When the Batch Foams: Surface-Active Leachate and the 90-Second Test

“It climbed out of the tank like a cake in an oven.” The batch was 8 cubic metres in a 10 cubic metre tank, dosed at the usual rate, and within four minutes the foam head had reached the rim. Half the polymer was in the foam, the batch below was under-dosed, and the shed floor took an hour to clean.

Foaming is not a polymer problem, though the polymer makes it visible. It is what happens when a liquid carrying surface-active material gets hit with high-shear mixing and a large quantity of fine solids.

Where the surfactants come from

Landfill leachate can carry surfactants from household products, and the concentration rises when the site takes commercial waste, restaurant waste or leachate from a composting operation. Volatile fatty acids at high concentration also act as mild surfactants, which is one reason young leachate foams more than old.

There is a second source that surprises people: the powder itself. Fine particles at the surface stabilise foam mechanically, in the same way that dust stabilises foam in a river. A grade with a high fines fraction will hold a foam head longer than a coarse grade at the same dose.

So the foam is a three-part system: surfactant to lower surface tension, fine solids to stabilise the film, and energy from mixing to generate the bubbles. Remove or reduce any one and the foam collapses.

The 90-second shake test

Before dosing, half-fill a 500 mL bottle with the batch liquid, cap it, and shake hard for fifteen seconds. Set it down and time how long the foam takes to fall below 5 mm. Under 90 seconds, foaming is unlikely to be a problem. Over five minutes, expect trouble and plan for it.

The test costs a bottle and a minute, and it separates the streams that need a defoamer from the ones that do not. Run it whenever the inflow changes character: a new waste source, a tanker from a different site, a rain event after a long dry spell.

Add one variant: repeat the shake test after adding the intended dose of powder. If foam volume doubles, the powder is contributing to the stabilisation and a coarser grade will help.

Operating changes that beat defoamer

Reduce mixing energy at the start. Most of the foam is generated in the first two minutes when the mixer is at full speed and the batch is still thin. Starting at half speed and ramping up over five minutes cuts foam generation substantially without lengthening the batch much.

Leave headspace. A batch filled to 90 percent of tank volume has nowhere for foam to go. Cap the fill at 70 percent when the liquid is known to foam, and accept a smaller batch rather than a cleanup.

Dose below the surface. Powder or slurry added at the vortex, rather than dropped on top, is wetted without being thrown into the foam layer. A dip tube costs almost nothing and prevents the situation where half the dose ends up in the froth.

If a defoamer is needed, a silicone antifoam at 20 to 50 mg/L usually collapses the head within a minute. The caution is that silicone antifoam can impair wetting of the powder if added before the powder, so add it after the batch is wetted, not before. Sites that add antifoam first and then wonder why the batch is slow have the order backwards.

The batch that foamed and still passed

Worth recording, because it misleads people. A foaming batch often looks like it is working harder than a quiet one: the volume expands, the surface moves, and the mixer torque rises. In one case the operator reduced the dose because the batch “seemed to be reacting strongly”, and the load failed. The foam was air, not gel.

The reliable check is the settled volume after thirty minutes, not the appearance during mixing. If the batch settles to a smaller volume than usual with a clear liquid layer, it was under-dosed regardless of how energetic it looked.

Before the next wet season, add the shake test to the incoming inspection for tanker loads, cap the batch fill at 70 percent for known foaming streams, and fit a dip tube. Where foaming is chronic, a coarser grade reduces both the dust and the foam, and the particle distribution is the specification to look at first.