But advisers have pooh-poohed claims a vetiver crop could absorb all the sewage in less than four hectares of hillside.
Andy Kirkwood of Te Vai Ora Maori had proposed the use of vetiver crops for land-based wastewater disposal, saying they could absorb all the treated wastewater in less than a quarter of the 16 hectares needed for traditional land-based disposal.
His solution would address the distaste for a sewage outfall expressed by the community, but also help the government’s project management unit in its struggle to find a bigger land block for wastewater dispersal.
He argued that case studies supplied by Vetiver research scientist Dr Paul Truong showed a vetiver disposal field would require “significantly” less land – about a quarter of that the unit was seeking. It could also be sited on sloping land, he added.
Yesterday, the Project Management Unit said even vetiver would still require a large block of land.
But Murray Wallis, the unit’s environmental lead, confirmed they had hired Dr Truong to conduct a feasibility study for the use of the vetiver crops. Truong provided him with the data he needed specific to Rarotonga to conduct modelling called “Effluent Disposal by Vetiver Irrigation”.
Based on this modelling, he said the amount of land needed for vetiver crops would be between 15 and 20 hectares – very similar to the area of land already sought to absorb the wastewater.
Kirkwood disagreed. He said the vetiver crops would need less space, could be planted on a slope, and land-based wastewater disposal could also help Rarotonga’s water supply system.
He said the grass could be used to slow surface water flows, trap organic material, and support infiltration where water quality was impacted by surface soil erosion during high rainfall.
“As less sediment enters waterways, water enters the treatment facility ‘cleaner’. With cleaner source water, coagulation and disinfection processes (either physical or chemical) are more efficient. There are direct economic benefits in terms of reduced operational and waterworks maintenance costs,” Kirkwood said in his email to the Project Management Unit.
“The Vetiver System also provides work and youth development opportunities. Due to the inaccessibility of the intakes, mapping and planting activities must be undertaken on foot. This work can also be planned to take advantage of in-country expertise that is not utilised during our tourism 'off-seasons'.
“Growers can also be engaged to provide vetiver nurseries using less-viable lands (steep, erosion-prone) or inter-crop using vetiver for improved cash-crop yields.”
Wallis said they were still looking for potentially suitable land for land-based disposal: “We are very keen to hear from anyone who has land in the Muri area they can offer to the project for land-based disposal of treated wastewater. Our specialists are currently reviewing the report, and will be continuing conversations with Dr Truong.”