What is a coagulant?

If your clarifier overflow is cloudy, your effluent fails on suspended solids or colour, or your sludge will not settle, the fix often starts with one chemical: a coagulant. Here is what a coagulant is, where it is used, the problems it solves, and how to choose the right one.

1. What is a coagulant?

Impurities in water come in three progressively finer forms: suspended, colloidal and dissolved.^[1] Larger particles settle on their own if given time. The finest ones — colloidal particles — do not. They are so small that they stay suspended for a very long time, and they carry a negative electric charge on their surface, so they push each other away, like magnets held with the same poles facing.

A coagulant is a chemical that removes that repulsion. Most coagulants are salts of aluminium or iron. In water they form positively charged, insoluble hydroxide flocs that neutralise the negative charges of the colloidal particles.^[1] With nothing keeping them apart, the particles can finally stick together.

Treatment then happens in two steps:

• Coagulation — the coagulant is mixed in quickly and thoroughly. The particles are destabilised and begin to clump into tiny “micro-flocs”.
• Flocculation — gentle mixing lets the micro-flocs collide and grow into larger, visible flocs. A polymer flocculant is often added at this stage to bridge them into bigger, stronger flocs.^[1]

The flocs are then separated from the water by settling in a clarifier, by dissolved air flotation (DAF), or by filtration.

Common types of coagulant

• Aluminium-based — aluminium sulphate (alum) and polyaluminium chloride (PAC).
• Iron-based — ferric chloride and ferric sulphate.
• Organic coagulants — cationic polymers such as polyamines, often used together with an inorganic coagulant.

2. Where are coagulants used?

• Drinking water treatment — removing turbidity and colour from river and reservoir water before filtration and disinfection.^[1]
• Industrial process water — clarifying raw water before it is used in the factory.
• Industrial wastewater — as the main physico-chemical treatment step, or to polish effluent after biological treatment, in industries such as textile and dyeing, food and beverage, paper and pulp, palm oil, rubber, chemical and petrochemical, metal processing, paint and ink, sugar and starch.
• Oily wastewater — helping to separate fats, oils and grease, often ahead of dissolved air flotation.^[2]
• Municipal sewage — improving solids removal and removing phosphorus.^[2]
• Sludge treatment — conditioning sludge before thickening and dewatering on belt presses, screw presses, filter presses and decanters.

3. What problems do coagulants solve?

Sources: colour and turbidity ^[1]; phosphate, oil and grease, metals and fine solids ^[2].

Meeting Malaysian discharge limits

In Malaysia, industrial effluent must meet the acceptable conditions in the Environmental Quality (Industrial Effluent) Regulations 2009.^[3] Standard A applies to discharges into inland waters within the catchment areas listed in the Sixth Schedule — the areas upstream of water supply intakes used for human consumption, including drinking water. Standard B applies to discharges into any other inland waters or Malaysian waters. The full list of limits is below; the parameters marked ● are the ones coagulation directly affects.

Note that aluminium and iron are on the list too. The coagulant itself must be dosed correctly: too little leaves the water cloudy, while too much wastes chemical, lowers the pH and can leave residual aluminium or iron in the treated water.^[1]

4. Choosing the right coagulant — and what ours do

Every water is different. Each coagulant works best within a particular pH range and at the right dose, and both depend on the water being treated.^[1] The only reliable way to choose is the jar test: samples of your actual water are treated side by side with different coagulants and doses, and the results are compared. It is described as probably the most important routine test at a plant using coagulation.^[1]

Accot coagulants

Used together with

• ACCOT Anionic Polymer — flocculant aid, added after the coagulant for larger, faster-settling flocs.
• ACCOT Cationic Polymer — for thickening and dewatering the sludge that coagulation produces.
• ACCOT Non-ionic Polymer — for oily or fatty wastewater and mineral suspensions.
• pH adjusters — Soda Ash or Caustic Soda 50% to raise the pH, Hydrochloric Acid 33% or Sulphuric Acid 50% to lower it — keeping coagulation in its best pH range. See What is a pH adjuster?