Flocculants for Water Treatment: Polymer Types, Charge, Molecular Weight and Selection

Flocculants for water treatment are high-molecular-weight, water-soluble polymers, in practice polyacrylamide in anionic, nonionic and cationic grades, that bridge destabilized particles into large floc after coagulation so the floc settles, floats or filters out. Their doses run from 0.01 mg/L (ppm) as a filter aid in drinking water to 20 kg per tonne of dry sludge solids in a centrifuge, so which charge, molecular weight and form fits which water?

Polyacrylamide flocculants come in 3 charge types (anionic, nonionic and cationic) and 4 commercial forms (dry powder, emulsion, dewatered emulsion and Mannich solution). They are also called polymer flocculants or polyelectrolytes, and "poly" or "floc" among operators. Flocculants form the second of the 16 families of water treatment chemicals.

Flocculants work by polymer bridging, helped by chains of 1 to 20 million g/mol, and doses carry two bases that never mix: mg/L in drinking water and kg per tonne of dry solids in sludge. Two thirds of US surface-water plants (66 %) dose a polymer (AWWA survey cited by EPA, 2024). NSF/ANSI/CAN 60 caps each certified product at its own maximum use level, most often 1 mg/L as product, and the acrylamide treatment technique caps residual monomer at 0.05 % dosed at 1 ppm (40 CFR 141.111).

Every value on this page is checked against primary sources (NSF listings, eCFR, EPA, AWWA, CEN, NHMRC, WHO); see our methodology and fact-checking process.

What Is a Flocculant in Water Treatment?#

A flocculant in water treatment is a long-chain polymer, usually polyacrylamide, that collects small destabilized particles into large, settleable floc during the gentle-mixing stage that follows coagulation.

US federal rules define flocculation as "a process to enhance agglomeration or collection of smaller floc particles into larger, more easily settleable particles through gentle stirring by hydraulic or mechanical means" (40 CFR 141.2).

A coagulant neutralizes particle charge in seconds at rapid mix; a flocculant then builds the floc that the clarifier or filter removes. The sequence of the two steps and the differences between them are explained on coagulation and flocculation in water treatment.

Where is a flocculant dosed in a treatment plant?#

A flocculant is dosed after the coagulant and any pH adjustment, into water mixed at moderate shear, because high shear breaks the polymer bridges that hold the floc together. A filter aid goes in just upstream of the filters (NHMRC Australian Drinking Water Guidelines). Velocity gradients show the difference in energy: about 1,000 s-1 at coagulation and about 100 s-1 in the flocculation tank (SNF Floerger, 2003). Flocculation detention runs about 5 to 20 minutes ahead of direct filtration and up to 30 minutes in conventional plants (Irish EPA).

What does a flocculant do in water and sludge treatment?#

A flocculant does 8 jobs in water and sludge treatment, listed below.

• Clarification aid in settling basins
• Settling aid in lime softening
• Floc-to-bubble binder in dissolved air flotation
• Filter aid
• Backwash water recovery
• Sludge thickening
• Sludge dewatering conditioner
• Industrial wastewater clarification

Each use sets the charge and form, compared below.

How Do Flocculants Work?#

Flocculants work in 2 ways: long polymer chains adsorb on several particles at once and bridge them, and cationic grades also neutralize negative charge in patches, while too much polymer reverses the effect and restabilizes the particles.

Polymer bridging#

Polymer bridging is the main mechanism of every polyacrylamide flocculant: a chain of 1 to 20 million g/mol adsorbs on several destabilized particles and ties them into floc far larger than charge neutralization alone produces. The bridge survives only at low shear, which is why flocculation runs at about one tenth of the velocity gradient of rapid mix.

Why does a flocculant need a coagulant first? The coagulant collapses the repulsion between particles, and the polymer then bridges what the coagulant has destabilized. Operator training texts give finished floc sizes of 0.1 to 3 mm after 15 to 45 minutes of flocculation in 3 compartments of decreasing speed.

Charge neutralization and patch flocculation#

Cationic polyacrylamide flocculants add charge neutralization to bridging: their positive groups attach to negatively charged organic particles and biosolids in patches, which is why cationic grades dominate sludge conditioning.

Patch flocculation differs from coagulation by chain length. Low-molecular-weight, high-charge cationic polymers, such as polyDADMAC and polyamine, neutralize charge without bridging far, so they count as coagulants.

What happens when a flocculant is overdosed?#

An overdosed flocculant restabilizes particles by charge repulsion and, in drinking water, blinds the filters, so the dose is set by jar test rather than raised when settled water turns cloudy.

