Te Mato Vai: Rarotonga’s sludge crisis

Lower residual discharge pipe. Avana, 15 Aug 2022.

Te Mato Vai: Rarotonga’s sludge crisis.
Compiled by: Andy Kirkwood, Justine Flanagan for Te Vai Ora Maori.

Contact: [firstName] @ islandbooth.com.

Masthead Lower residual pond discharge pipe. Avana, 15 Aug 2022.
Even in drought conditions, the Te Mato Vai residual storage ponds are overflowing chemically-treated water and sludge.


Published 20 Aug 2022. Updated 21 Sept 2023.


July-August 2022, the residual storage ponds at the Turangi and Avana drinking-water treatment facilities were observed to be discharging treated water and sludge to the bordering stream(s).

In the Environment Impact Assessment report accompanying the operational permit application, consultants advised that the risk of impact on biodiversity would be mitigated by any overflow only occuring at times of high-flow: that mixing with streamwater would dilute chemical concentration, specifically aluminium levels, to acceptable values.

It is now evident that overflow (and intentional discharge) occurs even during times of sustained low, or even zero-flow. At these times the Te Mato Vai residual: chemically-treated water and sludge, is the only fluid in the streambed.

The permit proponents have not acknowledged that impact is also relative to the force and frequency of the discharge. Intermittent changes to water quality such as an increase in suspended sediment also disrupt biodiversity distribution, behaviour and reproduction. More frequent discharge increases the likely magnitude of the disruption.


Permitting discharge

The residual from the Te Mato Vai drinking-water treatment system is a mix of natural sediment, treated water, and the chemical coagulant (polyaluminum chloride). The coagulant also contains secondary compounds that are a by-product of the coagulant manufacturing process.

In a court affidavit informing land-access negotiations, Te Mato Vai project managers GHD advised that the stream discharge of residuals is not standard practice in New Zealand or Australia.

62. In New Zealand and Australia, in most cases sludge supernatant is recycled back to the inlet of the water treatment plant…. Only the sludge becomes waste product as the supernatant is recycled…. So, for discharges of PACl (or dissolved aluminium) to the environment (through supernatant), there is no ‘standard’ practice.
- GHD, Nov 2019

  • Due to the cost of the coagulant chemical, and the uncertainty regarding long-term environmental impacts, the residual treated water (supernatant) is rather pumped back into the settling tank for reuse.
  • Settled sludge is typically de-watered onsite, may be further dried, and then used as capping material in landfill sites.


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 (NBSAP) 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 (2011), National Environment Service.

Although aluminium toxicity trigger values are cited in the Environment Impact Assessment (EIA), the accompanying baseline ecology report noted there is little published data on Rarotonga’s stream communities. As such, the promoted regulatory limits are largely based on laboratory assessment: of the short-term impact on specific — often on larger (commercially valuable), and predominantly temperate-climate species.

Spot environmental sampling was conducted over a week in January 2020. By this time, the Te Mato Vai construction processes had already disrupted the intake and treatment sites. The habitat and biodiversity sampling was not sustained, nor conducted as part of the six month operational trial of the new system.

  • Trial-operation of the Te Mato Vai system commenced in September 2020. The data collected did not include the level of dissolved aluminium at the point of discharge from the settling tank — the probable maximum value.
  • In November 2021, a permit was issued by the Rarotonga Environment Authority: authorising To Tatou Vai to discharge residuals at all ten of the Te Mato Vai treatment facilities.
  • The permit is valid for one year, and due for renewal November 2022.


The treatment facilities vary in configuration, with either two or three residual ponds at each site.

Avana drinking-water treatment facility, Rarotonga.

Above: Avana drinking-water treatment facility
From-left: PACl coagulant tower; dual-chamber concrete settling tank; (green) AVG sand filter towers; residual ponds; pressure header/water storage tank.


To maintain drinking-water quality, the sludge must be cleared regularly from the settling tank to prevent the sediment from mixing back into the process water. Prior to trial operation, it was estimated that the sludge would be cleared every 4-6 weeks, more frequently in high-flow/wet weather conditions. Firstly the upper layer of treated water is drained to the stream; and then the settled sludge-and-water flushed to an onsite holding pond.

