Application of online UV absorption measurements for ozone process control in secondary effluent with variable nitrite concentration

Ozone process control in secondary effluent used for elimination of trace organic compounds (TrOCs) requires the use of surrogates, such as the relative reduction of UV absorption at 254 nm (Delta UVA(254)) to adapt the ozone dose to a varying water quality. In the present study, a closed-loop process control based on two online UVA(254) measurements was successfully implemented and tested under realistic conditions with ozone doses from 0.2 to 1.05 mg-O-3/mg-DOC at a pilot scale ozonation system with subsequent coagulation filtration at a municipal wastewater treatment plant (DOC similar to 13 mg/L, UVA(254) similar to 27 m(-1), and nitrite peaks of up to 1.6 mg-N/L). It could be shown that measuring the UVA(254) at the ozonation effluent was superior to the measurement of UVA(254) at the filter effluent in terms of response time due to changes in water quality, whereas online measurement at the filter effluent showed a better agreement with laboratory data and a reduced maintenance interval due to less particles. Additional online nitrite measurement is not necessary as the ozone consumption by nitrite directly impacts Delta UVA(254). (C) 2016 Elsevier Ltd. All rights reserved.