Design Build Operate - The Mangawhai Community Wastewater Scheme - Lessons Learnt

The Mangawhai Community Wastewater Scheme was a design, build, transfer, operate (DBO) project encompassing collection, treatment and disposal. This paper describes some of the more interesting facets of the project and draws on these to develop a number of lessons that may help other designers in the future. Pressure sewer systems were installed in the low lying areas of Mangawhai Village. Anaerobic activity in pressure sewers was greater than anticipated and the selection of materials for these conditions is discussed.

Measurement and control systems are considered by the authors to be critical to long term operational efficiency. PLC controls and SCADA were developed in conjunction with the electrical and controls contractor who was appointed early in the construction process. The early involvement of the contractor allowed the setting of site standards that were applied across the various equipment supply contracts. These systems have recently been updated a process we consider essential to ensure operational reliability.

Those charged with setting contractual and regulatory limits need to give more consideration to whether the limits are indeed within the ability of the parties to influence them. A limit in the DBO contract on maximum influent NH3 was never met; more importantly it was not within the control of the council who were required to deliver effluent meeting this standard.