The plants discharge supernatant from the ponds to the streams. The water going into the ponds originates from the AVG backwashes (approximately once every day), or from the less frequent de-sludging of the settling tanks.
-GHD Mar 2021

Early estimates were that the sand filters would self-clean automatically every 4-7 days, however GHD later indicated the process occurs daily. Given the volume back-calculated by Watson of 16,000 litres per unit, when all plants are online the daily volume discharged would met the needs of 3,200 people, around a third of the island’s population.

The backwash ponds are sized to contain (nearly) three backwashes. In some facilities settling tank scour is combined with backwash water.

Although known to occur at least daily, the frequency of backwash cycles is largely unknown.

Monitoring comprises of a manual cup-and-string system. Operators leaving a facility raise a string tied to a short length of pipe (cup) which floats between the filter unit and the outflow pipe. When in flow, backwash catches the cup, which pulls down the string. By this method To Tatou Vai can only record that a (single) backwash cycle has occurred since the last inspection.

Flow through the deanting head to the bordering stream is unmonitored. If the decanting heads are raised, then the overflow is first between pond cells, and then to the stream.

Avana residual ponds and stream discharge points.

Above: Avana residual ponds and stream discharge points.
Since commissioning, the ponds appear to have been widened. Sludge drying platforms cut into the embankment above the ponds.


Optimally each treatment facility has a dedicated (larger) pond to receive the sludge; and a separate pond for the backwash from the sand filter towers — with a standby backwash pond that can be left to dry. At some sites however, the same pond is used to contain the residual from both the settling tank and the sand filters.

Sludge drying platform, Avana.

Above: Sludge drying platform and storage ponds, Avana 15 Aug 2022
Mid-right of frame are interim sludge storage containers.


All ponds feature a T-bar decanting arm and rip-rap overflow. When sludge is cleared, the decanting arms are first lowered to drain the upper layer of water. When the decanting arms are permanently lowered, and the pond is overfull, the water discharged to the stream includes suspended sludge-sediment.

Based on the configuration of the Avana treatment site, the decanting arms have been positioned directly opposite the inflow: from the settling tank and/or sand filter(s).

Inflow sediment plume, Avana residual ponds, 15 Aug 2022.

Above: Pond function, Avana 15 Aug 2022
The Inflows from the Settling Tank and AVG Filter are located directly opposite the Decanting Arms. When the arms are permanently lowered, suspended sludge flows directly to the stream.

T-bar decanting head, Avana sludge pond.

Above: Upper Pond T-bar decanting arm, 15 Aug 2022

The AVG backwashes occur automatically. Each pond has a decant arm which floats on the top of the pond, and slowly discharges the top water layer to the stream. This provides time for any solids in the AVG backwash that discharges to settle. … it is anticipated that the decant arms will be left floating on the pond surface free to drain once the operational EIA has determined the long term allowable dissolved aluminium levels that can be discharged without environmental harm.
- IBID (emphasis added)

It is critical to note:

  1. The proposed operation of the Te Mato Vai system includes stream discharge but only of clear water. The pre-trial ecology report and the trial monitoring did not evaluate or track sludge discharge.
  2. Leaving the decant arms permanently lowered presumes pond drainage/leakage and residence time will ensure the settling-out of suspended solids.
  3. The Aquatic Ecology report referenced trigger values for the short-term impact of dissolved aluminium. No studies were referenced on the long-term impacts of the proposed method of operation.
  4. The T&T EIA report advised that Dissolved aluminium entering the stream via the discharge will associate with sediment in the stream to form flocs that will settle in the region of the discharge [the streambed].
  5. Changes in streamwater pH (an increase in acidity) can remobilise the aluminium from settled floc. This occurs when plant material, such as algae, biodegrades.
  6. Aluminium has been found to bioaccumulate in freshwater species, such as those found in Rarotonga’s streams, including koura (freshwater prawn). (Tempero 2015)

PACl dose

The potential impacts of discharge on biodiversity have previously been quantified based on the initial coagulant dose rate, and the eco-toxicity of aluminium.

Over the life of the Te Mato Vai Project the coagulant dosing values have gradually increased.

