Zero Liquid Discharge and Crystallized Salt Resource Recovery Process for FGD Wastewater in Thermal Power Plants

Flue gas desulfurization wastewater from thermal power plants is a highly complex saline stream loaded with suspended solids, heavy metals, and high concentrations of chlorides and sulfates. Traditional chemical precipitation methods merely transfer pollutants between phases and fail to address the ultimate disposal challenge. The zero liquid discharge process combined with crystallized salt resource recovery provides a definitive solution, converting this problematic wastewater into reusable distilled water and marketable solid salts.

The treatment sequence begins with chemical softening in a high-rate clarifier. Lime and soda ash are precisely dosed into the equalized wastewater to raise the pH and precipitate calcium and magnesium ions as calcium carbonate and magnesium hydroxide sludge. Simultaneously, heavy metals such as lead, cadmium, and mercury co-precipitate or adsorb onto the floc surface. An anionic polymer flocculant aids in agglomeration, and the resulting sludge is dewatered and disposed of safely. The clarified supernatant now contains predominantly sodium chloride and sodium sulfate, with hardness reduced to less than 10 mg/L as calcium carbonate.

This softened water is then fed to a multistage membrane concentration unit. A first pass of ultrafiltration removes any residual suspended particles and colloidal silica, protecting downstream reverse osmosis membranes. A primary reverse osmosis system, often in a two-stage array, recovers roughly 75 to 80 percent of the water as permeate, which can be directly reused as cooling tower makeup or further polished for boiler feedwater. The remaining concentrate is directed to a high-pressure reverse osmosis stage, utilizing specialized brine-resistant membranes operating at pressures above 80 bar. This secondary membrane step raises the total system water recovery to over 95 percent, generating a highly concentrated brine with total dissolved solids exceeding 80,000 mg/L.

The concentrated brine then enters the heart of the zero liquid discharge system: a mechanical vapor recompression evaporator-crystallizer. The brine is heated in a plate or shell-and-tube heat exchanger, where water evaporates under vacuum. The produced vapor is compressed, raising its temperature and pressure, and then condensed on the heating side, releasing latent heat to evaporate more incoming brine. This design achieves exceptional energy efficiency. As water is continuously removed, the brine reaches supersaturation, and sodium chloride crystals form and grow in the crystallizer vessel. A controlled bleed of crystal slurry is withdrawn, and a centrifuge or pusher centrifuge separates the sodium chloride crystals from the mother liquor. The mother liquor is returned to the crystallizer, while the dried sodium chloride product achieves purity exceeding 97 percent, suitable for road deicing or industrial use. When sulfate concentrations are high, a sequential crystallization approach can separately harvest high-purity sodium sulfate by exploiting solubility differences at varying temperatures.

Distillate condensed from the evaporator is of extremely high quality, with conductivity below 20 microsiemens per centimeter, and is recycled directly to the power plant's demineralized water storage tank. The entire plant operates in a closed loop, achieving true zero liquid discharge. No wastewater leaves the site, all water is recovered, and valuable salts are transformed from a disposal liability into a revenue stream.