3. Coal Chemical Water Purification: The Key Challenge of Achieving Zero Liquid Discharge

Coal chemical projects are mostly located in western regions rich in coal but short of water. Their raw water is typically highly turbid, high-hardness, and high-salinity surface water or mine water. The water treatment equipment first treats the raw water through a combined process of "high-density sedimentation tank + ultrafiltration." The goal of this process is to reduce the feed water turbidity to below 1 NTU and remove most of the hardness, thereby eliminating scaling and fouling risks for the downstream reverse osmosis membrane system. Ultrafiltration serves as the protective barrier for the reverse osmosis, and its effluent SDI must be stably maintained below 3. Once this index is exceeded, silica scale and calcium-magnesium precipitates will rapidly destroy the separation performance of the membrane elements. The main difficulty of this pretreatment stage lies in the drastic fluctuations in raw water quality. Therefore, the system needs to rely on the linkage between intelligent chemical dosing systems and online monitoring instruments to precisely dose coagulants and coagulant aids, thereby providing reliable pretreatment assurance for downstream systems.

Gasification wastewater and coking wastewater contain high concentrations of phenols, polycyclic aromatic hydrocarbons, ammonia nitrogen, and toxic cyanides. Such wastewater is extremely difficult to treat. To treat this type of wastewater, the purification system first adopts an enhanced biochemical process of "anaerobic hydrolysis—anoxic denitrification—aerobic biological treatment" to remove most of the organic matter and ammonia nitrogen. Afterwards, the system further connects an "ozone catalytic oxidation + biological aerated filter" for advanced treatment. This advanced treatment stage oxidizes and decomposes residual macromolecular organic matter into biodegradable small molecules and filters them out. Among this, the selection of the ozone catalyst and the design of the reactor flow field are the key technical difficulties in ensuring the efficient utilization of hydroxyl radicals. This technical challenge directly determines whether the effluent can consistently meet the feed water requirements for circulating water makeup or membrane concentration. This biochemical and advanced treatment stage is the core link in degrading refractory organic pollutants.

Achieving zero liquid discharge for high-salinity wastewater is the final stage of coal chemical water purification. After being concentrated by membranes, the wastewater's salinity is increased to 8%–10%, and it then enters a mechanical vapor recompression or multi-effect evaporation crystallizer. At this point, the equipment faces severe challenges, including pitting corrosion from chloride ions and a sharp increase in slurry viscosity caused by the co-precipitation of mixed salts. To address this, the system must use titanium or super duplex stainless steel to resist corrosion. The system also needs to control the nucleation sites of calcium sulfate and silica scale through seed crystal methods, while simultaneously using an elutriator to achieve the separate purification of sodium chloride and sodium sulfate. These measures are the true key to achieving zero liquid discharge and resource recovery. How to maintain a high concentration factor under low energy consumption conditions and stably produce industrial-grade crystalline salts is the direction of long-term industry research.