Typical dosing rates for PACI are generally 14-24mg/L, however, the proposed dosing rate for Te Mato Vai sites are significantly lower at approximately 4-6mg/L.
- GHD, Nov 2019

PACl is generally dosed to reach concentrations within 7-8 mg/L on the inflow (Jia et al 2018), with residual aluminium concentrations in outflow observed in the range 10-84 µg/L. This indicates that at a typical dosage rate, the residual concentrations of aluminium would be below the derived long-term specific criteria for Rarotonga of 290 µg/L,
- GHD, Sept 2020

How much PACI to use
Through our trials we found an effective range of PACI doses for different weather conditions. The more turbid the water, the more PACI is used.

  • Dry weather: 15mg/L up to 20mg/L
  • Wet weather: 20mg/L up to 35mg/L

- GHD, April 2021

The PACl dose rates are set between 10 and 40 mg/L (as solid PACl). During dry weather, the PACl doses are set to mostly 15 or 20 mg/L and are increased during rain events.
- GHD in Watson, Sept 2021

As a manual system, coagulant dosing can only be adjusted by an operator accessing the treatment facility. This configuration results in a mis-match between turbidity and coagulation dosing in times of flow variability: under-, or over-dosing. The concentration of residual chemical in the sludge may be anticipated to also be affected by the dosing regime.

With the system now operational, the PACl dose is believed to be in excess of 40mg/L. Extrapolating from the inflow values attributed to Jia et al, the outflow concentration residual aluminium, in clear water, may be as high as 420µg/L.

Evaluating the relative merits of coagulant compounds, both alum and polyaluminum chloride were noted to affect the function of down-treatment filters.

Table 7.1: Advantages, disadvantages and risks of alternative treatment options

  • Mismatched dose rates may cause significant floc overflow to the sand filters resulting in the shutdown of the filters.
  • Under low to moderate turbidity conditions short-circuiting of floc may shorten filter runs.

- T&T, May 2021

The manual system makes mismatched dose rates a certainty; trial data indicates sustained periods of low to moderate turbidity.

Turangi streambed in dry conditions.

Above: Turangi streambed alongside sludge ponds, 30 July 2022
In dry conditions, the only water in the stream is from the residual intentionally discharged, or overflowing from the Te Mato Vai system.


Evaluating impacts

Regulatory evaluation of potential impacts on streamhealth presumed:

“It is not only residual (chemical or physical) contents, but also the force and frequency of flow that risks harm.”

  • the discharge to be the near-clear water from the upper half of the settling tank; or from the surface of the residual pond;
  • sludge (solids/sediment) would not be discharged to the stream, but excavated or pumped from the pond for transport to a permanent storage facility;
  • any unplanned overflow/overtopping would likely occur only when the stream was in high-flow / wet weather conditions — which would rapidly dilute the discharge; and
  • streamwater pH levels would remain constant (neutral, to mildly-alkaline).

During and immediately following very heavy rainfall events, discharge of water to the scour ponds or via the settlement tank’s high level overflow may immediately enter the waterways. The large volume of water present via overland flows and within the waterways would dilute aluminium concentrations such that no significant impact would be expected.
- GHD, Sept 2020

The ponds have been designed so that water is directed around the ponds during heavy rainfall. In addition, any discharge to the stream will be diluted by the addition of rainwater. A rain event causing surface flooding will also result in high flows in the stream. This means a discharge to the stream will be subject to significant dilution.
- T&T, May 2021

Both in an Official Information Act request, and later in a submission to the Rarotonga Environment Authority, community group Te Vai Ora Maori queried the anticipated residual discharge volume; and how often discharge would occur:
“It is not only residual (chemical or physical) contents, but also the force and frequency of flow that risks harm.”

The Te Mato Vai Project Management Unit (GHD) refused to supply the requested data; and operators To Tatou Vai did not address this request in the formal response to submissions.

Left: Upstream reference site. Right: Upper pond discharge point.

Avana streambed, 15 Aug 2022
Left: Site A reference streambed, above the discharge points.
Right: Site B: Upper Pond stream discharge point. White pipe, bottom-left of frame.


Sludge characterisation

June to August 2022, Rarotonga experienced a period of meteorological drought, leading to operators To Tatou Vai issuing water conservation notices. Despite the low-flow conditions, the residual ponds at Turangi and Avana were found to be discharging residuals to the stream.

