Water Treatment: Chemicals and Pathogens

Water Treatment: Chemicals and Pathogens.
Compiled by: Andy Kirkwood, Justine Flanagan, 2019.

Contact: [firstName] @ islandbooth.com.


This is a working draft. Updated 3 Dec 2020.


The Te Mato Vai Project to update Rarotonga’s intakes and water main. Chemicals have been proposed as part of treating the source water – coagulation: to remove particles; and disinfection: to kill or inactivate micro-organisms.

This page details the chemicals proposed for the treatment process; storage, transportation and handling risk, human health impacts and environmental fate.

Also provided are details of common illness causing micro-organisms (pathogens), and standard water testing methods.


Water Treatment Chemicals

Safety Data Sheets

Safety Data Sheets (SDS) detail the transportation, storage and handling requirements for industrial chemicals and compounds. The content of an SDS can vary depending on local regulations and safety requirements for each country.



Coagulation is a chemical process that involves neutralization of charge.

For the Te Mato Vai Project, facilities have been constructed for dosing a chemical coagulant. Once combined with the chemical, the water travels slowly from one end of a concrete settlement tank to the other. The chemical enables smaller particles to clump together to form larger particles that sink to the bottom of the tank. Particles that are less-dense than water float to the surface (and are removed by the sand filter).

Dosing of coagulants is more effective when water contains a large amount of sediment. In clear (low-turbidity) water, particles may be too far apart and do not come into contact. If dosed to clear water the risk is that levels of dissolved aluminium in finished water will be in excess of recommended values.

PolyAluminum Chloride / PAC / PACl

Aluminum Chlorohydrate (Intl./US-preferred synonym) / PolyAluminum Chloride
TTV (annual) 75 ~ 100 tonnes.

Safety Data Sheets: Chempro, Ixom (Solipac), ICL, Anchem, GEO.

Aluminium Sulfate/Sulphate / Alum

Aluminium Sulfate/Sulphate
Initially proposed, but no longer specified coagulant.

Safety Data Sheet: ‘Tui Hydrangea Blue’.



Flocculation is a physical process and does not involve neutralization of charge.


Disinfection Chemicals

Disinfection is about reducing the microbial content of water: removing, killing or inactivating bacteria, viruses and protozoa/parasites.

Calcium Hypochlorite / CaHCl

Calcium Hypochlorite
TTV (annual) 8 ~ 10 tonnes
Calcium hypochlorite should be stored dry and cold, away from any acid, organic materials, and metals.

Safety Data Sheets: Radox, Argo (US), Argo (JP).

  • Calcium hypochlorite will lose 3 to 5 percent of available chlorine per year. Reduce deterioration by maintaining a 30 to 60 day stock supply of dry material and mixing fresh solution daily.…
  • Many fires of spontaneous origin have been caused by improperly stored calcium hypochlorite.
  • Never store calcium hypochlorite where it may be subject to heat or allowed to contact organic material.
  • Calcium hypochlorite releases chlorine fumes when exposed to heat. It may build up pressure in sealed storage containers if containers are exposed to sunlight or other heat sources.

Drinking Water Plant Operator Certification Training. Module 25: Hypochlorite (2006)

Calcium Hypochlorite (transportation) Safety Lesson.

Source: Chlorine Safety: The Rest of the Story (Presentation). G Lohse, 2013.



Anolyte / Electrolysed Water
The electrolysis of salt solution to produce a hypochlorous acid + sodium hydroxide solution.
Safety Data Sheets: Envirolyte

Anolyte can also be used in an ice slurry to clean biofilm from drinking-water network pipes – this is referred to as ‘ice pigging’.


Comparison between Calcium Hypochlorite and Anolyte

A key distinction is between calcium hypochlorite and anolyte is the chemical formula of the two disinfectants. Although they sound the same, and both are chlorine compounds, the properties of hypochloric acid (HCl) are not the same as those hypochlorous acid (HOCl).

