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 i n biologically purified effluents from mills with integrated waste pap er treatment plants and/or TMP lines. For this purpose, waste water sa mples were subjected to ozonization or a combined ozone/UV treatment a t varied temperatures and pHs. The influence of ozone on the biodegrad ability of biologically part-treated effluents was studied by means of the Zahn-Wellens test. For specific ozone consumption rates <2.5 g O- 3/g eliminated COD, COD elimination rates of up to 82% and DOC elimina tion rates of up to 6410 were obtained. Ozone consumption was consider ably higher for a combined ozone/UV treatment or treatment at pH >9 or at elevated temperatures (40 degrees C). For colour stripping of the waste waters and for destroying the surfactants contained in them, ozo ne additions of up to 80 mg O-3/l waste water were required. The bioav ailability of effluents that have been part-treated biologically in tr ickling 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 O-3/l waste water. On the other hand, in fully biologically treated effluen ts (COD <500 mg/l), ozonization brought about a rise in the BOD5/COD r atio from an initial <0.05 to a maximum of 0.37-an observation which c learly demonstrates an increase in biodegradability. (C) 1997 IAWQ.