Water treatment depends on a precise combination of chemicals, each performing a specific role at a specific stage of the treatment process. Disinfectants destroy pathogens. Coagulants aggregate suspended particles into removable flocs. pH adjusters maintain the chemical balance that makes other treatments effective. Corrosion inhibitors protect infrastructure. Scale removers maintain system efficiency.
This guide covers the top 10 water treatment chemicals plus chlorine, the most critical of all, with their functions, mechanisms, typical applications, and relevance to Australian water treatment practice.
Whether you are managing a municipal water treatment plant, an industrial wastewater system, or an agricultural water supply, this reference covers the chemicals your system most likely depends on.
Use the quick reference table below to find what you need, or read through the full profiles for each chemical.
Water Treatment Chemicals: Quick Reference Table
| Chemical | Primary Function | Treatment Type | Key Application |
|---|---|---|---|
| Chlorine | Disinfection | Drinking water, wastewater | Municipal water supply, swimming pools |
| Sodium Hypochlorite | Disinfection | Drinking water, wastewater | Water sanitation, algae control |
| Sodium Chlorite | Disinfection precursor (→ ClO₂) | Drinking water | Municipal water systems |
| Poly Aluminium Chloride (PAC) | Coagulation / Clarification | Drinking water, wastewater | Turbidity removal, sludge reduction |
| Ferric Chloride | Coagulation | Wastewater | Phosphate removal, suspended solids |
| Ferric Sulphate | Coagulation | Industrial wastewater | Colloidal particle removal |
| Hydrochloric Acid | pH adjustment / Descaling | Industrial | Boilers, heat exchangers, pipes |
| Phosphoric Acid | Corrosion inhibition | Industrial | Metal pipe and tank protection |
| Sodium Molybdate | Corrosion inhibition | Industrial, cooling systems | Cooling towers, boilers |
| Citric Acid | Scale removal / Chelation | Industrial | Pipes, boilers, evaporators |
| Hydrogen Peroxide | Oxidation / Disinfection | Drinking water, wastewater | Odour control, organic removal |
Note: This guide covers 11 chemicals — the original top 10 plus chlorine, which is the most widely used water treatment disinfectant globally and the foundation of most municipal treatment programmes.
What is the Best Chemical for Water Treatment?-Exploring Top 10
Chlorine
Chlorine is the most widely used chemical in drinking water treatment globally, and the backbone of most municipal water treatment programmes in Australia and worldwide. It functions as a powerful disinfectant, killing bacteria, viruses, protozoa, and other pathogenic microorganisms that cause waterborne diseases including typhoid, cholera, and gastroenteritis.
In water treatment, chlorine is applied at multiple points in the treatment process — at the point of entry to the treatment plant (primary disinfection), and again before water enters the distribution network (secondary disinfection) to maintain a residual disinfectant concentration throughout the pipe network.
How it works: Chlorine reacts with water to form hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻). Hypochlorous acid is the more effective disinfecting agent, and its proportion relative to hypochlorite increases at lower pH levels — which is why pH control is important in chlorine-based disinfection programmes.
Typical dosage in Australian drinking water treatment: 1–5 mg/L, with 2–3 mg/L being the most common range for standard treatment. The Australian Drinking Water Guidelines (NHMRC) recommend maintaining a free chlorine residual of at least 0.2 mg/L throughout the distribution system to protect against recontamination.
Limitations: Chlorine reacts with naturally occurring organic matter in water to form disinfection byproducts (DBPs) including trihalomethanes (THMs) and haloacetic acids (HAAs), some of which are regulated due to health concerns at elevated concentrations. This is one reason why alternative or supplementary disinfectants — chloramines, chlorine dioxide, ozone, and UV — are increasingly used in combination with chlorine in advanced treatment plants.
Citric Acid
Citric acid is used in water treatment systems primarily as a scale remover and chelating agent — making it one of the most practically useful chemicals for maintaining the operational efficiency of industrial water infrastructure.
How it works: As a weak organic acid, citric acid forms stable complexes with metal ions — calcium, magnesium, iron, and manganese — through a process called chelation. This breaks apart the ionic bonds that hold limescale and mineral deposits together, dissolving them into a water-soluble form that can be flushed from the system. The chelation mechanism is particularly effective against calcium carbonate scale (the most common type in hard water areas) and iron oxide deposits.
Applications: Citric acid is used to descale boilers, heat exchangers, cooling towers, evaporators, pipes, and water storage tanks. It is applied as a cleaning solution — typically at concentrations of 5–15% in water — and left to circulate for several hours before being flushed from the system.
Advantages over mineral acids: Unlike hydrochloric acid or sulphuric acid, citric acid is biodegradable and does not produce hazardous fumes during application. It is classified as safe for use in food-grade and potable water systems when applied correctly, making it the preferred descaling agent in settings where contamination risk must be minimised. It is also non-corrosive to most metals at standard treatment concentrations, reducing the risk of equipment damage during the descaling process.
