A site budgeted disposal on input volume: 12,000 m³ a year of leachate, at a gate fee it had negotiated per tonne and converted at a bulk density of 1.0. The first year produced 15,600 m³ of cake and 16,380 tonnes at 1.05 t/m³. The difference between the budget and the invoice was 30 percent, and nobody had made an error. The site had simply assumed that solidifying a cubic metre produces a cubic metre.
Volume growth is the most reliably under-budgeted line in leachate solidification, and it is predictable enough to design for. This article sets out where the extra volume comes from, how to measure bulk density properly, three ways to reduce the penalty, and how to build the growth factor into a gate fee model.
Where the Extra 30 Percent Comes From
Three sources, each with a different fix. The polymer itself contributes solid volume: 20 kg/m³ of dry polymer at a particle density near 1.5 g/cm³ adds about 0.013 m³ per cubic metre, which is negligible. What is not negligible is the water the polymer binds. A gel at 20 kg/m³ holds 45 to 50 times its mass in water, and that swollen network occupies real space, so the cake volume exceeds the liquid volume by 10 to 20 percent before anything else happens.
Entrained air is the second and usually the largest source. A high-shear paddle at 1,400 rpm whips air into a viscous gel, and the trapped fraction runs 8 to 15 percent by volume in a typical batch. That air does not escape before the skip is closed, and it shows up as a cake that looks bulky and feels light. A gentle paddle, a longer quiescent period before discharge, or a de-aerating screen all reduce it.
Bulking is the third, and it is the one that can triple the factor rather than add 30 percent. Any site that blends in a dry amendment should model volume on the blend, not on the cake.
Measuring Bulk Density Properly
A bucket is not a measurement. Bulk density has to come from a container of at least 0.5 m³, filled the way the skip is filled, settled for ten minutes, and weighed on a certified scale. The difference between a 10 litre bucket sample and a 1 m³ box sample on the same gel routinely runs 15 percent, because the bucket compacts under its own small mass and the box does not.
Take three measurements per campaign from different depths in the skip and use the mean. Record them against the batch number, because bulk density changes with dose: a 26 kg/m³ batch is denser than an 18 kg/m³ batch by 4 to 6 percent, and a site that changes dose without changing its volume factor will mis-budget the quarter.
Three Ways to Cut the Penalty
Raise absorption rather than dose. A grade that achieves the same gel strength at 18 kg/m³ instead of 24 produces a lighter, less bulky cake and uses less polymer, and the two savings compound. This is the single highest-leverage change and it starts with a jar test comparing grades at equal gel strength rather than equal mass.
De-air the batch before discharge. Ten minutes at 60 rpm instead of 1,400 rpm before the valve opens lets most entrained air rise, and it costs almost nothing except time. Sites that do it report cake density 6 to 9 percent higher and a visibly tighter load.
Dewater before solidifying where the leachate is dilute. Removing 20 percent of the water by filtration or a geotextile bag before dosing cuts both polymer demand and cake volume, and on a 12,000 m³ site it is often the difference between one and two skips a day.
Modelling the Gate Fee on the Right Number
Build the model as input volume times a growth factor times bulk density times the gate rate. For an unbulked gel at 20 to 24 kg/m³, use a growth factor of 1.25 to 1.30 and a density of 1.03 to 1.08. For a bulked blend at 1:1 with C&D fines, the factor is 3.0 to 3.3 and the density falls to 0.6 to 0.7, which changes the haul trip count as well as the tonnage.
Then check the contract. Gate fees quoted per tonne favour bulking; fees quoted per cubic metre or per load penalise it heavily. A site that bulks to pass a moisture test and then pays per load can find itself worse off than one that dose-tuned and shipped a denser cake.
None of this is hidden if it is measured once. Three bulk density readings, a growth factor in the model and a note in the batch file turn a 30 percent surprise into a planning assumption. The SAP grades we supply for leachate work are specified with a target gel strength at a stated dose, so the absorption-versus-dose trade can be tested before the volume penalty is locked into a year of gate fees.
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.
