Land-based Disposal of Wastewater using Vetiver for Muri Septic, 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 Septic, 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 2 Sept 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 650,000L per day.

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 Dr 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

A Vetiver Solution Disposal Field might only require 1-2ha of sloping land (assuming direct scalability based on the Boonah footprint). Ideally the disposal site would be north-facing (to maximise transvaporation/sun exposure/plant growth). Suitable sites may be located in the Avana or Turangi Valleys.

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 2.6cm 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 investigations evaluate the combined qualities of both the soil and the characteristics of the plant to determine the effective adsorption 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.
Nitrogen and Phosporous uptake of vetiver grass vs. other commonly used disposal field crops.

Nitrogen and phosphorous absorption of vetiver grass vs. other commonly-used wastewater disposal field crops.
Grown hydroponically the grass is even more effective.Source Effectiveness of Vetiver Grass versus other Plants for Phytoremediation of Contaminated Water (2019).

 

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).
  • In Aug 2019, the PMU engaged Dr Paul Truong of Veticon Consulting to conduct a feasibility assessment into the use of a vetiver system for Muri.
  • A decision on a septic system for Muri is scheduled to be made at the end of 2019.
 

Appendix (A) Greywater Treatment: Kigali, Rwanda

In August 2019, a case study for urban-rural greywater treatment was provided through the International Vetiver Network.

The presentation for the project demonstrates methods for the management and utilisation of greywater in Kigali, Rwanda, many of which are applicable to the challenges facing Rarotonga.

Above Greywater Treatmemt Presentation: Kigali, Rwanda. Source Via ITVN.

‘Reverse Technology Transfer’

A challenge facing advocates for sustainable, biological systems for water management is the need to revisit cultural expectations. Moving away from hard-engineering (steel and concrete), to the used of plants and bacterial agents/microbes.

A similar challenge has been faced in the use of Oral Rehydration Solution to manage gastro-illness: when what is perceived as a ‘third-world’ solution is relevant to a ‘first-world’ problem.

In his project providing low-cost sanitation solutions for Haiti, Roger Giezten has observed:

Another unexpected problem that I have encountered, is that some Haitians reject the [vetiver] latrine.

So far I have found that the people with no toilet or latrine at all, universally embrace it.

The ones who reject this have been men in leadership positions, who have nicer toilets at their homes, and feel the vetiver latrine demeaning to the people they serve. They have made this decision without actually asking the people themselves. It appears that it is pride which causes them to reject this solution even when no other solutions are in sight.
The Vetiver Latrine. Roger Giezten, May 2019. Accessed 17 Aug 2019.

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

Lagoon and Reef

  • Impact of storm waves and storm floods on Hawaiian reefs. Paul Jokiel, University of Hawai'i, June 2006.
    Flood events erode watersheds and deposit sediments on the reefs that can kill corals and block recruitment of new coral colonies. Fresh water delivered by storms also can lower local salinity to levels that are lethal to corals and other reef organisms. Nutrients and pollutants associated with fresh water runoff are transported onto coral reefs during such storms.
  • The 2014 coral bleaching and freshwater flood events in Kane'ohe Bay, Hawai'i. K Bahr et al. PeerJ 2015.
    A freshwater ‘kill’ event occurred during July 2014 in the northern part of Kane'ohe Bay that reduced coral cover by 22.5% in the area directly impacted by flooding. A subsequent major bleaching event during September 2014 caused extensive coral bleaching and mortality throughout the bay and further reduced coral cover in the freshwater kill area by 60.0%.
  • Impact of Three Bleaching Events on the Reef Resiliency of Kane'ohe Bay, Hawai'i. K Bahr et al. Frontiers, Dec 2017.
    Continued exposure to bleaching stress will eventually lead to reduced reef complexity, coral cover and biodiversity; therefore, providing lower quality habitat for fishes and marine invertebrates and consequently fewer ecosystem goods and services for dependent human communities.
  • 15 July 2019: Thirty years of unique data reveal what’s really killing coral reefs Science Daily - Accessed 29 July 2019.
    Study is world’s longest record of reactive nutrients, algae concentrations for coral reefs. Coral bleaching is not just due to a warming planet, but also a planet that is simultaneously being enriched with reactive nitrogen from sources like improperly treated sewage, and fertilizers.
  • 4 May 2019 Study says Hawaii reefs provide $835M in flood protection. AP News.
    “Most people have no idea how valuable coral reefs are for coastal protection.”
    Reefs act as submerged breakwaters, dissipating up to 97% of wave energy offshore.

