Comprehensive Wastewater Zero Liquid Discharge Process for Steel Plants

Integrated steel plants generate large volumes of comprehensive wastewater from ironmaking, steelmaking, and rolling processes. This wastewater contains suspended solids, oil, heavy metals, and salts. Treating this wastewater through a complete treatment process to achieve zero liquid discharge is the key to the industry's green upgrading.

The comprehensive wastewater first enters a coarse screen and an equalization tank, then flows to a high-efficiency clarifier. In this clarifier, we dose lime, flocculant, and coagulant aid to co-precipitate suspended solids, colloids, and some heavy metals. The clarified effluent is filtered through a V-type filter or sand filter to reduce turbidity. Subsequently, the effluent enters an ultrafiltration system. The UF membranes reduce the effluent turbidity to below 0.2 NTU, remove bacteria and colloids, thereby creating excellent feed water conditions for reverse osmosis.

The core desalination unit employs a multi-stage fouling-resistant reverse osmosis system. Its permeate is directly reused in the plant's industrial-grade fresh water network, replacing a large amount of primary water. The RO concentrate contains high salinity and refractory organic matter and requires further concentration and volume reduction. The system typically adopts a combination of high-efficiency RO or nanofiltration with high-pressure RO to increase the concentrate salinity to over 8%, while recovering permeate, achieving an overall water recovery rate of over 97%. The terminal high-salinity brine enters an evaporation and crystallization system, such as an MVR evaporator or a multi-effect evaporator. This system produces condensate for reuse, while the crystallized mixed salts are handled as solid waste, thereby achieving zero wastewater discharge across the entire plant.

Our zero liquid discharge process integrates self-cleaning filters, fouling-resistant UF membranes, energy-recovery RO units, and evaporative crystallizers, and meets the water quality requirements of different steel plants through customized design.