Sawdust at 14 Cents a Service, Gel at 31: Where the Gap Closes

Put the two numbers side by side and the argument seems over. A cubic metre of sawdust from a local mill costs a site about €25 and covers roughly 180 services at 5.5 litres of sawdust each, which works out near 14 cents. A gel dose at 30 g per service, on powder at €9/kg, is 27 cents before you count the sachet. The gel looks twice as expensive, and in a composting operation with a free sawdust supply it genuinely is.

But 14 cents is a material cost, and the comparison that decides a contract is cost per service including everything the material touches. Once you put that frame around it, the gap closes fast — and sometimes reverses.

The number people compare and the number that matters

Sawdust does not absorb much. Its liquid holding capacity is roughly 0.3 to 0.5 grams per gram, so 5.5 litres of sawdust holds about 2 to 2.5 litres of urine and mostly by capillary action, not by any chemical change. A 30 g gel dose handles 5 to 6 litres and locks it into a solid. To match one gel service, you need two to three times the sawdust volume, and that volume has to be bought, stored dry, carried to the unit and then carried out again as wet, heavy material.

That last point is where the ledger changes. Wet sawdust at 60 per cent moisture weighs around 700 g per litre of capacity used. A gelled 5-litre service adds 30 g plus the urine’s own mass, and the urine mass is the same either way. The difference is the 4 to 5 kg of sawdust that arrives dry and leaves wet, and every kilo of it is charged at the disposal rate.

Running the full ledgers

Take a 40-unit site, 300 services per unit per week, over 30 weeks.

The haulage line is the one that gets left out of tender comparisons, and it is larger than the material difference by a factor of three. It also has a second-order cost: a wet sawdust bin is heavy, and heavy bins get lifted less often or lifted badly, which is how injuries and damaged bins happen.

When sawdust genuinely wins

Two situations. If the site composts on location and the sawdust is a by-product of an operation already happening there, the haulage line disappears and the carbon story is genuinely better. And if the toilet is a dry composting unit designed around bulking agent, adding gel changes the compost chemistry and can create anaerobic pockets — a real failure mode that has ruined more than one compost batch.

The failure case worth telling is a festival that switched from sawdust to gel in a composting toilet block without changing the unit design. The gel sealed the surface of the heap, the pile went anaerobic, and the compost tested positive for listeria-type indicators at the end of the season. The gel was not faulty; the unit was designed for a bulking agent and got a solidifier.

How to present it to a client

Do not argue about cents per service. Present three columns — material, mass hauled, storage — and let the client’s own disposal rate fill in the third. Most will find that the crossover sits at a haulage cost of about €30 per tonne; below that, sawdust is competitive; above it, gel wins by a wide margin.

Ask for the absorption figure in saline, not in water, because the sawdust comparison is usually run on urine that is more concentrated than a lab sample. The SAP powder product range gives capacity under load as well as free swell, and under load is the number that predicts whether a bin will still be liftable after a weekend.