PAC Powder vs Alum Water Treatment | ETP Coagulant Switch Guide

Alum (aluminium sulphate, Al2(SO4)3·18H2O) dissolves in water and hydrolyses to form aluminium hydroxide flocs. These flocs adsorb suspended solids, colloidal colour and organic matter, enabling gravity settling in clarifiers. Alum requires the raw water to be within a relatively narrow pH band — typically 6.5 to 7.5 — for optimal coagulation. Outside this range, lime or caustic soda must be added to raise pH, or acid to lower it, increasing chemical cost and operational complexity.

PAC powder (poly aluminium chloride) is a pre-hydrolysed inorganic polymer coagulant with the general formula [Al2(OH)nCl6-n]n. Because PAC is partially hydrolysed during manufacturing, it releases active aluminium species more rapidly upon dissolution, achieving faster floc formation and higher charge density per unit of aluminium dosed. Powder PAC typically contains 28–30% Al2O3 compared to 16–18% Al2O3 equivalent in alum, meaning less product weight is needed to achieve the same coagulation result.

The practical consequence for plant operators is that PAC powder doses are typically 30–60% lower than equivalent alum doses for the same raw water turbidity. PAC also performs effectively across a broader pH range (6.0–8.5), reducing the frequency and volume of pH correction chemicals. For high-colour textile effluent or seasonal monsoon turbidity spikes, PAC often achieves compliance limits that alum cannot meet without excessive dosing.

Not all alum is the same. Ferric alum (also called ferrous alum) contains iron as an inherent impurity, typically 0.3–0.5% Fe, which gives the crystals a yellowish to brownish appearance. Ferric alum is the most widely used and lowest-cost alum grade in Indian industrial ETPs. It coagulates effectively for general wastewater clarification where iron staining of the treated water or final product is not a concern.

Non-ferric alum is refined to iron content below 0.004%, producing white crystalline lumps suitable for potable water treatment, paper sizing, pharmaceutical process water and food-grade applications. Non-ferric alum is supplied as a premium grade where even trace iron would cause product rejection — for example, white paper stock, bleached textile fabric or drinking water that must meet IS 10500 colour standards.

When comparing PAC powder vs ferric alum specifically, PAC eliminates the iron staining risk entirely because it contains negligible iron. When comparing PAC vs non-ferric alum, the cost differential narrows because non-ferric alum is already a premium product — yet PAC still offers lower dosing rates and broader pH tolerance. Many plants that previously used non-ferric alum for colour-sensitive applications have migrated to PAC powder to reduce both coagulant dose and pH correction chemical consumption.

Sludge characteristics also differ between grades. Ferric alum produces slightly higher sludge volume due to iron hydroxide precipitation alongside aluminium hydroxide flocs. Non-ferric alum sludge is cleaner but still exceeds PAC sludge density on a per-litre-of-water-treated basis. For large ETPs where sludge handling and disposal represent 15–25% of operating cost, denser PAC sludge translates to measurable savings in centrifuge time, filter press cycles and landfill charges.

Jar test results from Indian ETP operators consistently show PAC powder achieving equivalent or better turbidity removal at 40–70% of the alum dose. For a typical textile effluent with inlet turbidity of 200–400 NTU, ferric alum doses of 200–400 mg/L may be required, whereas PAC powder doses of 80–200 mg/L often achieve the same outlet clarity. The absolute dose depends on pH, temperature, alkalinity, TDS and the specific contaminants present — which is why site-specific jar testing is essential before switching coagulants.

Colour removal is where PAC powder most clearly outperforms alum. Reactive dye effluent from cotton dyeing operations often contains chromophores that resist alum coagulation even at high doses. PAC's higher charge density and polymeric structure bind dye molecules more effectively, frequently achieving CPCB colour limits (100 Pt-Co for inland surface water discharge) where alum-treated effluent still fails compliance. Paper mill backwater and pulp mill effluent show similar advantages for PAC in colour and COD reduction.

Temperature sensitivity favours PAC in Indian conditions. Alum coagulation efficiency drops noticeably in cold weather (below 15°C), which affects northern Indian municipal plants during winter. PAC maintains coagulation performance at lower temperatures because its pre-hydrolysed structure is less dependent on water temperature for activation. For plants in Gujarat, Maharashtra and South India where ambient water temperatures are higher, both coagulants perform adequately — but PAC's lower dose still reduces chemical inventory and handling costs.

Storage and handling considerations differ between the two products. Alum is supplied as crystalline lumps or powder in 50 kg HDPE bags and has an indefinite shelf life when kept dry. PAC powder also comes in 25–50 kg bags but is hygroscopic and must be stored in a dry, ventilated warehouse to prevent caking. Liquid PAC (10–18% Al2O3) is an alternative for plants with automated dosing systems, though this article focuses on powder PAC vs alum as the most common comparison for Indian bulk buyers.

Compare PAC powder and alum on a per-m³-of-water-treated basis. Alum typically needs higher dosing and tighter pH control, while PAC often achieves similar turbidity/colour reduction at lower dosing. Include the impact of pH adjustment chemicals and sludge handling to see the true operating cost driver.

In a typical 500 m³/day ETP (jar-test dependent), alum dosing often starts in the 200–350 mg/L range while PAC dosing can begin around 80–200 mg/L for similar clarity. When dosing is optimised, lower coagulant consumption can reduce overall chemical load and decrease sludge volume, improving dewatering efficiency.

Switch from alum to PAC when: (1) alum dosing exceeds ~250 mg/L and still fails to meet discharge turbidity/colour limits; (2) pH adjustment chemicals become a major recurring expense because alum needs tight pH control; (3) sludge volume strains dewatering equipment capacity; (4) iron staining from ferric alum affects product quality in paper, textile or food applications; or (5) seasonal raw water variability causes frequent dose rework that PAC handles more consistently.

Stay with alum when: (1) alum dosing achieves stable compliance at doses below ~150 mg/L; (2) you have established sludge handling/reuse processes; (3) jar testing and program transition are not feasible right now; or (4) procurement terms lock in long-running alum supply. In borderline cases, Drashti Chemicals can run jar-test support to compare PAC and alum side-by-side on your actual effluent sample.

For alum dosing, prepare a 5–10% stock solution by dissolving crystals in water in a HDPE tank with mechanical agitation. Dose the solution into the rapid mix chamber or flocculator inlet using a metering pump. Monitor pH continuously and maintain 6.5–7.5 for ferric alum and 6.8–7.2 for non-ferric alum. Retention time in the flocculator should be 15–30 minutes before settling.

For PAC powder dosing, prepare a 5–10% stock solution — PAC dissolves faster than alum but ensure adequate mixing to prevent undissolved particles from entering the clarifier. Dose PAC into the same rapid mix point as alum would be applied. PAC tolerates pH 6.0–8.5 but performs best at 6.5–8.0. Because PAC flocs form faster, reduce flocculator retention time slightly and monitor for carryover of fine flocs into the outlet weir.

Always conduct jar tests when changing coagulant type, dose or raw water source. Test at least five dose levels (including zero dose as blank) and evaluate floc size, settling rate, supernatant clarity and residual aluminium. Drashti Chemicals technical team can visit Gujarat and Maharashtra plants for on-site jar testing, or provide jar test kits and protocols for remote sites.