Coagulants vs flocculants: the short answer
A coagulant destabilises. A flocculant assembles. Those two verbs carry the whole distinction, and most dosing trouble comes from treating them as interchangeable.
Coagulants are small, highly charged, fast-acting chemicals — metal salts or low-molecular-weight organic polymers — that collapse the electrostatic repulsion keeping colloidal particles suspended, so the particles can stick together on contact. Flocculants are very long-chain, high-molecular-weight polymers; they do little to a stable colloid, but once a coagulant has destabilised it a flocculant adsorbs onto several particles at once and bridges them into a large, dense, settleable floc. Flocculation acts on the product of coagulation and cannot substitute for it.
What is a coagulant? Charge neutralisation and destabilisation
Clay, silt, humic colour bodies, oil droplets and most bacteria carry a net negative surface charge. Counter-ions gather around each in a diffuse cloud — the electrical double layer — and repulsion between like-charged particles exceeds the attractive van der Waals force at the separations involved. That is why raw-water turbidity does not simply settle out in a holding tank.
A coagulant attacks it two ways, usually at once:
- Charge neutralisation. Hydrolysed aluminium or iron species carry high positive charge, adsorb onto the negative surfaces, compress the double layer and drop the repulsive barrier, so collisions result in attachment. It dominates at lower dose and lower pH, and is effectively the only mechanism cationic organic coagulants use.
- Sweep flocculation. At higher dose the metal salt exceeds the solubility of its hydroxide and precipitates bulk aluminium or ferric hydroxide, which enmeshes colloids and adsorbs dissolved organics.
What is a flocculant? Bridging and agglomeration
A flocculant is a water-soluble polymer of molecular weight in the millions of daltons — a chain thousands of units long that uncoils on dissolution into an extended configuration hundreds of nanometres across. Segments adsorb onto a particle surface while the rest of the chain reaches into solution and attaches to a second, third and fourth particle. The result is polymer bridging: particles tethered into one aggregate by polymer chains rather than by direct surface contact.
Because bridging needs a chain to span a gap, flocculants are acutely sensitive to overdose: saturate every surface site and none remain for the far end of a chain to land on, leaving particles individually wrapped in polymer and better dispersed than before you started. Nor is the biggest floc the target: the Irish EPA manual puts the useful range at roughly 0.1 mm to 3 mm and warns the best floc "will not necessarily be the largest, since large flocs often settle slowly and are easily broken".
Why coagulant and flocculant are dosed in sequence
They work on different timescales and need different mixing energy, so they are almost never added at the same point. Coagulant hydrolysis is essentially instantaneous; if it is not dispersed through the whole flow in that time, part of the water sees a large local overdose and part sees none. Hence the flash mix: high energy, contact of seconds only. Flocculation is the opposite problem — bridging needs collisions, but the aggregate is fragile and excess shear tears it apart faster than it forms, so mixing is gentle, prolonged and tapered.
Any pre-oxidant goes in ahead of the coagulant, so its precipitates join the solids load the coagulant must condition. See our guide to potassium permanganate uses and dosing: potassium permanganate converts soluble iron and manganese into filterable oxides that coagulation then has to carry.
Chemicals used in water treatment coagulation
Aluminium coagulants
Aluminium sulphate — alum — is the reference coagulant. The anhydrous salt is Al2(SO4)3, CAS 10043-01-3; water-treatment grades are hydrated, usually the 14-hydrate (CAS 16828-12-9). Coagulation consumes bicarbonate alkalinity:
Al2(SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2
Six moles of bicarbonate go per mole of alum, so pH falls as dose rises. The US EPA's composite correction programme handbook gives a working rule of about 0.5 mg/L of alkalinity consumed per mg/L of commercial alum, with 5 to 10 mg/L left after the reaction to hold pH. In soft water, alkalinity must be supplemented with calcium hydroxide, caustic soda or soda ash.
