Ferric alum vs PAC vs alum: which coagulant should your plant use?

No. All three are aluminium coagulants, but ferric alum and alum are both aluminium sulphate and differ mainly in iron content and grade, while PAC is a different chemical.

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2001; Bureau of Indian Standards, 2018

Ferric alum is the Indian trade name for IS 299:2012 alumino-ferric: essentially Al2(SO4)3·18H2O, sold solid or liquid, and not ferric chloride. Grades 4 (solid) and 5 (liquid) are for drinking water, and a public-sector tender asks for "Ferric Alum per IS 299:2012 (Grade 4)" (Northern Coalfields, 2016). Check whether a certificate cites the 2012 standard alone or with its 2015 amendment: the 2012 standard capped iron at 0.04% for Grade 4 and 0.02% for Grade 5, and Amendment No. 1 (2015) raised these limits to 0.7% and 0.35%.

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2015; Northern Coalfields Limited (Coal India subsidiary), Khadia Project, 18 Jul 2016

Non-ferric alum is IS 260:2001, which calls it a purer aluminium sulphate than alumino-ferric; its technical grade allows at most 0.01% iron. Its grades are technical (textile and paper), pure (pharmaceutical) and analytical reagent, and none is for drinking water.

Source: Bureau of Indian Standards, 2001

PAC (polyaluminium chloride, written PACl in some of the sources below) is pre-hydrolysed, so it arrives partly neutralised and less acidic than alum. IS 15573:2018 specifies it as liquid or powder for drinking-water treatment.

Source: Bureau of Indian Standards, 2018; Health Canada, 2021

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2015; Bureau of Indian Standards, 2001; Bureau of Indian Standards, 2018; Health Canada, 2021; Stantec Consulting Services Inc. (presentation hosted by PNWS-AWWA), 2023; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; US Environmental Protection Agency, 1999

On Al2O3, the active content, because a lower dose of a stronger product is not by itself a saving. At the IS minimums, 17.5 mg/L of 16% Grade 4 ferric alum adds 2.8 mg/L of Al2O3 (17.5 × 0.16). So does 10 mg/L of 28% powder PAC, or about 29.5 mg/L of 9.5% medium-basicity liquid PAC (2.8 ÷ 0.095). These are minimums, so take the Al2O3 from each delivery's chemical analysis, which CPHEEO asks plants to check.

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2018; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

The other figure normally given for PAC is basicity, how far the aluminium is pre-neutralised: 50% basicity means 50% pre-neutralised relative to alum (Health Canada). IS 15573 sets minimums of 35% for medium and 64% for high, but other schemes differ: a Stantec presentation by David Pernitsky calls 50% medium, 75% high and 85% aluminium chlorohydrate (ACH). Ask a supplier for the number, not the label.

Source: Health Canada, 2021; Bureau of Indian Standards, 2018; Stantec Consulting Services Inc. (presentation hosted by PNWS-AWWA), 2023

Ferric alum is essentially the same aluminium sulphate as filter alum, so start from the alum figures for both. By CPHEEO's arithmetic, 666 parts of filter alum, Al2(SO4)3·18H2O, use 300 parts of alkalinity as CaCO3: about 2:1, or 0.45 mg/L per mg/L (300 ÷ 666). A Tennessee operator course (TDEC) gives 0.5 for alum written as the 14-hydrate, whose lower molecular weight (about 594) accounts for most of the difference. The Texas regulator TCEQ gives about 5 mg/L of alkalinity per 10 mg/L of its regular-grade alum, which it defines as the 14-hydrate. The Irish EPA manual prints 5.3 mg/L per mg/L, about ten times the others, so do not use it. Use CPHEEO's figure, a fair first estimate for ferric alum, which IS 299 describes as the same 18-hydrate.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; Texas Commission on Environmental Quality (TCEQ), 2022; Environmental Protection Agency, Ireland, 2002; Bureau of Indian Standards, 2012

In CPHEEO's 50 mg/L alum design example, that is about 22.5 mg/L of alkalinity (50 × 300 ÷ 666), and the example also doses 25 mg/L of lime. CPHEEO wants enough alkalinity left that treated water is not corrosive; where raw water cannot supply it, add hydrated lime, or soda ash where hardness must not rise.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

PAC uses less: Health Canada ranks PACl below alum, and TDEC gives 0.18 mg/L per mg/L for ACH; we found no primary figure for other PAC grades. Qiu et al. (2019) found PACl lowered pH much less than alum or ferric chloride across the doses tested. On paper-mill wastewater at equal aluminium doses (Miranda et al., 2020), pH fell most with the lowest-basicity products: at the highest dose, to 5.4 with alum and 6.0 with a 37%-basicity PAC, against 6.5–6.8 with an 85%-basicity PAC and a polyaluminium-polyamine hybrid.

