Explain UV disinfection for water and wastewater treatment.

UV disinfection uses ultraviolet light (254 nm wavelength) to inactivate microorganisms by damaging DNA/RNA, preventing reproduction. UV dose (fluence) = intensity x time, typically 40 mJ/cm2 for drinking water. Low-pressure lamps are monochromatic and energy-efficient; medium-pressure provide broader spectrum. Advantages: no chemical addition, no byproducts, effective against Cryptosporidium resistant to chlorine. Limitations: no residual protection, effectiveness reduced by turbidity and dissolved organics (UVT should be >75%).