Flocculant Polymer Types by Charge#

Flocculants used in water treatment come in 3 charge types of polyacrylamide, anionic, nonionic and cationic, each with its own copolymer chemistry and CAS number, plus a few natural and mineral aids. EN 1407:2023 covers the anionic and nonionic grades and EN 1410:2023 the cationic grades.

Table 1 compares the 3 charge types and the natural and mineral aids by chemistry, CAS number, EN standard, mechanism and role.

Polymers have no EC number (EN 1407:2023, 4.1.7).

1. Anionic polyacrylamide flocculants#

Anionic polyacrylamide flocculants are copolymers of acrylamide and sodium acrylate (CAS 25987-30-8) whose negative chains bridge flocs that a metal coagulant has already neutralized. Very-high-molecular-weight anionic grades are generally the most effective coagulant aids, including in precipitation softening (Veolia Water Technologies Handbook, chapter 5). Anionic polymers are also the most used polymer type in US groundwater plants (AWWA WATER:STATS, cited in EPA's fourth Six-Year Review, 2024).

2. Nonionic polyacrylamide flocculants#

Nonionic polyacrylamide is the acrylamide homopolymer (CAS 9003-05-8), a near-neutral chain that flocculates by bridging alone and is standardized together with anionic grades in EN 1407:2023. No separate EN standard exists for nonionic polyacrylamide, and the anionic and nonionic grades share one class page on this site.

3. Cationic polyacrylamide flocculants#

Cationic polyacrylamide flocculants are copolymers of acrylamide with a quaternized aminoester, such as ADAM-MeCl (CAS 69418-26-4) or MADAM-MeCl (CAS 35429-19-7), that bridge and neutralize negatively charged organic solids. EN 1410:2023 names 13 exemplary CAS numbers for the class. Their ester groups hydrolyze above pH 6, so cationic solutions stay most stable below pH 6, preferably at pH 5.5 (SNF emulsion handling guide, 2013). In drinking water, cationic grades are less commonly used than anionic or nonionic grades (EPA, 2024).

Coagulant or flocculant? Where alum and coagulant polymers fit#

Alum is a coagulant, not a flocculant: aluminum sulfate destabilizes particles at rapid mix, and a polyacrylamide flocculant then grows the floc, so searches for "alum flocculant" describe a coagulant and a flocculant used in sequence.

Is a polymer a coagulant or a flocculant? Molecular weight and charge density decide. Low-molecular-weight, high-charge cationic polymers, such as polyDADMAC (10^5 to 10^6 g/mol) and polyamine, are coagulants; high-molecular-weight polyacrylamides are flocculants. Natural and mineral aids, such as chitosan, bentonite and activated silica, also sit with the coagulants. Metal salts, coagulant polymers and coagulant aids are compared on coagulants for water treatment.

Molecular Weight and Charge Density of Flocculants#

Flocculant molecular weight is typically 1 to 20 million g/mol under EN 1407:2023 and EN 1410:2023, 10 times or more that of a coagulant polymer such as polyDADMAC (10^5 to 10^6 g/mol), and that chain length is what lets one molecule bridge many particles. The Australian guidelines (NHMRC) give a wider span of 1 to 30 million g/mol for polyacrylamide. Polyacrylamide forms a gel at about 20 g/L (2 %) and above (EN 1407:2023), so every flocculant is diluted far below that strength.

Charge type sets what the polymer attaches to: anionic grades for mineral particles, cationic grades for organic particles and biosolids, and nonionic grades where bridging alone is enough (SNF Floerger, 2003). Charge density, the share of charged monomer in the chain, tunes the balance between bridging and patch neutralization within each type.

Flocculant Forms and Polymer Make-Down#

Flocculants are sold in 4 forms, dry powder, emulsion, dewatered emulsion and Mannich solution, and every form is diluted and activated on site before it is dosed.

Dry powder, emulsion, dewatered emulsion and Mannich solution#

Dry polyacrylamide carries up to 90 % active polymer and keeps for over 3 years unopened, while emulsions carry 20 to 55 % active polymer in about 30 % hydrocarbon oil and keep for 6 months. Dewatered emulsions hold less than 6 % water, and Mannich polymer solutions arrive by tanker at 4 to 6 % active and must be used within 4 weeks.

Table 2 compares the 4 forms by active polymer, shelf life, storage and make-down.

Every form is injected at 0.1 to 0.25 % active, in dilution water with free chlorine below 4 mg/L.

