Ferric chloride, aluminium sulphate and polyaluminium chloride (PACl) are the workhorse coagulants in water and wastewater treatment. This page covers selection, dose-response chemistry, jar testing, pH window management, sludge yield and complete dosing-system design.
Three mechanisms, one outcome: making suspended particles agglomerate.
Suspended particles in water carry a small negative surface charge that keeps them mutually repulsive and therefore in stable suspension. Coagulants are positively-charged metal salts (Fe³⁺, Al³⁺) that hydrolyse rapidly on contact with water to form polynuclear hydroxide species. These positively-charged hydrolysis products attach to the negatively-charged particles, neutralise their surface charge (zeta potential approaching zero), and trigger inter-particle collision and floc formation. Three concurrent mechanisms drive coagulation: charge neutralisation (low coagulant dose, low pH), sweep flocculation (excess hydroxide precipitate physically entraps particles — the dominant mechanism in most plants), and adsorption / inter-particle bridging (relevant when polymeric coagulants are co-dosed).
Delivered as a 40% w/w solution, ferric chloride is the most widely used coagulant in industrial and municipal practice. The solution is dense (1.4–1.5 kg/L), acidic (pH 0.5–1), and aggressively corrosive — demanding fully chemical-resistant infrastructure.
Best for: high-organic effluent (food, brewery, dairy), phosphorus precipitation, oily wastewater (DAF), high-colour surface water.
| Tank material | FRP, HDPE or PP-lined steel |
| Bund volume | 110% of largest tank |
| Pipework | HDPE, PP, PVDF or PVC-U |
| Pumps | Solenoid or motorised diaphragm; Hastelloy/PTFE wetted parts |
| Min temp (storage) | −15°C (40% solution); higher for higher concentrations |
| PPE | Full face shield, gauntlets, chemical apron |
Alum (Al₂(SO₄)₃·14H₂O) is the historic standard for drinking-water clarification. Delivered as a 48–50% w/w solution (8% as Al₂O₃), it is less corrosive than ferric, has narrower effective pH range, and produces lighter (less dense) flocs.
Best for: drinking-water plants, low-turbidity surface water, sites with established lime feed for pH control. Less common in industrial wastewater where ferric performs better with high organics.
| Alum | Ferric | |
|---|---|---|
| pH window | 5.5–7.5 | 5.0–9.0 |
| Floc density | Light | Heavier |
| Sludge production | Lower | Higher |
| Cold water | Poor (<5°C) | Good |
| Phosphorus | Acceptable | Excellent |
| Residual concern | Al (regulatory) | Fe (aesthetic) |
PACl is a pre-hydrolysed polymeric form of aluminium, supplied as a 10–20% solution as Al₂O₃. Pre-hydrolysis means the polymer is already in a highly charged form, requiring less reaction time and consuming less alkalinity than monomeric alum.
The product is sold under various trade names with different basicity (degree of pre-hydrolysis, expressed as percent OH bound). Higher basicity (60–80%) consumes less alkalinity but produces larger flocs slower. Lower basicity (40–50%) behaves more like alum.
Best for: drinking-water plants in soft-water regions where alum alkalinity demand is a problem; cold-climate municipal plants; high-turbidity events; any process where reducing sludge yield or chemical Operating expenditure matters.
When alum hits water, hydrolysis goes through a series of mono-, di- and polynuclear species. Each step consumes alkalinity. PACl ships at the polynuclear stage already — the hydrolysis is done before it reaches your reactor. Less alkalinity consumed; less pH disturbance; faster reaction.
10 mg/L PACl (as Al₂O₃) typically replaces 30–40 mg/L alum. Always verify by jar testing — basicity and product chemistry differ.
No design calculation replaces a properly-run jar test on your actual water.
Collect representative samples covering low/typical/high turbidity events. Measure pH, alkalinity, conductivity, temperature, turbidity, TSS, TOC, colour.
1L jars on a six-paddle stirrer. Dose ascending coagulant doses (e.g., 10, 30, 50, 80, 120, 180 mg/L). Standard mixing: 200 rpm for 60 seconds.
Reduce to 30 rpm for 15 minutes. Observe floc formation, density, settleability.
Stop mixer; settle 15 minutes; sample top 200mL. Measure turbidity, residual coagulant, pH. Plot dose-response curve.
Identify lowest dose meeting target effluent quality (Coagulant Optimum Dose, COD). Re-test at COD ±20% to confirm. Repeat for cold/warm season conditions.
FRP or HDPE day tanks sized for 14–28 days’ consumption at design dose. Bunded to 110% volume; vent to scrubber for acid coagulants.
Polished day tank (24–72 hrs consumption) sized to absorb dose variability. Level transmitter, low-low alarm to interlock dosing pumps.
Duty/standby motorised diaphragm pumps. Wetted parts: PVDF or PTFE for ferric/alum; PE/PP for PACl. VFD for flow-paced control.
Quill or static mixer at point of maximum turbulence. Inject ahead of a high-shear zone (G-value >1000 s⁻¹) for fast charge neutralisation.
Flow-paced (proportional to raw-water flow), compound-loop (flow x feedback turbidity), or feed-forward from turbidity sensor. See dosing control strategy.
Emergency shower & eyewash within 10 m. Containment kerbs. Spill kit. Acid-resistant flooring. Bund leak detection. Pressure-relief on dosing line to prevent burst.
Beyond optimum, charge reversal can re-stabilise particles. Symptoms: turbidity worsens with increasing dose. Cure: reduce dose; retest by jar test.
KLa, reaction rates and floc densification all slow at <10°C. Alum particularly affected. Mitigation: switch to PACl or ferric for winter; lengthen flocculation time.
Ferric and alum precipitate in pipes during prolonged shutdown. Specify pipe heat-tracing, recirculation loops, or daily flush cycles.
Each mg/L of ferric consumes ~0.9 mg/L alkalinity (as CaCO₃). Soft water needs supplementary lime/NaOH dosing to maintain coagulation pH. Monitor and budget.
The companion dose after coagulation — polymer selection, charge density, preparation.
Read MoreFeedback vs feed-forward vs cascade control; PID tuning for coagulant dosing.
Read MoreDiaphragm vs peristaltic vs solenoid — the decision matrix for coagulant duty.
Read MoreThe full physico-chemical treatment sequence and reactor design.
Read MoreSend us your influent characterisation, target effluent quality and any existing dosing infrastructure. We will return coagulant selection, dose-response curves from jar testing, dosing-system P&ID and a chemical-Operating expenditure projection.
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