COMPARISON
PAC vs Ferric Chloride: Choosing Between the Two Workhorses
PAC and ferric chloride are the two coagulants most plants end up choosing between. Both work by charge neutralisation and hydrolysis, both remove turbidity and phosphorus, and both are commodity chemicals with wide supply. The differences that matter are in floc character, corrosion, colour and the total cost of running each - not in the per-tonne price tag.
Head to head
| Property | PAC | Ferric chloride (FeCl3) |
|---|---|---|
| Working pH window | roughly 5.5 - 8.5 | wide, tolerates lower pH |
| Floc character | larger, denser, settles well | heavy, fast-settling, shear-sensitive |
| Phosphorus binding | good, ratio-based dosing | strong, often dose-efficient for tight P limits |
| Effluent colour | clean | yellow-brown tint possible on overdose |
| Corrosion | mild; ordinary materials suit | aggressive; needs resistant pumps and tanks |
| Handling | bagged solid or liquid, low hazard | liquid, corrosive class; stringent containment |
| Sludge | modest volume | heavier hydroxide sludge at high doses |
Where ferric chloride wins
Ferric earns its keep on hard phosphorus limits - the ferric-phosphate chemistry binds tightly, and plants chasing very low effluent TP often reach the target with less chemical than an aluminium programme demands. It also tolerates acidic, sulphide-bearing streams better, which makes it a familiar face on industrial effluents with odour and low pH.
The price per tonne frequently looks lower than PAC, and on a pure commodity basis it often is. The offsetting costs are equipment and maintenance: corrosive-duty pumps, coated tanks, careful spill management, and the colour risk - an iron overdose paints the effluent yellow-brown in a way that discharge permits and downstream users notice.
Where PAC wins
Drinking water and any discharge with an aesthetic component suit PAC: clean effluent colour, low residual aluminium when dosed correctly, wide pH tolerance with less alkalinity consumption per unit of turbidity removed, and handling that ordinary plant materials shrug off. Sludge from a PAC programme is usually easier on dewatering than iron-heavy sludge of the same solids load.
Operationally, PAC forgives more: bagged inventory that stores for a year, dosing gear that does not fight corrosion, and a floc that tolerates the mixing conditions found in real, imperfect tanks. For plants without dedicated corrosive-chemical infrastructure, that operational margin often decides the comparison before the jar test does.
Running both, deliberately
A growing pattern is not choosing at all: PAC for routine duty and seasonal swings, with ferric dosing capability installed for tight phosphorus periods or industrial load spikes. The jar test on your actual water, at your actual targets, with both chemicals on the bench - that afternoon decides whether the second system earns its capital cost.
Frequently asked questions
Which coagulant removes phosphorus more efficiently?
Ferric chloride generally binds phosphorus more strongly per unit of metal, so very low effluent TP targets often arrive cheaper on ferric. PAC still meets most municipal limits comfortably; run both through a jar test at your target and compare chemical cost per removed kilogram of phosphorus.
Can I mix PAC and ferric chloride in the same tank?
No - store and dose them separately. Their solutions react and foul equipment, and blended product has no predictable performance. If the plant runs both, keep separate storage, dosing lines and injection points, and never let them meet anywhere but the water.
Is ferric chloride cheaper than PAC overall?
The invoice says often yes; the plant account says it depends. Corrosion-grade dosing equipment, heavier sludge handling and the colour-management risk can consume the per-tonne advantage. Price the whole year - chemical, maintenance, sludge disposal - before deciding.