Dry Feed or Slurry Feed: Choosing Where the Polymer Meets the Leachate

Two sites run the same polymer at the same dose and get different results. The first tips 25 kg bags straight into a 12 m³ batch through a 200 mm eductor throat and screens 6 percent out as fish-eyes at discharge. The second pre-slurries at 1.5 percent in a 2 m³ cone-bottom tank, pumps it in, screens 0.4 percent, and spends forty extra minutes a day on labour. Neither is wrong. They answer different questions, and the answer turns on batch size, crew, and the cost of a rejected load.

Where the polymer meets the leachate decides how much of it actually works. This article compares dry feed and slurry feed on dispersion quality, labour, equipment and failure modes, then covers the injection point question that sits behind both.

What Dry Feed Buys and What It Charges for It

Dry feed is fast and simple. A bag dumped into an eductor throat or drawn into the vortex of a high-shear paddle disperses in 20 to 40 seconds, needs no mixing tank, and leaves nothing to clean at the end of the shift. On batches under 6 m³ with a skilled operator, it produces acceptable gel at the lowest capital cost of any method.

It charges for that in three currencies. Dust at the bag dump is the visible one, and it is a respiratory and housekeeping issue rather than a product problem. Fish-eyes are the expensive one: powder that contacts water on the surface of a clump gels immediately and seals the interior, so 5 to 8 percent of the dose can end up doing nothing. And dry feed is operator-dependent in a way that is hard to audit, because the difference between a good and a bad batch is often where the bag was emptied rather than how much went in.

If the fish-eye rate is above 3 percent on screening, the feed method is costing more than the polymer saved.

Slurry Feed and the 1.5 Percent Rule

A slurry tank makes dispersion a controlled step. Polymer is added to water under gentle agitation to reach 1.0 to 2.0 percent solids, held 15 to 20 minutes to wet out fully, and then pumped into the batch. At 1.5 percent the slurry is pourable, pumpable through a progressive cavity or diaphragm pump, and stable for 45 to 90 minutes before it begins to thicken beyond useful viscosity.

Beyond 2.5 percent the slurry becomes a paste that will not pump without dilution water at the suction, and beyond 3 percent it sets in the line. Below 1 percent you are moving water rather than polymer and the batch volume calculation starts to matter. Hold the tank under slow agitation at 30 to 60 rpm; a fast paddle entrains air and shears the slurry into a froth that pumps badly and doses unevenly.

The trade is 30 to 45 minutes of labour per shift, a second tank, and a cleaning routine. In exchange the dose becomes accurate to within 2 percent, because you are metering a volume rather than counting bags, and the fish-eye rate drops to a level where screening becomes unnecessary.

Injection Point: Before or After the Vortex

Whichever feed you choose, the injection point decides whether the polymer meets moving water or still water. Injecting into the vortex core of a running paddle gives instant dilution and the best dispersion; injecting at the tank wall or over the top gives a clump that sinks and sits on the floor of the tank, where it hydrates slowly and reappears as a soft layer in the skip.

For slurry, inject into the suction side of a recirculation loop rather than directly into the batch, so the loop provides the mixing energy and the batch tank only has to hold. For dry feed, drop the bag into the eductor with the water flow already established at 1.5 to 2 bar, and never start the water after the powder, which is the single most common cause of a blocked eductor.

Matching the Method to the Crew and the Batch

Three decision rules cover most sites. Batches above 8 m³, crews with turnover above 30 percent a year, and gate fees above $25/t all point to slurry feed, because the accuracy and the audit trail are worth the labour. Batches under 6 m³, a stable crew, and a site that runs campaigns rather than continuous operation point to dry feed with a good eductor and a screening step. Sites in between should run slurry for the main plant and keep a dry feed kit for emergency spills, where speed matters more than dispersion quality.

Whichever you run, measure the fish-eye rate for a month. It is the one number that tells you whether the feed method is working, and it is almost never on a site’s dashboard. The SAP grades we supply for leachate work are available in mesh ranges selected for eductor dispersion as well as for slurry preparation, because a grade that wets out too fast is as much of a problem in dry feed as one that wets out too slowly.