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

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 (ΔUVA₂₅₄) to adapt the ozone dose to a varying water quality. In the present study, a closed-loop process control based on two online UVA₂₅₄ measurements was successfully implemented and tested under realistic conditions with ozone doses from 0.2 to 1.05 mg-O₃/mg-DOC at a pilot scale ozonation system with subsequent coagulation filtration at a municipal wastewater treatment plant (DOC ∼ 13 mg/L, UVA₂₅₄ ∼ 27 m^-1, and nitrite peaks of up to 1.6 mg-N/L). It could be shown that measuring the UVA₂₅₄ at the ozonation effluent was superior to the measurement of UVA₂₅₄ 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 ΔUVA₂₅₄.