Land-based Disposal of Wastewater using Vetiver for Muri, Rarotonga, Cook Islands: Response to Mei Te Vai Ki Te Vai PMU Project Update (22 May 2019)

Land-based Disposal of Wastewater using Vetiver for Muri, Rarotonga, Cook Islands.
A Kirkwood, J Flanagan, 2019.
[www.islandbooth.com/comm/190522-vetiver-sewerage-land-disposal.html].
Author: Andy Kirkwood, Justine Flanagan (ed).

Contact: [firstName] @ islandbooth.com.

Masthead Tapere map showing potential treatment and disposal site zone for proposed Muri reticulated septic system. From Pouara in the north-east round to Kauare in the south-west. Source PMU, MTVKTV Presentation, 22 May 2019.

 

Published 27 May 2019. Updated 4 June 2019.

 

On 22 May 2019 the Mei Te Vai Ki Te Vai Project Management Unit (PMU) presented to the Community a progress update on the wastewater infrastructure proposed for Muri, Rarotonga, Cook Islands.

The audience for the presentation was representative of the Community and included traditional leaders, MPs, business owners, and families from Vaka Takitumu and around the island.

The clear preference of the Community was for Land-based disposal of treated septic wastewater.

The Ocean Outfall option (disposing of treated wastewater over the reef) was rejected by the Community on a number of grounds. Issues raised at the meeting are to be detailed separately.

Specialists attending the meeting to respond to technical questions from the Community were:

  • Dr Murrary Wallis, Environmental Lead (Soil Scientist/Horticulturalist) (GHD)
  • Martin Smith, Civil Engineer (GHD)
  • Matt Blacka, Coastal Engineer (UNSW)
  • Dr Lara Ainley, Marine/Coastal Ecology, Biodiversity (MMR)

At the meeting we provided the PMU with a research paper on the Toogoolawah sewerage treatment system in Queensland, Australia (PDF) [1]. The paper provides technical detail on a system that uses vetiver grass for wastewater treatment. Vetiver grass is already grown in the Cook Islands.

A vetiver system to treat the volume of wastewater proposed by the PMU would require less than a quarter of the land, and allow for significant future population growth.

This document provides information for the Community about the Vetiver System and discussion with the PMU specialists at the presentation meeting.

Land-disposal Scale Reference: Nukupure Park

As a scale reference for the proposed system, 1 hectare (ha) is around 10,000sqm — the size of Nukupure Park rugby field.

The PMU have estimated that 18ha will be required for Land-disposal of treated wastewater for the developed Muri coastal area [2]:

  • Treatment plant: 2ha / 2 × Nukupure Park
  • Disposal field for feeding treated waste into the soil: 16ha / 16 × Nukupure Park

The criteria the PMU for land that is suitable land for treatment and disposal includes:

  • accessible from an existing access/road,
  • within a 3km radius of the Muri coastal area,
  • not within the coral sand zone, and
  • on a gentle slope (up to 15% gradient / 20m run: 3m rise).

See Tapere Map for potential treatment and disposal site zone.

The land may not need to be in a single block/piece. The system could be modular and distributed across different lands, connected by pipes. However, locating the treatment plant close to Muri; and as one piece would be more cost-effective and easier to monitor.

Population Size / Wastewater Volumes for Muri

In response to the question of the population size and volumes of wastewater to be treated by the Muri system, Martin Smith provided a population estimate of 3,500-4,000 over peak tourist season.

Information published by the PMU previously also notes that:

If other areas around Rarotonga need to be connected to the system in future, more land will be needed. [3]

Dr Murray Wallis provided a disposal capacity estimate of 650 cubic metres / 650,000L.

The Toogoolawah research paper was unclear on size of the population that the 300,000L system would service, and how the system might scale. Since the meeting the lead vetiver researcher Paul Truong has answered these questions.

  • In Australia the average wastewater generated is 150L/person/day for full sewerage connection.
  • A more recent (2014) vetiver sewerage system for Boonah, Queensland is a 4ha treatment and disposal site that services a population of 15-20,000.

Above Vetiver septic wastewater treatment/disposal field for Boonah, Queenland.
Source Veticon Consulting.

PMU Specification — Vetiver Solution

Based on the PMU’s specification — and only considering the amount of land required — a vetiver treatment system would be 20 × more efficient.

  • PMU Specification Population: 3,500-4,000 | Land required: 16ha
  • Vetiver System Population: 15-20,000 | Land required: 4ha

There are significant capital cost and ongoing maintenance benefits in using a plant-based solution rather than industrial/technological treatment. For a vetiver system wastewater is pumped to an irrigation system to water the plants. The vetiver are maintained by cropping the grass 1-2 times a year. No proprietary systems, no specialist technical training or servicing.

considering the amount of land required, a vetiver system would be 20 × more efficient.

Vetiver treatment would sustain significant growth (up to a peak population of 20,000), and could be implemented on a 4ha site. To service a smaller population, less land would be required.

The deep root system of the plant make it suitable for use on steeper slopes. This expands the potential sites to include inland hillside areas such the Avana and Turangi valleys.

The PMU also mentioned that the treatment and disposal system for Muri would likely be rolled-out to other locations elsewhere around Rarotonga, Aitutaki and the Pa Enua.

Taupo District Land Treatment Scheme / Ryegrass

Responding for the PMU, Dr Murray Wallis mentioned an overland sewerage treatment system in Taupo, New Zealand. In the Taupo system a 200ha+ area of ryegrass pasture is irrigated with treated wastewater using a sprinkler system. The grass is harvested and sold as animal feed/hayledge.

