A winter storm inflow surge: dosing against the rain gauge

At 03:40 the rain gauge on the north fence passed 30 mm, and by the time the day operator arrived the sump had climbed 700 mm. Nothing in the leachate had changed except its volume, but that was enough to put the yard three hours behind and leave a tanker idling at the gate with the driver asleep in his cab.

Storm inflow is the one event where a leachate pad can be technically correct and still come out badly. The dose per cubic metre goes down, the total polymer goes up, and the batch takes longer at exactly the hour when you have the least patience. Here is how the sites that cope actually run it.

Forty millimetres in six hours is not forty millimetres of leachate

Take a four-hectare site with a capped active cell and a partially restored flank. Forty millimetres of rain is 1,600 m³ of water falling on the property. With a runoff coefficient between 0.25 and 0.6 depending on cover, compaction and how much of the surface is cracked, you are looking at 400 to 950 m³ arriving at the sump, spread over six to eighteen hours depending on slope length and drain capacity.

The lag is the useful part. A gauge reading at midnight tells you what the sump will look like at breakfast. Sites that watch the gauge and nothing else tend to react to the sump level two hours too late, when the batch plan is already wrong. Sites that plot gauge against sump rise for a season learn their own lag, usually to within two hours, and that is enough to start a batch early rather than dig out of a backlog.

The dose per cubic metre falls while the polymer bill rises

Diluted leachate is easier to gel per unit. Conductivity dropping from 18 to 11 mS/cm, ammonia halving, and suspended solids washing through all push the required dose down, typically from 2.4 kg/m³ to somewhere near 1.9. Operators see that number and relax. Then they multiply by volume.

A dry-weather day of 150 m³ at 2.4 kg/m³ is 360 kg of polymer. A storm day of 500 m³ at 1.9 is 950 kg. The mass balance is not subtle, but it arrives at the same time as cold water at 8 °C, which slows wetting and stretches a six-minute batch to twelve or fifteen. Two effects compounding on one shift is what empties a store room.

There is a second trap. Rapid dilution can make a batch look set when the surface has skinned and the core is still pourable. If the cake is going to a landfill for a load test rather than a paint filter check, that skin is the difference between an accepted load and a rejection at the gate.

Staging polymer before the peak, not during it

The forecast gives you two days of warning. Use both. Move pallets under cover and off the floor, check that the bags nearest the door are the oldest lot, and pre-batch slurry into a stirred tank so the peak hours run on liquid rather than on bag-cutting. A pad that can hold four hours of slurry at surge rate rides out most storms without anyone working at speed in the rain.

Split the surge into three passes rather than one giant batch. A 30 m³ batch that goes wrong costs you an hour and a half and a lot of polymer; three 10 m³ batches let you correct the dose after the first one, when you can still see how the floc is forming. Keep a 30 percent reserve untouched until the gauge stops moving. The reserve is not for the storm. It is for the second storm, which on a wet winter often arrives before the ground has drained.

The site that dosed on volume alone

A site in a wet coastal climate ran its storm plan on a simple rule: double the dose volume, keep the dose rate. The arithmetic worked on paper for the first twelve hours. On day two the store ran dry at 16:00, the haulier had already sent four trucks, and two loads sat overnight in open containers because there was no polymer left to finish them. The rejected load alone cost more than a year of holding an extra tonne in stock.

The fix was not more storage. It was a one-page table taped inside the dosing cabin: gauge reading, expected sump rise, polymer needed, batches required, and the hour at which the store would run out if nothing changed. The last column is the one that made operators act early.

If your site has never plotted gauge against sump rise, start with the next rain event and record both every two hours. A single season of that data turns storm dosing from improvisation into a schedule, and the leachate solidification guide sets out the batch and dose checks worth running alongside it.