Residents view options for sewage system upgrade

May 5th, 2013
[by David Armstrong]

It may be a matter of 'out of sight, out of mind', but Motueka's Wastewater Treatment Plant needs urgent attention and upgrading it to an acceptable level won't be cheap.

The plant, also known as the sewage works at the northern end of Thorpe Street, is at the end of its reliable life, and its upgrading can no longer be deferred or delayed, Tasman District Council says.

Alternative plans for an upgrade to properly handle present and future needs were presented to interested residents during open days yesterday and on Friday, including bus trips to view the area and understand the situation first-hand.

The open days were run by the council's consultant on this huge project, civil engineering company MWH. Residents who availed themselves of the opportunity to see the current problems and scope the upgrade options were clearly impressed by the amount of detail and understanding that is going into planning.

For those who may have doubted that large amounts of money were necessary for this project, it became clear that the council is taking the future wastewater treatment needs of Motueka very seriously.

Following the heavy rains of two weeks ago, the ponds are still overflowing directly into the wetlands beside the Motueka River south branch as input exceeds current capacity, as the following photo at the overflow area shows.

With groundwater and stormwater adding to the load, the plant was receiving about five times the volume it could handle over the worst of the rain event, and raw sewage spilled out of the oxidation ponds ar.

So what can be done about it? MWH engineer told the viewing group that although the very long term plan (30 years or more) will be to build a new site further inland, but until then an upgrade of capacity and new methods of waste discharge is urgently needed and work must begin within a year.

Adding to the pressure is the changing shape of the adjacent coastline and river path, a need to plan for rising sea levels, and the fact that the present discharge paths are now irretrievably clogged with algae.

The treatment plant was first constructed in 1980, and in 1989 an aerated lagoon was added to increase treatment capacity for waste water from Kaiteriteri and Riwaka. The original discharge was to river and land, but the tidal River discharge was abandoned when the River mouth moved north.

Due to insufficient capacity in the sand soakage beds, they progressively clogged with solids carried over from the oxidation pond. For the past two years the treated waste water has been largely discharged uncontrolled via overflow into the wetland.

A working party, which will report to the council's Engineering Services Committee, is looking for a solution which will be in the best interests of the community, for the treatment of waste water from Motueka, Riwaka and Kaiteriteri. The working party includes Councillors, the Motueka Community Board, District Health Board, Department of Conservation, Fish and Game, Wakatu Inc and Tiakina te Taiao.

The working party's preferred option is to discharge to land through what are known as Rapid Infiltration Basins (RIBs), each about the size of half a rugby pitch. A trial of a small RIB has been under way.

The RIB takes the wastewater that has initially been treated at the inlet stage and allows the remaining solids to settle for a few days, then the water is flushed out to either another RIB (if there is a series of them) or straight to groundwater.

It is constructed so that its base can be cleaned easily. And if there is a large rain event that causes adjacent flooding, it can be refurbished without great cost. To keep costs low, the RIB would be designed so that in big rain events some of the discharge could temporarily be direct to surface water.

Also to keep costs down, the working party's preferred practical option is a "mix and match" of land and surface water disposal. The cost is likely to be in the region of $7 million to a bit over $10 million, and will need to be funded by borrowing.

For those who can follow maps and diagrams, the proposed option is shown in this drawing. (The Motueka River is directly above the diagram, the square current pond is in the bottom left segment and the "polishing ponds" are in the upper right segment. The Thorpe Street entrance is bottom left.)

The working party is asking for feedback from residents during this month, and will put their resultant recommendation to Council in June, to be approved by August 1st. It will then go to the final concept design and resource consent processes.

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