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NOVEMBER 2009

Dunedin City Council had requested consultants submit proposals to design an effluent disposal system (EDS) for the town of Allanton, estimated population of 650 in 2030.  Following the receipt and evaluation of tenders the Council accepted an alternative proposal from Harrison Grierson to design instead, a pressure sewer system (PSS) for the town.  This alternative will produce project lifecycle cost savings of approximately 25% over equivalent EDS systems.  The estimated lifecycle cost of the PSS system is $4.9 million.

Effluent disposal systems work on the principle that effluent from existing septic tanks is collected in a new gravity sewer network and then pumped to the local treatment plant, which in the case of Allanton is in Mosgiel.  The disadvantages of EDS systems are that reliance is made on old septic tanks which frequently are not in good condition and that gravity systems leak as they age increasing inflow and infiltration of stormwater into the sytem.

Whereas, pressure sewer systems have the advantage of easy sewer alignments and the pipes do not have to be installed deeper than 1 metre below the ground.  Each house is provided with a grinder pump and the wastewater from each house is pumped directly to the treatment plant through a common, public pressure main.  Pressure sewer systems will not suffer from inflow and infiltration due to the fact the system is presssurised and this means lower flows at the treatment plant and lower treatment costs.  This is a far more sustainable approach to providing first time sewerage to outlying communities.

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