Bulking Wet Cake With Shredded Refuse: Ratios That Hold a Load Test

A cake at 78 percent moisture will not pass a paint filter no matter how much polymer went into it, because there is more water than the network can bind. The fix on many sites is not more polymer but a second material: shredded construction and demolition fines, dry refuse or compost oversize, blended in at a controlled ratio. Get the ratio right and a failing cake passes in twenty minutes. Get it wrong and you triple the volume you pay to landfill without solving the free-liquid problem.

Bulking is a moisture and structure intervention, not a chemical one, and it has its own arithmetic. This article works through the ratios, the moisture maths behind them, what you are allowed to blend under a landfill permit, and the load test that tells you whether the blend will hold.

The Moisture Arithmetic Behind a Ratio

Take one tonne of cake at 78 percent moisture: 780 kg of water and 220 kg of dry solids. Take one tonne of shredded C&D fines at 18 percent moisture: 180 kg of water and 820 kg of solids. Blend them one to one and you have two tonnes at 960 kg of water, or 48 percent moisture.

The same arithmetic at other ratios gives the working range. At 1:0.6 the blend lands at 55.5 percent. At 1:1.4 it lands at 43 percent. Below 1:0.5 the blend stays above 60 percent and usually still fails, because at that point the bulking agent is being coated rather than blended. Above 1:1.5 you are mostly landfilling the bulking agent, and the gate fee reflects it.

The target is 45 to 52 percent for most acceptance criteria, which puts the practical ratio between 1:0.8 and 1:1.2 for a 78 percent cake. A 70 percent cake needs only 1:0.5, and a 65 percent cake often needs no bulking at all if the free-liquid test passes.

Why Bulking Works, and Why It Inflates Volume

Bulking works by three mechanisms at once. It dilutes the water, it creates a continuous solid matrix that interrupts the pathways free water would take to the filter mesh, and it increases the surface area available for evaporation during handling. The third effect is real but slow, and it should not be counted on for a load that ships the same day.

The volume penalty is severe and often under-estimated. Cake at 78 percent moisture has a bulk density near 1.05 t/m³, so one tonne occupies 0.95 m³. C&D fines at 18 percent moisture bulk at roughly 0.45 t/m³, so one tonne occupies 2.22 m³. The 1:1 blend is two tonnes in 3.17 m³, a bulk density of 0.63 t/m³, and it occupies 3.3 times the volume of the original cake. If the gate fee is charged per tonne, bulking is neutral on mass and expensive on haul trips. If it is charged per cubic metre, bulking can triple the cost.

Choose a dense bulking agent wherever the permit allows it. Crushed glass cullet, screened bottom ash and dried grit all bulk at 0.8 to 1.1 t/m³ and deliver the same moisture reduction in half the volume.

What You Can Legally Blend, and What You Cannot

The permit decides this, not the chemistry. Most non-hazardous landfill permits allow blending with a material that is itself acceptable at that site and that does not change the waste classification. C&D fines are commonly accepted; putrescible material, anything containing free liquids, and anything with a different waste code usually are not.

Two rules keep a site out of trouble. Blend ratios and bulking agent sources go into the site’s written procedures before the first load, not after a rejection. And every blend gets recorded with the source, the batch, the ratio and the resulting moisture, because a regulator who finds bulking agent in a load with no record will treat it as unauthorised dilution.

The Load Test That Decides Whether It Holds

Bench tests on a blended sample are necessary but not sufficient, because blending in a bucket is more thorough than blending in a skip. Run the paint filter on the bench blend, then repeat it on a sample taken from the skip after the loader has mixed it, and expect the skip sample to read one grade worse.

A simple field check sits between the two: fill a 20 litre bucket, invert it on a clean concrete slab for ten minutes, and look for a wet ring wider than 100 mm. That correlates well with a fifteen-minute filter result and takes almost no time. If the ring is wider, add 0.2 to the ratio and re-test rather than adding polymer, which does nothing for a cake that is already over-saturated.

Bulking is the right tool when the problem is water volume and the wrong tool when the problem is gel continuity. If the cake tears rather than flows, the dose is short and blending will only disguise it. The SAP grades we supply for leachate work are specified with a target moisture at the dose you plan to run, so the decision to bulk or to dose can be made on numbers rather than on how the skip looks at four in the afternoon.