Anionic or cationic polyelectrolyte: which one does your plant need?

The difference is the charge. In water, anionic polyelectrolyte is negative and cationic is positive. A polyelectrolyte (often shortened to 'poly' or 'PE') is a long-chain organic molecule with side groups that can carry a charge in water. Ireland's EPA manual lists four kinds: anionic, cationic, non-ionic and amphoteric (both charges).

Source: Environmental Protection Agency (Ireland), 2002

A grade is described by three things: molecular weight, the sign of its charge and its charge density (US EPA, 2024). Commercial polyacrylamide runs from 100,000 to more than 10 million daltons. A grade with higher molecular weight flocculates better, up to a point. Above about 18 million daltons the chains tend to entangle and catch fewer small particles (Xiong et al., 2018).

Source: U.S. Environmental Protection Agency, 2024; npj Clean Water (Nature Portfolio), 2018

CPHEEO says cationic polyelectrolyte can work as a coagulant or as a coagulant aid, and anionic mainly as a coagulant aid. A polymer added after the coagulant works mainly by bridging: one chain adsorbs onto several particles. Bridging needs a long-chain, high-molecular-weight polymer that is non-ionic or of low charge density (US EPA, 2024).

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; U.S. Environmental Protection Agency, 2024

Source: U.S. Environmental Protection Agency, 1987; SUEZ (Degrémont Water Handbook), 2026; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Environmental Protection Agency (Ireland), 2002; U.S. Environmental Protection Agency, 2020; World Health Organization, 2026; PeerJ (Hisar & Oehlmann, Goethe University Frankfurt), 23 Nov 2023

In surface-water clarification the flocculant works together with a coagulant such as ferric alum or PAC. For this job the SUEZ (Degrémont) water handbook says the most effective polymer is usually anionic, more rarely non-ionic or very slightly cationic. It says anionic also tends to be the most effective option for municipal wastewater treated with a mineral coagulant. Some industrial effluents are different. For effluent from surface treatment, steel mills or gas washing, a cationic polymer on its own at 0.5 to 5 g/m³ (mg/L) is often preferred.

Source: SUEZ (Degrémont Water Handbook), 2026

The traditional order is pH correction first, then the metal coagulant, then the poly. CPHEEO (Part B) says other orders, polymer before coagulant among them, sometimes work better. The sources do not agree on the delay after the coagulant. Ireland's EPA gives one to several minutes, so microflocs can form. The CPHEEO design manual gives about 8 to 10 seconds after it, with the poly dosed downstream of the coagulant point in a weir mixer or near the outlet of a mechanical flash mixer. US EPA puts high-molecular-weight anionic or non-ionic aids at a point of gentle mixing. Check the order on your own water with a jar test.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Environmental Protection Agency (Ireland), 2002; U.S. Environmental Protection Agency, 2020

You do not always need poly. For drinking-water treatment, Ireland's EPA cites a Water Research Centre study of five waters that found no benefit from it with dissolved air flotation. It also says to review the need above 12°C, when coagulants work better. For the coagulant itself, see the PAC vs alum guide.

Source: Environmental Protection Agency (Ireland), 2002

• Drinking water (Australian Drinking Water Guidelines): typically 0.05–0.3 mg/L as a coagulation, flocculation or clarification aid, and 0.01–0.1 mg/L as a filter aid.
• SUEZ: 0.05 to 0.5 g/m³ (mg/L) for surface-water clarification and up to 2 g/m³ in very heavily loaded water. For industrial effluent after a mineral coagulant, up to 2 g/m³ of anionic polymer.
• UK drinking-water limit for polyacrylamide (Drinking Water Inspectorate): an average of no more than 0.25 mg/L of active ingredient, and never above 0.50 mg/L. The limit is the same for anionic, non-ionic and cationic grades.

Source: National Health and Medical Research Council (Australia), 2011; SUEZ (Degrémont Water Handbook), 2026; Drinking Water Inspectorate (UK), 12 Nov 2025

None of the government sources in this guide gives a figure for an ETP or STP, and SUEZ is a vendor handbook. So you have to find your own dose. US EPA says to start very low and raise the dose slowly until the filtered water improves. When two combinations give similar results, Ireland's EPA prefers the one with the lower polyelectrolyte dose.

Source: SUEZ (Degrémont Water Handbook), 2026; U.S. Environmental Protection Agency, 2020; Environmental Protection Agency (Ireland), 2002

The US and UK limits are set on active polymer. Dry polymer can be as much as 90–95% active, while emulsions commonly run as high as 25–50% (US EPA, 1987). Compare products on the same basis.

