Current Discharge Situation and Policy Pressure
Circulating cooling water becomes concentrated through evaporation, producing a high-strength blowdown stream. Conventional open recirculating systems typically run at 3–5 cycles of concentration, which fixes a substantial discharge volume per unit of cooling duty.
Policy pressure has increased. China's dual-carbon target, announced in September 2020 — carbon peak before 2030, carbon neutrality before 2060 — elevated industrial water saving and emission reduction to national strategy level, with subsequent measures tightening discharge and water-use quotas.
Concentration Ratio: Definition and Breakthrough
Concentration ratio (CR) is the core indicator of water-saving performance, defined as the ratio of a conservative ion concentration — usually Cl− or conductivity — in the circulating water to that in the make-up water.
Chemical treatment is capped near 3–5 cycles for several reasons: inhibitor effectiveness falls as ion strength rises, corrosion risk grows with chloride, and blowdown is needed to keep dissolved solids within the range the chemical programme can hold.
Electrochemical treatment removes hardness ions directly through cathodic precipitation, which changes the limiting condition. Cycles of concentration well above the chemical ceiling become operable because the scaling constraint has been decoupled from dissolved-solids accumulation.
Blowdown Evaporation and Consumption
Even at greatly increased concentration ratio, a small high-strength blowdown remains. Achieving true Zero Liquid Discharge (ZLD) requires handling that residual stream.
Evaporation-based consumption routes the small blowdown volume to a dedicated evaporator — a low-temperature evaporator or multi-effect system — using waste heat or low-grade heat from the cooling system itself to separate water from dissolved solids.
Self-balancing full-membrane technology is the alternative deep-treatment route, combining ultrafiltration, reverse osmosis and further membrane stages to recover most of the water and concentrate the remainder for evaporation or crystallisation.
Chemical vs Electrochemical ZLD Pathways
The two pathways differ in annual chemical consumption, achievable concentration ratio, blowdown volume, energy demand and capital profile. The chemical path carries an annual chemical load measured in tens of tonnes for a mid-size system and remains tied to a discharge permit; the electrochemical path trades that recurring chemical and discharge burden for electrical load on the cells.
Economic Analysis
For a system circulating two million tonnes per year, the difference in water balance alone is material: a chemical system at CR = 4 discharges roughly 100,000 tonnes per year, while an electrochemical system at a much higher CR reduces that to a small fraction, with corresponding make-up water savings.
A ten-year lifecycle comparison should account for capital, electricity, chemicals, water purchase, discharge fees and avoided cleaning downtime. The relative weight of each depends heavily on local water and discharge tariffs.
Conclusions and Outlook
Zero liquid discharge for industrial cooling has moved from a question of technical feasibility to one of economic rationality. Routes that break the historical 3–5 cycle constraint change the arithmetic, and the crossover point depends on site-specific water and discharge costs rather than on technology maturity alone.