Application of Ultraviolet Absorption Measurement for Closed-loop Control of Tertiary Ozonation

To cope with the occurrence of organic micropollutants (OMPs) in the urban water cycle, different technologies have been tested to upgrade wastewater treatment plants for OMP removal. Measures are either based on adsorption onto activated carbon or ozonation. To ensure an economic and effective operation of the OMP removal, an automatic control system, which adapts the dosage to the varying water quality of the secondary effluent, is necessary. An online OMP measurement is not possible because of the high analytical afford, thus other surrogates have to be used for control purposes instead. One promising surrogate is the reduction of the ultraviolet absorption at 254 nm (delta UVA254) at the ozonation stage, which correlates very well with the OMP removal. In this study, the results of the successful application of the delta UVA254 for a closed-loop control at an ozonation pilot plant for OMP removal are presented with a focus on implementation issues like coping with delay time and the choice of measurement points. OMP removal was assessed for three different delta UVA254 setpoints, of which two of them also were performed as an advanced ozone process. It could be shown that changes of the ozone demand, e.g. by a varying concentration of the dissolved organic carbon (DOC) or nitrite, can be detected and countered by an adaption of the applied ozone dose.

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