Polymer make-down and activation#

Polymer make-down activates dry polyacrylamide at 0.2 to 1 % active and emulsion at 0.5 % active, ages the solution, then dilutes it to 0.1 to 0.25 % active at the injection point. A neat emulsion is never fed. The 3 make-down variables carry these numbers.

• Concentration: 0.2 to 1 % active for dry polymer, 0.5 % active for emulsion, 0.1 to 0.25 % active at injection.
• Aging: about 30 minutes for dry polymer in wastewater service, 1 to 4 hours of gentle mixing in drinking-water service (NHMRC).
• Dilution water: free chlorine below 4 mg/L, hardness up to about 400 mg/L, temperature above 4 °C (39 °F) and below 38 °C (100 °F); hydraulic emulsion units need at least 414 kPa (60 psi).

Mixing energy, aging tanks and inversion of emulsion polymers are covered under polymer make-down and activation.

Which Flocculant for Which Application? Selection by Particle Type and System#

Flocculant selection depends on 3 factors, the charge of the solids, the system and the form the plant can make down, and a jar test on the actual water or sludge confirms the grade and the dose.

By particle type: mineral solids versus organic solids#

Mineral solids such as clay, silt and metal hydroxide floc usually respond to anionic or nonionic polyacrylamide, while organic solids and biosolids usually need a cationic grade, a rule of thumb that a jar test must confirm.

Drinking water clarification, softening and filter aid#

Drinking-water plants dose 0.05 to 0.3 mg/L (ppm) of polyacrylamide as a flocculation aid and 0.01 to 0.1 mg/L as a filter aid (NHMRC), usually an anionic or nonionic grade, and a polymer aid can cut the metal coagulant dose by 40 to 80 % (EPA, 2024). Lime softening clarifiers flocculate for 30 minutes or more, and very-high-molecular-weight anionic grades improve their settling rate.

Sludge thickening and dewatering#

Sludge thickening and dewatering use high-molecular-weight cationic polyacrylamide dosed per dry solids: 1 to 10 g/kg (2 to 20 lb/dry ton) of active polymer on a belt filter press and 2.5 to 20 kg/t (5 to 40 lb/dry ton) in a centrifuge (EPA 832-F-00-057 and 832-F-00-053). Centrifuges target 95 % solids capture. Water-works sludge needs less: 0.5 to 2 kg/t of dry solids for thickening and 1 to 4 kg/t for dewatering (NHMRC).

Industrial wastewater, flotation and oily water#

Industrial wastewater has no generic flocculant dose: the grade and dose come from a jar test, and the polymer is fed in a flocculation tank after the coagulation tank at a velocity gradient of about 100 s-1. In flotation of oily water, one study found that 2.5 mg/L of a cationic polyelectrolyte treated 70 to 280 mg/L of oil, where alum needed 28 to 50 mg/L, and that 24 mg/L of PAC plus 1.5 mg/L of an anionic polymer removed 99 % of oil and grease.

Membrane pretreatment#

Ahead of reverse osmosis, anionic or nonionic flocculants are preferred over cationic grades, because cationic polymer that carries over reacts with anionic antiscalants into a gum-like gel that fouls the membranes. The flocculant belongs in the clarifier and filters, ahead of dechlorination, acid, antiscalant and the 5 µm cartridge filter (DuPont FilmTec Reverse Osmosis Technical Manual).

Table 3 lists the usual charge, the reason and the key number for each application.

Cationic grades dominate solids handling and anionic or nonionic grades dominate potable and membrane duty; all wastewater programs are compared on wastewater treatment chemicals.

Flocculant Dosage Ranges by Application#

Flocculant doses span two bases: drinking water runs at 0.01 to 0.3 mg/L (ppm) of polyacrylamide, while sludge conditioning runs at 0.5 to 20 kg of polymer per tonne of dry solids (1 to 40 lb/dry ton), and the two bases never mix.

Table 4 gives the typical dose, basis and source for each system.

A dewatering dose in kg/t of dry solids is never quoted for potable clarification. In drinking water, the product dose is also capped by the product's NSF/ANSI/CAN 60 maximum use level and by the monomer limit of 40 CFR 141.111.

How is a flocculant dose converted between active polymer, product and feed rate?#

A sludge flocculant dose in lb/dry ton equals 2,000 times the polymer flow times its active fraction, divided by the sludge flow times its solids fraction. A drinking-water feed rate follows lb/day = mg/L x MGD x 8.34 / purity (Florida DEP, 2014). The 2 worked examples below state every input.

