Sodium dihydrogen orthophosphate is a sodium salt of phosphoric acid used in water treatment, where it helps purify water for human consumption. As a coagulant aid and corrosion inhibitor it assists in clarifying water and protecting distribution pipes, and its buffering action stabilises pH during treatment. The compound also appears in food as an acidity regulator and emulsifier, and in detergents and industrial processes. Like all phosphates in the environment, its discharge into waterways can contribute to eutrophication, so its use is managed. In the treatment works it is one of the invisible chemicals standing between the reservoir and the tap, ensuring the water runs clear and safe.