Rarotonga: Alternatives to Chemical Water Treatment

Settlement Tank - Turangi.

Rarotonga: Alternatives to Chemical Water Treatment. Compiled by: Andy Kirkwood, Justine Flanagan, 2019.
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Masthead Settlement Tank, Turangi, Rarotonga: 4 Sept 2019.
The split tank could be used to trial non-chemical coagulation methods.

 

This is a working draft, final proofing in progress. Updated 3 October 2019.

 

This page provides a digest of non-chemical methods of water treatment in response to the Te Mato Vai Project upgrade of Rarotonga’s water treatment systems and network.

International Principles of Drinking Water Safety recommend multiple-barriers against contamination. The solutions below may be combined based on the investigation of source water quality at each intake, and the integrity of the water delivery network.

Chemical-free Drinking Water

Worldwide, communities who have chemical-free drinking water have a number of things in common:

  1. They are geographically-blessed: plenty of rainfall; located in areas naturally rich in water resources.
  2. Protection and care of source water resources is paramount.
  3. Environmentally-conscious: not allowing heavy pollution from industries or intensive agriculture.

Rarotonga can have clean and safe water – without chemicals.

  1. The island has high annual rainfall and abundant groundwater supply.
    The majority of the supply is collected surface water; aside from the Totokuitu intake which may also be fed by a mineral spring.
  2. Catchment areas are generally remote from population centres; and communities have implemented cultural or legal restriction on the activities permitted.
    Ra’ui (restrictions on activities) are set by traditional leaders; and the Environment (Takuvaine Water Catchment Management Plan) Regulations 2006.
  3. There is no significant pollution-generating industry; agricultural practices are moving from synthetic fertilisers and sprays to organic farming methods (and there is no significant farming above the intakes).

The results of the 2014 Watercare source water tests show that there is significant diversity to the water quality in each of the intake valleys. This indicates that different treatment methods may be required at each plant.

[In Rarotonga] Seven raw water intakes were sampled under either wet weather conditions or after a rainfall event. Total Manganese exceeded the MAV (maximum acceptable value) set in the Drinking Water Standards New Zealand (DWSNZ) at the Rutaki and Avatiu raw water intakes during wet weather conditions. Total Iron and total Manganese exceeded the GV (guideline values) set in the DWSNZ at several raw water intakes during wet weather conditions as well. An investigation into an appropriate form of Manganese and Iron removal needs to be undertaken.
-Te Mato Vai Project Raw Water Quality Review Rarotonga (2014). Watercare New Zealand.

 

Rarotonga’s Water Catchment Areas

On the island of Rarotonga, the water catchment areas are inland, generally distant from settlement, and are not subject to industrial or agricultural pollution.

Rarotonga's water catchment areas (2006).

Rarotonga’s Water Catchment Areas (2006) (Enlarge).
Water intakes (dark blue squares) are located inland away from areas of settlement.
The intake for the largest catchment – Avana/243ha – is 4km inland from the main ring road. Source Water Supply System Description: Cook Islands.

In geological terms, the interior is described as ‘over-steep’ (near vertical) as the island is relative young, and the angular volcanic dykes that form the ridgelines are largely unweathered.

In heavy rain, soil, organics such as leaf litter, and surface minerals wash down the slopes and into the waterways. This leads to a temporary change in water quality. Anecdotal reports claim that within three hours of surface flooding, waterways run clear – even when streams may still be clouded below the intakes where water runs more slowly off the foothills.

Above: Aerial footage of the Ngatoe water treatment facility. Rarotonga is only 8-9km in diameter, and the intake points are located at elevation, 3km+ inland of settlement.

Clean and Safe Water

Water treatment systems ensure that water is both ‘clean’: suitable for drinking, washing, etc.; and ‘safe’: free of organisms, elements or chemicals that might cause illness.

Key metrics used when describing water quality include:

  • turbidity: how clear/cloudy the water is; due to the presence of particles that are generally too small to observe with the naked eye.
  • microbial content: numbers and types of organisms present in the water; including bacteria, viruses and parasites/protozoa.
  • metals/minerals: detection of elements and chemicals that affect the taste, usefulness for washing; or have a negative impact on human-health.

