**5. Other applications**

Some water treatment facilities require the oil component control in wastewater [9, 93]. Organic pollutants can occur in the cooling water, in the technical-use water, in a boiler-feeding water, and due to the leaks of oils into the steam condensate. For online pollutants' monitoring, the fluorescent sensors are broadly used. The method is highly sensitive. Depending on the type of oil, the detection limits may vary from 1 to 100 ppm. Most of the oils contain some polycyclic aromatic hydrocarbons (PAHs) [94], capable of generating blue or violet fluorescence being exposed to UV irradiation. Usually the excitation light wavelength corresponds to 254 nm, while the detection operates in a 360 nm spectrum range.

Finally, one of the first known applications of fluorophores in water treatment should be mentioned. It is associated with water leaks detection in industrial pipelines. To solve this problem, some fluorescent indicators, for example fluorescein, have been merely added to the circulating water [11].
