Never overdose
Excess aluminium carries through and precipitates on the membrane. Aluminium fouling is difficult to clean and sometimes irreversible.
Application — desalination
PAC for desalination pre-treatment
Coagulation ahead of ultrafiltration and reverse osmosis is what keeps membranes alive. PAC reduces SDI, drops organic load and cuts the cleaning frequency that dominates membrane operating cost.
What pre-treatment is actually protecting
Reverse osmosis membranes are the single most expensive consumable in a desalination plant. They foul from three directions: particulate matter, organic matter, and biological growth that feeds on the organics. Pre-treatment exists to reduce all three before the feed ever reaches a membrane.
Coagulation with PAC removes colloidal and fine particulate material that would otherwise pass a conventional filter and register as a high Silt Density Index. Most RO manufacturers want SDI15 below 3 to 5 at the membrane inlet, and hitting that consistently on Gulf seawater without coagulation is difficult.
PAC also removes a meaningful fraction of natural organic matter, which matters more than the particulate story. Organics are what biofilm feeds on, and biofouling is the failure mode that is hardest to reverse once established.
Get these wrong and you damage membranes
Critical operating constraints
Watch residual aluminium
Membrane manufacturers typically want residual Al below about 0.05 mg/L at the inlet. Confirm the figure for your specific membrane.
Control pH tightly
Aluminium solubility is strongly pH-dependent. Drift outside the window and soluble Al passes through to the membrane.
Adequate rapid mix
Poor flash mixing leaves unreacted coagulant in solution, which reaches the membrane rather than being filtered out.
Coordinate with antiscalant
Some antiscalants interact with residual aluminium. Check compatibility with your chemical supplier before changing dose.
Monitor SDI continuously
SDI is your early warning. A rising trend means pre-treatment is losing ground before differential pressure shows it.
“Under-dosing costs you membrane life slowly. Over-dosing costs you membranes quickly. This is one application where the jar test is not optional.”
FAQ
Common questions
Yes, widely — as a coagulant in pre-treatment ahead of media filtration, ultrafiltration and reverse osmosis, to reduce SDI and organic load reaching the membranes.
Typically low, often 0.5 to 5 mg/L as Al, because the objective is protecting membranes rather than clarifying heavily turbid water. Overdosing is a genuine risk here — establish the dose by jar test and monitor residual aluminium.
Overdosed PAC can. Residual aluminium carrying through to the membrane precipitates as aluminium hydroxide fouling, which is difficult to clean. Keep residual Al below the limit your membrane manufacturer specifies.
Both are used. PAC generally gives lower sludge volume and avoids the iron-fouling risk; ferric can be more effective on some organic-heavy seawaters. It depends on your feed — trial both if the decision is significant.
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