The above values are affected by seasonal variations and weather conditions that affect water flows. The 2014 Watercare analysis of Rarotonga’s raw/source water describes conditions as: dry weather, wet weather or after rain. One recommendation made in the report was that the Te Mato Vai system should stop water collection (‘abstraction’) and/or vary treatment processes based on the conditions.

Consider undertaking selective abstraction and/or treatment if abstraction is to occur across all flow conditions.
Te Mato Vai Project Raw Water Quality Review Rarotonga (2014).

Visiting the Turangi intake in Sept 2019 selective abstraction has not been implemented.

  • Collection stops only when waterflow exceeds the volume that can be processed by the AVG sand filtration units or when demand falls/storage tanks are full.
  • Proposed chemical treatment does not automatically change to match source water quality; the dosage must be changed manually. With coagulation and disinfection chemicals, the amount not ‘used-up’ by the treatment process enters the public water supply. If the water quality deteriorates water standards will be impacted until the dosage can be changed.

Rain events are the most common cause of changes in source water quality, and the more extreme the event the greater the likelihood of a breakdown in treatment barriers. Rain events present treatment plant operators with two problems simultaneously. First, changes in water quality require the process control parameters to be re-optimised. The speed with which this can be done will determine how well the barrier to contaminants is maintained. Second, at the time when maintaining optimum treatment is at its most difficult, the concentrations of contaminants in the source water are often at their greatest.
-Introduction to Drinking Water Contaminants, Treatment and Management. (NZ) Ministry for the Environment. 2008.

A manual chemical treatment system runs the risk of either over–, or under– dosing.

  • under-dosing will negative impact treated water quality;
  • over-dosing will result in residual chemical levels that may be unacceptable to consumers or impact on the environment and biodiversity.
Chlorine affects water taste and odour at concentrations equal to or greater than 0.3mg/L.

Chlorine Dosage: Chlorinated water has a distinctive taste and odour that is detectable in concentrations greater than 0.3mg/L, due to residual chlorine levels or interaction between the chlorine and organic or inorganic material in the water supply.
Source: (PDF-Presentation) Principals and Practices of Drinking-Water Chlorination. World Health Organisation, 2017.

 

The Dutch Secret: Clean, Safe Drinking-Water — Without Chlorination

References below from: The Dutch Secret, Smeets, et al, 2008.

In the 1970s dutch chemist Joop Rook discovered that chlorine disinfection of the municipal water supply resulted in toxic by-products. From 1976 chlorine has been progressively removed from Dutch water treatment systems. In 2005 the last chlorine disinfection processing plant was replaced by a new-generation UV system. Chlorine is no longer used for primary disinfection or as a residual disinfectant in the distribution network. The Netherlands has a population of over 17 million, and approximately one-third of the public water supply is collected from river water (surface water).

Drinking water treatment needs to be tailored to the hazards in the source water to provide safe water leaving the water treatment plant. This requires assessment of source water quality and treatment efficacy.
-The Dutch Secret

The Dutch approach is to:

  1. Use the best water source available.
  2. Use physical treatment such as sedimentation, filtration and UV treatment.
  3. Prevent ingress of contamination during distribution [no broken pipes, maintain network pressure].
  4. Prevent microbial growth in systems by using biologically stable water and constructing the network from biologically stable materials.
    [Biologically stable water and pipes prevent microbial growth due to lack of organisms and nutrients in the treated water; and networks constructed of materials that are smooth and inorganic such as high-density plastics.]
  5. Prevent significant health consequences by monitoring for timely detection of any systems failure.

Surface Water Contamination Controls

Control measures act as a barrier to contamination where intakes are fed by stream and river water.

  • The intake point can be changed during contamination events or water shortage.
    Likely contamination events in Rarotonga’s intake valleys include surface flooding or pollution hazards (e.g. contruction work above the intakes, slips, tree uprooting).
  • Filtration and/or UV treatment before storage reduces microbial and chemical hazards.
  • Stored water reduces hazards due to natural processes: sedimentation, die-off, and dark inactivation: lack of light preventing micro-organism growth and reproduction.
    Chemically-treated water should not be stored due to the risk that undesireable compounds may form.
 

