Hypersaline RO Concentrate at 80,000 mg/L: Choosing a Granule That Still Swells

A zero liquid discharge plant near a chemical park was sending 60 cubic metres a day of reverse osmosis concentrate to a lined evaporation pond. TDS was 78,000 mg/L, chloride 42,000, calcium 1,900. The site wanted to stop using the pond and solidify the concentrate into skips instead. The first granule tested took 11 grams of water per gram of polymer, which made the exercise pointless at any price.

Hypersaline streams are the edge case where most absorption data becomes fiction. This article explains what 80 g/L of dissolved solids does to a polyacrylate network, which structural features still work at that concentration, and when the honest answer is that you should evaporate rather than absorb.

What 80 g/L TDS Does to a Standard Polyacrylate

At 80 g/L, the external solution is close to 1.4 mol/L in ions. The osmotic driving force that normally pulls water into the network is almost gone, and what remains is opposed by the elastic energy of the crosslinks. A conventional lightly crosslinked sodium polyacrylate settles at 8 to 15 g/g, and much of that is water held in the voids between particles rather than truly absorbed.

There is a second effect that matters more in practice: the granule does not wet. High ionic strength reduces the hydration of the polymer surface, so a dry particle dropped onto concentrate floats or forms a skin and stays dry inside for minutes. In a stirred tank this shows up as a crust on the impeller and a liquid that never thickens, even at 8 percent dose.

Surface Crosslinking and Charge Density: What Still Works

Two levers remain at high salinity. The first is surface crosslinking: a densely crosslinked shell resists the collapse that otherwise seals the particle, and it keeps a pathway open for water to reach the core. The second is a higher proportion of non-ionic or sulfonate groups, which are less sensitive to divalent cations than carboxylate.

Grades built this way will reach 25 to 40 g/g in 80 g/L brine, against 10 to 15 for a standard hygiene grade. That is still a poor number, and it changes the economics completely: at 30 g/g, one tonne of polymer binds 30 tonnes of water, so 60 cubic metres a day needs about 2 tonnes of polymer a day. At 1,900 dollars a tonne, that is 3,800 dollars a day to deal with water you could evaporate for less.

Dose and Cost per Tonne at 20x Absorption

Suppose you accept the absorption and want to know the dose. A cubic metre of concentrate weighs 1.05 tonnes and carries 80 kg of salt and 970 kg of water. At 30 g/g, one tonne of water needs 33 kg of polymer, so a cubic metre needs about 32 kg, or 3 percent by weight. That is 32 kg per cubic metre, which is 30 to 60 times the dose for municipal sludge.

The gel that results is also heavy and low in solids: about 3 percent polymer and 5 percent salt, with the rest water. A skip that holds 12 tonnes holds 400 kg of original salt. If the disposal route charges per tonne, you have converted a liquid problem into a transport problem with no volume reduction at all.

When No SAP Works and You Should Evaporate Instead

The practical cut-off is around 50,000 mg/L TDS with calcium under 1,000 mg/L. Above that, absorption stops being a solidification method and becomes a very expensive way to make wet salt. Three alternatives usually win: a mechanical vapour recompression evaporator if you have waste heat, solar drying in a covered greenhouse if you have land and climate, or spray drying if the salt has resale value.

Where SAP still earns a place is at the margin: patching a leak, stabilising a skip load for transport, or handling the 2 cubic metres a week that the evaporator cannot take during maintenance. For those jobs a 25 kg bag lasts a month and the cost is irrelevant. Sizing a daily duty on hypersaline concentrate is a different decision, and the arithmetic above is the one to take to your finance team.

If your stream sits between 20,000 and 50,000 mg/L, a brine-tolerant grade can still make sense, and the SAP granules for high-TDS liquids are worth testing on a 20 litre sample before you commit.