Which method is ineffective in reducing nitrate levels in water?

Boiling water is ineffective in reducing nitrate levels because the process of boiling primarily raises the temperature of the water to eliminate pathogens and other microorganisms. However, nitrates are inorganic compounds that do not evaporate at boiling temperatures. Instead, they remain dissolved in the water, and boiling will not remove or reduce their concentration.

In contrast, methods like filtration and reverse osmosis are effective for reducing nitrates. Filtration can physically separate particles and some dissolved substances, while reverse osmosis employs a semipermeable membrane to remove a wide range of contaminants, including nitrates, from water. Chlorination, while primarily used for disinfection, does not specifically target nitrates either, but doesn't effectively address nitrate removal.

Thus, boiling does not address the chemical nature and solubility of nitrates, making it an ineffective method for their removal from water.