Protecting Source Water

Vegetative Erosion Control

Organic matter (OM) enters waterways after surface flooding. Soil erosion can be mitigated through the use of vegetation to slow water flow above the intake point.

Vetiver Diagram.

Vetiver grass for surface water management and erosion control (Enlarge).
Source The International Vetiver Network / Fleur de Point.

Planted to the contours of the catchment valleys, and at the junction points of the tributaries feeding the main stream, vegetation such as vetiver grass would slow flows, trap and hold leaf litter, and direct surface water into the ground down through the root system (infiltration). The result is a reduction in the amount of silt entering the waterways.

Community Catchment Management

In 2006 the communities of the Takuvaine catchment drafted regulations governing the management, use and access to the area. These regulations could be used by other community groups to discourage activities that might compromise water quality.

Entrance signage and provision of facilities for tourists walking the cross-island tracks could also safeguard these areas. A low-cost, no-chemical system to provide facilities for visitors is to install vetiver/plant-based latrines.

Household Water Storage

Household collection of rainwater or onsite storage of the mains supply will both reduce demands on the water treatment plants for back-up supply and enable greater resilience in drought conditions.

Progressing with household storage systems might include;

  • confirming that roof cladding and existing storage tanks are suitable for long term water collection,
  • installing intermediary filtration (between mains supply and storage tanks) to remove network sediment,
  • ‘first-flush’ diverter systems to avoid mixing of roof organics, spray drift. and dust with stored water, and
  • installation of tanks that allow for easy inspection of sedimentation levels and cleaning.

A companion concern with household water storagae systems (when supply is chemically-treated) that secondary compounds might be formed in combination with organic matter. Most household tanks are above-ground which can also elevate water temperature and increase the potency of chemicals such as chlorine solution.

 

Diversion and Turbidity Detection: Don’t Collect Muddy Water

Turbidity is a measurement of the level of sediment/particles in water — or ‘cloudiness’.

A turbidimeter-controlled system would stop water collection when turbidity increases and resume collection when turbidity levels decrease. Turbidimeter controls can be battery-powered and recharged by solar panel or water turbine.

The proposed chemical-dosing equipment for the Te Mato Vai System is battery-powered; and a float/ballcock control diverts water when tanks are full.

Kemtrak Turbidimeter.

Turbidimeter Control Unit. Source Kemtrak.

Flood events are unlikely to result in island-wide water shortage. With the completion of the Te Mato Vai Project there will be 2-3 days of stored water supply (15 million litres). Storage tanks are planned for 8 of the 10 water treatment plants. Collection and treatment would resume as soon as the waterways clear; and the storage tanks would begin to refill.

When only clear water is collected, the need for coagulation chemicals would either be reduced or unnecessary.

Intake valleys also have their own micro-climates; it can be raining in one valley but dry in another. Sustained, heavy, island-wide rain is infrequent.

When only clear water is collected, the need for coagulation chemicals would either be reduced or unnecessary.

Centrifugal Water Filtration

A self-cleaning water-driven centrifuge can be used to separate heavier particles and could be implemented above the intake point.

 

Reducing Turbidity: Sedimentation, Coagulation, and Flocculation

Sedimentation is the process of removing suspended and dissolved solids from raw (untreated) water.

Chemical Coagulation: Polyaluminum Chloride

Polyaluminum chloride (PACl) has been proposed to encourage sedimentation to occur. To Tatou Vai (water authority) estimates 100 tonnes of PACl will be required per year.

Concerns raised by the public about the use of PACl include:

  • chemical cost: starting estimate for disinfection and coagulation chemicals is $350-500,000 per year,
  • human-health impacts resulting from exposure of aluminium compounds / occupational safety and health,
  • impacts on the environment and biodiversity
    As an example both the Avana and Turangi streams drain into Muri lagoon and the only estuarine environment on the island,
  • the onsite storage of PACl and sand-filtration waste ’sludge’ in unfenced, open-air ponds that are close to natural waterways; and
  • PACl sludge processing and disposal methods; which have not been disclosed.

Settlement Tanks: Gravity Separation

In the new Te Mato Vai settlement tanks, water takes three hours to move from one end to the other. Heavier particles settle to the bottom and lighter particles rise to the surface; clearer water is collected from the ‘middle’.
Monitoring of the effectiveness of the settlement tanks has been initiated as part of training To Tatou Vai staff (Sept 2019).

