The Month We Cut the Dose by 20 Percent and the Bill Fell 9

Thirty bins a day, six days a week, 500 g each. That is 78 kg of polymer a month at the site in question, roughly 310 in material, on top of a haulage bill that had been climbing for a year. Nobody had questioned the 500 g — it was on the card, and the card had been right when the bin held 90 kg.

Two things had changed since the card was written. The kitchen had moved to smaller 120-litre bins, and the menu had shifted towards grilled dishes with less sauce. The typical load was now 48 kg, and 500 g was nearly double what it needed.

What the dose was doing to the bill

Polymer at 500 g holds roughly 150 kg of water at full absorption — far more than the bin contained. In practice, the excess powder stayed unswollen or formed a hard mass with a dry core, and the load went to the weighbridge heavier than it needed to be because every litre of liquid was now bound inside it rather than draining away.

At 0.19 per kilo on a per-kilo contract, 20 kg of extra retained water per bin is 3.80 per bin, 22.80 a day, close to 600 a month. That is the hidden cost of over-dosing on a weight-based contract: you pay for the polymer, and then you pay again to haul the water it holds.

The four-week trial

• Week one: dose at 400 g, weigh every bin at close, log rejects and drips.
• Week two: dose at 380 g on the three heaviest bins, 400 g elsewhere.
• Week three: hold at 380 g and add a second half-dose at close on the two wettest bins.
• Week four: return to 400 g across the board and compare.

Records kept: bin weights, gate flags, drip complaints, and staff comments. Nothing more elaborate than a clipboard by the door.

The result, including the part that surprised us

Polymer use fell 24 percent. Haulage fell 6 percent, because average bin weight dropped by about 9 kg. Total monthly saving came to roughly 9 percent of the combined waste cost — less than the material saving alone would suggest, because the polymer was never the biggest line.

The surprise was the complaint count. Drips did not increase at 400 g; they fell. The heavier dose had been creating a hard crust with wet material underneath, and the crust was breaking at the tip. A slightly lower dose produced a more uniform gel that travelled better.

The one thing that did get worse was set time: an extra five to ten minutes at 400 g in cool weather. The site covered it by dosing fifteen minutes earlier, which cost nothing.

The part that did not work

One thing the trial got wrong: the site cut the dose on the banqueting bin at the same time, because it looked identical on the card. It was not. Banqueting waste is wetter and heavier, and the cut produced two gate flags in a week before the dose went back up.

The lesson is to run the trial per bin type rather than per site. A dose that is 20 percent too high on a salad bin can be 10 percent too low on a stew bin, and only weights will tell you which is which.

How to run the same trial without a consultant

Pick one bin type, not the whole site. Weigh it daily for a week at the current dose, then drop 15 percent for a week, then 25 percent. Watch three numbers: gate flags, visible free liquid at the tilt test, and weight per lift.

Stop when the flags start. Then go back up one step. The optimum is almost always one step above the point where things go wrong, not two, and the margin costs a fraction of what the over-dosing was costing. Most sites that run this find they can cut 15 to 20 percent without any change in outcome, and they keep the cut because they have the weights to prove it was safe. The dose optimisation guide has a blank version of the same log.