures after observations by council staff, consultants and contractors.”

A problem with odour several weeks ago, which promoted some complaints, was traced to a minor construction error in the biofilter. This had now been rectified.

“Now it is down to good maintenance practices. The new screens are vastly superior to the old Stanley Road screens. We are probably removing a lot of the BOD [biological oxygen demand] at that stage and the grit separation system is very efficient. We are taking a lot of the grit out which in the past would have gone out into the bay and settled at the end of the diffuser pipe. About 400 kilograms of grit is being removed every fortnight or so.”

Industrial wastewater from Bernard Matthews and the Awapuni industrial area were now going through the industrial milliscreen before discharge through the outfall pump station.

HEB Structures, main contractor for the project, is completing minor mechanical and civil works and is expected to leave the site by the end of February.

The plant will be officially opened on 22 March.