Explain Zero Liquid Discharge (ZLD) system design for industrial wastewater.

ZLD eliminates wastewater discharge through treatment and recovery. Typical process train: primary treatment, biological treatment (if biodegradable), ultrafiltration, reverse osmosis (70-85% recovery), concentrate treatment via evaporator and crystallizer. Design considerations: feed water quality analysis, scaling potential (LSI, S&DSI), material selection for corrosion, energy requirements (50-100 kWh/m3), and salt disposal. High-recovery RO (HERO) with pH adjustment increases recovery. Capital-intensive but required for textile, pharma, and power plant effluents in water-stressed regions.