PAC (polyaluminium chloride), which the EPA indexes under CAS 1327-41-9, is pre-hydrolysed and partially polymerised, arriving already loaded with high-charge polymeric aluminium species. It consumes substantially less alkalinity than alum and holds up better in cold water, where alum hydrolysis slows; the same handbook puts its usable range at roughly pH 4.5 to 9.5. Grades differ in Al2O3 content and in basicity (the hydroxide-to-aluminium ratio), so specify both rather than a generic "PAC". Aluminium chlorohydrate (ACH), CAS 12042-91-0, is the high-basicity extreme: EPA cites a producer specification of 23–24% Al2O3 at 75–90% basicity. Potassium alum (CAS 7784-24-9) serves specialist duties.
Iron coagulants
Ferric chloride (FeCl3, CAS 7705-08-0; hexahydrate 10025-77-1) and ferric sulphate (Fe2(SO4)3, CAS 10028-22-5) behave analogously but precipitate ferric hydroxide:
2FeCl3 + 3Ca(HCO3)2 → 2Fe(OH)3 + 3CaCl2 + 6CO2
Iron works over a much wider pH range — the EPA handbook gives pH 4 to 12 for ferric chloride against about 5 to 8 for alum — is stronger on highly coloured, high-TOC water, and gives a denser, faster-settling floc. It consumes more alkalinity per unit mass, though — about 0.75 mg/L per mg/L of ferric chloride by the same handbook's rule. Ferric chloride solution is severely corrosive and, per the Irish EPA manual, "attacks almost all metals and concrete", so wetted materials and containment must be specified for it. Excess residual iron also stains laundry and fixtures.
Organic coagulants
Organic coagulants are cationic polymers working purely by charge neutralisation: no metal added, no alkalinity consumed, no hydroxide precipitate. PolyDADMAC — poly(diallyldimethylammonium chloride), CAS 26062-79-3 — carries a permanent, pH-independent positive charge. Polyamine coagulants are epichlorohydrin–dimethylamine polymers, higher in charge density and stronger on low-turbidity water.
They are weak on colour and dissolved organic carbon, so on coloured water they supplement rather than replace a metal salt. Monomer content is also regulated in drinking-water service: in the United States, 40 CFR 141.111 sets treatment techniques of 0.05% acrylamide dosed at 1 ppm and 0.01% epichlorohydrin dosed at 20 ppm (or equivalent). Specify potable-approved grades.
Flocculant chemistry for water treatment
The dominant flocculant family is the polyacrylamides (PAM; homopolymer CAS 9003-05-8, from acrylamide monomer CAS 79-06-1), in three ionic characters.
- Anionic PAM is the workhorse for mineral systems and for water clarified with metal coagulants: after alum or ferric dosing the floc surfaces carry net positive charge from adsorbed hydrolysis products, which an anionic chain binds readily. It also reaches the highest molecular weights — NSF 60 lists anionic grades at 4 to 30 million daltons against 4 to 20 million for cationic and non-ionic — so it gives the strongest floc.
- Cationic PAM suits organic and biological solids that remain negatively charged — sludge dewatering, food, beverage and paper effluent.
- Non-ionic PAM bridges by hydrogen bonding alone — the fallback for high-salinity water, where ionic chains collapse.
Dry powder grades need an engineered make-up system: an eductor wetting every particle individually, low make-up concentration, and an ageing period with slow stirring, because an unhydrated chain will not bridge. Where a blend fits better than a single chemistry, our polyelectrolytes and coagulants ranges cover formulated grades within the wider water treatment chemicals catalogue.
How to select a coagulant and flocculant
Raw water quality. Low-turbidity, low-colour water favours charge neutralisation with an organic coagulant or high-basicity PAC. High colour and high TOC favour an iron salt or alum in sweep mode.
pH and alkalinity. The hard constraints. The Irish EPA manual gives tighter working windows by water type: typically pH 6.5 to 7.5 for lowland surface waters, about 5.5 to 6.5 for highly coloured upland waters. Soft, low-alkalinity water pushes you towards high-basicity PAC, ACH or an organic coagulant.