Aluminium concentrations in sludge are likely to be significantly higher than in the supernatant. In normal conditions, [sludge] will not be discharged into the stream (i.e. it will be removed off-site and taken to an appropriate disposal area).…

  • Based on the neutral pH levels within the streams, the maximum value over the long-term that could be tolerated is in the range of 290-630 µg/L.
  • The highest concentration of aluminium to which the aquatic community can be exposed briefly without resulting in an unacceptable effect is in the range of 570-1100 µg/L.
  • In a scenario where pH decreases within the receiving environment (below 6.5) the maximum acceptable concentration would decrease to 78 µg/L.

-Water Treatment Plants: On-site Discharges Environmental Impact Assessment. Tonkin+Taylor, May 2021.

  • Operational monitoring has been of dissolved aluminium levels; in water samples collected 50m downstream of the discharge point.

  • Since the initial (2014) estimate, the PACl dose has increased nearly ten-fold; stream discharge and overflow occurs in zero-flow conditions; and sludge is discharged to the streambed in dry conditions.
  • Sludge is now accumulating in the streambed, and may be washed downstream when rain returns.
  • Streamwater pH may also be affected by seasonal algal growth, and then decay. Growth uses up hydrogen, making water more alkaline; decay can make water more acidic/decrease the pH. Changes in streamwater chemistry are more likely to be pronounced in Rarotonga given the extreme variability in surfacewater flows (streams can be ephemeral), along with the fluctuation in groundwater inputs that are richer in dissolved mineral content.

Secondary compounds

A characterisation of the sludge (chemical analysis) was included in the later application for a long-term storage site. The characterisation identified secondary compounds in the coagulant chemical with potential to impact agricultural soils/plant health. Noting that the characterisation was for land application, and did not assess the potential impacts of the sludge on freshwater chemistry or biodiversity.

The Sulphate concentrations measured in the PACl sludge samples were high, at 140 to 576 mg/kg, compared with a highly variable range of <1 to 33 mg/kg in the five soil samples.…
The sulphate values are higher than optimal for plant growth. The level of available sulphur in the sludge will provide a luxury level of sulphur for plant uptake. The high sulphur level would be expected to lower the soil pH when applied to natural soil.
-Characterisation of PACl Sludge Samples Water treatment management in Rarotonga. GHD, May 2021.

Even considering a more moderate comparison, the sulphate levels in the sludge are five-fold those found in the environment. Lowering the soil pH… makes the soil more acidic.

The sludge characterisation report was not included in the on-site discharge permit application. Although sludge is to be stored onsite, there was no intent for the operational discharge of sludge directly to the stream.

The consultants preparing the required reports did not detail the potential impact of sludge being discharged, despite pond failure (overtopping) being reported at the Takuvaine facility on two occassions prior to commissioning.

Left: Detail from Avana sludge platform. Right: Sludge in Turangi streambed.

Left: Avana sludge platform, 15 Aug 2022
Right: Sludge in streambed below Lower pond, Turangi 30 July 2022.


Emergency response

Without a long-term containment facility, operators To Tatou Vai have attemped a number of methods of managing the accumulated sludge.

  • Temporary storage of the sludge in plastic containers, geotextile bags and gabion baskets.
  • Transfer of stockpiled sludge to the under-utilised Papua residual ponds.
  • Existing ponds appear to have been increased in size/dimension.
  • Sludge drying platforms/beds have been cut into pond embankments. These platforms have been formed to drain back into the ponds. The increased catchment increases the risk of overflow in wet weather conditions.
  • An application was made to form additional residual ponds at Turangi. However construction consent was not granted as an Environment Impact Assessment had not been submitted to the Rarotonga Environment Authority.
  • An (unsuccessful) application for an interim sludge storage facility on private land in the Takuvaine valley. Landowners objected to development as wider family/tribal consent had not been obtained.
Left: Collapsed plastic sludge storage containers. Right: Degraded division between sludge ponds.

Left: Sludge storage containers.
Right: Deteriorating division between the upper and lower residual ponds, Turangi 30 July 2022.


Insufficient localisation

…the ponds required as part of the TMV treatment process are engineered designs, and are not just ‘holes in the ground’.
- GHD, Nov 2019.

The current predicament can largely be attributed to a lack of investigation. Relying primarily on summary reports and borrowing the treatment method from a dissimilar (temperate) climate has resulted in a system with high operational overheads. The management of residuals was also not adequately considered as part of specifying the treatment method.

Algae in the Avana settling tank, 15 Aug 2022.