Comparison Calcium Hypochlorite (Chemical) Anolyte (Electro-chemical)
Formula HCl HOCl
Safety Dangerous good 100% safe
Disinfection contact time (in solution) 20-30 minutes (minimum) 10-60 seconds
Disinfection By-products (DBPs) Potentially toxic by-products; trihalomethanes, etc. Salt and water.
Storage and handling PPE: Gloves, dusk mask/repiratory device, safety glasses, overalls
Onsite drenching station and eyebath.
Gloves (in the case of sensitive skin).
Effective against Bacateria and viruses. Bacteria, viruses and protozoa/parasites, e.g. Giardia and Cryptosporidium.
Volume Effective at very small doses; in drinking-water 0.2–5.0ppm. Requires dosing at 1–4% per volume.
History In use for over 100 years. Properties of ‘activated’ electrolytic solution discovered in 1972.

Hypochlorous acid is new, but old

Hypochlorous acid has only relatively-recently (2007) been commercialised as an effective antimicrobial agent; as a pharmaceutical drug for treating infection; under the brand name Microdacyn. This has been possible by finding a method of forming a stablised solution.

Hypochlorous acid is more effective that other common antimicrobial agents such as hypochlorite and hydrogen peroxide. Human white blood cells synthesise hypochlorous acid as part of the immune response.



Neutralising chemicals balance a solution so that there is no excess of either acidic or alkaline ions; neutralisation results in a solution of salt + water.

For the Te Mato Vai Project; the chemical sodium thiosulphate has been proposed for rountine use to de-chlorinate.

5.2 Physical methods

Chlorine is neutralized upon reaction with air, sunlight and contacting surfaces. Chlorine readily reacts with organic and inorganic impurities in soil, paved surfaces, water and wastewater. Physical dechlorination relies on the breakdown of chlorine and chloramines as water travels from the discharge point to the receiving water body.
-Dechlorination Of Drinking Water Discharged To Waterways. Water Services Association of Australia. 2019

Sodium Thiosulphate

  • Sodium Thiosulphate/Thiosulfate (neutralises chlorine)
    Safety Data Sheets: ‘Antichlor’
  • To prepare solution: 100L water : 15.2kg Sodium Thiosulphate.
    Example formula: The free available chlorine (FAC) of the super-chlorinated water to be neutralised is 20mg/l (i.e. 20g/m3).
    The super-chlorinated water is being discharged from the main at a flow rate of 100L/min.
    The required flow rate of the 15% neutralising chemical solution is: 2.4 L/hr.
    Source: Watercare: Code of Practice for Disinfection of Water Systems, No. COP-04 Ver. 3 - 20 June 2017.

It is important to note that sodium thiosulphate [when used to dechlorinate] will also bind the chlorine in chloramines, thereby releasing ammonia. The pH of the water will dictate whether the ammonia released will exist in the form of toxic ammonia, or the less toxic ammonium ion. At a pH above neutral (pH=7), the release will increase the toxic ammonia concentration and can have a detrimental effect on the fish. At a low pH, the ammonia will shift to the less toxic ammonium ion. Sodium thiosulphate will also remove oxygen from the water which should be aerated before it enters the culture system.
-(PDF) The Ins and Outs of Dechlorination. C MacQuarie and S Wilton. (Aquaculture) Hatchery International, Sept 2002.

The toxicity of chloramine was observed in accidental spills of treated drinking-water in Canda in the late 1980s and early 1990s (where chloramine is used as a form of residual disinfectant rather than chlorine).

5.1 Chemical methods

The sulphur-based compounds [including sodium thiosulphate] are all inhibited by the general traits of not being fully effective in dechlorinating chloramines as well as their noticeable impact on DO (dissolved oxygen) levels and OH&S needs.

Any use of these chemicals instead of ascorbic acid [vitamin C] or sodium ascorbate would require careful justification and is not recommended as the best practise for dechlorinating drinking water.
-Dechlorination Of Drinking Water Discharged To Waterways. Water Services Association of Australia. 2019

Above: Obtaining consent to use sodium thiosulphate / The Many Hats of GHD.


Water Treatment Residual Reuse

Waste sludge from coagulation or fitration processes may itself require processing. One reuse of processed aluminium sludge (such as PACl) is to treat septic wastewater.

