Coagulation and Flocculation in Water Treatment: How They Work and How They Differ

Coagulation and flocculation in water treatment are two linked steps: coagulation destabilizes colloidal and suspended particles within seconds at a rapid-mix point, and flocculation grows them into floc of 0.1 to 3 mm over 15 to 45 minutes of gentle mixing so they can be settled, floated or filtered out. Both steps run in every conventional surface-water plant, so why does a plant need two steps instead of one?

The answer lies in 4 mechanisms: charge neutralization, sweep floc, electrostatic patch flocculation and polymer bridging. Metal salts such as aluminum sulfate (also spelled aluminium sulphate) and ferric chloride drive the first two, prehydrolyzed aluminum and cationic polymers the third, and high-molecular-weight polyacrylamide the fourth. Coagulants and flocculants are two of the 16 families of water treatment chemicals.

This page covers the US definitions, the 4 mechanisms, the hydrolysis reactions and their alkalinity cost (0.50 mg/L as CaCO3 per mg/L of dry alum, 0.92 mg/L per mg/L of ferric chloride), the velocity gradient G of about 1,000 s-1 at rapid mix and about 100 s-1 in the flocculator, values by system, control targets and 7 common mistakes, with 4 worked examples. Coagulation rarely runs alone: 66 % of US surface-water plants also dose a polymer, mostly cationic (US EPA Six-Year Review 4, 2024).

Every value on this page is checked against primary sources (eCFR, US EPA, NHMRC, Health Canada, Irish EPA, EUR-Lex); see our methodology and fact-checking process.

What Are Coagulation and Flocculation in Water Treatment?#

Coagulation and flocculation are not the same process: US rules define coagulation as destabilizing colloidal and suspended materials with coagulant chemicals and mixing, and flocculation as gently stirring the destabilized particles into larger, more easily settleable floc (40 CFR 141.2). Both sit inside "conventional filtration treatment", which the same section defines as coagulation, flocculation, sedimentation and filtration.

Coagulation: destabilizing particles in seconds#

Coagulation in water treatment is the rapid-mix step in which a positively charged chemical, such as alum, ferric chloride or polyDADMAC, collapses the negative charge of colloids so that they stop repelling each other and start to agglomerate into microfloc. Charge neutralization happens very rapidly, so rapid-mix design at about 1,000 s-1 is critical. Coagulation serves 3 targets: turbidity, color and natural organic matter.

The 13 coagulants in use span aluminum and iron salts, prehydrolyzed aluminum, cationic polymers and natural products. Their classes, dose ranges and selection rules are compared on coagulants for water treatment.

Flocculation: growing settleable floc by gentle mixing#

Flocculation is the slow-mixing step that turns destabilized particles into larger, more easily settleable floc by gentle hydraulic or mechanical stirring, typically for 15 to 45 minutes in 3 compartments of decreasing speed. The velocity gradient falls to about 100 s-1, because strong shear breaks growing floc. Flocculation is used in 5 places in water and wastewater treatment.

• Conventional clarifiers ahead of rapid gravity filters
• Direct filtration, with floc going straight onto the filter
• Dissolved air flotation
• Ballasted flocculation with microsand
• Sludge thickening and dewatering

The flocculant of choice is polyacrylamide in anionic, nonionic or cationic form. Polymer types, charge density and molecular weight are compared on flocculants for water treatment.

Coagulation vs flocculation: 6 differences#

Coagulation differs from flocculation in purpose, speed and mixing: coagulation neutralizes particle charge in seconds under intense mixing of about 1,000 s-1, while flocculation builds floc over minutes under gentle mixing of about 100 s-1. Table 1 compares the two steps on 6 attributes.

A coagulant is a metal salt or a low- to medium-molecular-weight cationic polymer (polyDADMAC runs 10^5 to 10^6 g/mol), while a flocculant is a polyacrylamide of 1 to 20 million g/mol (EN 1407:2023 and EN 1410:2023). NSF/ANSI/CAN 60 lists most of both under the single product function "Coagulation & Flocculation".

Which comes first, coagulation or flocculation?#

Coagulation always comes first: the coagulant is dosed at the rapid-mix point, flocculation follows in a slow-mix basin, and sedimentation or flotation and then filtration remove the floc. The 4 steps of conventional filtration treatment run in this order (40 CFR 141.2).