Temperature. Cold water slows hydrolysis and settling; pre-hydrolysed aluminium products are the standard answer.
Sludge and dewatering. Residuals scale with metal coagulant dose, so a thickener- or press-limited plant should look at high-basicity products or a dual programme before anything else.
Jar testing: how coagulant dose is actually determined
No supplier, including us, can tell you your dose. Coagulant demand moves with turbidity, colour, temperature, alkalinity and algal activity. The jar test is how the number is determined; the Irish EPA manual calls it "probably the most important routine test carried out at a water works employing coagulation and flocculation". ASTM D2035 is the formal standard practice; the manual's Appendix C is public-domain.
Apparatus and stock solutions
A multiple stirrer unit driving paddles in four or more one-litre vessels at 20 and 200 rev/min. Glassware cleanliness is not optional: the test is a visual comparison. Make stock solutions so that 1 mL into one litre gives a convenient dose — 1 mL of a 0.1% solution is 1 mg/L.
The standard procedure
- Measure one litre of well-mixed bulk raw water into each vessel.
- Apply rapid stirring at 200 rev/min for 1 minute.
- Add the coagulant doses to all vessels simultaneously, using a dosing rack; staggered addition invalidates the comparison.
- Continue rapid stirring for 1 minute.
- Reduce to 20 rev/min for 15 minutes, recording time to first floc and final floc size.
- Allow 30 minutes of settling.
- Decant the supernatant and measure turbidity, colour and pH.
To evaluate a coagulant aid or flocculant, add it after the coagulant and its 1-minute rapid stir, give a further minute of rapid stirring, then slow-stir and settle as before. Testing a flocculant by adding it with the coagulant will mislead you.
The three-test sequence for a new or changed water
For routine control, jars bracketing the current dose suffice. For a new source, or after a flood or algal bloom, the manual sets out three tests. First, with no pH adjustment, vary dose widely and plot the coagulation curve to establish coagulant demand. Second, using that demand dose in every jar, adjust pH in roughly 0.5-unit steps across about pH 5.5 to 8.5 with pre-titrated acid or alkali, and plot colour and turbidity against pH to find the optimum coagulation pH. Third, hold pH there and vary dose again to fix the optimum dose, narrowing from 10 mg/L steps to 2 or 3 mg/L.
Reading the jars
The winner is the dose producing the most rapidly settling floc in the shortest time — not the biggest floc. The manual describes a good result as "coarse dense granular particles which, while settling down, show streaks of clear (not dull or smoky) water between individual particles." Where two combinations perform alike, take the lower polyelectrolyte dose. The manual warns its value "lies in predicting trends rather than in predicting exact coagulant concentrations", since jar mixing is horizontal while upflow tanks mix vertically.
Dosing and mixing regimes
Mixing intensity is expressed as the velocity gradient G (s−1); the product Gt characterises a flocculation stage. Virginia tabulates rapid-mix pairs from 1,000 s−1 at 20 seconds down to 600 at 60, puts flocculation Gt "ordinarily in the range of 20,000 to 200,000"; minimum flocculation detention there is 30 minutes. Utah requires rapid-mix G of at least 750 s−1 within 30 seconds, flocculators capable of 25 to 80 s−1, and permits tapered flocculation falling from 100 to 10 s−1.
| Stage | Purpose | Typical G | Typical contact time |
|---|---|---|---|
| Rapid (flash) mix | Disperse coagulant before hydrolysis completes | 750–1,000 s−1 | Seconds; under 30 s |
| Flocculation, first stage | Build microflocs into floc | Upper end, e.g. 100–50 s−1 | Part of 20–30 min total |
| Flocculation, final stage | Grow and toughen floc without breaking it | Lower end, e.g. 25–10 s−1 | Part of 20–30 min total |
Tapered energy, usually three baffled compartments with declining mixer speed, exists because formed floc is easily destroyed and broken floc overloads the filters. These are design envelopes, not doses: treat every dose figure in the literature as a starting point for a jar test, never a setpoint.
