Alum Sludge From a Water Works: Why It Behaves Nothing Like Wastewater Sludge

A 60 megalitre-a-day treatment works produces about 4.5 tonnes of alum sludge a day at 96 percent moisture, and the contractor who took the disposal contract assumed the granules he used at a sewage works would behave the same way. They did not. At the same 1.5 percent dose the sludge stayed liquid, the polymer sat in grey lumps on the surface, and the tanker driver refused the load. The pH was 5.9, the sulphate was 640 mg/L, and the aluminium was still active.

Alum sludge is a different chemical environment, not a weaker version of the same one. This article explains what the residual coagulant does to swelling, gives bench numbers from three water works, and sets out a jar protocol that will tell you in 40 minutes whether a grade is worth trialling at full scale.

Residual Aluminium and a pH of 5.9 Suppress Swelling

A polyacrylate network swells because the carboxylate groups along the chain repel each other once they are neutralised. In a sludge at pH 5.9, a meaningful share of those groups is protonated and no longer carries a charge. The network still takes water, but the driving force drops sharply. Add 30 to 80 mg/L of residual Al(III), which is trivalent and binds three carboxylates at once, and the surface of the granule skins over within seconds.

That skin is what the operator sees as lumps. Water has to diffuse through a partly collapsed shell, so the 30-second absorption you measured in tap water becomes a 6 to 10-minute process, and in a 90-second mixing tank the granule never gets there. Sulphate makes it worse: at 600 mg/L it raises the ionic strength to roughly 0.02 mol/L, enough to screen the charge without any divalent cation being present.

Dose Comparison: 4.2 Percent Against 1.2 Percent

Three works, three sludges, one 30/60 mesh sodium grade. A digested sewage cake at 22 percent dry solids took 1.2 percent of wet weight to reach a non-dripping consistency. A lime-conditioned waterworks cake at 28 percent dry solids took 0.9 percent. The alum sludge at 4 percent dry solids took 4.2 percent, and even then only reached a stiff paste rather than a stackable solid.

The lesson is about the water-to-solids ratio, not the chemistry alone. At 96 percent moisture there are 24 kg of water for every kilo of solids, and the granule has to compete with a continuous aqueous phase rather than wet the surface of a damp solid. Granules are the wrong tool for a 4 percent stream. Thicken to 8 to 12 percent in a gravity belt or decanter first, then dose at 1.5 to 2 percent on the thickened stream, and the consumption per dry tonne falls by two thirds.

Sulphate, Chloride and What Leaches Back Out

Absorption is reversible under load. A granule holding alum sludge water will give part of it back when the cake is stacked 2 m high in a trailer, and what comes back is not clean water: it carries sulphate, aluminium and a portion of the original colour. In one trial the filtrate TDS rose from 1,900 to 2,400 mg/L after 24 hours under 1.5 m of stack.

If the receiving site has a leachate consent, that matters. Two practical responses work. Dose 10 to 15 percent above the visual endpoint so the gel has reserve capacity, or blend 2 to 3 percent hydrated lime with the granules so the released liquor is neutralised and the aluminium is re-precipitated in place. The second option adds roughly 12 dollars per tonne but usually removes the argument at the gate.

A 40-Minute Jar Protocol for an Unfamiliar Sludge

Fill four 500 ml jars with 400 g of sludge, measured by weight not volume. Add 0, 0.6, 1.2 and 2.0 percent granule by weight. Stir at 150 rpm for 90 seconds, the same shear as your tank, then stop and observe at 2, 10 and 30 minutes. Record three things: whether a clear water layer separates, whether a 20 mm ball holds its shape when rolled in a gloved hand, and the volume of liquor that drains from 100 g on a paper filter after 10 minutes.

Pick the lowest dose that gives a shape-holding ball and under 10 ml of drainage. Then repeat that dose with the plant’s actual mixing equipment on 200 litres, because a jar impeller and a paddle in an IBC do not deliver the same energy. Sludges from water works vary more week to week than sewage sludge ever does, so keep the jar set on the shelf and re-run it whenever the raw water source changes.

For alum and ferric sludges, ask suppliers for a grade with a higher charge density and a coarser cut, since fine powder simply skins over faster. The water absorbing polyacrylate granules page lists the mesh options that matter here.