Vetiver Cyclic Dry-Wetland: Te Mato Vai Betterways

Vetiver Cyclic Wetland.

Vetiver Cyclic Dry-Wetland: Te Mato Vai Betterways.
Compiled by: Andy Kirkwood. April 2021.

Contact: [firstName] @

Masthead Vetiver Cyclic Wetland (Australia). Veticon Consulting.


This is a working draft. Published 26 April 2021. Updated 25 Sept 2023.


Betterways is a series by Te Vai Ora Maori showcasing methods of remediating the Te Mato Vai drinking-water treatment system for Rarotonga, Cook Islands.

Converting residual storage ponds to vetiver cyclic wetlands will reduce the risk of contamination of freshwater ecosystems by the by-products of the Te Mato Vai drinking-water treatment system.

Drinking-water treatment by-product

The new Te Mato Vai drinking-water treatment system has three waste by-products (residuals):

  • sludge: a combination of streamwater, the chemical coagulant polyaluminum chloride (PACl), and organic material/soil;
  • supernatant: the near clear, chemically-treated water which is drained to the stream from the settling tank when clearing the sludge; and
  • backwash: the layer of fine organic particles trapped by the sand filter(s).

When PACl is continually dosed to the settling tank, all three by-products will contain residual levels of aluminium and chloride. Sludge characterisation (Mar 2021), also found significant levels of sulphur-based compounds due to the PACl manufacturing process.

Operation will likely require the periodic re-sterilisation of settling tank. This process will require the stream discharge of de-chlorinated water. This was the process used in Sept-Oct 2020 to dispose of the initial commissioning/cleaning water.

The Te Mato Vai Project Management Unit (operated by New Zealand firm GHD), has sought to reassure intake Landowners that the system is ‘safe’. That the onsite storage ponds are sufficiently-sized to contain residuals; and that stream discharge will not impact stream health.

The long-term disposal of sludge will be subject to a separate environment impact assessment (EIA), and will require some form of processing, transportation, and construction of a containment facility.

Each Te Mato Vai treatment facility includes 2-3 sludge storage ponds, and up to seven formal stream discharge points.

Above: Model Te Mato Vai water treatment facility (Ngatoe).
Each Te Mato Vai treatment facility includes 2-3 residual storage ponds; and up to seven formal stream discharge points depending on the pond configuration.

Ngatoe: Model site

The Ngatoe site was used as a model by the project managers when evaluating the efficacy of the new Te Mato Vai water treatment system, but represents the exception in terms of configuration:

  • The site is located in the drier south-west of the island.
    The area reeives less rainfall, and is less prone to overflow as result of surfacewater inputs.
  • Single chamber settling tank: less treatment throughput, less sludge production.
  • Dedicated scour pond.
  • The scour pond is not shaded.
  • Single sand filter: fewer backwash cycles.
  • Two dedicateed backwash ponds: the ponds can be cycled to enable drying.

A trial presuming the Ngatoe facility as exemplar would fail to identify the operational issues that now impact operation of the Te Mato Vai system.


Onsite storage pond configuration

The configuration of the Te Mato Vai onsite storage ponds is non-standard.

Optimally there will be numberous ponds. One pond is in active use, while the other(s) are left to dry as the first stage of processing (dewatering) the sludge.

Due to the topography at the Te Mato Vai sites there may be only a single scour/sludge pond, and a separate backwash pond for each sand filter.

Where a single pond is used for both scour and daily backwash, the sludge cannot dry.

Avana residual ponds and stream discharge points.

Above: Avana is one of the largest and most productive water catchments. The facility has a dual bay settling tank and two sand filters ― but only a combined scour-backwash pond. Each backwash adds 16,000L to the active pond, and the settled sludge cannot dry.

Pond specification

  • The as-built Te Mato Vai sludge storage ponds are either 1.5 or 2.0 meters deep with a 300mm freeboard (clearance between the maximum surface water level and ground height).
  • Ponds vary in capacity from 41.6–180m3.
    The larger scour ponds are sized relative to settlement tank volume (some facilities have dual settling tank bays).
    The filter backwash ponds are smaller and sized to contain two backwashes + 30%.
    Some facilities have combined scour-backwash ponds.
Sludge pond section detail.

Above: Sludge pond section detail. Te Mato Vai PMU.
Source: Information Pack: Stage 2 Commissioning (p.127). GHD. Nov 2019. Turangi Intake Proposed Temporary Ponds. April 2021.