Source: U.S. Environmental Protection Agency, 2024; Drinking Water Inspectorate (UK), 12 Nov 2025; U.S. Environmental Protection Agency, 1987

For sewage and other organic sludge, the answer is usually cationic. US EPA calls cationic polyacrylamides the most widely used sludge-conditioning polymers, because most sludge solids carry a negative charge. Finer or more hydrated sludge needs more charge. SUEZ says mineral sludge needs anionic, and US EPA (1987) adds three cases for anionic:

Source: U.S. Environmental Protection Agency, 1987; SUEZ (Degrémont Water Handbook), 2026

• Sludge conditioned with lime at high pH. Cationic suits a pH a little above or below neutral, though in some cases it has worked up to pH 12.
• Sludge with large amounts of iron or aluminium from coagulant dosed upstream. It may need two polymers, or at least an anionic one.
• Belt presses. A cationic polymer dosed after an anionic or non-ionic one sometimes improves cake release from the upper belt.

Source: U.S. Environmental Protection Agency, 1987

Dewatering doses are given in kg of polymer per tonne of dry solids (kg/t DS). None of these ranges names the polymer type:

Source: SUEZ (Degrémont Water Handbook), 2026; National Health and Medical Research Council (Australia), 2011; U.S. Environmental Protection Agency, 2000

• General: 0.5 to 10 kg/t DS (SUEZ).
• Drinking-water plant sludge: 0.5–2 kg/t to thicken, 1–4 kg/t to dewater (Australian guidelines).
• Belt filter press, g/kg = kg/t (US EPA, 2000): raw primary 1 to 5, raw waste activated sludge (WAS) 1 to 10, anaerobically or aerobically digested primary + WAS 2 to 8.
• Centrifuge, lb per dry ton (× 0.5 gives kg/t): US EPA's 2000 fact sheet gives undigested primary 5–30 (2.5–15 kg/t), undigested WAS 15–30 (7.5–15 kg/t) and undigested primary + WAS 5–16 (2.5–8 kg/t). The 1987 US EPA manual gives solid-bowl doses of raw primary 2–7 (1–3.5 kg/t) and raw primary + WAS 4–15 (2–7.5 kg/t).
• CPHEEO's 20 MLD water treatment plant design: 1 kg/t DS for the centrifuge.

Source: SUEZ (Degrémont Water Handbook), 2026; National Health and Medical Research Council (Australia), 2011; U.S. Environmental Protection Agency, 2000; U.S. Environmental Protection Agency, 1987; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

For screw presses, no source in this guide gives a typical range. US EPA's 1987 manual reports only results from individual plants and pilots. CPHEEO's sewerage manual says a polyelectrolyte is hard to specify by formula and advises laboratory tests before buying. It also says 0.1% solution is mostly injected into the feed line ahead of the machine. Mix sludge and polymer gently: US EPA says turbulence shears floc.

Source: CPHEEO, Ministry of Urban Development, Government of India, 2012; U.S. Environmental Protection Agency, 2000; U.S. Environmental Protection Agency, 1987

1. Use clean make-up water. Poly works less well in hard water (Ireland EPA), and making it up with chlorinated water can reduce its effectiveness (Australian guidelines).
2. Work out the strength on the product as supplied, not on its active content: 1 kg of powder in 99 litres of water makes 1% (Ireland EPA), so 1 kg in 999 litres makes 0.1%.
3. Start the stirrer first. Then sprinkle the powder in slowly, or feed it through an eductor, so each grain wets separately. If the powder clumps, a jelly skin forms around the dry powder inside ('fish eyes'). These can last hours and foul dosing-pump valves.
4. Mix slowly until it dissolves, then keep mixing gently to age the solution. US EPA (1987) says at least 60 more minutes and the Australian guidelines say 1–4 hours; SUEZ gives 30 to 60 minutes in all for the polymer to mature. Follow the data sheet. Avoid brisk stirring, which degrades the polymer (SUEZ).
5. Dilute before dosing. Ireland's EPA dilutes a 0.5% stock to 0.05–0.1% working solution. SUEZ makes powder up at 2 to 5 g/L (0.2–0.5%) and dilutes it in line to 0.02–0.1 g/L (0.002–0.01%) for clarification, or 0.5–1 g/L (0.05–0.1%) for sludge. A dilute solution disperses more easily, but it should not add a significant volume to the flow.

Source: Environmental Protection Agency (Ireland), 2002; National Health and Medical Research Council (Australia), 2011; U.S. Environmental Protection Agency, 1987; SUEZ (Degrémont Water Handbook), 2026

Made-up solution does not keep. A 0.5% stock usually lasts two to three days, and working solution should be made daily. At first the solution improves as the chains uncurl. Then it deteriorates, fastest in hard water and with high-molecular-weight, low-charge grades (Ireland EPA).