1. Worked example 1, sludge dose: sludge at 100 gpm (378.5 L/min) and 2.0 % solids, polymer solution at 2.0 gpm (7.57 L/min) and 0.25 % active. The dose is 2,000 x 2.0 x 0.0025 / (100 x 0.020) = 5 lb/dry ton of active polymer, or 2.5 kg/t.
2. Worked example 2, potable feed: 0.2 mg/L of active polymer at 10 MGD (37,854 m³/d). The feed is 0.2 x 10 x 8.34 = 16.7 lb/day (7.6 kg/d) of active polymer; dividing by the active fraction of the product gives the product mass.

The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.

How Is Flocculant Dose Controlled and Tested?#

Flocculant dose is controlled in 3 layers: a jar test picks the grade and dose, plant signals such as settled turbidity, filter headloss and cake solids track it, and solution quality keeps the polymer working.

Jar testing for flocculant selection and dose#

A flocculant jar test adds the polymer after the coagulant, mixes at 250 rpm for 10 seconds and 40 rpm for 5 minutes, and judges floc size and settled clarity after about 10 minutes of settling (SNF Floerger, 2003). A 0.5 % polyacrylamide stock keeps for 1 week (Irish EPA), and a 1 % stock equals 10,000 mg/L. The jar test runs in 4 steps.

1. Make a fresh 0.25 % or 0.5 % polymer stock and use it within a week.
2. Dose the coagulant first, then the polymer in steps (1 mL of 0.25 % stock in a 500 mL jar gives 5 ppm, a supplier jar-test figure).
3. Mix and settle at the set speeds and times.
4. Compare floc size, settling speed and settled turbidity.

Interpretation and dose calculation are covered in the jar test procedure.

Plant control: turbidity, headloss, cake solids and capture#

Plants control flocculant dose by its result: settled and filtered turbidity and filter headloss in drinking water, and cake solids, filtrate or centrate clarity and solids capture of 95 % on a centrifuge in sludge dewatering. Belt press operators check feed, cake and filtrate total solids at least once per shift, and centrifuge operators check centrate and cake solids each shift (EPA fact sheets).

In drinking water, the result has a federal benchmark: combined filter effluent turbidity at or below 0.3 NTU in at least 95 % of monthly measurements and never above 1 NTU (40 CFR 141.173). Turbidity in water is measured in NTU. Overdosed polymer clogs filters, so headloss is read together with turbidity (NHMRC).

What degrades a flocculant solution?#

Oxidants, ferrous iron and alkaline dilution water degrade flocculant solutions: free chlorine must stay below 4 mg/L in make-down water, and 0.5 ppm of ferrous iron degrades a polyacrylamide solution in about 2 hours. Cationic solutions at 1 g/L last about 1 hour in low-alkalinity water and 10 minutes in high-alkalinity water, while anionic solutions keep 8 hours at 1 g/L and 8 days at 5 g/L (SNF emulsion handling guide, 2013).

Which Standards and Regulations Apply to Flocculants?#

Flocculants for drinking water are governed in 3 layers: NSF/ANSI/CAN 60 product certification, the acrylamide treatment technique and its foreign counterparts, and the ANSI/AWWA B453-24, EN 1407:2023 and EN 1410:2023 product standards.

NSF/ANSI/CAN 60 maximum use levels for polyacrylamide#

NSF/ANSI/CAN 60 maximum use levels for polyacrylamide are product-specific and stated as product, most often 1 mg/L (2,599 product-function rows), then 3 mg/L (946 rows) and 3.5 mg/L (229 rows) (NSF listings, 2026-10-21). The Polyacrylamide category lists 177 companies, 3,544 products and 4,114 product-function rows (NSF listings, 2026-10-21), and individual product MULs run from about 0.2 to 160 mg/L (NSF listings, 2026-10-20). The 2020 text of the standard gives a typical use level of 1.0 mg/L for dry and 4.0 mg/L for emulsion polyacrylamide; a typical use level is a reference value, never a product MUL.

Every listing carries the footnote that certified polyacrylamide products comply with 40 CFR 141.111 for percent monomer and dose. EPA reports a lot with 760 mg/kg residual acrylamide certified at a MUL of 0.6 mg/L (Six-Year Review 4, 2024). How MULs are set and checked is explained on NSF/ANSI/CAN 60.

Acrylamide monomer limits by jurisdiction#

Acrylamide from polyacrylamide flocculants is controlled in the United States by a treatment technique, not an MCL: the polymer may contain at most 0.05 % residual monomer when dosed at 1 ppm, certified to the State each year (40 CFR 141.111), and the MCLG is zero.