Pond operation

  • The residual water filters down into the soil, or evaporates from the surface of the pond.
    Drying by natural methods is of limited effectiveness due to the high rainfall and sheltered locations of the inland and elevated intake sites.
  • The ponds are unlined, but are gradually sealed by the sludge which forms the secondary compound gibbsite (aluminium hydroxide).
  • When the ponds are full, solids will be cleared by sucker truck or excavator. Sludge viscocity ranges from mud-cake to porridge.
  • Until a permanent storage facility can be constructed, geotextile-lined containment cages have been installed at some intake sites.
  • Annual rainfall in Rarotonga is around 2000mm in coastal areas, but increases to 4000mm in the water catchments. In storm events, surface flooding is common. In January 2022, a tropical depression resulted in monthly rainfall of 934mm, recorded at the Totokuitu monitoring station.

The Takuvaine sludge pond was reconstructed twice in 2020: in June, and during storm events in August. As the PACl dosing system was not operational, overflow was only rainwater and clay sediment from the pond.

Short-term containment

Without a suitable long-term containment facility, To Tatou Vai first trucked sludge between the treatment sites, to the under-utlised Papua facility; and has since begun stock-piling the material in above-ground geotextile lined cages.

Papua sludge ponds and sludge storage cages, Nov 2021.

Above: Papua sludge ponds and sludge storage cages, Nov 2021
Before clearing, surface water is drained to the stream by lowering the (white) decanting heads.


Stream discharge

The stream discharge of drinking-water treatment residuals is not practiced in New Zealand or Australia.

  • The water component of the sludge is managed by allowing storage ponds to dry, or by dewatering.
  • Supernatant is commonly recycled back into the treatment process. Along with preserving ecological values, this is also due to the expense of the coagulation chemical.
  • De-chlorinated sterlisation water — a by-product of the cleaning the settling tanks — is not discharged to natural waterways, but to ground or a municipal stormwater system.

In the Cook Islands, stream discharge is unlawful: the Public Health Act 2004 prohibits sludge and other by products from water treatment plants, from being directly or indirectly deposited or discharged, or to seep, into a waterway.

Discharging to the stream risks chemical and bacteriological contamination. The force of the flow may also affect the distribution, population-size, and migratory behaviour of biodiversity, have erosion impacts, and increase the level of sediment in streamwater.

Along with operational discharge, the settling tank also has an overflow that returns water to the stream when the tank is full. (This configuration may explain the Takuvaine stream die-off observed by a resident family in December 2019.)


Long-term risks

As the sustained, but sporadic, stream discharge of residuals is not standard practice, there is no research on the long-term impacts on biodiversity and ecosystem function.

The risk is magnified by the number of treatment facilities. There are ten new waterworks to manage, on all of Rarotonga’s main freshwater streams. Access to the facilities is mainly by unsealed road, and may include stream-crossings. Some sites are inaccessible in storm events, and upstream blockages risks diverting stream flows and inundating the treatment sites.

Rarotonga water catchments showing Te Mato Vai stream discharge points.

Above: Rarotonga water catchments showing Te Mato Vai stream discharge points.
Shaded are the Avatiu, Takuvaine, Turangi, and Avana biodiversity and ecosystem function protection areas — proposed by the Global Environment Fund in 2022.

Bodies of freshwater in the Cook Islands are extremely limited, with no large lakes or rivers, only wetlands, streams and a few small freshwater lakes present. Freshwater biodiversity is therefore extremely limited. The Cook Islands National Biodiversity Strategy and Action Plan lists only nine native and four introduced fish species, two native and three introduced gastropods, and six native and one introduced crustacean.
Cook Islands 4th National Report to the Convention on Biological Diversity. National Environment Service 2011.

A report prepared as part of the operational use of PACl EIA details the risks to the aquatic environment due to contamination (at that time, presumed to be aluminium and chlorine compounds):

  • Degradation of water quality with these effects flowing through to riparian vegetation, macroinvertebrate assemblages and fish communities.
  • Riparian vegetation dieback, changes in community structure and composition, encroachment of weed species leading to bank destabilisation, erosion and sedimentation of waterways and degradation of water quality.
  • Loss of aquatic flora leading to an overall reduction in species diversity and abundance and decrease in availability of habitat for aquatic fauna.
  • Reduction in diversity and abundance of macroinvertebrate communities, potentially limiting their availability as a food source.
  • Increase in susceptibility of fish to disease, predation and death and overall reduction in fish species diversity and abundance.

Te Mato Vai Aquatic Ecology Baseline Report. Pratt L., GHD Sept 2020.

A six month PACl trial (Sept 2020-Mar 2021), mentions ‘environmental monitoring’, however this was the recording of streamwater chemistry and water quality values: aluminium, turbidity, E. coli. Samples were not taken at the mid-level drain where chemical concentrations will be higher; downstream stream sampling was 25m from the discharge point.

