Water and wastewater treatment chemicals

What each treatment chemical does, where it is used, and how to choose the right grade and dose for your plant.

Why chemical choice matters

Chemicals are a running cost that never stops. The wrong coagulant, an over-dosed polymer or a poor-quality hypochlorite can raise the monthly chemical bill, produce more sludge and still miss the outlet limit. The right product, grade and dose come from a jar test on your own water or effluent, not from a catalogue. Below is each group of chemicals, what it does and what to watch.

Coagulants

Aluminium sulphate (alum): The traditional coagulant for raw water and clarification. It consumes alkalinity and lowers pH, so waters with low alkalinity may need a pH correction chemical alongside.
Poly aluminium chloride (PAC): A pre-hydrolysed aluminium coagulant that works over a wider pH range, consumes less alkalinity and often gives a faster floc and less sludge than alum. Widely used for raw water and many effluents. Grade is stated as percent aluminium oxide.
Ferric chloride and ferric sulphate: Iron coagulants that work over a wide pH range and are good for phosphate and sulphide removal and for heavy loads. They are corrosive and stain, so storage and dosing need suitable materials.
Ferrous sulphate: A lower-cost iron salt, often used with lime or with an oxidant. It needs oxidation to ferric to work well as a coagulant.
Organic coagulants (polyamines): Liquid cationic polymers that neutralise charge. Used alone or with inorganic coagulants to cut the dose and sludge, especially in textile and oily effluents.

Flocculants (polyelectrolytes)

Anionic, cationic and non-ionic polyacrylamides: Long-chain polymers that bind the micro-flocs into larger, settleable or floatable flocs. Anionic polymer is common after inorganic coagulants, cationic is used for sludge dewatering and some organic effluents, and non-ionic for specific waters.
Powder and emulsion forms: Powder is economical and needs a make-up unit with good wetting and ageing time. Emulsion is easier to feed but usually costs more per kilogram of active polymer.
Dosing and make-up: Polymers are made up as a dilute solution, aged before use and fed gently, because high shear breaks the chains and wastes the product. See dosing systems.

pH correction and precipitation

Caustic soda and soda ash: Raise pH, support precipitation of metals and supply alkalinity.
Lime (hydrated lime): Low-cost alkali for precipitation, softening and sludge conditioning. It adds solids and needs good slurry handling.
Sulphuric and hydrochloric acid: Lower pH, neutralise alkaline streams and control scaling. Need acid-resistant dosing and safe handling.
Sulphide and organosulphide precipitants: Remove heavy metals such as copper, zinc, nickel and others where hydroxide precipitation is not enough.

Oxidants and disinfectants

Sodium hypochlorite: The most common liquid disinfectant for treated water and sewage and an oxidant for iron, manganese and sulphide. Strength falls with time, heat and light, so storage and stock age matter.
Calcium hypochlorite: A solid form that stores longer and suits small plants and remote sites.
Chlorine dioxide and bromine products: Used where chlorine is less effective, for biofilm control or for taste and odour issues.
Potassium permanganate and hydrogen peroxide: Oxidants for iron and manganese, sulphide, odour control and advanced oxidation steps such as Fenton treatment.

Membrane (RO and UF) chemicals

Antiscalants and dispersants: Prevent scale such as calcium carbonate, calcium sulphate and silica deposits on RO membranes. Chosen from the feed water analysis and the plant recovery.
Sodium metabisulphite: Removes residual chlorine ahead of RO membranes, which chlorine would damage.
Cleaning chemicals (CIP): Acidic cleaners remove scale and metal oxides, and alkaline cleaners remove organics and biofilm. The cleaning order and pH limits follow the membrane supplier guidance.
Biocides and preservatives: Non-oxidising biocides for membrane systems and preservatives for shutdowns.

Biological treatment support

Nutrients: Urea and DAP supply nitrogen and phosphorus where industrial effluent lacks them, so that the biomass can grow.
Defoamers: Control foam in aeration tanks and clarifiers. A defoamer treats the symptom, so also look for the cause, such as overload or surfactants.
Bio-cultures and enzymes: Seed and support the biomass during start-up, after a toxic upset or for hard-to-degrade effluents.
Decolourants: Reduce colour in textile and dye effluents where coagulation alone is not enough.
Odour control agents: Oxidising or neutralising products for sulphide and amine odours.

Which chemical for which job

Job	Usual chemicals
Raw water clarification	PAC or alum, with anionic polymer
Textile or dye effluent	Inorganic coagulant with organic coagulant or decolourant, then polymer
Oily or food effluent in a DAF	Coagulant with cationic or anionic polymer
Sludge dewatering	Cationic polymer
Heavy metals	Alkali for hydroxide precipitation, sulphide precipitant if needed
Treated sewage disinfection	Sodium hypochlorite or calcium hypochlorite
Iron and manganese in borewell water	Oxidant such as hypochlorite or permanganate, then filtration
RO scaling and cleaning	Antiscalant, dechlorination chemical, acid and alkaline cleaners
pH correction	Caustic or acid
Weak biological activity	Nutrients, bio-cultures, and a check of the cause

How we select and supply

Jar test first: We test your water or effluent with candidate chemicals to select the product, grade and dose, before supply.
Dose sheet and procedure: Written dose rates for your flow and inlet conditions, with mixing instructions and safety data sheets for your operators.
Grade and batch quality: We specify concentration or active content and supply with a certificate of analysis for each batch.
Scheduled delivery: Supply on a schedule that matches your consumption so the plant does not run dry.
Review: We review consumption against outlet results and adjust the dose when the inlet or season changes.
Safe handling: Compatible storage, labelling, spill control and personal protective equipment, as stated in the safety data sheets.

Common questions

Which coagulant is best, alum or PAC?

PAC works over a wider pH range, uses less alkalinity and often produces less sludge, but costs more per kilogram. Alum is cheaper and works well in many raw waters. A jar test on your water decides which gives the lower total cost.

How do I know the right chemical dose?

From a jar test, then refined on the plant. The dose changes with the inlet quality and season, so it is reviewed regularly rather than set once.

Which chemical is used for sludge dewatering?

A cationic polymer is common for biological sludge on belt presses, screw presses and decanters. The grade and dose come from a trial with your sludge.

Why does sodium hypochlorite lose strength?

Hypochlorite breaks down with time, heat and sunlight. Buy in quantities you can use soon, store cool and shaded, and check strength when you receive stock.

Do you supply chemicals on a regular schedule?

Yes. We supply on a schedule that matches your consumption, with a certificate of analysis for each batch.