In Australian industrial and municipal water systems, citric acid is commonly used for periodic maintenance cleaning cycles on boilers and heat exchangers — particularly in regions with high water hardness, such as parts of Western Australia and South Australia where calcium carbonate scale accumulation is rapid.
Ferric Chloride
These flocs make it easier to remove contaminants through filtration or sedimentation. Wastewater treatment plants commonly use ferric chloride to remove phosphates and suspended solids, which helps prevent water pollution.
Since it binds these particles together, ferric chloride improves water clarity and quality. It also helps control odours and reduce harmful nutrients in water systems. Its effectiveness, reliability, and versatility make ferric chloride an essential chemical for maintaining clean and safe water in industrial and municipal settings.
Ferric Sulphate
These flocs settle quickly, making it easier to remove contaminants through filtration or sedimentation.
Plus, ferric sulphate works well for treating industrial wastewater and removing stubborn colloidal particles that are hard to separate. It improves water clarity and reduces pollutants like heavy metals and organic matter. By enhancing the coagulation process, ferric sulphate helps prevent blockages and keeps water systems running efficiently.
Its effectiveness, especially in challenging wastewater conditions, makes it a reliable choice for maintaining clean, treated water in industrial and municipal systems.
Hydrochloric Acid
This is commonly used as a pH adjuster and descaling agent in the water treatment industry.
Hydrochloric acid, or HCl, effectively lowers the pH of alkaline water, making it more suitable for industrial processes. When water becomes too alkaline, it can cause corrosion or reduce treatment efficiency.
HCl helps balance the pH, ensuring proper system operation.
Additionally, hydrochloric acid dissolves limescale and mineral deposits that form in boilers, heat exchangers, and pipes. When breaking down these deposits, it maintains smooth water flow, prevents blockages, and improves system efficiency. Its powerful properties make hydrochloric acid essential for maintaining clean, efficient, and well-functioning water treatment systems in various industries.
Hydrogen Peroxide
As hydrogen peroxide works, it decomposes into water and oxygen, leaving no harmful residues behind.
This makes it an eco-friendly alternative to chlorine and other chemical disinfectants. Water treatment facilities use it to control odours, remove pollutants, and improve water quality without adding toxic by-products.
Its versatility and environmental safety make hydrogen peroxide an excellent choice for maintaining clean water in both drinking water systems and wastewater treatment processes.
Phosphoric Acid
Phosphoric acid protects metal pipes and tanks by creating a layer of phosphate salts that prevents rust and corrosion. This protective layer shields infrastructure, especially in systems where water contains corrosive elements.
Since the latter reduces the risk of corrosion, phosphoric acid extends the lifespan of pipes, boilers, and storage tanks. It also helps maintain water flow efficiency by preventing blockages caused by rust build-up. This is what makes it valuable for industries that rely on metal infrastructure. Also, phosphoric acid ensures that systems remain durable, reliable, and safe for long-term use.
Poly Aluminium Chloride (PAC)
Poly Aluminium Chloride (PAC) is one of the most versatile and widely used coagulants in both municipal drinking water treatment and industrial wastewater management. In Australia, PAC has largely replaced traditional alum (aluminium sulphate) as the preferred coagulant in many treatment plants, particularly for treating turbid surface water sources.
How it works:
PAC is a pre hydrolysed aluminium based coagulant. When added to water, it releases positively charged aluminium polymer species that neutralise the negative surface charge on suspended particles, colloids, clay particles, organic matter, and microorganisms.
Once destabilised, these particles aggregate into visible flocs that can be removed by sedimentation or filtration.
Advantages over alum:
PAC operates effectively across a wider pH range (5.5 to 8.5 compared to alum’s 6.0 to 7.8), making it suitable for a broader range of source water conditions.
It produces lower sludge volumes, typically 25 to 40% less than alum, reducing disposal costs.
It is also effective at lower water temperatures, where alum’s coagulation efficiency drops significantly.
Typical dosage:
5 to 80 mg/L depending on raw water turbidity, colour, and organic loading.
For highly turbid water following floods or bushfire runoff events in Australia, doses at the upper end of this range may be required.
Applications:
Municipal drinking water clarification, industrial wastewater treatment for suspended solids removal, stormwater treatment, paper and pulp industry effluent treatment, and food and beverage processing wastewater.
Sodium Chlorite
When activated, sodium chlorite produces chlorine dioxide, a powerful disinfectant that kills bacteria, viruses, and other harmful microorganisms.
Water treatment plants use chlorine dioxide for its strong oxidising properties, making it highly effective at purifying drinking water. Unlike traditional chlorine, chlorine dioxide does not create harmful by-products, making it a safer option.
Sodium chlorite helps maintain water quality by ensuring thorough disinfection without unwanted residues. Not to mention that this is truly valuable for municipal water systems, providing clean, safe water for communities.
Sodium Hypochlorite
Sodium Molybdate
It forms a protective layer that shields equipment like pipes, cooling towers, and boilers from rust and corrosion. This protective barrier helps prevent damage caused by exposure to oxygen and moisture, extending the lifespan of critical infrastructure.
