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16 January 2021


Land or ocean outfall for treated wastewater?

Tuesday 28 May 2019 | Published in Environment


Land or ocean outfall for treated wastewater?
Land based option. 19052708

The question of whether treated wastewater should be dispersed of on land or into the ocean was the focus of a public meeting in Muri last week.

Environmental scientist Dr Murray Wallis and his colleagues at the Project Management Unit (PMU) informed the well-attended meeting of progress in the Mei Te Vai Ki Te Vai project, which is now investigating data for the design of vital information to support the wastewater treatment plant, and whether treated wastewater would be disposed of on land or into the ocean.

During question time marine biologist Dr Teina Rongo addressed the meeting “wearing my fisherman’s hat” and said that in all of the Pacific island countries in which he had resided that utilised ocean outfall to dispose of treated wastewater, “the story is not a good one”. He said coral in each of those island states was in poorer condition than coral found in Rarotonga’s waters.

Muri pu tapere, Keta Williams, said he was against the option of ocean outfall. He said in 20 years’ time the residents in Ngatangiia would be living with the decisions made today.

Manavaroa Mataiapo Tutara, Dr Philip Nicholas, said he has been researching water chlorination and asked if this form of water disinfection is used in Rarotonga, would it cause problems for those drinking it as well as for eventual wastewater treatment. The PMU team replied that it would not as the dose would be tiny. Their website at vaikitevai.com says that based on World Health Organisation recommendations this would mean the addition of two litres of chlorine solution to every 10 million litres of water.

The PMU’s vaikitevai.com website says land-based disposal for a new reticulated wastewater system for the developed Muri coastal area would need a total of 18 hectares (44.5 acres) of land; about 2 hectares (5 acres) for the treatment plant, and at least 16 hectares (39.5 acres) for drip-feeding treated wastewater into the soil. “If other areas around Rarotonga need to be connected to the system in future, more land would be needed.”

“The PMU is very keen to hear from anyone who has land they can offer to the project, and will assess any land offered to see if it is suitable.”

Land-based disposal involves either spraying treated wastewater onto land using an irrigation system, or drip-feeding it beneath the soil using irrigation pipes. The PMU has concluded that the sub-soil drip-feed option is more suitable for Rarotonga, their website states.

“Land disposal enables plants to reuse the remaining nutrients in treated wastewater, and the soil has a natural ability to filter out pathogens. A well-designed system enables pathogens to decay within the ground, preventing them from getting into streams or aquifers. A wide range of plants can be grown on the land being fed by the treated wastewater. Land use options include golf courses, trees, and farming certain crops that absorb nutrients.”

The PMU says land disposal is used successfully in places around the world, such as the subsoil drip feed systems in Omaha, Auckland, which distributes tertiary treated wastewater over 20 hectares of land and a golf course. In Pauanui, Coromandel, a subsurface drip distributes treated wastewater to parks, planted road median strips and a grassed air field.

Ocean outfall involves dispersing treated effluent deep into the ocean beyond the reef, using a special ‘diffuser’. The ocean currents disperse and dilute the treated wastewater, and the sea life consumes any remaining nutrients and organic matter it contains.

The PMU said an ocean outfall option for the developed Muri coastal area would need about 2 hectares (5 acres) for the treatment plant and a pumping station to send treated wastewater to the outfall. “One advantage of an outfall is that it could cope with future growth, without needing more land.”

“Ocean outfalls typically have minimal environmental impact because the ocean has a very high capacity to disperse treated wastewater,” the PMU says.

The first of two examples in the Pacific is the Kinoya treatment plant in Suva, Fiji, which incorporates a biological trickling filter that is similar to the concept designs prepared for Rarotonga. The second is in Sogi, Samoa, which manages wastewater collected from Samoa’s central business district.

At an earlier Mei Te Vai Ki Te Vai public consultation meeting held at the USP campus in Takamoa in December 2017 Finance secretary Garth Henderson emphasised the importance of consultation meetings, an avenue for the PMU to reach out to the public and for the community to give their views and feedback on the direction the project needs to take to ensure the best decisions are made to improve wastewater management on Rarotonga.

“We will keep doing outreach to you and from you because we need to broaden our knowledge base to raise awareness and help us progress this project,” Henderson said.

“We will make the hard decisions, but we want to hear more from our community over many more consultation meetings, grow our knowledge base so we can come up with a technical solution that is socially and culturally acceptable as well as economical.”

- Lawrance Bailey