Source: Environmental Protection Agency (Ireland), 2002

• Clarifier: if a lot of floc goes over the launder weirs, the floc is too light for the detention time. A higher coagulant dose or a polymer aid may make it heavier. But fine floc washing over the weir can instead mean too much alum. Change one thing at a time (CPHEEO).
• Filters: excess polymer causes short filter runs and mudballs (US EPA) and can clog the beds. Inspect the beds regularly and track headloss (Australian guidelines).
• Belt press, too little: free water drains poorly in the gravity zone. Sludge overflows there or squeezes out in the pressure section.
• Belt press, too much: sticky sludge can blind the belt and cause cake-release problems. Over-flocculated sludge may drain so fast that the solids spread unevenly. US EPA says over-conditioning is as bad as under-conditioning.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; U.S. Environmental Protection Agency, 2020; National Health and Medical Research Council (Australia), 2011; U.S. Environmental Protection Agency, 1987

The concern is residual acrylamide monomer. It dissolves in water and is not adsorbed by solids, so it stays in the treated water (Ireland EPA). Conventional treatment does not remove it (WHO). Limits in drinking water:

Source: Environmental Protection Agency (Ireland), 2002; World Health Organization, 2026

• WHO guideline value: 0.0005 mg/L (0.5 µg/L). Australia: 0.0002 mg/L.
• European Union: 0.10 µg/L (Drinking Water Directive (EU) 2020/2184). This value is calculated from the polymer's maximum-release specification, not measured in the water.
• India: the 2012 text of IS 10500 sets no acrylamide limit. CPHEEO sets no numeric dose limit, but it says the toxicity of any polyelectrolyte must be checked before use.

Source: World Health Organization, 2026; National Health and Medical Research Council (Australia), 2011; Official Journal of the European Union (EUR-Lex), 16 Dec 2020; Bureau of Indian Standards (copy hosted by CPCB), 2012; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Acrylamide is controlled through the product's monomer content, the dose, or both (WHO). WHO notes that the maximum authorised polymer dose is generally 1 mg/L. At 0.05% monomer, that gives at most 0.5 µg/L in theory. Practical levels may be 2–3 times lower for anionic and non-ionic grades, and may be higher for cationic. The US caps dose and monomer together: 0.05% monomer dosed at 1 ppm (1 mg/L), or equivalent. The UK allows no batch of anionic, non-ionic or cationic polyacrylamide above 0.020% free monomer on active ingredient, and the supplier must state a monomer limit for each batch.

Source: World Health Organization, 2026; Legal Information Institute, Cornell Law School (text of US regulation), 1991; Drinking Water Inspectorate (UK), 12 Nov 2025

Ask the supplier for each batch's monomer figure. Check your own country's rules, and use the lowest dose that works. Where the dose is the control, WHO says overdosing should always be avoided.

Source: World Health Organization, 2026

• Store powder dry. It normally keeps 12 months or longer (Ireland EPA), but moisture cakes it and makes it unusable (US EPA). Liquid and emulsion grades usually keep 4–12 months if protected from frost.
• Handling: avoid swallowing polyelectrolyte, getting it in your eyes and prolonged skin contact. Follow the safety data sheet for the product (Ireland EPA). Collect a spill dry first, then wash the area with plenty of water, warm if possible.
• Materials: fibreglass-reinforced plastic, polyethylene, polypropylene, PVC, stainless steel and coated steel suit polyacrylamide (Australian guidelines). Galvanised steel, natural rubber and copper alloys are the most likely to be affected, and some polyelectrolytes also attack mild and carbon steel and aluminium (Ireland EPA).

Source: Environmental Protection Agency (Ireland), 2002; U.S. Environmental Protection Agency, 1987; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; National Health and Medical Research Council (Australia), 2011

Rise Chemical India makes both types at its own factory in Bharatpur, Rajasthan. Anionic polyelectrolyte (anionic polyacrylamide) comes as powder or granules in 25 kg HDPE bags, the minimum order in India is 250 kg, and sealed bags keep up to 24 months stored dry. For cationic polyelectrolyte, tell the office what sludge you are dewatering and ask for the grade, packing and minimum order. The same factory makes ferric alum, a coagulant the anionic grade can follow in a clarifier, with a 5 tonne minimum order. All three are sold in India and offered for export, priced per order. The company is ISO 9001:2015 certified. Its office at Plot No. 1/6, M-11 Kirti Nagar Industrial Area, Delhi 110015 handles every enquiry and order. Send an enquiry or call or WhatsApp +91 95829 99838.