Reduction in residual COD in biologically treated paper mill effluents by means of combined ozone and ozone/UV reactor stages

Lab-scale tests have been conducted on the reduction of residual COD in biologically purified effluents from mills with integrated waste paper treatment plants and/or TMP lines. For this purpose, waste water samples were subjected to ozonization or a combined ozone/UV treatment at varied temperatures and pHs. The influence of ozone on the biodegradability of biologically part-treated effluents was studied by means of the Zahn-Wellens test. For specific ozone consumption rates < 2.5 g O₃/g eliminated COD, COD elimination rates of up to 82% and DOC elimination rates of up to 64% were obtained. Ozone consumption was considerably higher for a combined ozone/UV treatment or treatment at pH > 9 or at elevated temperatures (40°C). For colour stripping of the waste waters and for destroying the surfactants contained in them, ozone additions of up to 80 mg O₃/l waste water were required. The bioavailability of effluents that have been part-treated biologically in trickling filters or anaerobic stages to COD > 500 mg/l and BOD₅/COD > 0.2 cannot be positively influenced by ozonization with up to 1.8 g O₃/l waste water. On the other hand, in fully biologically treated effluents (COD < 500 mg/l), ozonization brought about a rise in the BOD₅/COD ratio from an initial < 0.05 to a maximum of 0.37 - an observation which clearly demonstrates an increase in bio-degradability.