Turbid water contains fine suspended particles, colloids and dissolved organic matter that do not settle on their own. Coagulation and flocculation are the chemical steps that turn these into settleable or filterable solids. They are central to drinking water treatment, industrial process water and wastewater treatment. This article explains how the main chemicals work and how to choose between them.
Why particles do not settle
Particles such as clay, silt, bacteria and natural organic matter in water typically carry a negative surface charge. Like charges repel, so the particles remain dispersed as a stable colloid. Particles smaller than about 1 µm settle so slowly that, for practical purposes, they never settle in a treatment plant.
Coagulation: neutralising the charge
A coagulant is usually a salt of a trivalent metal, aluminium or iron. When added to water, it hydrolyses to form positively charged species and metal hydroxide precipitates. These neutralise the particle charges, allowing particles to approach and stick together, and they also trap particles in the growing hydroxide precipitate, a mechanism called sweep flocculation. Coagulation requires rapid, intense mixing for a short time so that the chemical is dispersed before hydrolysis is complete.
Flocculation: building bigger flocs
After coagulation, gentle mixing encourages the small destabilised particles, called microflocs, to collide and grow into larger flocs. Polymeric flocculants accelerate this by bridging between particles with long molecular chains. Large, dense flocs settle quickly or are removed by flotation or filtration.
The main coagulants
| Coagulant | Typical form | Effective pH range (approx.) | Notes |
|---|---|---|---|
| Aluminium sulfate (alum) | Solid or liquid | 5.5 to 7.5 | Widely used and economical. Consumes alkalinity |
| Polyaluminium chloride (PAC) | Liquid or powder | 5.0 to 8.5 or wider | Pre-hydrolysed. Lower alkalinity consumption. Effective in cold water |
| Ferric chloride | Liquid, about 40% solution | 4 to 11 | Wide pH range. Corrosive. Good for phosphate removal |
| Ferric sulfate | Solid or liquid | 4 to 11 | Less corrosive than chloride. Used in drinking and wastewater |
| Ferrous sulfate | Solid | Higher pH, often with lime or oxidation | Low cost. Needs oxidation to ferric form |
Ranges are indicative and depend on water chemistry.
Aluminium sulfate (alum)
Alum, Al₂(SO₄)₃·xH₂O, is the traditional coagulant. Each mole of aluminium hydrolyses and consumes alkalinity, so waters with low alkalinity may need lime or soda ash to keep pH in range. Performance falls in cold water. Commercial grades are specified by their aluminium oxide (Al₂O₃) content and by limits for insoluble matter and iron.
Polyaluminium chloride (PAC)
PAC is a family of pre-hydrolysed aluminium chloride polymers with the general formula Aln(OH)mCl3n−m. Its key specification parameters are Al₂O₃ content, commonly around 10 to 18% for liquids and about 28 to 31% for powders, and basicity, the degree of pre-neutralisation, expressed as a percentage. Because PAC is partly pre-hydrolysed, it consumes less alkalinity than alum, depresses pH less, works across a wider pH range, forms flocs faster and performs better in cold or low-turbidity water. It also tends to produce less sludge. The trade-off is a higher price per kilogram, although cost per cubic metre treated is often competitive.
Ferric salts
Ferric chloride and ferric sulfate work over a wide pH range and form dense flocs. They are effective for removing natural organic matter, arsenic and phosphate, and are widely used in wastewater. Ferric chloride solutions are highly corrosive and require suitable storage materials. Residual iron can cause colour if dosing is poorly controlled.
Flocculants and coagulant aids
- Polyacrylamides (PAM) are high-molecular-weight polymers available as anionic, cationic or non-ionic types. Anionic PAMs are common in drinking water and mineral processing. Cationic PAMs dominate sludge dewatering. Drinking water regulations commonly limit residual acrylamide monomer, so grades intended for potable water carry a low monomer specification.
- PolyDADMAC is a cationic polymer used as a primary coagulant or coagulant aid, particularly to reduce metal coagulant doses.
- Activated silica and natural polymers, such as starches, are occasionally used as aids.
Jar testing: finding the right dose
Coagulant choice and dose depend on turbidity, alkalinity, pH, temperature and organic content, and they change with the seasons. The jar test is the standard way to optimise them:
- Fill a series of beakers with raw water.
- Add increasing doses of coagulant, and pH adjustment if needed.
- Mix rapidly for about one minute, then slowly for 15 to 20 minutes.
- Allow to settle, typically for 20 to 30 minutes.
- Measure turbidity, pH, colour and residual aluminium or iron in the supernatant.
The best dose gives the lowest turbidity and acceptable residual metal at the lowest cost. Repeat the test with a flocculant to see whether it improves settling.
Residual aluminium and drinking water standards
Poorly controlled aluminium coagulation can leave dissolved aluminium in treated water. Many standards set limits or operational targets for aluminium. For example, the Indian drinking water standard IS 10500 specifies an acceptable limit of 0.03 mg/L and a permissible limit of 0.2 mg/L in the absence of an alternative source. Correct pH control is the main way to keep residual aluminium low.
Specifying and buying coagulants
- Specify active content, such as % Al₂O₃ or % Fe, and basicity for PAC.
- Set limits for insoluble matter and heavy metals, particularly for drinking water use.
- Confirm whether the product must comply with a national standard or a drinking water additive certification in your market.
- Consider liquid versus solid forms: liquids are easy to dose but cost more to transport. Powders are compact but need make-up equipment.
- Check storage and material compatibility, since many coagulants are corrosive.
FAQs
What is the difference between a coagulant and a flocculant?
A coagulant neutralises particle charge so particles can stick together. A flocculant, usually a polymer, bridges these small particles into larger flocs that settle faster.
Why use PAC instead of alum?
PAC is pre-hydrolysed, so it consumes less alkalinity, works over a wider pH range, performs better in cold water and often produces less sludge.
What does PAC basicity mean?
Basicity is the degree of pre-neutralisation of the aluminium chloride, expressed as a percentage. It influences performance, stability and alkalinity consumption.