Dosing at 5 °C: Why a Leachate Batch Takes 18 Minutes Instead of 6

A landfill in northern England ran its first batch of the winter on 4 February: 12 m³ of leachate at 4.8 °C, COD 21,000 mg/L, ammonia 1,150 mg/L, dosed at 22 kg of polymer per cubic metre with the same recipe that had set firm in six minutes the previous August. At eighteen minutes the paddle was still turning in a thin soup. The operator added another 4 kg/m³, the batch finally passed the paint filter, and the run cost 18 percent more than the summer average for identical volume.

Cold weather rarely fails a batch the way operators expect. It does not simply slow a reaction; it changes the order in which things go wrong, so the correction that works in July makes February worse. What follows is what happens to leachate and polymer between 0 and 8 °C, how to split wet-out from set when timing a batch, and which recipe changes recover gel strength without buying a premium grade.

Viscosity Moves First, and Further Than Most Operators Expect

Water at 20 °C has a dynamic viscosity near 1.00 mPa·s. At 5 °C it is about 1.31, at 0 °C about 1.52. A 30 to 50 percent rise sounds modest until it is applied to the only step that matters in the first minute: separating individual polymer particles before they start to swell. A particle that stays dry inside while its neighbours form a skin becomes a fish-eye, and fish-eyes do not recover at any dose. They sit in the batch as pale lumps that pass through the pump and reappear in the skip as soft spots.

The second effect is hidden in the mixer curve. For a fixed impeller speed, dropping temperature raises viscosity, which lowers the Reynolds number and therefore the delivered mixing energy at exactly the moment more is needed. A paddle that produces a clean 120 mm vortex in August may show 40 mm in February at the same 1,400 rpm. Operators read that shallow vortex as a full tank rather than a stalled one. Check it before the first bag is opened, not after.

The Eighteen-Minute Rule: Two Clocks, Not One

Wet-out and set are separate events with separate timings, and treating them as one number is the root of most winter under-dosing. A 0.5 to 1.0 mm grade under a 1,400 rpm paddle in a 12 m³ batch wets out in roughly 20 to 25 seconds per kilogram at 20 °C. At 5 °C budget 45 to 50 seconds per kilogram. Set, the point where the batch stops flowing as a liquid, runs 4 to 6 minutes warm and 12 to 18 minutes cold.

A practical protocol uses three checkpoints rather than one. At three minutes, confirm no dry pockets against the tank wall and no powder raft on the surface. At eight minutes, draw a rod and watch the drain film: a cold batch that is progressing leaves a film that breaks into threads, while one that is not leaves a smooth continuous sheet. At fifteen minutes, run the tilt test below. If the batch has not reached thread-breaking by fifteen minutes at 5 °C, the cause is almost always dose or particle size, not patience.

Recipe Changes That Recover Strength Without a New Grade

Four adjustments cover most cold-weather shortfalls, and they can be combined. First, drop one mesh band: a 0.2 to 0.5 mm grade exposes roughly three times the surface area per gram of a 0.5 to 1.0 mm grade and wets out in half the time, at the cost of more dust at the bag dump. Second, warm the dilution water to 25 to 30 °C and never above 35 °C, because the surface of a particle that hydrates too fast gels before the core is wet, which produces the same fish-eye by a different route. Third, raise dose by 6 to 10 percent, confirmed against a jar test run at the sump temperature rather than in a warm office. Fourth, cut batch size from 12 m³ to 8 m³ so the impeller keeps its vortex and the batch turns over three to four times a minute.

Insulation belongs on the list too. A 15 m length of uninsulated 50 mm hose lying on frozen ground will take the batch from 8 °C to 3 °C before it reaches the skip, and the gel that looked acceptable at the tank outlet will bleed free liquid at the gate. Lag the line, drain dead legs after each batch, and never let a part-full tank stand overnight without agitation at 60 rpm.

Three Signs a Cold Batch Has Not Finished Setting

Surface sheen is the first: free liquid collecting at the rim within 60 seconds of stopping the paddle. Tearing rather than stretching is the second, tested by drawing a 12 mm rod through the mass, where a finished gel stretches 100 mm before breaking and an unfinished one snaps at 20 to 30 mm. Temperature is useless as a progress check because the hydration reaction is barely exothermic at this scale, so use a tilt test instead: fill a 500 mL sample jar, cap it, lay it at 45 degrees, and watch for 30 seconds. A surface that deforms under its own weight is a batch that will fail the paint filter eight hours later.

Winter dosing is a timing problem before it is a chemistry problem, and the fix is usually a stopwatch, a smaller batch and a thermometer on the dilution line rather than a more expensive polymer. If you are writing a cold-weather protocol for this season, the SAP grades we supply for leachate work are specified by mesh, absorption in 0.9 percent saline and set time at 5 °C, which are the three numbers a February batch actually depends on.