Upstream membrane plant: how permeate changes the solidification job

Permeate is the clean-looking stream. It comes off the RO skid at maybe 8 percent of the feed volume, clear, low in suspended solids, and easy to mistake for the easy job. On most sites it is the hardest liquid on the pad to turn into a stable cake.

The reason is simple once you see the analysis. Reverse osmosis removes water and leaves the salts behind in the concentrate, but it also passes a fraction of the dissolved gases and small organics into the permeate. What comes out is a liquid with very little to floc around, very high chloride relative to its solids, and almost no buffering capacity.

What the membrane takes out and what it leaves in

A well-run RO stage might reject 95 to 99 percent of the COD and most of the divalent ions, but chloride passage is typically 85 to 95 percent, and ammonia passage depends heavily on pH. At pH 6.5, ammonia is largely rejected; at pH 5, it passes. So permeate quality moves with the feed pH, which moves with the season and with whatever the upstream plant is doing.

That is why two permeate samples a month apart can need doses that differ by 30 percent. The visual appearance does not change. The conductivity may barely change. But the ammonia and the small organic acids that pass the membrane are exactly the fractions that interfere with gel formation.

Why permeate gels badly

Polymer needs something to hold onto. In raw leachate, the suspended solids, colloids and divalent ions give the chains something to bridge. Permeate has had most of that removed. The result is a batch where the polymer swells, the liquid stays cloudy, and the mass never develops the structure that lets it pass a paint filter test with confidence.

There is also a chloride effect. At 20,000 mg/L and above, the ionic strength is high enough to collapse the polymer’s expanded network. The gel forms, but it holds less water per kilogram than the same product does in a weaker solution. The dose curve flattens: adding 20 percent more polymer buys you 5 percent more performance.

Blending, and when not to gel at all

Blending permeate with a small volume of raw leachate or concentrate is the most common fix. Even 15 percent raw leachate by volume gives the polymer something to bridge and restores a workable dose curve. The blend ratio needs to be tested rather than assumed, because the benefit plateaus and then reverses as the blend becomes its own problem.

The other question is whether permeate needs solidifying at all. If the site has a permitted discharge or a reuse route for dust control, gelling a clean stream is an expensive habit. The calculation is straightforward: litres per day, cost per cubic metre of gelling, and the cost of the alternative route. Sites often find that only the concentrate genuinely needs the pad, and the permeate is better sent elsewhere.

The plant that gelled permeate for a year

A site with a two-stage RO plant had been solidifying permeate since commissioning because the original permit listed it as leachate. Dose had crept from 1.4 to 2.9 kg/m³ over eighteen months, and nobody could explain the drift. The membrane had been replaced twice, and each replacement changed the passage characteristics enough to shift the permeate composition.

A six-week trial blending 20 percent raw leachate brought the dose back to 1.7 kg/m³ and cut polymer use by roughly 40 percent. The plant also discovered that its permeate ammonia was six times higher at pH 5.2 than at 6.8, which explained the seasonal pattern nobody had connected to the upstream acid dosing.

Start by getting a full analysis on the permeate itself, not on the combined stream. Ammonia, chloride, conductivity, pH and TDS are the five numbers that decide the dose, and most labs will report them on a single sample. The SAP powder specifications show which grades hold performance at high ionic strength, which is the property that matters most when the liquid is mostly chloride.