1. Coagulation
2. Flocculation
3. Sedimentation
4. Filtration

Direct filtration omits sedimentation and flocculates for only about 5 to 20 minutes (Irish EPA, Water Treatment Manual: Coagulation, Flocculation and Clarification). Settling basins and overflow rates are covered under sedimentation in water treatment.

Why Do Particles in Water Need Coagulation?#

Particles in water need coagulation because colloids, such as clay, silt and humic matter, carry a negative surface charge that keeps them dispersed, and only a destabilizing chemical lets them collide and stick. That charge is measured as zeta potential, the potential at the boundary between the Stern layer and the diffuse layer, which indicates the energy needed to bring particles together (Hach coagulation guide LIT2141). Zero is the theoretical ideal; most utilities run slightly negative after coagulation.

Only cationic chemicals serve as primary coagulants, because fresh-water solids are negatively charged. Cold water adds a second barrier: at 0 °C (32 °F) water is twice as viscous as at 25 °C (77 °F), so particles settle more slowly (Irish EPA). Coagulation and flocculation remove about 60 to 70 % of natural organic matter, a generic figure that varies by site. How exactly do coagulants overcome that charge?

How Do Coagulation and Flocculation Work? 4 Mechanisms#

Coagulation and flocculation work through 4 mechanisms: charge neutralization by cations, sweep floc formed by precipitating metal hydroxide, electrostatic patches laid down by polynuclear aluminum and cationic polymers, and bridging by long-chain polyacrylamide. The first three act at rapid mix; bridging acts in the flocculator.

1. Charge neutralization#

Charge neutralization is the mechanism of every coagulant: trivalent aluminum and iron cations are about 700 times more effective than sodium ions at collapsing the negative charge of particles in drinking water. The Schulze-Hardy rule predicts 1 : 100 : 1,000 for mono-, di- and trivalent ions in theory, while drinking-water practice gives Na+ : Ca2+ : Al3+ = 1 : 60 : 700 (Hach guide LIT2141).

What happens when the dose overshoots neutral charge? The particles turn positive and restabilize. A positive zeta potential signals that overfeed, and an overdose of a cationic polymer such as polyDADMAC re-disperses particles in the same way (NHMRC).

2. Sweep floc (enmeshment in metal hydroxide)#

Sweep floc forms when the metal-salt dose exceeds the solubility of its hydroxide, so aluminum or ferric hydroxide precipitates as a floc that enmeshes particles as it grows. Alum works by charge neutralization at low dose and slightly acidic pH, and by sweep at higher dose near neutral pH; the Australian Drinking Water Guidelines give pH 5.5 to 6.2 for enhanced coagulation and 6.5 to 7.5 for sweep.

Sweep flocculation depends on hydroxide solubility. Aluminum hydroxide reaches its minimum solubility at about pH 6.2 at 20 °C (68 °F) and about pH 6.7 at 5 °C (41 °F) (Health Canada, 2021). Ferric hydroxide is less soluble than the aluminum hydroxides.

3. Electrostatic patch flocculation#

Patch flocculation happens when highly charged cationic species, such as the Al13 polycation of polyaluminum chloride or a polyDADMAC chain, attach to part of a particle surface and create positive patches that bind to bare negative areas of other particles. Polyaluminum chloride delivers preformed polynuclear aluminum (the Al13 Keggin ion and larger species) that ends as aluminum hydroxide (NHMRC). Basicity measures prehydrolysis: alum 0 %, medium-basicity PAC about 50 % and ACH 83 %.

4. Polymer bridging#

Polymer bridging is the flocculation mechanism of polyacrylamide: chains of 1 to 20 million g/mol adsorb onto several destabilized particles at once and tie them into large floc. High molecular weight and a nonionic or low charge density favor bridging; cationic grades add patch flocculation. Anionic grades suit mineral particles and cationic grades organic particles, and overdosing restabilizes the suspension. Very-high-molecular-weight anionic polyacrylamide is generally the most effective coagulant aid (Veolia Water Technologies Handbook, chapter 5).

