Iron or Aluminium? Choosing and Dosing Flocculants Without Wasting Chemical

Flocculants on a water-treatment site all do one thing: they grab suspended particles in solution and link them into flocs heavy enough to settle or float out. Beyond that the catalogue fragments. The two dominant families are the iron-based series and the aluminium-based series, each with its own high-polymer derivatives, and picking between them, then dosing them correctly, is where most of the operating cost is won or lost.

The aluminium side

Aluminium sulfate is the reference material. It is extremely soluble in water, insoluble in ethanol, acidic in solution, and it hydrolyses to aluminium hydroxide. It will not dissolve in pure sulfuric acid, only coexist with it, so its solubility in sulfuric acid is the same as in water. At room temperature it crystallises with 18 molecules of water as the octadecahydrate, which is what industry mostly ships. That product is about 51.3% anhydrous aluminium sulfate and will not self-dissolve even at 100 C. It weathers slowly, is reasonably stable, and on strong heating breaks down to alumina and sulfur oxides, with decomposition starting around 770 C. The commercial solid is grey-white flake, granule or lump; a green tint means low-iron salts, and surface yellowing means those salts have oxidised. It is non-toxic, though the dust irritates eyes.

Where it earns its keep: paper sizing, coagulation of fine and colloidal suspended solids in water supply and wastewater, turbidity removal, precipitation and colour fixing, foam fire-extinguishing formulations with baking soda, mordanting and tanning, oil decolourising, and as a raw material for artificial gemstones, high-grade ammonium alum and other aluminates.

The iron side

Polyferric sulfate is a pale-yellow amorphous powder, very soluble in water; a 10% by weight solution is red-brown and transparent, and the solid is hygroscopic. It is used across drinking water, industrial water, industrial wastewater, municipal sewage and sludge dewatering. Ferric sulfate in its own right shows up as a silver-analysis and sugar-determination reagent, a dye and ink intermediate, a disinfectant and polymerisation catalyst, and an electrolyte component for zinc-nickel-iron and zinc-iron-cobalt alloy plating.

Naming the polymers without getting lost

The abbreviations are worth keeping straight because the products do not behave alike. PAC is polyaluminium chloride. PAF and PAFC sit in the polyaluminium-ferric family, PAFS is the polyaluminium-ferric-sulfate type, and PFS is polyferric sulfate. In practice, under the same conditions, the dose of a polyferric product is roughly comparable to solid PAC and only about a third to a quarter of the dose of solid aluminium sulfate. If a plant is converting from a liquid product, work back from the corresponding concentration; a weight ratio of 1:3 is the usual starting assumption.

Make-down and dosing

Raw water varies, so there is no substitute for a beaker test or an on-site trial before fixing a dose. Standard practice is to charge the dissolving tank at 10-30%, add tap water, stir to complete hydrolysis, then let it stand until the liquid turns red-brown before diluting to working strength. Waterworks often make down a 2-5% solution for direct dosing; industrial wastewater plants usually run 5-10%. Prepare on the day of use, use tap water, and do not be alarmed by a slight precipitate.

Once made down, the solution is pumped to a measuring tank and metered into the raw water. Where a plant previously used another coagulant, the old dose is a reasonable reference point, but the only defensible number comes from jar testing against floc appearance.

The three stages inside the coagulation basin

Hydraulics decide as much as chemistry. In the coagulation stage the reagent hits the raw water and fine flocs must form almost immediately; the water looks worse before it looks better, and the mixing has to be violent. In a beaker that means 250-300 rpm for 10-30 s and never more than 2 min.

Flocculation is the opposite: gentle turbulence and enough time for flocs to grow and coarsen, typically 10-15 min, with a visible mat of floc gathering and sinking near the end and a clear layer forming on top. A standard jar protocol stirs at 150 rpm for about 6 min, then at 60 rpm for about 4 min until the suspension stabilises again. As a rough field rule, give the rapid mix somewhere between 30 s and 5 min depending on water temperature, hold the flocculation basin for about 10 min, and allow up to 15 min of total flocculation retention before the water reaches the separator.

Storage and handling deserve a line as well. Solid products should be kept dry, because polyferric sulfate in particular picks up moisture quickly and cakes in the bag. Where a site buys both an aluminium salt and an iron salt, keep the make-down systems physically separate; cross-contamination changes the hydrolysis behaviour of both and the jar results stop matching what the plant does.

Get those three stages right and the chemical choice matters far less than most suppliers admit. Get them wrong and no amount of extra PAC or PFS will rescue the effluent. Plants that track BOD and COD removal against dose, rather than dosing to a fixed setpoint, typically land on a lower and steadier consumption.