Q._________ is/are an ideal disinfectant for waste water.

U-V Rays are the ideal disinfectant for waste water: UV irradiation kills/inactivates microorganisms without altering water quality and without leaving any chemical residue.

Disinfection is the final step in waste-water treatment, meant to destroy disease-causing microorganisms before the treated water is reused or released. Comparing the options: boiling is impractical for the very large volumes of municipal waste water; chlorination is effective but chlorine can react with organic matter to form harmful residual by-products and leaves a chemical residue; ozonisation is also effective but is comparatively costly and its effect does not persist. UV irradiation, by contrast, is regarded as the ideal disinfectant because it is a chemical-free physical p …

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 …

Biological treatment of wastewater uses microbes to decompose organic waste, and its main purpose is to reduce the water's Biochemical Oxygen Demand (BOD) before discharge.

In secondary (biological) sewage treatment, aerobic microorganisms (in activated sludge tanks/aeration tanks) consume and break down the dissolved and suspended organic matter in wastewater, converting it into microbial biomass, CO2 and water. This significantly lowers the wastewater's BOD -- the amount of oxygen microbes would otherwise consume decomposing that organic matter -- which is important because a high-BOD effluent released untreated into a river/lake would deplete dissolved oxygen and harm aquatic life. Sedimentation (options a and c) is …