See also: PACl on Trial



Pathogens are illness-causing micro-organisms.

The pathogens listed are those commonly referenced in the design of water treatment systems. Despite misleading media releases; there has been no substantiated incidence of mass waterborne illness in Rarotonga, or the Cook Islands.



  • Escherichia coli (E. coli) - Indicator.
    E. coli are …commonly found in the lower intestine of warm-blooded organisms (endotherms). Most E. coli strains are harmless.
    As part of a water treatment sytem, the presence of E. coli indicates there may be contamination of the source water or a failure within the treatment process or distribution network.
    Disinfection typically kills or deactivates all E. coli, so live E. coil is a cause to investigate. The water manager will then seek to contain contamination: repair systems or remove contamination sources.
  • Escherichia coli O157:H7 - Pathogen.
    is one of the Shiga toxin–producing types of E. coli. It is a cause of disease, typically foodborne illness, through consumption of contaminated and raw food, including raw milk, leafy greens (if irrigated with contaminated water), and undercooked ground beef.
    The digestive tract of cattle lack the Shiga toxin receptor globotriaosylceramide, and thus, these can be asymptomatic carriers of the bacterium (can carry without exhibiting any symptoms).
  • Leptospira / Leptospirosis - Pathogen.
    High humidity and neutral (6.9–7.4) pH are necessary for their survival in the environment, with stagnant water reservoirs —bogs, shallow lakes, ponds, puddles, etc.—being the natural habitat for the bacteria.… Like most bacteria, Leptospira require iron for growth.
  • Legionella pneumophila - Pathogen.
    Within endemic regions, about 4% to 5% of pneumonia cases are caused by L. pneumophila. internalization of the bacteria appears to occur through phagocytosis. Once internalized, the bacteria surround themselves in a membrane-bound vacuole that does not fuse with lysosomes that would otherwise degrade the bacteria. In this protected compartment, the bacteria multiply.



Protozoa / Parasites

  • Giardia lamblia / Giardiasis.
    Chief pathways of human infection include ingestion of untreated sewage, a phenomenon particularly common in many developing countries; contamination of natural waters also occurs in watersheds where intensive grazing occurs.
  • Cryptosporidium / Cryptosporidiosis.
    Cryptosporidium is highly resistant to chlorine disinfection. Ultraviolet light treatment at relatively low doses will inactivate Cryptosporidium. One of the largest challenges in identifying outbreaks is the ability to verify the results in a laboratory. The oocytes may be seen by microscopic examination of a stool sample, but they may be confused with other objects or artifacts similar in appearance.

Particle Size

Filtration is used to remove micro-organisms and organic material from the water.

  • TTV system is active gravity valveless (AVG) sand filtration: effective filtration specification TBC.
  • TTV have advised that micro and ultra filtration will not be used (11 July 2019, pers. comm. BM).
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)


[Q] How does the new system deal with flooding events — turbidity detection [*] [**] / switch over to stored water?
TTV: The new process has been designed robust-enough to deal with high turbidity events – in the biggest floods the treatment process can ‘switch off’ (using a float valve arrangement I understand) whereby highly turbid water can be diverted back to the stream.
TVOM (Post Turangi visit 4 Sept 2019): There is no turbidity detection, treatment stops when the storage, AVG and settlement tanks back-up, but will continue to supply/drain to the network. The rate as which water is treated is limited by the AVG rapid sand filtration units.
Automatic Valveless Gravity (AVG) filtration rate, per unit: 2.5 L/sec/m2 – 9m3/hr/m2 (large AVG units can filter up-to 125,000 L/hr) Filtec AVG.


Standard Water Quality Tests

Classification/subjects: Te Mato Vai, To Tatou Vai, chlorination, coagulation, disinfection, anolyte, calcium hypochlorite, polyaluminum chloride, drinking water standards, monitoring water quality, Rarotonga, Cook Islands, South Pacifc, water monitoring, pathogens, waterborne illness.


References and Further Reading

Working Draft. Updated: 3 Dec 2020.


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