Protecting our Oceans:
Opua Low Pressure Sewer Scheme

For many years, sewage treatment in the town of Opua in the Bay of Islands was primarily by way of on-property septic tank systems and drainage fields. As the population grew and treatment facilities aged, there were ongoing issues with systems failing to meet environmental discharge standards.

Challenging topographical and geological conditions made the construction of a gravity sewer reticulation system prohibitively expensive. This led to the Far North District Council issuing an invitation to tender in 2012 for the Design and Construction of a Low Pressure Sewer Scheme. The Scheme would service 160 properties with houses or other dwellings, and provide facility for the connection of a further 30 properties, should they be developed in the future.

United Civil Construction won the contract and were faced with overcoming several significant challenges in order to deliver the project successfully. This included the acceptance of a high level of design risk, and extremely challenging topography which made property works very difficult. Additional challenges included inflexible timeframes, funding constraints, and working in an urban area which required extensive consultation with often absent owners.

Key features:

  • An 81m difference in elevation required a network design incorporating innovative engineering to deliver a cost effective solution.
  • Challenging topographical and geological conditions, including narrow and winding roadways on ridge lines, limited machinery access (requiring extensive works by hand) and the available room to construct trunk rising mains.
  • Highly challenging design, due to the large difference in elevation, and the need to limit the number of booster pump stations and minimise sewage retention times.
  • With the objective of improving water quality and reducing any risk to public health, United Civil ensured both its design solution and construction method were developed whereby the risk of overflow, even in cases of extended power outage, were minimised to the point that such events would be considered extremely unlikely
  • Many properties had absent landowners (who were renting out their homes) for much of the year, making it particularly challenging to keep such landowners informed of works on their property (and agree as to scope of such works), whilst at the same time accommodating the requirements of temporary residents of the property.

That the Opua Sanitary Sewer Scheme was completed within tight and inflexible deadlines, on budget and without incident, serves as a testament to the technical expertise, project management and risk management skills of United Civil staff. The project went on to win at the New Zealand Contractors Federation Hirepool Construction Awards in 2013.

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