Q.Without altering water quality, ________ is/are an ideal disinfectant used in waste water treatment.

U-V irradiation disinfects waste water physically (by damaging microbial nucleic acids) without adding chemicals, so it leaves water quality unchanged -- unlike chlorination or ozonisation.

Disinfection is the final step of waste-water/sewage treatment, aimed at killing remaining pathogenic micro-organisms before treated water is released or reused. Chlorination, though cheap and widely used, is a chemical process -- it leaves residual chlorine, can react with organic matter to form disinfection by-products, and alters the taste and odour of water. Ozonisation is also a strong oxidising chemical treatment that can alter water chemistry. Boiling is energy-intensive and impractical at the scale of waste-water treatment plants. U-V rays, in contrast, disi …