The trial did not include sustained monitoring of biodiversity or habitat. Both the baseline ecology report and subsequent trial monitoring were conducted after the Te Mato Vai construction processes which included temporary redirection of stream flows and significant earthworks, processes likely to have disrupted ecosystems.

  • The US Environmental Protection Agency Freshwater aluminum guidelines recommend 24 months of monthly sampling of streamwater chemistry values to account for seasonal variation.
  • The PACl trial did not include monitoring of ecological values or biodiversity.
  • Research from New Zealand found that aluminium bio-accumulates in the tissues of freshwater fish and macroinvertibrates.
  • Intermittant discharge can trigger avoidance behaviours.
    Detecting a change in water quality, freshwater species may be displaced from the mixing zone, or downstream areas of slower moving water where waste sediment will settle.
  • Residual aluminium in the dischage will coagulate the streamwater: gradually forming a layer of sludge in the stream bed, which results in benthic smothering.

See also: Chemically Treated Drinking-Water: Environmental Impacts.

Setting aside critique of the process leading to the construction of a compromised system, the challenge is now to remediate the infrastructure. The brief is to implement a method of operation that safeguards streamhealth — and is engineered for tropical conditions.


Vetiver Systems: Phytoremediation

Phytoremediation uses the growth characteristics of specific plant species to contain or process contaminants in soil or water.

Vetiver Phytoremediation: Water treatment residual processing

Above: Vetiver Phytoremediation: Water treatment residual processing

Vetiver grass (Chrysopogon zizanioides) is a fast-growing, non-invasive, tropical grass used in applications including domestic and industrial effluent treatment and leachate management. The grass grows in dryland conditions, water-logged soils, and over water on rafts/pontoons.

A vetiver system has been proposed for the land disposal of wastewater as part of the planned Muri septic reticulation.


Cyclic wetland

Video: Cyclic Wetlands: Te Mato Vai Sludge Ponds. Andy Kirkwood on CITV, June 2021.

Community group Te Vai Ora Maori propose converting the residual ponds to cyclic vetiver wetlands. ‘Cyclic’ as the ponds are flooded, and then allowed to dry-out. This is good fit with operational process (sand filter backwash cycles, and sludge removal/tank cleaning). Cyclic flooding also maximises the growth of the grass.

Te Mato Vai: Vetiver Cyclic Wetland Concept

Above: Te Mato Vai: Vetiver Cylic Wetland Concept (Ngatoe)
In the remediated treatment process, the risk of stream discharge will be buffered by enhancing the function of the storage ponds.

  • The grass will be planted around the perimeter and inside the pond — potentially on raised beds.
  • Increasing the drainage potential of the ponds (infiltration), reduces the need for stream discharge.
  • Nutrient and water is used for root and leaf growth. The climatic conditions in Rarotonga are ideal. During peak season, the leaves grow up to 20cm/week; and roots can grow 1-2cm/day.
  • The dense and (3m+) deep root system grows directly downward stablising the sides of the pond, increasing drainage, and reducing the likelihood of overflow. The root system filters out and binds minerals — including aluminium. Microbes living in the soil further purify the water.
  • As a perennial plant, the root system continues to grow-and-decay. This sustained increase in organic matter sustains improved soil structure providing supplementary water storage without requiring earthworks.
  • Planting design will be guided by operational process including periodic removal of sludge by excavator or suction truck.
  • Although the ponds may have characteristics similar to a natural wetland, residuals do not provide habitat for biodiversity.
    The lack of structure, the disruptive inflows of residual, and the highly-variable water and sediment levels are not favourable to aquatic species.

Wetland planting will not entirely avoid the need for stream discharge; nor for a permanent sludge containment facility. These companion issues can however be addressed by altering the treatment process to reduce, and ideally eleminate, the generation of hazardous waste. See: Te Mato Vai Betterways: Diversion.

Better residual processing with vetiver grass.

Above: Better residual processing
Vetiver increases infiltration/drainage: purifying residual water as it filters down into the soil. The leaves of the plant increase the effective surface area of the pond to improve drying times. Photo: Veticon Consulting, Australia.

Planting plan

Design of the vetiver beds will varying to suit the configuration of each site. Some locations have dedicated, separate scour ponds, and a backwash pond for each sand filter. Sites such as Turangi and Avana collect both residuals to a single pond, and are unable to dry — even in drought conditions.

The two potential options are:

  • A double perimeter: preserving the capacity of the existing ponds to contain the current volume of dry solids.
  • Perimeter and raised beds: maximising drainage and water processing potential, but with reduced solids storage capacity.

