Zero Liquid Discharge and Salt Separation Crystallization Process for High-Salinity Wastewater in Chemical Plants
High-salinity wastewater generated from chemical manufacturing, coal-to-chemicals and petrochemical processes presents a formidable treatment challenge. This stream typically contains extreme concentrations of dissolved salts, often exceeding 20,000 milligrams per liter of total dissolved solids, along with organic contaminants, silica and heavy metals. A zero liquid discharge approach with salt separation crystallization transforms this hazardous waste into reusable water and commercially valuable individual salt products.
The treatment train begins with chemical conditioning and precipitation. The wastewater is collected in an equalization basin to homogenize flow and composition. It then enters a high-rate chemical softening reactor where milk of lime is added to raise the pH above 11. Under these conditions, magnesium precipitates as magnesium hydroxide, and silica is co-adsorbed onto the floc. Soda ash is dosed in a second reactor to target calcium removal, forming calcium carbonate solids. A coagulant and anionic polymer are injected, and the mixed liquor flows into a lamella clarifier where dense sludge settles rapidly. The clarified supernatant has hardness reduced to below 20 milligrams per liter as calcium carbonate and is virtually free of silica and suspended solids.
The softened water is then directed to a multi-stage membrane system for pre-concentration. Ultrafiltration removes any residual fines, providing feed with a silt density index below 3 to the downstream reverse osmosis unit. A primary reverse osmosis array, configured in three stages for high recovery, extracts approximately 80 percent of the water as permeate. This low-salinity permeate is sent to the plant's process water tank. The primary concentrate passes through a high-pressure reverse osmosis stage using brine-resistant spiral-wound membranes at pressures up to 90 bar. This secondary membrane step boosts overall water recovery to over 95 percent, producing a brine with salinity exceeding 100,000 milligrams per liter.
The hypersaline brine now enters the evaporative crystallization section, which is the key to both zero liquid discharge and salt separation. A forced-circulation mechanical vapor recompression evaporator concentrates the brine close to the saturation point. The slurry is then transferred to a specialized crystallizer where controlled cooling or heating is applied depending on the salt mixture. In a typical process for a mixed sodium chloride and sodium sulfate system, the brine is first cooled to selectively crystallize sodium sulfate decahydrate, exploiting its sharp solubility decrease at lower temperatures. The sodium sulfate crystals are harvested by a pusher centrifuge, dried and packaged for industrial use. The mother liquor, now enriched in chloride, is reheated and sent to a second crystallizer where sodium chloride is precipitated by evaporation. The sodium chloride product reaches purity above 99 percent, suitable for chlor-alkali production.
All distillate from the evaporators and crystallizers is condensed and polished through a mixed-bed ion exchange unit, yielding high-purity water with conductivity below 5 microsiemens per centimeter. This water is recycled as boiler feedwater. The entire integrated plant operates with no liquid discharge to the environment. Solid waste is limited to the small volume of chemical sludge from pretreatment, which is dewatered and landfilled. This process converts a costly waste problem into a resource recovery operation, aligning with circular economy principles.