How the Taupo disposal field operates is detailed in a report by Mott MacDonald New Zealand [4].

Ryegrass and vetiver are both types of grass, however there are significant differences between the two plant species and how they function as part of a disposal system.

Evaluating the feasibility of Land-based disposal must consider the specific characteristics of the plantings that will be used to treat the wastewater.

Evaluating the feasibility of Land-based disposal must consider the specific characteristics of the plantings that will be used to treat the wastewater.

Cropping Vetiver — for the Vetiver System

Dr Wallis said that he would prefer a crop that would make productive use wastewater nutrients and fluid.

Vetiver has an end-use relevant to Rarotonga. The treatment system is also a nursery that provides plants for other Vetiver System applications.

The Vetiver System (VS) includes using the plant as a low-cost solution to soil and moisture management (for agriculture), as erosion prevention, and also to retain slopes. The grass was promoted to the sugarcane farmers in Fiji in the 1950s for this purpose. [5]

In Rarotonga, residential development and agriculture is moving away from the flat, low-lying coastal areas to the sloping interior. Vetiver provides a cost-effective alternative to hard-engineering slope-management methods. For example, instead of a concrete block retaining wall, vetiver hedge-rows can be used to stabilise a slope, manage surface water run-off and drainage.

Example application: Nukupure Park erosion control

As a method of erosion control, forming a vetiver hedge-row requires plantings at a spacing of 15cm (6-8 plantings per meter). Each planting is a bunch of three shoots (tillers). [6]

To stabilise the length of Nukupure Park foreshore area, with a single vetiver hedge row:

144m × 6 plantings/m × 3 tillers/planting = 2592 tillers

A mature parent vetiver plant can be divided/split into 15 tillers.

2592 ÷ 15 tillers/plant = 173 mature plants (rounding-up)

To plant a single hedge-row along the length of Nukupure Park would require the harvesting of approx. 173 mature vetiver plants.

The above calculation is provided only to illustrate the number of tillers required for an example application of a single hedge-row. Site investigation is required to determine the number and spacing between hedge-rows for erosion control purposes.

The variant of vetiver commonly-used is sterile. It produces no viable seed, runners/stolons, or rhizomes. It can only be propagated by culm-division; splitting from the parent plant. Mature plants can be split after a year of growth. Harvesting from a ‘vetiver patch’ or floating pontoon is preferred (and easier) than digging out of the soil.

Other secondary uses of the plant as a crop that are relevant to the Cook Islands include: perfume/massage oil (roots), handcrafts (roots/leaves) and mulch or animal feed (young leaves).

Vetiver Wastewater Uptake - Conceptual Models

A key take-away from the presentation was that the PMU will be investigating potential treatment and disposal sites. Soil depth, composition, water flows and weather patterns (wind and rainfall) are some of the variables that impact on the effectiveness of a disposal field.

In a disposal field, the bacteria and microbes in the soil, along with the plants grown on the land use the nutrients in the wastewater. Fluid is likewise used by organisms and plants — to grow. Fluid can also evaporate from the surface of a disposal field - greater evaporation results where irrigation is closer to the soil surface.

One common misunderstanding is that crops simply ‘soak-up’ nutrients — nutrients are also converted by the plant or by bacteria and microbes that live in the soil and plant root systems.

One common misunderstanding about the role of plantings is that crops simply ‘soak-up’ nutrients. Depending on the specific plant species, nutrients are also converted; either directly by the plant or by bacteria and microbes that live in the soil and plant root systems.

For example, plants convert — rather than simply store — carbon dioxide. The process of photosynthesis is where plants convert carbon dioxide gas into sugars (sugar/carbohydrate = carbon + water). Oxygen is a by-product of this process and released back into the atmosphere.[7]

Vetiver uses the fluid and nutrients from wastewater to grow. Vetiver root growth can be up to 12.6cm per day. Root systems grow to a depth of 2-4m in the first year, and the dense bushy leaves grow to a height of 2m over the same time.[8]

Vetiver improves fluid absorption and nutrient update in a disposal field.

Vetiver can thought of a ‘super soil’ in the way that it supplements and improves fluid absorption and nutrient uptake in a disposal field.

For a Vetiver System, site investiations evaluate the combined qualities of both the soil and the characteristics of the plant to determine the effective absorption rate.

  • In ideal conditions, 1kg of dry shoot biomass (1kg of foliage/leaves) will absorb 6.8L of liquid per day. [9]
  • The dense root system of the vetiver plant acts as an additional medium for beneficial bacteria and microbes that treat wastewater.

Mei Te Vai Ki Te Vai PMU Response

  • The PMU has said that no decisions have made on the method for disposing of treated wastewater (May 2019).
  • A decision on a septic system for Muri is scheduled to be made at the end of 2019.
  • At the 22 May 2019 presentation, the PMU has said they will consider and respond to the proposal of a Vetiver System as a disposal field option.

This page will be updated with the PMU response.

Classification/subjects: vetiver grass, Chrysopogon zizanioides, septic systems, sewerage, wastewater, Muri, Rarotonga, Cook Islands, South Pacific islands, Mei Te Vai Ki Te Vai, wastewater treatment, effluent, land-disposal, ocean outfall.

 

References and Further Reading

References

Wastewater Treatment

Cook Islands Government

News

 

Updated: 6 June 2019.

 
 

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