Table 5 compares the acrylamide controls of 6 jurisdictions by instrument and control.

NSF test data from 2019 to 2021 show 307 analyses with 216 detections, a median of 49 mg/kg and a maximum of 760 mg/kg of acrylamide in polymer, against a treatment-technique equivalent of 500 mg/kg. EPA's fourth Six-Year Review (EPA-815-R-24-023, February 2024) found that a revision of the acrylamide treatment technique was not warranted. Conventional treatment does not remove acrylamide (WHO), so control sits with the product and the dose.

AWWA and EN product standards for polyacrylamide#

Polyacrylamide for drinking water is specified by ANSI/AWWA B453-24 in North America and by EN 1407:2023 for anionic and non-ionic grades and EN 1410:2023 for cationic grades in Europe. Both EN standards cover the solid commercial form.

Boiler steam that contacts food falls under a separate rule: 21 CFR 173.310 lists only the acrylamide-sodium acrylate resin, with at most 0.05 % acrylamide monomer, while cationic and nonionic polyacrylamide are not listed in 21 CFR 173.310. Every B-standard edition is listed under AWWA B-series standards.

Complete List of Flocculants: Polyacrylamide and Its Grades#

The complete list of flocculants on this site is one entity, polyacrylamide, in 3 charge grades, each below with its CAS number, EN standard, function, systems and 21 CFR 173.310 status.

Table 6 lists the entity and its 3 grades in charge order.

Grades 1a to 1c are covered on the class pages. Every other chemical is filterable by CAS, function and NSF status in the water treatment chemical database.

Who Supplies Flocculants and Polymer Programs?#

Coagulants and flocculants together form the largest product segment of water treatment chemicals, 38.2 % of a USD 35.8 billion market in Grand View Research's 2025 estimate, and flocculants are bought by active polymer content, form and certification rather than by volume.

A quote for polyacrylamide depends on charge and grade, active content (dry up to 90 %, emulsion 20 to 55 %), packaging from 23 kg (50 lb) bags to tankers, freight distance, contract term, and the specification: an NSF/ANSI/CAN 60 listing and ANSI/AWWA B453-24 or EN 1407:2023 and EN 1410:2023. Distributors and manufacturers by region are listed under water treatment chemical suppliers.

Public procurement records show what buyers paid at a given date and place; they include delivery and local terms and are not a current price.

Plants buy polymer in 2 ways. A polymer supplier delivers a grade to specification, while a program provider runs jar tests, sets up make-down and runs dewatering trials before a grade is fixed.

How to compare program providers is covered under water treatment companies.

Are Flocculants Safe in Drinking Water and at the Plant?#

Certified flocculants are safe in drinking water at or below each product's NSF/ANSI/CAN 60 maximum use level, because the hazard lies in residual acrylamide monomer, which the treatment technique caps, while at the plant the main risks are the monomer in the product and slippery spills. The polyacrylamide polymer has no harmonised entry in CLP Annex VI, and its classification follows the supplier SDS. The residual monomer acrylamide (CAS 79-06-1) is harmonised as Carc. 1B (H350), Muta. 1B (H340), Repr. 2 (H361f) and STOT RE 1 (H372). Polyacrylamide is not listed under EPA RMP (40 CFR 68.130) or OSHA PSM (29 CFR 1910.119).

What is the most commonly used flocculant?#

Polyacrylamide is the flocculant of water treatment, sold in anionic, nonionic and cationic grades, and two thirds of US surface-water plants (66 %) used a polymer in the AWWA survey cited by EPA in 2024. Those polymers, coagulant polymers included, were predominantly cationic, and 13 % of groundwater plants used one.

What is a polyelectrolyte?#

A polyelectrolyte is a water-soluble polymer that carries ionizable groups along its chain, such as anionic and cationic polyacrylamide flocculants and the cationic coagulants polyDADMAC and polyamine.

Which chemicals must never be mixed with flocculants?#

Flocculants must never meet oxidants in concentrated form or anionic antiscalants in the same line, because chlorine breaks the polymer chains and cationic polymer reacts with antiscalant into a membrane-fouling gel. The 4 incompatibilities are listed below.

• Oxidants: free chlorine above 4 mg/L in dilution water degrades the chains.
• Ferrous iron: 0.5 ppm degrades a solution within about 2 hours.
• Anionic antiscalants: they form a gel foulant with cationic polymer ahead of RO.
• Other emulsions: emulsions from different suppliers are not mixed.