The Chemistry of Coagulation in Equations#

Metal coagulants react with the alkalinity of the water: each hydrolysis reaction forms a metal hydroxide floc and releases carbon dioxide, which lowers both alkalinity and pH. Sodium aluminate is the exception.

Hydrolysis reactions of alum, ferric chloride and sodium aluminate#

Three hydrolysis reactions explain most coagulation chemistry: alum and ferric chloride consume bicarbonate alkalinity and release CO2, while sodium aluminate releases caustic soda and adds alkalinity. The reactions are listed below in text form.

• Alum: Al2(SO4)3·14H2O + 3 Ca(HCO3)2 → 2 Al(OH)3 + 3 CaSO4 + 6 CO2 + 14 H2O
• Ferric chloride: FeCl3 + 3 HCO3- → Fe(OH)3 + 3 CO2 + 3 Cl-
• Sodium aluminate: Na2Al2O4 + 4 H2O → 2 Al(OH)3 + 2 NaOH

Dry alum doses use the Al2(SO4)3·14H2O basis of 594.4 g/mol.

Why does coagulation consume alkalinity?#

Coagulation consumes alkalinity because metal hydrolysis releases acid: each mg/L of dry alum consumes 0.50 mg/L of alkalinity as CaCO3 and each mg/L of ferric chloride consumes 0.92 mg/L. Sodium aluminate adds 0.54 mg/L as CaCO3 per mg/L of Na2Al2O4 at 88 % (NHMRC). On an aluminum basis the factor is 5.56 mg/L as CaCO3 per mg/L of Al.

Prehydrolyzed coagulants consume less. Health Canada ranks them from least to most: ACH and polyaluminum silica sulfate, PACl, polyaluminum sulfate, alum and ferric sulfate, then ferric chloride. Low raw alkalinity therefore caps metal-salt doses, a point developed under alkalinity of water.

Worked example 1: a 30 mg/L alum dose#

A 30 mg/L dose of dry alum consumes 15.0 mg/L of alkalinity as CaCO3, releases 13.2 mg/L of CO2, delivers 2.73 mg/L of aluminum and forms about 7.8 mg/L of aluminum hydroxide solids. Worked example 1 uses a plant flow of 37,854 m³/d (10 MGD), a water with 40 mg/L of alkalinity as CaCO3, and 30 mg/L (ppm) as Al2(SO4)3·14H2O at purity 1.00.

The factors come from the Minnesota Rural Water Association Water Works Manual, chapter 12; turbidity solids are excluded. Daily hydroxide solids follow as 7.8 x 37,854 / 1,000 = 295 kg/d, or 7.8 x 10 x 8.34 = 651 lb/day.

The 25 mg/L of alkalinity left covers this dose. Commercial strengths and the same dose as liquid-alum volumes are on the aluminum sulfate (alum) in water treatment page.

Worked example 2: the same dose as ferric chloride#

A 30 mg/L alum dose equals 16.5 mg/L of anhydrous ferric chloride in metal equivalents, and that ferric dose consumes about 15.2 mg/L of alkalinity as CaCO3. Worked example 2 uses the US EPA equivalence 10 mg/L alum = 5.5 mg/L FeCl3 = 9.1 mg/L FeCl3·6H2O (EPA 815-R-99-012, 1999, Table 3-1), which scales linearly.

The equivalence is chemical, not a performance promise, so a jar test confirms the dose. Ferric chloride works at pH 4 to 11.

Residual iron is judged against the non-enforceable US secondary standard of 0.3 mg/L (40 CFR 143.3). Ferric strengths, NSF limits and handling are on the ferric chloride in water treatment page.

Velocity Gradient G and Gt: How Mixing Intensity Is Specified#

Mixing intensity in coagulation and flocculation is specified as the velocity gradient G, in s-1, and as the product Gt of G and mixing time, so a rapid mixer and a flocculator are designed to different G values rather than to different stirrer speeds. Gt is dimensionless.

What is the velocity gradient G?#

The velocity gradient G is the mixing-intensity parameter of coagulation and flocculation, expressed in s-1: about 1,000 s-1 disperses a coagulant at rapid mix and about 100 s-1 grows floc in the flocculator (SNF Floerger, Coagulation-Flocculation, 2003). A high G spreads the coagulant through the flow before hydrolysis ends; a low G lets particles meet without tearing the floc apart. Bounding Gt means a lower G needs a longer time.

