Options for wastewater disposal

Tuesday July 04, 2017 Written by Published in Local
Some crucial decisions yet to be made about the disposal of the island’s wastewater will affect future generations such as these two youngsters, seen relaxing by the lagoon, with the ocean beyond. One option is disposal at sea. 17070345 Some crucial decisions yet to be made about the disposal of the island’s wastewater will affect future generations such as these two youngsters, seen relaxing by the lagoon, with the ocean beyond. One option is disposal at sea. 17070345

One of the key components of the Mei Te Vai Ki Te Vai project is the design of wastewater infrastructure for the Muri/Avana area.


This involves designing a reticulation system in which wastewater is collected from the majority of properties in the area and transported through pipes to a single off-site location for treatment and then disposal.

The wastewater reticulation system for Muri/Avana has received funding from New Zealand, including the $8.8 million dollars announced by New Zealand prime minister, Bill English, during his visit to Rarotonga last month.

The Muri/Avana area is the initial focus stage of the design given the importance of addressing the water quality of Muri Lagoon. but other areas around the island are likely to follow.

The current concept design stage of the project involves looking at the options for how the wastewater is collected, to what level it is treated, and the method of disposal. These are fundamental decisions that form the foundation of the design. A draft concept design will be presented to the Cook Islands Government at the end of this month.

Mei Te Vai Ki Te Vai Project Management Unit (PMU) project manager Evan Mayson says the evaluation of options includes a range of considerations including public health, environmental impact, engineering feasibility, operation and maintenance, social impact and acceptability.

One of the key decisions made at the concept design stage is how the treated wastewater will be disposed as part of this new wastewater reticulation system. From a long list of options, there are three options being considered further, each with their own challenges. 

The first option is land disposal which involves spraying or drip feeding treated wastewater onto land. One of the biggest challenges with this option is finding suitable land for this purpose. For treating the wastewater from the Muri/Avana alone, a near flat area around the size of Rarotonga’s airport (about 40 hectares) would be required. In the absence of available of land, or the community pooling sufficient land together, the government would need to consider acquiring properties for land-based disposal of wastewater.

The second option is disposing treated wastewater through a gap in the reef when currents will provide dispersion. A pipe could be placed on the floor of the Ngatangiia harbour mouth.

The third disposal option is discharging treated wastewater past the reef, deeper into the ocean (approximately 30 metres), known as ‘ocean outfall’. This would involve tunnelling through and beyond the reef. The installation of the pipe will be challenging constrained by geology.

All the options require further investigations. The possible discharge to the sea requires information on ocean currents and modelling. The key issues for the land disposal option is the availability of suitable parcels of land.

The extent to which wastewater needs to be treated, differs depending of the disposal option. Ocean outfall would at a minimum require screening to remove debris, whereas land disposal and tidal discharge are likely to require secondary treatment, which involves using biological processes to remove dissolved organic matter (including nutrients).

Mr Mayson ¬is encouraging the community to share their thoughts on the options and the considerations that need to be addressed, as part of the project’s commitment to consider community values as part of the decision-making process. “This is about working together to design a system that works well and for that, we all need to play a part.

“Our job as technical experts is to look at the environmental science and the engineering solutions to develop a functional solution.

Your job as community members, is to raise the issues that are important to you so that they can be taken into consideration.”

If you would like to have your say on the options for the wastewater reticulation system, come along to the community workshop 6pm tomorrow night at Muri Meeting House.

If you are unable to attend the workshop, members of the PMU will be available to discuss the project from 10am to 12pm tomorrow outside BCI Bank in Avarua and from 3pm to 6pm on Thursday outside Muri Meeting House.

You can subscribe to news updates on the project website vaikitevai.com/subscribe or call 28851 for more information.

                - Release

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