Biological treatment studies were carried out with several Kraft pulp bleaching wastewaters both in continuous-flow and batch activated sludge reactors and substrate degradation was examined. The results of different studies indicated comparable findings. In the first study, bleaching effluents from the chlorination and extraction stages of the last pulp bleaching (Process: D/C-E/D-D-E-D, prebleaching with oxygen) were taken. Substrate removal has been evaluated using the parameters TOC, DOC, COD, AOX, BOD5 and Color (A436) and UV254. In the second study, spent bleaching effluents from a Kraft pulp mill using a conventional C/E-H-D/E-D bleaching were used. COD and Color (A436) was used for substrate determination. In the third study, the bleaching effluent from the same process was used at another time interval and substrate removal was followed by the measurement of COD and BOD5 and the spectrophotometric changes in the visible and UV regions. In all wastewaters a high amount of nonbiodegradable substrate was left over as expressed by residual TOC, DOC, COD. In general, the nonbiodegradable fraction of these wastes was about 30-60 % of the initial substrate. The dehalogenation of chlorinated compounds was either negligible or occurred not at all. Additionally, the specific absorption ratios A436/CODs, A346/CODs, UV272/CODs, UV254/CODs and the relative UV absorption ratio UV254/UV272 were measured in biotreated wastes and compared to the raw waste. These measurements proved to be a very useful tool since they provided information on specific pollutant groups. It was seen that UV absorbing substances were removed to a lesser extent than the total of organics measured by either TOC or COD.

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