Sodium molybdate works well in both hot and cold water systems, making it versatile for different conditions. Industries tend to use it to maintain system efficiency, reduce maintenance costs, and avoid downtime caused by corrosion-related failures.
Its non-toxic nature also makes it safer for the environment than other inhibitors, ensuring long-lasting and reliable water treatment solutions.
Water Treatment Chemicals in Australia: Regulatory Context and Industry Application
Australia’s water treatment chemical landscape is governed primarily by the Australian Drinking Water Guidelines (ADWG), published by the National Health and Medical Research Council. These guidelines set the maximum acceptable concentrations of treatment chemicals and their byproducts in drinking water, and specify which chemicals are endorsed for use in Australian potable water treatment systems.
Key regulatory points relevant to chemical selection in Australia:
- Chlorine and sodium hypochlorite:
Endorsed for use in Australian drinking water treatment since 1983. The ADWG specifies a free chlorine residual of ≥0.2 mg/L throughout the distribution system, with typical dosing at 1 to 5 mg/L at the treatment plant. - Disinfection byproduct limits for THMs and HAAs apply.
- PAC (Poly Aluminium Chloride):
Widely used in Australian municipal water treatment, particularly for high turbidity source water events following bushfires and floods, both common in Australian conditions. - PAC is effective across the wide pH range (5.5 to 8.5) typical of Australian surface water sources.
- pH management:
Australian water sources vary significantly in natural pH and alkalinity, from the soft, low alkalinity catchments of Queensland and Victoria to the hard, alkaline groundwater sources common in South Australia and Western Australia. - Chemical selection for pH adjustment must account for source water chemistry, which directly affects the performance of coagulants and disinfectants.
- State specific discharge standards:
Industrial wastewater treatment chemical programmes in Australia must comply with state EPA discharge standards, which vary between jurisdictions. - The pH range for discharge to municipal sewers is generally 6.0 to 9.0, with stricter limits applying in sensitive receiving environments.
- Treatment chemicals, particularly acids and caustics used for pH adjustment, must be dosed with reference to these discharge limits.
- Bushfire and flood impacts on water chemistry:
Australian water utilities face periodic challenges with elevated turbidity, dissolved organic carbon, and heavy metal concentrations following bushfires and floods. - These events significantly alter the chemistry of source water and typically require increased coagulant doses (PAC, ferric chloride), enhanced activated carbon dosing for taste and odour control, and adjusted disinfection programmes to manage elevated DBP formation potential.
Conducting Chemical Performance Analysis Using AI-Driven Tools
The time for passive monitoring of chemical performance is over. It is time to take proactive steps to ensure the optimal functioning of our water treatment systems. Since we have stepped into the ‘AI Era,’ adopting AI-driven tools is the best way to revolutionise how we approach water treatment, maximising efficiency, minimising costs, and safeguarding our environment. Incorporating the industry expertise is simply another word for ‘streamlining work’ in the water treatment industry.
FAQs about Water Treatment chemicals
The most commonly used water treatment chemicals are chlorine and sodium hypochlorite (for disinfection), alum and poly aluminium chloride or PAC (for coagulation and turbidity removal), lime and caustic soda (for pH adjustment), ferric chloride and ferric sulphate (for wastewater coagulation), hydrogen peroxide (for oxidation and disinfection), phosphoric acid and sodium molybdate (for corrosion inhibition), and citric acid (for scale removal and chelation).
Australian drinking water treatment is governed by the Australian Drinking Water Guidelines (NHMRC). Approved chemicals for use in potable water treatment in Australia include chlorine and sodium hypochlorite, poly aluminium chloride, alum, lime, hydrofluorosilicic acid (for fluoridation where applicable), and various pH-adjustment chemicals. All chemicals must comply with ADWG limits for residual concentrations and disinfection byproducts in the treated water.
Wastewater treatment plants commonly use ferric chloride and ferric sulphate as coagulants for phosphate and suspended solids removal; polyacrylamide flocculants to assist sedimentation; sodium hypochlorite or chlorine dioxide for effluent disinfection; hydrochloric acid and sodium hydroxide for pH adjustment to meet discharge standards; and defoamers and odour control chemicals for operational management.
Coagulants — such as alum, ferric chloride, and poly aluminium chloride — destabilise suspended particles by neutralising their surface charge, causing them to begin aggregating. Flocculants — typically long-chain polymers such as polyacrylamide — then bridge these destabilised particles together into larger, faster-settling flocs. Coagulation comes first; flocculation follows. Both steps are often required for effective clarification of turbid water.
Yes. Hydrogen peroxide is used in water treatment as an oxidising agent and disinfectant. It effectively breaks down organic contaminants, bacteria, and odour-causing compounds. Its key advantage is that it decomposes into water and oxygen, leaving no harmful residues — making it an environmentally preferable option compared to chlorine-based disinfectants in applications where disinfection byproduct formation is a concern.
