UV Absorption can be used to detect TOC (and other parameters like COD/BOD) in water, by the absorption of light at 254nm by Natural Organic Matter in the water.
The exact solution will depend upon your requirements. Do you need continuous measurement to have early detection, and therefore the capacity to change the water treatment process, or do you need sample measurement for verification purposes?
Photonic Measurements provides a range of UV254 measurement devices, including continuous measurement probes, portable probe systems and a portable cuvette system for sampling.
A short overview of the products and how they can be used is shown here