Dewatering Digested Sludge to a 35 Percent Cake: Dose Math Before the Incinerator

At a works serving 180,000 people, the belt press had run at 78 percent cake moisture for two years and the fluidised-bed incinerator kept refusing the feed. The transfer screw plugged twice a week, the bunker drained a brown liquor into the sump, and the autothermal balance only closed with supplementary gas. Raising the press polymer dose changed nothing. What worked was smaller than anyone expected: 2 percent coarse polyacrylate granules dropped into the cake screw, plus a moisture target rewritten on a dry-solids basis.

Binding water in sludge is not the same as drying it, and that distinction decides whether the dose you are quoted makes any sense. This article works through the arithmetic for a digested cake headed to a furnace, the moisture targets that actually matter at the burner, and the three places where polymer helps against the three where it quietly burns budget.

The Water in a 78 Percent Cake Splits Into Three Fractions

One tonne of that cake carries 780 kg of water and 220 kg of solids. Roughly 55 to 60 percent of the water sits in the voids between flocs and moves the moment the cake is sheared. Another 30 percent is trapped inside the floc structure and only leaves under pressure or evaporation. The remainder is surface-bound and will not move at all without heat. A press stops working at 22 to 26 percent dry solids because the flocs compress to a point where the voids close, not because the polymer ran out.

Granules can only take the first fraction. In a transfer screw with 40 to 60 seconds of residence, shear releases perhaps a fifth of that free water, which is why in-line dosing lands at 1.5 to 2.5 percent of wet weight rather than the 6 or 8 percent people assume after reading a free-swell number. Anything above 3 percent mostly coats the outside of the cake and turns the belt house into a slipping hazard.

Dose Arithmetic From 1.8 Percent of Wet Weight

Take a plant at 10 tonnes an hour. At 1.8 percent, that is 180 kg of granules per hour. The granules will not see deionised water: the filtrate runs 8 to 12 mS/cm with 4 to 6 g/L of dissolved salts, and a standard 30/60 mesh sodium polyacrylate returns 110 to 140 g/g in that liquor, not the 300 g/g on the sales sheet. So the realistic capacity is about 22 tonnes of water an hour, against 7.8 tonnes of free water arriving every hour.

Read that comparison carefully. The polymer cannot dry the cake. It can bind 25 to 30 percent of the free water, which is exactly the margin between a cake that drips on the bunker floor and one that does not. The correct design target is not a moisture percentage at all: it is the three to five points of water above the press limit that cause drainage, screw slip and liquor in the pit.

What Binding Water Does to Calorific Value, and What It Cannot Do

Adding 20 kg of dry polymer per tonne adds mass with no heating value, so the lower calorific value per wet tonne falls by roughly 2 percent. The gain comes from elsewhere. Cake that no longer bleeds can be stored in a bunker for eight hours without a leachate pool, and the furnace no longer has to evaporate liquor that was previously drained off and re-injected. In practice plants recover 0.3 to 0.6 MJ/kg of net benefit from eliminating that re-injection loop.

If the furnace genuinely needs 40 percent moisture to burn without support fuel, no granule will get you there. That is a thermal drying job, and the honest answer is a dryer. Where the furnace can accept 60 to 63 percent cake, granules are the cheaper fix by a wide margin: 18 to 25 dollars per tonne of cake against a dryer capex that rarely closes on a plant this size.

Run the Trial on One Bunker, Not the Whole Works

Take three doses, 1.2, 1.8 and 2.5 percent, and run each for one shift through the existing screw. Sample five points across the bunker at 1, 4 and 8 hours. Two tests carry the decision: a drop test from 1.2 m onto concrete, which should leave no pooled ring after ten minutes, and a drain test where 2 kg sits on a 1 mm mesh for 30 minutes and the collected liquor is weighed. Below 15 ml, the cake is transport-stable. Above 60 ml, the dose is under-specified no matter what the certificate says.

Then convert to money. At 2,200 dollars per tonne of granules, 18 kg per tonne of cake costs about 40 dollars. Compare that with the supplementary gas, the sump pump hours, the plugged screw downtime and the gate fee on liquor that has to go back through the works. Most sites find the polymer pays back before the second week, but the payback lives in the handling numbers, not in the moisture reading.

Grades for this duty sit in the coarse range because fine powder blows out of an open screw and clogs the boot. If you are specifying a granule for cake conditioning, start from the bulk polyacrylate granules range and ask for a brine absorption figure, not a DI figure.