Dosing by Flush Volume: Recalculating Gel for 4.5-Litre Cisterns

A facilities manager in a 1970s office block replaced 9-litre cisterns with 4.5-litre dual-flush valves and expected his gel consumption to fall by half. It rose by 30 per cent. He had been dosing per use, and the number of uses had not changed — but the composition of the liquid in the trap had, and his dose table had quietly become wrong in a way that took three months to notice.

This is the most common arithmetic error in building-scale urine gelling. The dose is per litre of liquid, and a flush toilet adds litres that the dose table never sees.

What one visit actually puts in the bowl

A single void is 250 to 350 ml of urine. A full flush adds 4.5 to 6 litres. On a conventional toilet, one visit therefore produces around 5 litres of liquid at roughly 6 per cent urine. Five visits, three of them flushed, produce something close to 15 litres at 10 to 12 per cent urine, plus paper, plus whatever cleaning chemical the contractor used at 18:00.

Compare that with a waterless urinal or a dry toilet, where five visits produce about 1.5 litres at 100 per cent urine. The polymer sees a completely different job in each case: a small volume of concentrated electrolyte, or a large volume of dilute electrolyte with a much higher total salt load. A dose that works in the second case may be wildly excessive in the first, and a dose that works in the first will leave a puddle in the second.

Why the dilute case needs more, not less

The counterintuitive part is that dilution does not make the job easier. Absorption capacity in urine is limited by the ionic strength of the liquid: the charged groups on the polymer chain are shielded by sodium, chloride, urea and creatinine, so the network cannot expand as far as it would in pure water. Diluting urine with flush water lowers ionic strength, which helps per gram, but it multiplies the total volume by fifteen, which hurts far more.

Net effect, measured on three SAP grades in a 10 per cent urine solution: capacity fell to roughly 55 to 65 per cent of the pure-urine figure, while the volume to be handled rose by an order of magnitude. The correct conclusion is that flush toilets need a dose set by total liquid, with a modest uplift for the dilution penalty, typically 1.4 to 1.8 g per litre of total liquid depending on grade.

A working table for common cisterns

The figures below assume a mid-range crosslinked polyacrylate at 1.6 g per litre and a 300 ml void. They are a starting point for a trial, not a specification.

Two things in that table surprise people. The waterless urinal, which looks like the low-consumption option, uses almost no powder per visit but needs a grade that tolerates near-saturated urine and the seal liquid already in the trap. And the dual-flush toilet’s average consumption is not the midpoint between 3 and 6 litres; in practice, in an office with mostly male staff, it lands near 2.2 litres because the half flush is used most of the time.

The measurement that settles it

Put a 20-litre graduated bucket under the trap of one representative fixture for a working day, or fit an inline flow meter for a week. Multiply by the fixture count and you have a real daily liquid volume. Divide by the dose you intend to use and you have a consumption figure you can defend in a budget meeting.

One site did this and found that 40 per cent of its liquid came from a single cleaner’s mop bucket, tipped down the toilet every evening at 19:00. No dose table would have predicted it. If you are rebuilding a dose schedule from scratch, the SAP powder product range lists capacity in 0.9 per cent saline as well as in deionised water, and the saline figure is the one closer to what a flushed toilet actually contains.