Typical G and time values by stage#

G falls about tenfold from rapid mix to flocculation, and flocculation time grows from minutes in high-rate systems to up to 30 minutes in conventional plants. Table 4 lists the values by stage.

Worked example 3: checking Gt for a flocculation stage#

A flocculation stage run at G 80 s-1 for 10 minutes reaches Gt 4.8 x 10^4, inside the 4 x 10^4 to 8 x 10^4 range of the DAF design guideline, while G 50 s-1 for 4 minutes reaches only 1.2 x 10^4. Worked example 3 uses the Water Corporation DS-211 guideline (G 50 to 120 s-1, 4 to 15+ minutes, Gt 4 x 10^4 to 8 x 10^4) and 2 cases.

• Case A: G 80 s-1 x 600 s = 4.8 x 10^4, inside the range
• Case B: G 50 s-1 x 240 s = 1.2 x 10^4, below the range

At G 50 s-1, case B needs 4 x 10^4 / 50 = 800 s (13.3 min). These values apply ahead of dissolved air flotation (DAF).

Typical Coagulation and Flocculation Values by System#

Coagulation and flocculation values differ by system: conventional drinking-water plants flocculate for up to 30 minutes, direct filtration for about 5 to 20 minutes, and ballasted flocculation for 3 to 5 minutes in the reactor at overflow rates of 49 to 196 m/h (20 to 80 gpm/ft²). Table 5 compares 6 systems.

Floc blanket clarifiers run at 1 to 3 m/h (0.4 to 1.2 gpm/ft²): the high end suits lowland, high-alkalinity, low-color water and the low end colored water with weak floc (Irish EPA). Dissolved air flotation saturates at 5 to 6 atm (about 73 to 88 psi). Basin types are compared under clarifiers.

What Changes Coagulation and Flocculation Performance?#

Coagulation and flocculation performance changes with 4 variables: the chemical and its dose, the coagulation pH and alkalinity, water temperature, and the turbidity and organic load of the raw water. Each variable shifts the optimum dose.

Coagulant and flocculant choice#

The chemical sets the mechanism: aluminum and iron salts work by charge neutralization and sweep floc, prehydrolyzed aluminum and cationic polymers by charge neutralization and patches, and polyacrylamide by bridging. Table 6 lists 7 chemicals with their drinking-water dose basis.

A polymer coagulant aid can cut the metal coagulant dose by 40 to 80 % (US EPA Six-Year Review 4, 2024).

Coagulation pH and alkalinity#

Coagulation pH decides between aluminum and iron: aluminum coagulants work best at pH 6.5 to 7.5 in lowland surface water, while iron coagulants handle soft colored water at pH 4.0 to 5.0 and hard water at pH 8.0 or above. Highly colored upland water shifts the aluminum optimum to pH 5.5 to 6.5 (Irish EPA Advice Note 15).

The pH is set with acid or alkali, never with extra coagulant, because overdosing to depress pH raises dissolved residual metal. Lime, caustic soda and soda ash for pH and alkalinity correction are compared on pH adjustment chemicals.

Water temperature#

Cold water slows coagulation and settling: water at 0 °C (32 °F) is twice as viscous as at 25 °C (77 °F), and the optimum pH of aluminum coagulants shifts by about 0.4 units between 2 and 20 °C (35.6 and 68 °F). The optimum falls as water warms, an effect not seen with iron, and below 5 °C (41 °F) alum is more affected than iron (Irish EPA, citing Pontius 1990). One utility doses PAC below 12 °C (54 °F) and alum above, an example of what a plant can do rather than a rule.

Raw-water turbidity and organic matter#

Raw-water turbidity and organic matter set the coagulant approach: above 60 NTU a cationic polymer alone normally clarifies, while low-turbidity water needs a metal salt plus polymer to build enough floc (Veolia Water Technologies Handbook, chapter 5). Between 10 and 60 NTU, an inorganic coagulant plus a cationic polymer is the usual pairing.

Units and limits are explained under turbidity in water. Organic matter often drives the dose more than turbidity: US rules require a Step 1 TOC removal of 15 to 50 % by source TOC and alkalinity (40 CFR 141.135).