Common failure modes
Overdosing and restabilisation. Both underdosing and overdosing reduce solids removal. Past the charge-neutralisation optimum, adsorbed coagulant reverses the particle surface charge and the suspension restabilises; with a flocculant, surface saturation removes the vacant sites bridging depends on. The symptoms — poor settling, rising filtered turbidity, higher metal residual — look like underdosing, which is why adding more makes it worse. Bracket the current dose in jars before changing anything.
Pin floc. Small, discrete, slow-settling particles that persist through the flocculator into the clarifier. Usual causes: insufficient coagulant, coagulation pH outside the window, inadequate flocculation time or G, cold water, or a flocculant underdosed, under-aged or sheared. Diagnose with a pH sweep, not by dose alone — if turbidity removal worsens as you add alum and pH falls, the answer is alkalinity, not more coagulant.
Floc carryover. Visible floc leaving the clarifier, loading the filters and shortening runs. Causes include excessive shear in the final flocculation stage or the transfer to the clarifier, hydraulic overload, a sludge blanket risen too far, or an overdosed high-molecular-weight polymer giving a large, light floc.
Sludge and residuals implications
Metal coagulants add hydroxide solids on top of the raw-water solids, so residuals rise with dose and with sweep operation; gelatinous aluminium hydroxide sludge dewaters poorly, while ferric sludge is denser. Pre-hydrolysed products and dual metal/organic programmes cut residuals by delivering the same destabilisation with less precipitate. Conditioning for dewatering is a separate polymer selection, tested on the real sludge at the machine's mixing energy, since bench drainage tests under-predict the dose a belt press needs.
Handling, storage and safety
The detail is grade-specific. Always work from the supplier's current safety data sheet for the exact grade received, over any general guidance including this page.
In outline: aluminium sulphate is severely irritating to eyes, respiratory tract and gut and its solution is a medium-strong acid — avoid dust, store dry, keep away from bases and strong oxidants. Ferric chloride is corrosive, irritating and harmful to aquatic life; it decomposes on contact with water to form hydrogen chloride, decomposes above 200 °C to chlorine and hydrogen chloride, reacts violently with bases, and attacks metals, generating hydrogen. Keep it dry, closed, away from strong bases, and bunded with no drain access. Dry polyacrylamide is of low acute toxicity but becomes extraordinarily slippery when wet, making spills a fall hazard. Provide eye protection, gloves and chemical-resistant clothing for the grade, dust control, and eyewash and shower at dosing points.
Selection table: matching chemistry to application
| Application | Primary coagulant | Flocculant | Notes |
|---|---|---|---|
| Drinking water, lowland surface source | Alum or PAC | Anionic or non-ionic PAM | Target pH typically 6.5–7.5; confirm by jar test |
| Coloured, high-TOC upland water | Ferric sulphate, ferric chloride or alum | Anionic PAM | Lower pH band, around 5.5–6.5 |
| Soft, low-alkalinity water | High-basicity PAC or ACH | Anionic PAM | Minimises alkali supplementation |
| Low turbidity, residuals-limited plant | PolyDADMAC or polyamine, reduced metal dose | Anionic PAM | No hydroxide sludge from the organic fraction |
| Sludge thickening and dewatering | Often none, or ferric chloride with lime | Cationic PAM matched to the machine | Test at the machine's mixing energy |
Specifying coagulants and flocculants for supply
Enquire on the parameter set rather than the product name: Al2O3 content and basicity for aluminium coagulants; Fe content and physical form for iron coagulants; ionic character, molecular weight band and charge density for polymers; plus the duty and whether the application is potable. Contact our technical team with your raw water analysis and jar test data.
