RO Concentrate as a Sidestream: Sizing Solidification for 18 Percent of Feed

A plant treating 400 m³/day of leachate at 75 percent recovery produces 100 m³/day of concentrate, and that 100 m³ is where the whole treatment chain either closes or fails. The concentrate arrives at COD 8,800 mg/L, ammonia 2,400 mg/L and conductivity 52 mS/cm, having started the day at 2,200, 600 and 14. It needs 32 kg/m³ to gel, which is 3.2 tonnes of polymer a day, or roughly $6,900 before anyone has moved a skip.

Reverse osmosis is a volume-reduction machine, not a treatment step, and the concentrate is the bill it defers. This article covers why concentrate behaves so differently from raw leachate, how to size a solidification sidestream in three numbers, when blending back is smarter than dosing harder, and where the salt ends up.

Concentrate Is Not Leachate With the Water Removed

At 75 percent recovery everything that did not pass the membrane is concentrated fourfold. That includes the divalent ions that scale the membrane, which is why antiscalant is dosed upstream, and it includes the antiscalant itself. The concentrate therefore carries a polymer demand set not just by four times the COD and four times the ammonia, but by a ionic strength that has crossed the threshold where absorption collapses.

There is a second effect that surprises operators. The organics left in concentrate are the ones the membrane rejected, which are the high-molecular-weight humics and the recalcitrant fraction. These do not gel, but they raise viscosity. A concentrate at 8,800 mg/L COD often measures 3 to 5 mPa·s against 1.5 for the raw leachate, and that slows wet-out in the same way cold water does.

Sizing in Three Numbers: Flow, Concentration Factor, Absorption

Flow is the first number and it comes from the recovery rate. At 400 m³/day and 75 percent recovery the sidestream is 100 m³/day, and it varies with membrane condition: fouled membranes drop to 68 percent recovery and push the sidestream to 128 m³/day, which is a 28 percent increase in polymer demand with no change in feed.

The concentration factor is the second, and it is not simply one over recovery minus one. Use measured values from the concentrate sample rather than calculated ones, because ammonia and chloride often concentrate higher than the bulk factor due to membrane passage effects.

Absorption in the concentrate is the third, and it must be measured in the concentrate, not inferred. Typical values sit at 12 to 20 g/g against 40 to 55 in the raw feed, which puts dose at 28 to 40 kg/m³. Multiply flow by dose and you have the daily kilograms; multiply by 330 operating days for the annual tonnage.

Blending Back to Flatten the Dose Curve

The cheapest intervention on most plants is not a better polymer but a mixing tee. Blending 100 m³ of concentrate into 400 m³ of raw leachate gives 500 m³ at a weighted conductivity near 21 mS/cm, where absorption is roughly 30 g/g rather than 14. Dose across the combined stream lands near 22 kg/m³ instead of 32, and the total daily polymer falls from 3.2 tonnes to 2.8 tonnes even though the volume treated has risen 25 percent.

The blend has to be controlled, not accidental. Target a conductivity band of 18 to 25 mS/cm at the mixer, measure it inline, and let the concentrate flow be modulated against it. Above 30 mS/cm the dose curve steepens and the saving disappears. Blending also changes the raw leachate treatment performance upstream, so check that the membrane still meets its recovery target before committing.

Where the Salt Ends Up

Solidifying concentrate does not remove salt; it immobilises it in a cake that goes to landfill, where it will leach again over decades. That is acceptable where the receiving cell is lined and the leachate is recaptured, and it is a slow salt loop rather than a disposal route. Sites that do this should track chloride in the cake and in the receiving cell’s leachate, because a doubling over five years means the loop is tightening.

The alternatives are evaporation of the concentrate, which moves the salt into a residue that can be managed separately, or dedicated brine disposal where a permit exists. Neither is cheap, and both are easier to justify once the solidification cost is calculated honestly at 30 kg/m³ rather than at the raw-leachate figure.

Sizing this sidestream is mostly a matter of measuring in the right liquid. SAP tested in RO concentrate up to 60 mS/cm gives that absorption figure directly, and it is the only one worth putting into a sidestream budget.