Step 2 sets the point of diminishing return at 0.3 mg/L of TOC or less per 10 mg/L of added alum, as covered under enhanced coagulation and TOC removal.

How Are Coagulation and Flocculation Tested and Controlled?#

Coagulation and flocculation are controlled in 3 layers: jar tests set the chemical, dose and pH, online signals such as streaming current track charge, and settled and filtered turbidity prove the result.

Jar testing: 3 protocols compared#

A jar test is a bench-scale simulation of full-scale coagulation, flocculation and sedimentation, and the US EPA protocol runs 1 minute at 100 rpm, 30 minutes at 30 rpm and 60 minutes of settling. Table 7 compares 3 protocols.

The best floc is coarse, dense and granular, not simply the largest, and where results are equal the lowest polymer dose should win (Irish EPA). Stock solutions, dose steps and interpretation are covered in the jar test procedure.

Worked example 4: from jar dose to plant feed rate#

A 30 mg/L jar-test dose of dry alum at 37,854 m³/d (10 MGD) becomes 1,136 kg/d (2,502 lb/day), or about 1,760 L/d (465 gal/d) of commercial liquid alum. Worked example 4 uses these inputs.

• Jar dose: 3 mL of 20 g/L alum stock in a 2 L jar = 30 mg/L as dry alum
• Plant flow: 37,854 m³/d (10 MGD), purity 1.00 on the dry alum basis
• Liquid alum: 48.5 % dry alum equivalent, about 0.645 kg of dry alum per L (Chemtrade product profile)

The feed formulas give 30 x 37,854 / 1,000 = 1,136 kg/d and 30 x 10 x 8.34 = 2,502 lb/day; 1,136 kg/d / 0.645 kg/L = about 1,760 L/d. The chemical dosage and feed rate calculator runs the same conversion for any coagulant, flow and strength.

Online control and performance targets#

Online coagulation control tracks particle charge with a streaming-current detector, whose reading of zero marks neutralized charge, while compliance is judged on turbidity after the filters. Salinity, conductivity and pH swings strongly influence the signal, and siting the sensor is the greatest challenge (Hach guide LIT2141). Three turbidity targets judge coagulation.

1. Rule: combined filter effluent of 0.3 NTU or less in at least 95 % of monthly measurements, never above 1 NTU (40 CFR 141.173, Surface Water Treatment Rules)
2. Goal: settled water below 1 NTU 95 % of the time when raw water is 10 NTU or less, below 2 NTU when raw water is 20 NTU or less, and filtered water below 0.1 NTU (EPA 815-R-20-004, 2020, Table 6-1)
3. EU operational value: 0.3 NTU in 95 % of samples at the plant, none above 1 NTU (Directive (EU) 2020/2184, Annex II Part A point 3)

Meeting these targets also depends on filter operation, not on coagulation alone. The full chemical program of a utility is laid out under drinking water treatment chemicals.

Coagulation and Flocculation in Wastewater Treatment#

Coagulation and flocculation in wastewater treatment remove phosphorus, suspended solids and colloidal organics and condition sludge, with aluminum and iron salts precipitating phosphate and polyacrylamide flocculating the solids. Coagulation and flocculation serve 3 wastewater goals.

• Chemical phosphorus removal with aluminum or iron salts
• Primary and tertiary clarification, including ballasted flocculation at 80 to 95 % TSS removal
• Sludge thickening and dewatering, with 1 to 10 g of active polymer per kg of dry solids (2 to 20 lb/dry ton) on a belt filter press (US EPA 832-F-00-057); see sludge thickening and dewatering polymers

Phosphorus removal follows a 1 : 1 molar stoichiometry: 0.87 mg of aluminum or 1.80 mg of iron per mg of phosphorus, equal to 5.24 mg of FeCl3 per mg of P. The pH should stay above about 6.2 to protect the biology.

Ten States lists aluminum salts, iron salts or lime, with or without polymer, and recommends jar or pilot testing (Recommended Standards for Wastewater Facilities, 2014 edition). Feed points and dose ratios are worked out on chemical phosphorus removal.