Organic coagulants / Biocoagulants

Organic coagulants could either replace or supplement chemical coagulants (to reduce the amount of chemical required).

Moringa oliefera and vetiver grass are both already grown in Rarotonga.

Electro Coagulation (EC)

In electro coagulation, water passes over a charged plate which attracts and separates finer suspended particles.

 

Filtration

Filtration removes larger particles from raw water and can also disinfect through the action of beneficial micro-organisms that grow in the filter medium/material.

Particle Type Size (microns)
sands to gravels 100 – 100,000
silts 10 – 100
algae 1 – 70
protozoan cysts 2 – 11
bacteria 0.2 – 12
colloids/ clays 0.006 – 10
virii (viruses) 0.01 – 0.1

Source: AVG Filters: The no-power means of filtration for drinking water. M Evan, R Hayes (2014).

Sand Filtration

In a sand filter, water moves down through a graded medium. Larger particles are trapped at the surface and in slow-sand filtration systems, micro-organisms within the sand may also disinfect the water.

The Te Mato Vai system includes self-cleaning active valveless gravity (AVG) rapid-sand filtration.

Turangi WTP: AVG Sand Filters and Sludge Pond.

Twin AVG Sand Filters (left) and Sludge Pond for onsite storage of settlement tank and sand filtration wastes. Turangi WTP, 4 Sept 2019.

Sand filtration effectively removes larger pathogens such as the protozoa/parasites Giardia and Cryptosporidium. Sand filters typically trap particles larger than 1 micron in size.
Note: Awaiting confirmation from PMU regarding AVG filter specification of the installed system.

 

Disinfection

Disinfection is concerned with reducing the microbial content of water: deactivating or killing bacteria, viruses, and protozoa.

The widely-used chemical method of disinfection is chlorination; which has been used internationally for over 100 years. UV irradiation or ozonation at the water treatment plant has been discounted by the Te Mato Vai engineers due to the cost of providing power to the intake sites; and that these physical disinfection methods do not provide residual disinfection.

What is Residual Disinfection?

The model proposed by residual disinfection is that water leaving the treatment plant is over-dosed, and that the remaining chemical continues to work on organisms in the network, in household plumbing, ‘right to the tap’.

The Effectiveness of Residual Chemical Disinfection

The Te Mato Vai Project proposes calcium hypochloride solution to disinfect water. As the chemical comes into contact with organisms it is ‘used-up’.

The effectiveness of residual disinfection will be negatively impacted by:

  • integrity of the new network (breaks in pipes, low-water pressure);
  • extended network detention times (for example, water piped from Avana-to-Nikao has an estimated age of 72hrs); and
  • variable standards of household plumbing.

Increasing the chemical dose to provide disinfection to those furtherest from the intakes may result in inacceptable taste impacts nearer to the intakes — resulting in drinking-water than no-one wants to drink.

The dosage recommended by the World Health Organisation has been found ineffective in containing illness outbreak resulting from major contamination events. A study into waterborne illness outbreaks in the EU found that nearly half of the incidents leading to contamination occurred within the water distribution network. (Risebro - 2007 in Smeets et al, 2008).

Chlorine residual was not sufficient to prevent the outbreaks for cases [of backflow and cross-contamination], nor for other significant causes (e.g. repairs, leakage and low pressure)…
…disinfection during distribution was an insufficient barrier. … Most outbreaks occurred in a system that applied a chlorine residual during distribution.

Most complaints from residents about water quality are about colour/turbidity.
Chlorine does not dissolve dirt.

Most complaints from residents about water quality are about colour/turbidity. Chlorine does not dissolve dirt. The most effective method of improving the colour/turbity of the water, is not to collect muddy water. (see Protecting the Source Water).

Detailed separately are the impacts of chlorinated supply on argiculture and organic farming.

Dark Storage

Lack of light prevents light-dependent micro-organism growth and reproduction. Typically water requires 60-day dark storage to maximise micro-organism die-off and settlement.

 

Household Disinfection

A more reliable method of disinfection – one that will result in consistentaly safer drinking water – is disinfection at the tap; with a combination of filtration or domestic UV irradiation.