Above: Te Mato Vai: Avana Settling Tank, Rarotonga, 15 Aug 2022.
Despite drought conditions only the upper chamber appears to be operational.
The white particles around the Outflow are likely filamentous algae which is too bouyant to be settled by the coagulant. Algae accumulating in the sand filters increases backwash frequency, compromising the function of the residual/scour ponds and resulting in overflow.

  • Weather: The frequency of discharge to the residual ponds increases dramatically in wet weather conditions as the ‘first flush’ of runoff washes surface sediment into streams. As the level of sediment in the source streamwater increases, so does the accumulation of sludge in the settling tanks and sand filters.
  • Drainage: Settled sludge forms the compound gibbsite (aluminium hydroxide). Inland clay-type soils are already poorly-drained, and the sludge further seals the sides and floor of the ponds.
  • Climatic extremes: Inland areas receive an excess of 4000mm of rain annually. Less-evident from this total are the cycles of meteorological drought and intense rainfall. These extremes increase the likelihood of pond discharge, washout, and overflow.
  • Surfacewater management: Compacted accesses concentrate and direct surfacewater flows. Ponds are typically located downslope: between the access roads and waterways. The open canals that carry water to the ponds also act as catchwater drains and direct access road runoff flows to the ponds.
  • Undersized: The storage capacity of the residual ponds was determined by settling tank and sand filter volumes. However, the engineering calculations did not seem to factor operational evaporation potential or seasonal rainfall variability.
  • Limited evaporation potential: Broadly-speaking, the ponds have minimal surface area, are sheltered from the wind, and may be shaded for part of the year.
  • Groundwater inputs: The ponds have been excavated from platforms cut into steep terrain. The resulting groundwater inputs / hydrostatic pressure further reduces pond drainage potential.
  • Groundwater nutrient: In dry conditions, the streamwater balance shifts to nutrient-rich groundwater. Settling tanks are uncovered and the combination of sunlight, warmth, and nutrient provide an environment favourable to surface algal growth. The plant material compromises the function of the sand filters, decreasing the interval between the automatic self-cleaning (backwash) cycles. Each backwash contributes an additional 16,000L to the residual ponds.
  • Operational cycling: Optimally, one pond should be operational while another is allowed to dry. The most pressure is placed on facilities with fewer and/or smaller ponds. Increased water retention may be anticipated during periods of wet weather, but also seems to be an issue in dry and drought conditions.
  • Land mass and tenure: All land-access is subject to native claim. Much of the island is steep, undeveloped, inaccessible and/or unsuited to contain the waste from the treatment system. Prior to the introduction of chemical methods, there was no residual issue. The accumulated sediment contained only organic matter and was sought-after for agricultural application.
Sucker truck pumping sludge into the Papua residual pond.

Above: Pumping sludge into the Papua residual pond, May 2021.
Without a long-term disposal site, sludge has been transferred to less-utilised ponds.


Practical solutions

To minimise the risk of impact on the environment will require localisation of the infrastructure and treatment method. Ideal is a method of operation that does not result in the discharge of residuals to waterways.

If coagulant chemicals are not used, the sludge poses less of a risk to ecology, and will also be suitable for use in agriculture. It will not require specialised (and costly) amendment, processing or construction and operation of a dedicated long-term containment facility.

  • Sediment control: riprarian planting to stabilise streambanks and reduce the amount of sediment entering waterways above the intakes.
  • Physical treatment: pre-filtration and/or fine filtration. Retention areas above the intake will slow flows and allow coarse sediment to fall out of suspension.
  • Nutrient uptake: nutrient levels can be reduced by raw water passing through a vegetated area before entering the treatment facility.
  • Diversion/Selective abstraction: Stop water collection and treatment during periods of elevated turbidity/sedimentation to minimise sludge volumes.
  • Increase residual pond retention times: Offset decanting arms from residual inflows, and/or install baffles.
  • Improve pond function: (Bio-engineering) planting to increase storage capacity and increase water processing/uptake.
  • Remote turbidity monitoring and control, partial automation to optimise coagulant dosing.

References and Further Reading

See also

PACl on Trial, PACl Trial Data, Te Vai Ora Maori: Response To Water Treatment Plants: On-Site Discharges EIA, Te Vai Ora Maori - Facebook Page.


References ordered by publication date.


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Updated: 21 Sept 2023.


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