A supplementary enhancement is to plant vetiver hedgerows on the streambank downslope of the ponds to intercept and treat the surface water decanted to the stream when operators remove the sludge. The hedgerow would also minimise the impacts of any unplanned overwash.

Although vetiver can be grown overwater, the dedicated scour ponds are more likely to dry-out in periods of low rainfall. The combination, and dedicated backwash ponds are inundated at least daily.

Cyclic Wetland: Te Mato Vai Remediation Concept.

Above: Cyclic Wetland Concept.

Care and maintenance

  • Vetiver growth, and water-processing (evapotranspiration) is most vigorous in full sun, however the grass can tolerate periods of partial shade (e.g. sahding due to tree growth, seasonal variation in the position of the sun). Along with the grass, vegetation at the intake sites will need to be maintained.
  • Trimming every three months stimulates plant growth.
  • Cuttings are suitable for use as mulch/surface-cover and for composting Rather than accumulating in the foliage, metals including aluminium are adsorbed to the root system.
  • The grass is resistant to high levels of aluminium: it has been used in the treatment of tailings in aluminium (bauxite) mining, to contain landfill leachate, and has been determined to have an aluminium saturation threshold of 68%—80%; the root function of most plants are adversely affected by aluminium concentrations of less than 30%.
Vetiver grass in cyclic wetland.

Above: Vetiver grass: wetland application
Residuals discharged from the settling tanks and sand filters will flood the wetland areas. The cycle of inundation and drying increases plant growth rates.


Physical treatment methods: Better by-product

The volume of solids/sludge can be reduced by implementing diversion: only collecting the clearest streamwater for treatment. When streams become muddy, the distribution system switches to using stored supply. Less sediment entering the settling tank means less sludge, and fewer backwash cycles.

If PACl is not dosed, and if water treatment is only by physical methods, then the residual sediment will be organic silt — safe to re-use for growing crops. In this case the function of the grass is to stabilise the sides of the ponds and improve water-processing to minimise the risks of sludge deposition to the streambed.


Permitting On-site Discharges

A concept note on the cyclic wetland was submitted to the National Environment Service by Te Vai Ora Maori in reponse to the To Tatou Vai Water Treatment Plants: On-site Discharges Environment Impact Assessment.

TVOM Objection/Concept Note

Te Vai Ora Maori objection and wetland concept note June 2021

Te Vai Ora Maori also prepared a presentation on the cyclic wetland concept for the Rarotonga Environment Authority (REA). At the meeting to consider the TTV operational permit (Nov 2021) the REA members did not request any additional information, the presentation was not made.


To Tatou Vai response

The operators of the new water treatment system responded to the cyclic wetland concept in August 2021.

A concern raised was that pond maintenance could be compromised by an increase in (wetland) biodiversity.

This risk is offset by the low-forage potential of the vetiver cultivar (not a favoured food-source); and the rapid flood/drying cycle which negatively-impact the value of the residual ponds as a habitat.

Planting specification would be collaborrative; and account for access by maintenance vehicles and operational staff.

Response to the submissions to the EIA Report on Water Treatment Plants On-site Discharges To Tatou Vai, Aug 2021

Presentation to the Global Environment Fund

A revised vetiver cyclic wetland concept note was presented for consideration by the Global Environment Fund Project Management Team in February 2022. The GEF7 project seeks to mainstream actions that protect, maintain or enhance biodiversity and ecosystem services in riparian (natural freshwater waterways), wetland, and coastal environments.

Four of the Te Mato Vai catchment valleys have been proposed as biodiversity protected areas: Avatiu, Takuvaine, Turangi, and Avana. Given this status, it is prudent to consider infrastructure inteventions which safeguard environmental values.

Te Vai Ora Maori GEF 7 concept note: Te Mato Vai cyclic wetland Feb 2022

Classification/subjects: Te Mato Vai, To Tatou Vai, Global Environment Fund, GEF7, coagulation, flocculation, water treatment residuals, sludge, polyaluminum chloride, PAC, PACL, water standards, water quality, biodiversity, ecosystem services, Rarotonga, Cook Islands, South Pacific, environmental impact assessment, EIA, vetiver grass, phytoremediation, bio-engineering, nature-based solutions, NbS.


