The Cassette That Wouldn’t Empty: Gel Clumps in a 20-Litre Tank

The complaint came with a photograph: a cassette pulled from a campervan at a motorway disposal point, cap off, tilted, and nothing coming out. Inside, visible through the inspection hatch, was a translucent slug of gel sitting directly over the outlet, roughly the size of a fist. The owner had used the product correctly, he said. Two scoops per tank, every time.

Two scoops was, in fact, the problem. The tank was a 19-litre cassette that he emptied every second day after two people used it, so the actual liquid volume per charge rarely exceeded 12 litres — and in cool weather, in a van parked at 900 metres, the set had four hours to form before the next use.

Why clumps form in small tanks

Three conditions combine. First, powder poured onto a small volume of liquid has no room to disperse; it wets on the surface and forms a skin that slows further hydration, so you get a dense core surrounded by partially swollen particles. Second, the cassette geometry is unforgiving: a flat floor with a shallow sump and a 50 mm outlet, and the gel that forms first sits exactly where the outlet is.

Third, nobody agitates. In a portable toilet, the tank is walked on and the liquid sloshes. In a cassette, the tank sits still under a bench for two days and then gets carried by a handle at an angle. The result is a mass with a firm base and a liquid layer above it, and the outlet is at the bottom.

The dose ceiling nobody reads

Most suppliers publish a dose per litre and a maximum per container. The maximum exists because above a certain concentration the gel becomes stiff enough to resist the shear of a gravity drain, and because the swelling pressure can stress a cassette’s plastic weld. On a 19-litre cassette, a 60 g dose in 12 litres is 5 g per litre — three times the usual rate — and produces a gel with a consistency closer to set custard than to a slurry.

Correct dosing for a cassette is usually 25 to 35 g per charge, added to a litre or two of water first, swirled, and then topped up with the waste. That last step is the one people skip. Pre-dispersing in a litre of water costs fifteen seconds and prevents almost every clump we have seen.

If it is already blocked

Do not poke it with a screwdriver. Cassette walls are thin and the outlet spigot is the weakest point; a puncture there is a permanent leak. The reliable recovery is warm water, not hot, poured in and left for twenty to thirty minutes, followed by a gentle rocking of the tank on its side. Warm water lowers viscosity and lets the gel deform; hot water can warp the tank and, in some formulations, causes the gel to swell further.

If that fails, the cassette can be inverted over a drain and left overnight with the cap off. Gravity will do slowly what the outlet could not do quickly. A crew in Germany reports success with a 10 per cent salt solution, which collapses the gel by screening the charge on the polymer — chemically sensible, though it means the contents must then be disposed of as liquid waste rather than as a solid.

Designing the habit out of the problem

The owner in the photograph switched to a sachet sized for his tank, with a pictogram showing the sequence: add water, add sachet, swirl, use. His clumps stopped. That is not a coincidence — a pre-portioned sachet removes the scoop, removes the arithmetic, and makes the pre-dispersion step part of the action rather than an instruction on a label.

If you are buying for a rental fleet, standardise the sachet to the smallest tank in the fleet and instruct renters to use two in a larger cassette. One instruction, one SKU, no scoop in the van. The SAP powder product range includes grades with a slower, more uniform hydration profile that are better suited to still tanks; the fast grades that suit a serviced portable toilet are exactly the wrong choice for a cassette that will not be shaken.