PMU looking into vegetation options for land-based wastewater disposal

Updated: Nov 24, 2020

The PMU commissioned a study into how Vetiver grass can assist in land-based wastewater disposal. We wanted to understand if Vetiver grass could be used in land-based disposal.

What is Vetiver grass?

Vetiver grass is a deep-rooted, fast-growing plant, which originated in India. It thrives in tropical climates, and was introduced to the Cook Islands as a perfume in massage oil. Its deep and fast -growing root system is useful for absorbing nutrients from wastewater, reinforcing stream banks and filtering rain run-off.

Vetiver is a “green” solution. The Vetiver system will produce abundant grass that will need to be cut and removed from the treated wastewater disposal site. This is so nutrients don’t accumulate over time and cause contamination of the groundwater. The harvested grass can be used to make compost or mulch (ground cover) for gardens and for agriculture around the island.

What did the study conclude?

We gave Dr Truong, a Vetiver grass expert, soil and meteorological (weather conditions) data collected in Muri to carry out modelling called ‘Effluent Disposal by Vetiver Irrigation’ (EDVI).

Some interesting findings are:

  • Vetiver grass is effective at taking up water, nitrogen and phosphorus from soil. Vetiver can remove the nitrogen and phosphorus using a smaller land area, however the volume of water increases the amount of land needed.

  • A plot of Vetiver grass irrigated with treated Muri wastewater needs to be 15 to 20 hectares (ha) in size to soak up the land’s water and nutrients. Dr Truong’s calculations confirm the PMU’s land requirement calculation of at least 16ha (one hectare is 10,000 square metres, or a square with sides of 100m by 100m).

  • A sloping site of at least 5 per cent (5m rise per 100m site length) is recommended to help rain runoff the land surface. A sloping site is preferred so that the Vetiver grass can absorb the maximum amount of treated wastewater, rather than absorbing just rainfall, as a flat site would.

  • We still prefer to use underground irrigation pipes with trickle emitters (this is called subsurface irrigation). This option would work well with Vetiver, and help protect the environment.

What about using other crops?

We’re also looking into what other possible food crops could assist with a land-based disposal option of the wastewater. Planting food crops together with the Vetiver could be a good solution for the Muri wastewater. This gives landowners the opportunity to grow useful crops on the land irrigated with treated wastewater.

In our conversations with the community, they’ve suggested crops such as banana, pineapple and taro. We still need to do more research to understand which other crops may be suitable to mix with Vetiver as a combined solution that would benefit the community and land use.

The PMU is still looking for a similar amount of land (at least 16ha) for land-based disposal. We’re keen to hear from anyone with land the Muri area who is interested in offering their land to the project for land-based disposal of treated wastewater.

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