The clause sits on page four of a twelve-page haulage agreement: loads must show no free liquid under a paint filter test, and must not exceed 60 percent moisture. Most kitchens sign it without reading that far, then discover in week three that their bins fail. The test is not a hauler’s invention. It comes from a published method used for decades in hazardous and industrial waste, and once you know exactly what it measures, you can dose to it deliberately instead of hoping.
This article explains the method in plain terms, why a gelled bin can be 70 percent water and still pass, which other clauses in the same contract reject loads, and how to run a version of the test in your own yard for the price of a coffee filter.
What the Paint Filter Test Actually Measures
The standard method takes a 100-gram sample, places it in a conical paint filter with a 60-mesh screen, and holds it there for five minutes. If any liquid passes through the mesh, the sample fails. That is the whole test. It does not measure total water, it does not measure how wet the material feels, and it says nothing about how the load behaves when a bin is tipped or compressed. It measures whether liquid can drain out of the mass under gravity in five minutes.
That distinction is the entire reason solidification works as a compliance strategy. A gelled food waste load may be 70 percent water by mass and pass comfortably, because the water is inside a network and cannot drain. A load at 55 percent water can fail if that water is free in the bottom of the bag. Moisture percentage and free liquid are different numbers, and only one of them is being tested.
Moisture Limits Are Written for Sludge
The 60 percent figure in most contracts was borrowed from sludge acceptance criteria, where total moisture correlates reasonably well with free liquid. Food waste breaks that correlation badly. A bin of salad, melon and stock can sit at 80 percent moisture with almost no free liquid once gelled, while a bin of rice and bread at 50 percent moisture can release a litre of condensate. Ask your hauler whether the 60 percent limit is a contractual total-moisture limit or a proxy for the free-liquid test, and get the answer in writing.
If it is a hard total-moisture limit, and you are sending wet waste, the fix is source separation of the wettest fraction rather than more polymer. No dose reduces the water content of a melon.
The Other Four Clauses That Get Loads Rejected
Free liquid is the headline, but in practice loads are turned away for four other reasons. Bin weight above the contract limit, often 60 to 80 kilos for a 120-litre bin, which a gelled load can reach because density rises. Contamination, meaning glass, cutlery, packaging or bag-in-bin arrangements the site does not accept. Frozen loads, which cannot be processed and are treated as a separate rejectable category in cold snaps. And mis-declaration, where the waste code on the transfer note does not match what the driver sees.
All five are visible at the gate in under two minutes. That is why a driver’s photograph is usually enough to win the argument, and why a written dosing record is your only defence.
A Yard Test You Can Run Before You Sign
You do not need a lab. Take 100 grams of your typical gelled waste, place it in a paper coffee filter supported by a funnel, and stand the funnel over a clear measuring jug for five minutes. Look for any liquid in the jug. Run it three times: a Monday load, a Friday load, and a load after a weekend. If any of the three produces visible liquid, your dose is not yet compliant, and the weekend sample is the one that matters because it is the one your hauler sees after 60 hours.
Do this before signing, not after the first rejection. It takes twenty minutes, it costs nothing, and it tells you more about your dose than a datasheet will. When you know your free-liquid margin, you can also negotiate the clause sensibly instead of accepting a limit you cannot meet.
If your loads are failing a paint filter test, the issue is almost always dose per litre, mixing, or late liquid additions rather than the grade itself. Send us your sample results and bin volumes and we will recommend a dose with a margin for weekend holding; the higher-absorbency grades are on the SAP powder product range page.
Da maohua
Zhengzhou Flor New Materials Co., Ltd. specializes in SAP super‑absorbent polymer products. Serving personal hygiene, agriculture & horticulture, pet supplies and other industries, we leverage supply‑chain resources to provide particle‑size selection, performance matching and customized solutions based on clients’ practical requirements.