7 Common Mistakes in Coagulation and Flocculation#

The 7 most frequent coagulation and flocculation mistakes are overdosing past neutral charge, lowering pH with extra coagulant, mixing up dose bases, judging floc by size, feeding polymer wrongly, carrying cationic polymer into membranes and letting coagulants meet hypochlorite. Each mistake is listed below with its fix.

1. Overdosing past neutral charge: a positive zeta potential means overfeed, and excess coagulant or polymer restabilizes the particles.
2. Lowering pH with extra coagulant: this raises dissolved residual metal; plants should correct pH with acid or alkali.
3. Mixing up dose bases: alum consumes 0.50 mg/L of alkalinity as CaCO3 per mg/L of dry alum, never 5.3, which fits an aluminum basis only.
4. Judging floc by size: dense, granular floc settles better than the largest floc.
5. Feeding polymer wrongly: dilution water should hold less than 4 mg/L of free chlorine, and neat emulsion should never be fed; see polymer make-down and activation.
6. Carrying cationic polymer into RO membranes: carryover is a fouling risk, covered under reverse osmosis and membrane chemicals.
7. Letting coagulants meet hypochlorite: the mix releases chlorine gas, and hypochlorite with ferric chloride caused 13 % of 72 analyzed unloading accidents from 1953 to 2019 (INERIS, Chemical Engineering Transactions, 2025); see water treatment chemical compatibility.

Are Coagulation and Flocculation Chemicals Safe in Drinking Water?#

Coagulation and flocculation chemicals certified to NSF/ANSI/CAN 60 are safe in drinking water at or below each product's maximum use level, with aluminum coagulants capped so that finished water stays at or below 2 mg/L of aluminum and polyacrylamide limited by a monomer treatment technique. That 2 mg/L figure is a listing condition, not an EPA MCL; the US aluminum secondary standard is 0.05 to 0.2 mg/L and non-enforceable (40 CFR 143.3).

Coagulation is one of the 4 concept groups of water treatment chemistry concepts, next to scaling, corrosion and disinfection.

Maximum use levels are product-specific and stated as product. MULs, listings and the 2 mg/L aluminum condition are explained on NSF/ANSI/CAN 60.

Is flocculant safe for drinking water?#

Polyacrylamide flocculant is safe for drinking water when it meets the US treatment technique of at most 0.05 % residual acrylamide monomer dosed at 1 ppm, certified to the State every year (40 CFR 141.111). The EU value is 0.10 µg/L of acrylamide (Directive (EU) 2020/2184, Annex I Part B). NSF/ANSI/CAN 60 maximum use levels for polyacrylamide are most often 1 mg/L, then 3 and 3.5 mg/L as product (NSF listings, 2026-10-20).

What is a natural flocculant?#

A natural flocculant is a plant- or shell-derived polymer, such as a modified tannin, chitosan or moringa seed protein, and of these only modified tannins hold NSF/ANSI/CAN 60 listings for US drinking water, at 1.2 to 1.5 mg/L as product. Moringa seed extract can remove 83.2 to 99.8 % of turbidity at 125 to 150 mg/L in a laboratory study (PubMed 19403968). Plant-derived chemicals are grouped under natural coagulants.

Do chitosan and moringa have an NSF/ANSI/CAN 60 listing?#

No: NSF/ANSI/CAN 60 has no chemical category for chitosan or moringa (NSF listings checked 2026-10-20), so neither appears among certified drinking-water treatment chemicals.

What are the two types of flocculation?#

The two types of flocculation in plant practice are hydraulic flocculation, in which baffles or a floc blanket supply the gentle mixing, and mechanical flocculation, in which paddles or turbines stir the water in compartments of decreasing speed (40 CFR 141.2). Both aim at a G of about 100 s-1.

What are the steps of the water treatment process?#

A conventional surface-water plant removes particles in 4 steps, coagulation, flocculation, sedimentation and filtration, which 40 CFR 141.2 calls conventional filtration treatment.

Every stage and its chemicals are mapped under the water treatment process overview.

Does electrocoagulation use coagulant chemicals?#

Electrocoagulation doses no coagulant chemical, and in one study on a metalworking-fluid emulsion electrocoagulation-flotation reached up to 99 % removal, against 85 % for alum plus flotation. The metal ions come from the electrodes instead of a feed tank.