Source: Health Canada, 2021; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; International Journal of Environmental Research and Public Health (MDPI), 2019; Frontiers in Chemistry, 2020

Each water has its own optimum pH zone, which CPHEEO says narrows as dissolved solids and alum dose fall. Alum ranges differ by source: 5.5–8 (Irish EPA), 5.8–8.5 (TDEC) and 6.0–7.0 (Pernitsky). None of these sources gives a separate range for ferric alum, the same aluminium sulphate, so start from alum's and confirm by jar test. Where one source gives both alum and PACl, the PACl window is about as wide but higher: by 0.3 pH units in Health Canada's figures and by 0.5 in Pernitsky's (6.5–7.5 against 6.0–7.0).

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Environmental Protection Agency, Ireland, 2002; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; Stantec Consulting Services Inc. (presentation hosted by PNWS-AWWA), 2023; Health Canada, 2021; Bureau of Indian Standards, 2012

Health Canada says PACl can generally be used at higher pH and over a wider temperature range, at lower doses. It also notes a drawback: below its pH of minimum solubility (the pH at which the least aluminium stays dissolved), dissolved aluminium from PACl rises much more steeply than from alum.

Source: Health Canada, 2021

Cold water slows coagulation (Health Canada), and CPHEEO says the lowest effective coagulant dose usually needs 15 to 30 minutes of stirring in summer and 30 to 60 minutes in the colder months. The pH of minimum solubility also rises by 0.5 from 20 °C to 5 °C, for alum (6.2 to 6.7) and high-basicity PACl (6.4 to 6.9), so Health Canada asks for tests in cold and warm water. In jar tests simulating a winter storm at a US plant (7 °C water with added natural organic matter), Qiu et al. (2019) got below 0.20 NTU with PACl and 0.40 NTU with alum and ferric chloride. All three had an optimum near 75 mg/L, and PACl cost more.

Source: Health Canada, 2021; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; International Journal of Environmental Research and Public Health (MDPI), 2019

In low-turbidity water, too few particles collide for floc to grow. CPHEEO suggests stronger flocculation, recycling sludge or adding bentonite clay for the floc to form on, and US EPA notes that PACl can be the primary coagulant in such water.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; US Environmental Protection Agency, 1999

The raw water matters more than the coagulant. CPHEEO says sludge is directly proportional to raw-water suspended solids and calls sludge from the coagulant's own reactions minor. Its design example counts 100 mg/L of raw-water solids, 13 mg/L from 50 mg/L of alum (0.26 × dose, as in the TDEC course) and 25 mg/L from 25 mg/L of lime, so the lime dose adds sludge too. US EPA uses 0.36 × dose for alum and 0.5 × dose for ferric (iron-salt) coagulants. Ferric alum is an aluminium sulphate, so start from the alum factors, not the iron-salt one.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; US Environmental Protection Agency, 1999; Bureau of Indian Standards, 2012

For PAC, the only primary figure we found is TDEC's for ACH: 0.35 mg/L of sludge per mg/L, more than alum weight for weight, though TDEC notes its lower dose means less sludge overall. A vendor, Chimi Art, claims 15–30% less sludge with PAC but cites no study, so measure it in a plant trial.

Source: Tennessee Department of Environment and Conservation (TDEC), Fleming Training Center, 2022; CHIMI ART (vendor), 12 Jun 2026

IS 10500:2012 sets aluminium (as Al) at 0.03 mg/L acceptable and 0.2 mg/L permissible where no alternate source exists, and recommends applying the acceptable limit. CPHEEO's 2024 sampling schedule lists aluminium ≤0.2 mg/L among its daily critical process parameters at treated-water and distribution points. WHO sets no guideline value (it derives a health-based value of 0.9 mg/L, above practicable levels) but calls 0.1 mg/L achievable in large, well-run plants and 0.2 mg/L more reasonable for small ones, such as those serving under 10,000 people. The IS 10500 acceptable limit is tighter than both.

Source: Bureau of Indian Standards, 2012; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; World Health Organization, 2010; World Health Organization, 2011

All three coagulants add aluminium, so the residual depends on operation: WHO names coagulation pH, dose, mixing, paddle speed and filtration. For alkaline sources (pH above 7), Health Canada says PACl can lower the residual, citing Kimura et al. (2013), who found a basicity above 70% essential for this. Health Canada also warns that under-dosing to get a low residual can badly weaken pathogen removal and can even raise the residual aluminium.