News

  • 27 Apr 2017: Cook Islands PM Blames Noncompliant Landowners For Muri Lagoon Pollution Problem. Cook Islands News / Pacific Islands Report.
  • 16 Apr 2018: Nature-based Solutions for Septic Waste. Te Ipukarea Society / Cook Islands News.
  • 7 May 2018: Visitor’s Solution for Pollution Issues. Cook Islands News.
  • 16 July 2018: Engineers check ocean outfall option. Cook Islands News.
    Water, Waste and Sanitation Unit (WATSAN) director Jaime Short says Cook Islands residents produce 1.8 million litres of wastewater daily. Assuming there are 3000 visitors on the island at any one time, that number increases to 2.4 million litres daily.
  • 9 Nov 2018: Wastewater Systems to Blame for Nutrients. Cook Islands News.
  • 18 Nov 2018 Muri Lagoon Concerns Clarified. Cook Islands News.
  • 16 April 2019 Brown Clears the Air on Sanitation. Cook Islands News.
  • 28 May 2019 Land or Ocean Outfall for Treated Wastewater? Cook Islands News.
  • 28 May 2019 Land or Ocean Outfall for Treated Wastewater? Cook Islands News.
  • 14 Aug 2019 300-metre ocean outfall on table. Losirene Lacanivalu, Cook Islands News.
    Officials have been offered a two hectare block at Turangi, the old landfill site, for a wastewater treatment plant. But despite hiring a property specialist to scour the district, they can’t find the 16ha they would need to disperse the treated sewage into the ground. Unless they can find a block of land at least that size, they believe a sewage outfall pipe is the only option.
  • 16 Aug 2019 ‘Man in charge drank water from sewage outfall’. Melina Etches, Cook Islands News.
    The days of raw sewage going into the sea are gone: instead, treated wastewater would be piped out beyond the reef.
    Reeves understands the water would be high in nutrients which will evaporate and dissolve in the ocean and be consumed by bacteria.
    “There have been a lot of misinformation and scare tactics used with this issue.”
    Another option could be a treatment site closer to Muri, then to pump the treated wastewater to the old 2ha landfill site offered up by Pa Ariki in Turangi, which could be sowed with plants like bananas and pawpaw that absorb a lot of water.
    Done right, Reeves says, Rarotonga could have treated wastewater as clean as Omaha in New Zealand. “Evidently, the man in charge drank the water after treatment.”
  • 20 Aug 2019 As wastewater goes into the ocean. Andy Kirkwood in Cook Islands News.
    The reef absorbs 97 per cent of the wave action. A compromised reef would expose the Cook Islands government to liability for any damage to the properties along the Matavera coastline and joint action by owners of Ocean Escape, the new KAPS Petrol Station and further north Club Raro, Coastal Kitchen, and Tamarind House.
    It’s time that government washed its hands of this batch of consultants.
  • 22 Aug 2019 How much land [needed for Muri land-disposal of septic]?. Rashneel Kumar, Cook Islands News.
    Murray Wallis, the [Project Management 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.
  • 6 Sept 2019 Muri sewage report drives pursuit of clean treatment . Anneka Brown, Cook Islands News.
    There’s new hope for both the lagoon and ocean, with a proposal that Muri’s treated sewage could be dispersed on land in the valley behind Turangi.
  • 6 Sept 2019 Editorial: Searching together for a sewage fix . Jonathan Milne, Cook Islands News.
    A new environmental report published today reveals just how much work has to be done … the algal bloom is not entirely the fault of Muri resorts and their septic tanks; some of the contamination seeping into the lagoon from the aquifers beneath dates back to the ‘80s or ‘90s when Muri was used for intensive agriculture.
  • 27 Sept 2019 No sewerage outfall – leader. Anneka Brown, Cook Islands News.
    Pa Ariki responds to Turangi residents’ concerns about wastewater treatment plant and sewage outfall … has offered about 14 hectares of land to government in a circuit-breaking step that should remove the need for a sewage outfall pipe north of Avana.
 

Updated: 30 Sept 2019.

 
 

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