This is a working draft. Published 26 April 2021. Updated 4 Feb 2022.
Betterways is a series by Te Vai Ora Maori showcasing methods of remediating the new Te Mato Vai drinking-water treatment system for Rarotonga, Cook Islands.
Converting onsite storage ponds to vetiver cyclic wetlands will reduce the risk of contamination of freshwater ecosystems by residuals — the by-products of the new Te Mato Vai drinking-water treatment system.
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 periodic re-sterilisation of settling tanks — required if operational staff enter the tanks for cleaning or repair. This process includes 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 (off-site) 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.
Above:Typical Te Mato Vai water treatment facility (Ngatoe). Each Te Mato Vai treatment facility includes 2-3 residual storage ponds; and up to five formal stream discharge points.
Onsite storage ponds
The configuration of the Te Mato Vai onsite storage ponds is non-standard.
There are usually multiple ponds. One is in active use, while the other(s) dry-up as the first stage of processing the sludge. Due to the topography and size of the treatment sites, there is typically only a single scour/sludge pond, and a backwash pond for each sand filter.
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.
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 was 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.
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.
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.
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.)
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.
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.
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; and downstream stream sampling was 25m from the discharge point.
The trial did not include monitoring of biodiversity or habitat. Both the baseline ecology report and subsequent trial monitoring were conducted after the 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.
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 may result in benthic smothering.
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.
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 systems. The grass grows in dryland conditions, water-logged soils, and over water on rafts/pontoons.
Community group Te Vai Ora Maori propose converting the Scour (Sludge) and Backwash 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.
Above:Te Mato Vai: Vetiver Cylic Wetland Concept (Ngatoe) In the remediated treatment process, stream discharge will be avoided 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), avoids 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, increasing soil drainage, minimising the likelihood of pond overflow. The root system filters out and binds minerals — including aluminium. Microbes living in the soil further purify the water.
Planting design will prioritise the ned to clear of residual solids/sludge by excavator or suction truck.
Although the ponds are in function similar to a (natural) wetland by enabling the retention and gradual draining of water, they are not intended to provide habitat for biodiversity.
Wetland planting will not avoid the need for discharge; or 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 waste. See: Betterways: Diversion.
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 improving operation by decreasing drying times. Photo: Veticon Consulting, Australia.
Design of the vetiver beds will varying to suit the pond size and configuration at each site. The two potential options are:
A double perimeter: preserving the capacity of the existing ponds to contain 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 when removing the sludge. The hedgerow would also minimise the impacts of any unplanned overwash resulting from surface flooding.
Although vetiver can also be grown overwater, the larger scour ponds are more likely to dry-out in periods of low rainfall. The backwash ponds are inundated more frequently: every 2-7 days dependent on sediment loads.
Above:Cyclic Wetland Concept.
Care and maintenance
Growth is most vigorous in full sun, however the grass can tolerate periods of partial shade. Overhanding or shading vegetation at the intake sites will need to be maintained.
Quarterly trimming stimulates plant growth.
Cuttings are suitable for use as mulch and for composting (metals are absorbed to the roots).
The grass is resistant to high levels of aluminium: it has been used in the treatment of tailings in aluminium (bauxite) mining, and 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%.
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
If PACl is not dosed, and if water treatment is only by physical methods, then the remaining sediment will be organic silt — safe to re-use for growing crops or in compost.
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 fewer cleaning cycles, less sludge, and less residual.
Te Vai Ora Maori also prepared a presentation on the wetland concept for the environment permitting authority: Rarotonga Environment Authority (REA). Meeting in November 2021, the REA members (the majority being Rarotonga’s members of parliament) did not request any additional information, and the presentation was not made.
To Tatou Vai response to submissions
The operators of the new water treatment system provided a response to the submissions, and make reference to the wetland concept.
A revised vetiver cyclic wetland concept note was presented for consideration by the Global Environment Fund Project Management Team in February 2022. The GEF 7 project seeks to mainstream actions that protect, maintain or enhance biodiversity and ecosystem services in riparian (natural freshwater waterways), wetland, and coastal environments.
Environment Act 2003. PacLII. “Inland waters” means the waters and banks of any stream, river, or lake together with the bed (whether dry or not) of any stream, river or lake (for the purposes of this definition “bank” shall include all that area of land extending away from the stream, river, or lake and measured at right angles to a distance of 5 metres from the bank of that stream, river and lake);
Public Health Act 2004. PacLII. Material including sludge or other by products from water treatment plants is ‘Hazardous waste’. Section 54(2)(d) “prohibits hazardous waste to be directly or indirectly deposited or discharged, or to seep, into a waterway”.
Aquatic Macroinvertebrates: Biological Indicators of Stream Health. University of Kentucky.
Examples of aquatic macroinvertebrates include insects, worms, snails, mollusks, and crustaceans… macroinvertebrates are an integral part of the food chain. Without these creatures, a stream’s entire aquatic food web would collapse. Many macro-invertebrates feed on organic material such as leaves and algae.
Sludge Disposal Site in
Rarotonga — Environmental Impact Assessment. Tonkin+Taylor, July 2021. The EIA contains the GHD Characterisation of PACl Sludge Samples: The total sulphur and sulphate contents of the PACl sludge samples were consistently higher than the soil
samples. Sulphur is used in the production of the PACl product… the level of sulphur we measured in the PACl sludge was not enough to lower the pH.… The sulphur content of the sludge we sampled would in fact be
available at luxury levels (more than plant growth requirements)…
Sulphate sulphur could reduce the availability of some trace elements for plant growth…
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.
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.
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.