Making Slurry With Leachate Instead of Mains Water

The hose from the yard tap froze in January, so the operator filled the mixing drum from the nearest leachate line. The batch set in four minutes instead of nine. By the end of the week the crew had decided the new powder was “much faster” and had written a new dose into the log. Two months later, when the tap was repaired, the same dose produced a batch that was still liquid after half an hour.

What changed was the make-up water. Pre-hydrating powder in leachate rather than clean water alters both the rate and the extent of swelling, and the effect is large enough to rewrite a dose setpoint without anyone noticing.

Two mechanisms, opposite directions

First, ionic strength. Leachate at 15 mS/cm suppresses the osmotic pressure that drives water into the polymer network, so the final swelling ratio drops. A gel made up in leachate may reach only 60 to 70 percent of the volume it would reach in tap water. That argues for a higher dose.

Second, seeding and wetting. Leachate contains dissolved organics and fine suspended material that act as wetting aids, and the pre-hydrated slurry wets out faster and more evenly than dry powder does. A slurry made in leachate is often smoother and less lumpy than one made in hard tap water, and it disperses into the batch more readily. That argues for a lower dose, or at least for a faster batch time.

The two effects do not cancel neatly. In most leachate, the rate gain dominates early and the capacity loss dominates at the end, so the batch looks better for the first ten minutes and performs worse after an hour. That time signature is what fools operators.

When slurry in leachate is the right choice

If the site is dosing into a tanker or a drum where mixing energy is limited, a pre-hydrated slurry disperses far better than dry powder, and making that slurry in leachate avoids importing clean water to a site that has none. Remote landfills, closed sites with no potable supply, and winter operations where the yard tap is unreliable all fall into this group.

If the receiving batch is already at high ionic strength, the capacity penalty is small in relative terms, because the gel was going to be suppressed anyway. The marginal loss of pre-hydrating in leachate rather than water is maybe five percent of a number that has already fallen by forty.

Where it goes wrong is on weak leachate, or on a batch that needs maximum absorption per tonne. There the penalty is real, the dose creeps up, and the extra polymer is spent compensating for a decision made about plumbing rather than chemistry.

The concentration question nobody asks

Slurry concentration matters as much as the liquid used. A 2 percent slurry is pourable and disperses quickly. A 5 percent slurry is a thick paste that will not flow through a 25 mm line and will set in the hose if the pump stops. Most sites settle on 1.5 to 3 percent, mixed with enough shear to break lumps and then used within the shift.

Pre-hydrated slurry also has a shelf life measured in hours, not days. Once the particles are fully swollen, they do not re-disperse; a drum left overnight becomes a jelly block. That is fine if the batch is immediate, and a nuisance if the plan was to mix in the morning and dose all day.

A field comparison worth running once

Take two identical 20 litre buckets, each with 15 litres of the same leachate. In one, add dry powder at the site dose and stir at 300 rpm. In the other, add the same mass pre-slurried in one litre of the same leachate and stir identically. Record time to visible thickening, time to set, and free liquid after thirty minutes on a paint filter.

The result is usually a clear split: the slurry route sets faster and more evenly, and the dry route ends up with slightly better free-liquid performance. Which one matters depends on whether the constraint is time or acceptance. Sites with a two-hour turnaround per load should take the speed. Sites paying per tonne at a facility with a strict free-liquid limit should take the capacity.

Either way, write the make-up liquid on the batch sheet. “Water” is not a specification, and the difference between mains water at 300 microsiemens and leachate at 18,000 is the difference between two dose setpoints. If the site has to use leachate, note it in the resin selection too, since some grades hold capacity better in high-conductivity make-up than others.