Vault Toilets at a Three-Day Festival: Gel Dose per Thousand Uses

Four hundred vault units, three days, roughly 30,000 uses, and a pump-out contract priced per litre. The organiser had been adding 2 kg of powder per unit at the start of the event and still paying for full tanker loads on the Monday, because the gel had formed a layer across the top of the pit and the suction hose could not get under it.

Portable sanitation is a volume business with a chemistry problem inside it. The dose is easy to calculate and easy to get wrong in the same direction every time.

The dose math, done once

A use contributes about 250 ml of urine plus a little flush water, call it 300 ml. A thousand uses is therefore about 300 litres. At a working capacity of 200 g/g in urine, which is realistic for a sodium polyacrylate in a fluid with that ionic strength, 1.5 kg of powder takes up the whole 300 litres. Add a margin for the solids and the paper, and 1.2 to 1.8 kg per thousand uses is the band.

For the festival above, 30,000 uses across 400 units is 75 uses per unit, or about 22 litres each. That needs roughly 110 to 160 g per unit, not 2 kg. The organiser was overdosing by a factor of fifteen, which is why the pit had a gel layer and why the tanker could not empty it.

Dose also has to account for what else is in the pit. Paper, wipes and sanitiser all consume capacity, and a unit serving a food area collects more solid waste than one at a stage. If the event has a bar, expect 20 to 30 percent more liquid per use and dose accordingly. The one number that matters is not the unit count but the pump-out volume you were billed for last time; start from that and work back.

Odour is not a water problem

Urea hydrolyses to ammonia within a day or two, and the reaction is driven by urease-producing bacteria, not by free water. A polymer that locks up every drop of liquid will still smell of ammonia, because the ammonia is in the headspace and in the gel. Adding more powder does nothing for it and makes the pump-out harder.

The control levers are pH and biology, not absorbency. An acidic buffer holds the ammonia in the liquid as ammonium, which is far less volatile, and a biocide slows the hydrolysis. Some commercial products combine a polymer with an acidifier for exactly this reason, and the ones that only sell polymer are solving half the problem.

How the gel blocks a tanker

Dry powder sprinkled into a urinal floats, skins over, and forms a raft. Watered-in powder at the correct dose forms a soft mass that the hose can break up. The difference is in the application method, and it is why the same product can work at one site and fail at another.

Pre-hydrate in a bucket, or dose into the pit with a stirrer while adding water. Never broadcast dry powder onto a liquid surface and walk away. On the pump-out side, a macerator on the hose handles a soft gel and stalls on a raft, so the equipment choice reinforces the same discipline.

For a festival specification, write the dose per thousand uses, the hydration method, and a buffer requirement, then check one unit at the end of day one before committing to the rest. The polyacrylate powder grades page lists the absorption values used for this calculation, measured in saline rather than in water. A number taken from a distilled-water test will under-dose you by half and look fine on paper.