See also


References and Further Reading




  • 4 Jan 2020: The water debate continues. Te Ipukarea Society. Cook Islands News.
    Flocculation is a process of settling out particulate matter, for example dirt, from the water. Given that PACl is a chemical that produces a potentially toxic sludge as a byproduct of the flocculation process, this trial needed to be properly assessed for environmental impacts before it was approved.
    Another purpose of an EIA is to look at alternatives that may have a lesser negative impact. Therefore, from an environmental perspective, the logical thing to do to reduce dirt getting into the water is to first trial alternatives which provide a lower level of risk to our freshwater ecology.
  • 9 Feb 2020: What happens when we commission Te Mato Vai? Release, Te Mato Vai PMU.
  • 21 April 2020: How does polyaluminium chloride (PACl) work. Te Mato Vai PMU.
    We need to use PACl in Rarotonga because stream water contains high levels of contaminants — everything from bird poo to leaves and soil. This is true for water taken from streams, rivers or lakes around the world, not just in Rarotonga. Even stream water that looks clear still contains harmful protozoa and bacteria too small for people to see.
  • 2 May 2020: Andy Kirkwood: Sludge! Letters in Cook Islands News.
    A March 2020 expert review estimates the scale of the issue. The use of polyaluminum chloride (PACl) – a chemical that helps to remove dirt during rain – will generate 4,000 cubic metres of chemical sludge each year.
  • 25 Jun 2020: Turning on the taps Katrina Tanirau, Cook Islands News.
    Authorities acknowledge they need to be as transparent as the water they plan to pipe into people’s homes in Rarotonga.…
    Storm damage to the main water storage pond meant it had to be reconstructed, and that delayed the project.
  • 26 Nov 2020 LETTER: What are the effects of water treatment on Rarotonga’s streams? / Nutrient — more or less? Cook Islands News.
    Freshwater streams are not used as drains for water treatment residuals in New Zealand - or in Australia. There is no scientific research on the long-term effects of PACl waste on stream life.
  • 26 Nov 2020 LETTER: Landowner concerns with water treatment.. Cook Islands News.
    There are no aluminium tests, the filter is off, and people are drinking chemical water. How is this a trial?
  • 27 Nov 2020 MPs attend intake tour. Cook Islands News.
    Nine out of 10 intakes are undergoing trials where a chemical called polyaluminium chloride, or PACl, is added to the settlement chambers to remove small dirt particles and other contaminants found in water, such as bird droppings. Napa said the answers provided by officials during the tour didn’t adequately explain how PACl sludge and the by-product from sand filtration will be disposed in an environmentally responsible manner. “This is a major inland development that will affect our water supply, our streams that flow out to sea. Why didn’t this government first obtain an EIA? Everyone seems to be avoiding that question and ducking for cover.”
  • 28 Nov 2020 Time to turn on the tap, but it’s not that simple. Cook Islands News. Rarotonga’s geography, ecology, and weather are posing unique challenges. “It’s very different here. Water gets dirty very fast, but it cleans up fast.” Developing water treatment systems is never an easy process, says Free, and getting to this stage has been a lengthy process. “They always are, but this one, probably a little longer than most. There’re 10 plants. I’ve never worked on 10 plants at the same time.”
  • 8 Dec 2020 Government’s ecological report challenged. Cook Islands News. A desktop review relies on published reports. The consultants did not conduct any fieldwork on the sensitivity of Rarotonga’s freshwater ecosystems to pollution.
  • 22 Dec 2020 PACl trial update. GHD.
    Trial results so far have been positive, with the water treatment plant performing as expected and delivering a significant increase in water quality overall.
  • 26 April 2021 Rarotonga water treatment waste ‘no gift to growers’. Cook Islands News.
    NKA: “…if [an organic] grower is found to be using sludge, their organic certification will be revoked.”
    GHD: “Aluminium is not toxic to plants when the soil pH is near neutral or mildly acidic (the pH values are above pH 6). Most Rarotonga soils are above pH 6, and the silts and clays have very high natural buffering capacity meaning that their pH doesn’t change much when something more acidic is added. In fact, the PACl sludge we have tested is not acidic so it won’t make the soil more acidic.”
    Ministry of Agriculture: “the sludge will also contain aluminium (Al) and chloride (Cl) and possibly other elements such as heavy metals which can negatively impact plants and crops.…elevated levels of Al and Cl can have adverse effects on the growth of certain tree species and many vegetable crops such as tomatoes, lettuce, and melons.”
  • 26 April 2021 The potential for PACl sludge use. Te Mato Vai PMU.
    Initially PACl sludge was being considered for fill or capping material. At their request, and with the support of the Government, the PMU conducted preliminary tests to understand the composition of the PACl sludge and whether it would be suitable for reuse on the land.
  • 3 July 2021 Seeking solutions in nature for the planet’s problems. Cook Islands News.

News Archive: 2011–


Working Draft. Updated: 25 Sept 2023.


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