SOLVIA DesalCascade PRO is a cascade desalination system in which the salt load is distributed between ultrafiltration, two stages of electrodialysis and reverse osmosis. The concentrate is treated inside the scheme rather than sent away right after the membrane block.
The development is meant for the desalination and polishing of water of elevated salinity, where the fate of the concentrate has to be settled alongside the quality of the permeate. Instead of passing the whole salt load through reverse osmosis, the scheme distributes it between the stages: ultrafiltration, a first electrodialysis stage ahead of the membrane block, the reverse osmosis itself, and a second electrodialysis stage working on the mixed concentrate.
The final concentrate leaves the system only at the outlet of the second stage, while the diluate of that stage, after coagulation treatment, returns into the first equalising and mixing tank for repeated ultrafiltration. A linear scheme with a single outgoing salt stream turns into a circuit in which the concentrate stays a process stream: its composition is measured, corrected, concentrated further and partly returned into the cycle.
In an ordinary scheme reverse osmosis takes on the entire salt load of the raw water: the higher its salinity, the higher the osmotic pressure and the greater the working pressure needed to obtain permeate. At the same time the risk of scaling grows — carbonates, sulphates, compounds of calcium, magnesium and silica — and the membrane elements are cleaned more often.
The first electrodialysis stage removes part of the salts before the membrane block, and reverse osmosis receives an already partly desalinated stream: a lower working pressure, a lower risk of deposition, a steadier regime. The materials of the technology name the first stage as the means of bringing the salinity of the reverse osmosis feed to 1.5–3.5 g/l and of lowering the working pressure by no less than 30 % against treating the raw stream without it.
The concentrate of the first electrodialysis stage is not discharged in SOLVIA DesalCascade PRO: it goes into a mixing node, which also receives the concentrate of the reverse osmosis. The mixed stream passes control and preliminary treatment and is then fed to the second electrodialysis stage, which draws yet more water out of it. The final salt stream is formed only here, at the outlet of the second stage.
In one particular design example the recovery amounts to 88 % against 75 % for the prototype, the volume of the final concentrate to 12 % of the raw stream against 25 %, that is a reduction by 52 %, and the design working pressure of the reverse osmosis to 11.8 bar against 21.5 bar. These are the figures of a single calculation rather than a guarantee for any water: the actual values depend on the composition of the raw water, on silica, calcium, sulphates, organics and the regime required.
Concentrating salts by itself creates the conditions for a deposit, and calcium carbonate stands first in that row: at a certain combination of pH, calcium, alkalinity and salinity the solution becomes supersaturated, and deposits appear on the membranes and in the channels. In the mixed concentrate, therefore, pH, salinity and the calcium carbonate saturation index are measured, and where needed the pH is corrected and decarbonation is carried out before the second stage.
Calcium sulphate and dissolved silica behave differently: gypsum gives deposits that are hard to remove, silica is able to polymerise and to foul membranes and channels. The degree of calcium sulphate saturation and the concentration of dissolved silica are watched ahead of the second stage, and as hazardous conditions are approached, corrective treatment, antiscalants, partial softening or the removal of calcium, magnesium, sulphates, carbonates or silica are applied according to the water composition and the calculation.
Returning the diluate of the second stage untreated would accumulate organics and colloids in the circuit, and it is the ultrafiltration that suffers from that. The diluate therefore passes coagulation treatment before it returns into the first tank, and at an elevated content of organic carbon sorption on activated carbon is added. In the design example the interval between ultrafiltration cleanings amounts to 18–35 days against 3–4 days for the prototype.
The steady regime is held not by the layout but by the control: sensors of salinity, pH, dissolved silica, turbidity and organic carbon, together with calculation blocks for the risk of deposition, govern the flow of the recirculation stream and the flow of the final concentrate, the dosing of coagulant and antiscalant, the pH correction and the decarbonation. The aim of the control is to keep salts and fouling components from accumulating in the circuit.
SOLVIA DesalCascade PRO is SOLVIA's own technological development in the membrane and electrochemical treatment of water; the development holds patent protection in Russia. The solution combines ultrafiltration, two stages of electrodialysis, reverse osmosis, control of deposition, treatment of the recirculation stream and the governing of flows by sensor signals. Structural parameters, operating regimes, reagent doses, control algorithms and layout decisions are not disclosed in open materials and are selected for the particular water composition and site task.
No. ZLD systems aim at the full or nearly full elimination of liquid discharge and usually call for evaporation, crystallisation and high capital and operating costs. The role of the cascade scheme is an intermediate one — to cut the volume of the concentrate to a level that is easier to work with further on — so it serves as a separate stage ahead of deeper concentrate treatment or as part of a wider scheme for minimising liquid waste.
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