A more reliable method of disinfection – one that will result in consistentaly safer drinking water – is disinfection at the tap; with a combination of filtration or domestic UV irradiation.

The economic impacts of providing and maintaining household disinfection are offset by:

  • reducing the need to purchase and maintain chemical treatment systems,
  • consumers and businesses absorbing the economic impacts as onsite power would be used,
  • regulating the installation of household systems to enable replacement filters/bulbs to be provided at cost – bulk importation; and government subsidy for at-risk households.
UV Dynamics Minirack system.

Combination (dual) microfiltration and UV system. Source UV Dynamics.

Installation priority would be for the most-vulnerable members of the community; families with young families, the elderly and those in areas known to be subject to historically poor water quality.

Village Disinfection: Community Water Stations

The island is supplied at present (2019) with UV-treated supply through a network of 36 community village water filling stations. This supply is available free of charge to locals and visitors to the island.

The inconsistent standard of water from the community water stations has been used by To Tatou Vai to rationalise chemical disinfection. This conclusion is misleading as the water stations currently receive water from the old system, through the old network. The improved Te Mato Vai intake system and network will improve water quality.

Some water stations are maintained more regularly than others. Supporting a formal system of maintainance and cleaning will improve station treatment consistancy.

 

Keeping the Pipes Clean

One argument for chlorination is that the chemical prevents the formation of bacterial colonies (biofilm) inside network and household pipes.

This concern itself is a form of ‘residual’ — an understanding of biological systems and water disinfection circa 1950.

Friendly Biofilm

Contemporary research suggests that:

The removal of biofilm may be necessary if build-up were found to result in surface drag and slow water flows through the network. However increased surface drag is more common in corrodable concrete or metal pipes, rather than modern synthetic plastics. The entire Rarotonga ring main and trunk lines have been replaced as part of the Te Mato Vai Project: 2014–2019. Should biofilm accumulation become an issue, ice pigging could be used to peroidically clean sections of the network.

Ice Pigging

A salt water (brine) ice-slurry is fed through the pipe network. The ice crystals scour the interior of the pipe and displace any sediment. The slurry can be fed around any curves or angled joints. In Rarotonga, commercial ice production facilities used to produce shaved-ice for fish processing could be repurposed to produce the required food-grade slurry.

Ice pigging could be used to periodically clean sections of the network if flows are impacted by biofilm formation; cleaning the interior of the pipes reduces surface drag.

Above: Using brine solution ice slurry to clean network pipes.
Source: Water Environment Federation / Suez

Anolyte

Anolyte is an electrolysed salt solution (brine) that results in a fast-acting microbial disinfectant. It can be used in place of traditional chlorine disinfectants for storage tank disinfecton; and can also be frozen to produce a slurry suitable for pipe scouring (‘ice pigging’).

Although anolyte is a chlorine-compound; at the dosage required to achieve drinking-water microbial standards, it is non-toxic to biodiversity. In Malaysia, Thailand, India, and China analoyte is used as part of shrimp-farming processes; for the disinfection of pond-water (containing live shrimps).

 

Conclusion

There are natural and physical methods of delivering clean and safe water without the use of chemicals.

A contemporary, eco-friendly and biologically-harmonious approach to water treatment is relevant, acheivable and affordable; and reasonates with the cultural values of the people of Rarotonga.

See also: Te Vai Ora Maori: Non-chemical Water Treatment Methods.

 

References and Further Reading

See more: Cook Islands Chlorination References: Standards, Policy, Technical Reports and News Items.

 

Classification/subjects: Te Mato Vai, To Tatou Vai, chlorination, coagulation, disinfection, commissioning, surface water, drinking water standards, monitoring water quality, Rarotonga, Cook Islands, South Pacific.

Acknowledgements / Meitaki ma’ata

Dr Ian Calhaem for information on anolyte. Engineers David Sloan and Ross Dillon for technical detail on the new Te Mato Vai System. Evan Mayson and Tangi Taoro from the Project Management Unit for organising a tour of the Turangi Water Treatment Plant. Robinson Vanoh from the South Pacific Vetiver Network and Marie Clapisson from Fleur de Point for the vetiver diagram.

Working Draft. Updated: 3 October 2019.

 
 

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