Source: World Health Organization, 2011; Health Canada, 2021

Ferric alum adds iron too. CPHEEO calls it brown to dark brown with a high iron content, while IS 299 Amendment No. 2 (2021) describes the drinking-water Grade 4 as pale yellow to light brown. CPHEEO lists iron ≤0.3 mg/L beside aluminium, so log both.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Bureau of Indian Standards, 2021

CPHEEO says to choose the coagulant from jar tests run under identical conditions, and that only a jar test gives the exact dose. Run ferric alum, alum and PAC on one sample and record each dose both as product and as Al2O3, taking the Al2O3 content from that delivery's chemical analysis (CPHEEO asks plants to check the analysis of every coagulant delivery). Repeat daily when raw water changes significantly (CPHEEO), and in cold and warm water (Health Canada). The jar test guide covers the IS 3025 (Part 50) procedure.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Health Canada, 2021

At the Barekese plant in Ghana (raw water mostly 10–30 NTU), alum averaged 70.4 mg/L over 2013–2017 and PAC 33.2 mg/L from December 2016, less than half (33.2 ÷ 70.4 ≈ 0.47). The flocculated water was no clearer with PAC: 3.5–6.1 NTU, against 2.0–6.4 NTU with alum. The study's own jar tests put the PAC optimum at 15 mg/L, with 150/25 rpm mixing and pH 7.5–8.0, so the plant's PAC dose, mixing and pH had not been optimised. A vendor, HyChron, claims a 30–50% lower PAC dose but cites no study.

Source: IWA Publishing (Water Practice and Technology 16(4):1215–1223), 14 Jul 2021; HyChron (vendor), 2026

Price per kg alone misleads, because the products differ in Al2O3 and in the lime they need. Work from your own jar-test doses:

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2018; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

1. Coagulant cost per m3 = dose (mg/L) × price per kg ÷ 1,000, since 1 mg/L is 1 g per m3.
2. Add the lime or soda ash dose the jar test needed to hold pH and leave some alkalinity, costed the same way. The 2:1 ratio gives alkalinity as CaCO3, not a lime dose.
3. Add any polymer, such as anionic polyelectrolyte, and sludge disposal if you pay for it.
4. For daily use, dose (mg/L) × flow (MLD) = kg/day.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Cost it by season: CPHEEO's Nagpur case study assumes alum at 70 mg/L in the monsoon, 30 in winter and 10 in summer; these are that study's working figures, not typical doses. Freight cost also depends on Al2O3. At the IS minimums, a tonne of Grade 5 liquid ferric alum (8%) carries half the Al2O3 of a tonne of Grade 4 solid (16%), and a tonne of liquid PAC (9.5–10.2%) about a third of the Al2O3 in a tonne of 28% powder (9.5 ÷ 28 = 0.34; 10.2 ÷ 28 = 0.36). See the liquid vs powder PAC guide. For stock, CPHEEO advises all plants, particularly those under 5 MLD, to keep a safety supply of coagulant on hand, and calls three months' storage advisable.

Source: CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024; Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2018

• Ferric alum or alum: water with enough alkalinity, or a plant already dosing lime, where the jar test gives acceptable water at a lower cost per m3. See ferric alum. For drinking water, IS 299 has drinking-water grades (4 and 5) and IS 260 has none, so check the grade on the supplier's certificate.
• PAC: coagulation at higher pH and over a wider temperature range (Health Canada), low-turbidity water, where US EPA says PACl can be the primary coagulant, and low-alkalinity water, since it uses less alkalinity than alum. Run it against ferric alum in the same jar test. See poly aluminium chloride.
• Iron salts: for RO feed, where CPHEEO prefers ferric salts because the membranes do not tolerate aluminium, and for strongly coloured water, where it rates ferric coagulants generally better than alum. Ferric alum is aluminium-based, so for these waters test an iron salt such as ferric chloride.

Source: Bureau of Indian Standards, 2012; Bureau of Indian Standards, 2001; Health Canada, 2021; US Environmental Protection Agency, 1999; CPHEEO, Ministry of Housing and Urban Affairs, Government of India, 2024

Rise Chemical India makes ferric alum at its own factory in Bharatpur, Rajasthan, and sells it loose by weight as solid slabs of about 20 kg, with a minimum order of 5 tonnes. It also supplies PAC: powder at 28–32% Al2O3 and 70–85% basicity in 25 or 50 kg bags, minimum order 250 kg, and liquid at 10% Al2O3 in 35–50 kg carboys, 200–250 kg drums or tankers. The company holds ISO 9001:2015 and sells in India and for export; the price depends on the quantity and the destination. Its office in Kirti Nagar, Delhi handles enquiries and orders, so ask for a quote with the product and quantity you need.