Hold Tank or 200-Litre Drum: Where the Batch Should Actually Happen

Two crews, same landfill, same polymer, same liquid. One runs 6 m3 batches in a steel hold tank with a paddle stirrer and fills drums from the bottom valve. The other drops powder straight into each 200-litre drum, pushes a lance in through the bung and recirculates. Both get acceptable loads out of the gate. One of them spends forty minutes a batch cleaning, and it is not always the crew you would guess.

Two Vessels, Two Different Mixing Problems

A hold tank is a bulk-mixing problem. Typical working volume is 5-20 m3, the stirrer runs 1.5-3 kW at 60-120 rpm, and the whole batch must be homogeneous enough that the last drum filled is the same material as the first. That is harder than it sounds, because gel climbs the shaft and sets on the baffles. If your tank has no baffles, the batch rotates as a plug and the powder you added at the wall is still powder at the wall four minutes later.

The drum’s dead corner

A drum is a small-volume, high-viscosity problem. The mixing energy comes from the recirculation stream, typically 1-1.5 m3/h through a 20-25 mm lance, and the return must be aimed at the point furthest from the suction. Set the lance down the centre and you create a cone of moving gel with two stationary corners that never see polymer. Crews who fight lumpy drums almost always find the lance in the middle. Angle it 150 mm off the wall and the same pump does the job in six minutes instead of twelve.

Batch Size, Viscosity and the 8 m3 Line

Between 2 and 8 m3 of liquid per event, either method works and the choice is about labour, not chemistry. Above roughly 8 m3, the tank wins on labour alone: one fill, one dose, one mix, then decant into whatever containers the haulier wants. Below 2 m3, the drum wins because the cleaning overhead of a tank exceeds the mixing time it saves. That threshold moves with viscosity. A batch dosed to the point of stackability is thick enough that a drum cannot circulate it properly — if the gel will not turn over in the drum within eight minutes, you are not mixing, you are coating.

Watch the temperature too. A drum left in the sun reaches 40 °C and the gel sets faster than the same recipe in a shaded tank at 18 °C, which changes both the working time and the water you can lose to evaporation.

Cleaning is the hidden line item either way. A 10 m3 tank needs 40-80 L of wash water per batch plus a scrape-down every fifth batch, and that wash water is leachate again — it goes back to the sump and you pay to treat it twice. Drums need a rinse of perhaps 5 L each, but there are five of them, and a drum that is not rinsed before the next fill carries set gel into the new batch and seeds lumps.

Neither arrangement is free of failure. Tanks fail at the baffle and the bottom valve, where gel accumulates and eventually blocks a 100 mm outlet overnight. Drums fail at the bung thread, which strips after a few dozen cycles and then leaks on the truck. Keep two spare drums per ten in service and a spare valve body on the shelf.

If you are specifying a vessel and want the polymer side of it to be the fixed variable, the bulk grades we supply for tank or drum dosing are on our leachate solidification polymer page, including the coarser cuts that wet out faster in low-shear drums.