Iron or Aluminium? How Coagulant Chemistry and Jar-Test Discipline Decide Oily Wastewater Performance

Flocculants all do roughly the same visible thing: they take suspended particles that would otherwise stay in suspension and gather them into flocs heavy enough to settle or float. The differences that matter in a plant are buried in chemistry, and for oily wastewater they decide dose, sludge volume and whether the downstream separator copes.

Two families, and what each brings

Inorganic flocculants split into low-molecular and polymeric types, and the polymeric iron-based and aluminium-based families are the ones most used and most studied. Polyferric sulfate (PFS) and polyaluminum chloride (PAC) dominate practice.

PFS brings strong flocculation, fast settling, a wide application range, low corrosiveness to metal equipment and little secondary pollution. Its disadvantages sit on the other side of the ledger: more sludge, a more involved process and higher cost. PAC, by contrast, flocculates oily sewage noticeably better than other aluminium salts and generally better than polyferric flocculants, at lower dose and with less sludge — but floc settles more slowly, which can create its own secondary-pollution problem.

Composite salts close the gap

Composite inorganic polymeric flocculants were developed to keep the strengths of both. Polyaluminum ferric chloride (PAFC) is aluminium-dominated with iron assisting; it combines the characteristics of aluminium and iron salts and its coagulation performance usually beats both PAC and ferric chloride. On oilfield produced water, PAFC achieves oil-removal and suspended-solid removal above about 90%, which is enough to meet reinjection requirements.

Polyaluminum ferric sulfate (PAFS), made from ferrous sulfate with sodium nitrate as catalyst, works well on oily wastewater. Polyferric aluminum chloride sulfate, produced from bauxite, active calcium silicate, hydrochloric and sulfuric acid, combines the better properties of polyferric and polyaluminum types and treats oily wastewater better than PAC. Polyaluminum ferric sulfate chloride (PAF-CS) sits in the same composite group. What these formulations share is a higher degree of polymerization and higher basicity than their single-metal parents.

Mixing discipline matters as much as the salt

Dose is never a number you can take from a datasheet. Raw water varies, so commissioning or beaker tests have to be run on the actual water to fix operating conditions and dosage.

The three stages of coagulation each need their own hydraulic conditions. In the coagulation stage the dosed chemical is mixed rapidly with raw water to form fine flocs in a very short time; the water turns more turbid and needs intense turbulence. In jar tests that means stirring at 250–300 rpm for 10–30 s, and in any case no more than 2 min. In the flocculation stage flocs grow and coarsen, needing moderate turbulence and enough residence time, 10–15 min. A typical jar protocol stirs at about 150 rpm for roughly 6 min, then at 60 rpm for about 4 min until the flocs stay suspended. In the settling stage the flow must be slow; inclined-tube or plate settlers are normally used, and dissolved-air flotation is better still for floc separation. Jar tests here stir slowly at 20–30 rpm for 5 min, then settle for 10 min before reading residual turbidity.

Preparation and storage details that get missed

Before use, the product is made up at 10–30% in an alum-dissolving tank, stirred with tap water until fully hydrolysed, left to stand until a red-brown liquid forms, then diluted to the dosing concentration. Water plants often prepare a 2–5% solution for direct dosing; industrial wastewater treatment usually works with 5–10%.

Dose is best set by observing floc formation during production commissioning or jar tests. Under comparable conditions the dosage is roughly equivalent to that of solid polyaluminum chloride and about 1/3–1/4 that of solid aluminium sulfate; if a liquid product was used previously, convert from concentration, roughly a 1:3 weight ratio. Solution is generally prepared on the day of use with tap water, and slight sediment is normal. Dissolution and dosing equipment should be built from corrosion-resistant materials.

On storage: keep it dry, moisture-proof and away from heat — below 80 °C — with the packaging intact, and it keeps for a long time.

What the raw materials tell you about behaviour

The physical constants are not academic. Aluminium sulfate crystallizes at room temperature with 18 molecules of water as the octadecahydrate, containing 51.3% anhydrous aluminium sulfate, and does not self-dissolve even at 100 °C; it loses water on heating and at 770 °C begins decomposing into alumina, sulfur trioxide, sulfur dioxide and water vapour. Ferric salts decompose at 480 °C, and the commercial product usually carries about 20% water; some forms hold 9 molecules of water of crystallization with relative density 2.1, losing 7 molecules at 175 °C. These are the numbers that explain why storage conditions and make-up water temperature affect dose response.

Terminology worth keeping straight

Three words get used interchangeably on site and should not be. Coagulation is the mechanism: compression of the electric double layer, destruction of the ζ-potential and charge neutralization destabilize colloids so they aggregate into floc particles. Flocculation is the process view — floc particles aggregating into large flocs and settling through adsorption, cross-linking and enmeshment. A flocculant is simply the agent that causes colloids and suspended particles to coagulate and flocculate, hence the alternative name coagulant. Taken together, destabilization and flocculation form the complete process.

Practical notes for existing plants

For raw water with high colour, high COD and BOD, this product works very well when supported by auxiliary agents. Plants already using chemical coagulation generally need no major modification — adding an alum-dissolving tank is usually enough. Enhanced filtration, meaning sensible selection of filter-bed structure and filter aids, remains one of the more effective ways to lift final water quality.