Rain at 14:00: When an Event’s Toilet Tanks Fill Twice as Fast

The forecast said showers after four. They arrived at 14:00, hard, and stayed for fifty minutes. By 15:10 the sanitation supervisor at a 9,000-capacity agricultural show was looking at eight units with water above the seat line and a gel layer that had gone milky and loose. Nobody had used the toilets twice as often. The tanks had simply taken on a second liquid nobody had budgeted for.

Wet weather is the most predictable unpredictable thing in event sanitation, and it breaks gel programmes in a specific way: it adds volume without adding urine. A dose calculated per user is suddenly covering a tank that holds 30 to 50 per cent more liquid than the plan assumed.

Where the water actually comes from

Four routes matter. Surface water runs across saturated ground and enters through the door threshold of a unit standing in a dip. Wet clothing and waterproofs drip into the bowl. People sheltering from rain use the unit for ten minutes instead of two, and children play with the hand-wash tap. And the service crew, seeing a dirty unit, gives it a longer rinse than the schedule allows, then recharges without reducing the water volume.

The fourth route is the one that gets missed in the debrief, because it is self-inflicted. At the show above, two of the eight problem units had been rinsed for over a minute each and recharged to the 70-litre mark instead of 40. The gel in those two units was dosed at the correct grams for a 500-litre tank, but it was sitting in 620 litres of liquid at perhaps 20 per cent urine. Absorption capacity collapses when the liquid is mostly water with a little urine in it, not because the polymer is bad but because the ionic load per gram of polymer is different from what the dose table assumed.

Dosing for total liquid, not headcount

Every gel specification we have seen quoted per thousand uses is really a specification per litre, with an assumed urine volume per use. That assumption is usually 250 to 350 ml. When the tank gains 150 litres of rainwater, the effective urine concentration drops and the total volume rises, and both changes push in the same direction: you need more polymer per litre, not less.

A workable rain rule for a 900-litre unit: measure the free liquid depth at the inspection hatch before the visit, add the planned recharge volume, and dose at 1.8 to 2.0 g per litre of that total. It is a crude number, but it is far better than a per-use table that assumes dry ground. Crews who carry a dipstick marked in litres get to that number in under a minute.

The 90-minute recheck that saved a second day

At the show, the supervisor made one good call. Instead of pumping out the eight wet units, the crew added a supplementary dose of 400 g to each at 15:30, closed the lids, and came back at 17:00. Seven of the eight had firmed up enough to last until the evening collection. The eighth, which had taken the most surface water and also contained a full bag of paper towels, was pumped out and recharged.

That is the pattern worth copying: a mid-afternoon recheck on any day with more than 5 mm of rain, with a small top-up dose rather than a full pump-out. The cost is a few hundred grams of powder and twenty minutes of crew time. The alternative, pumping out eight units during peak attendance, means a tanker moving through a crowd and a queue that reaches the beer tent.

What to write into the event plan

Rain contingencies in sanitation plans usually cover matting, signage and extra hand-wash units. Add three numbers: the forecast threshold that triggers a recheck, the top-up dose in grams per unit, and the maximum free-liquid depth that is acceptable at the end of the day. Put them on the same page as the collection times, because the person making the call at 15:00 is not the person who wrote the plan.

For units that will stand on grass through a wet weekend, a slightly coarser grade with a higher absorption under load is usually the better choice than the finest powder on the price list; the SAP powder product range lists absorption under pressure alongside free swell, and the first figure is the one that matters when a tank is being walked on. Ask for both numbers in the quote, not just the headline ratio.