Light and Water are often associated with rainbows - inspiring songs..., but normally not with information. But indeed - a lot of information concerning organic impurities is hidden in this system. COD and TOC parameters are consisting of humic substances: proteins, detergents and other chemical compounds of households, tannins / lignins of the paper industry, organic impurities of the chemical industry and carbohydrates of dairies. Most of these substances can be measured by UV - spectroscopy. The ensemble of all these substances leads to a spectrum with an absorption of about 254 nm. This absorption is measured as the ISO/DIN SAC = Spectral Absorption Coefficient. New continuously working on-line probe systems are able to measure this SAC and the integral absorption close to 254 nm. This scanning UV/VIS - probe offers you an individual signal which is very close to the COD and TOC depending on your wastewater. The resulting parameters offer the advantage of delivering on-line data within minutes. When problems are signalled, event-triggered samples can be taken by automatic samplers for further detailed analysis and diagnosis, e.g. river - monitoring or outlet - monitoring of a WWTP. However, the range of possible applications is not confined to sewage outlets. This probe system measures a useful parameter at nearly each step of a wastewater treatment process. Environment friendliness, rapid availability in continuous operation, ease of use, flexibility, the absence of sample pre - treatment and the resulting avoiding of errors and costs are strong arguments for this system. The instruments and their investment costs should automatically (more than) pay for themselves by skilful, active utilisation of the information obtained. Somewhere, after the treatment, water is blue...