Two extensive mill-scale tests were performed to find out to what extent and in what way organic chlorine compounds (AOX, chlorinated phenols) are reduced in an activated sludge process. In an extended aeration activated sludge plant the reduction of AOX was 40 - 50% and that of chlorinated phenols 70 - 80%. In a high-load activated sludge plant the corresponding removals were only 20 - 30% and 35 - 55%. The removal of oxygen-consuming substances was also lower in the high-load plant. It is evident that most of the chlorinated organics, especially low molar mass compounds, are removed by microbiological action. About 0.05 - 0.1 kg AOX per tonne of pulp was removed from the system with the excess biosludge, to be mixed with primary sludge and finally either burned in a bark-fired boiler or used as landfill. Some of the chlorinated compounds volatilized, but no quantitative estimate of the amount could be made. At the extended aeration activated sludge plant the AOX discharge to the recipient was only 1.3 kg/t when birch was the chief raw material. The corresponding figure at the high-load plant was 3.1 kg/t; this is partly because of the lower reduction and partly because 65% of the fibre raw material was softwood. A detailed study revealed differences in the micro-organisms present in the aeration sections of the plants. For example, there were practically no protozoa present in the high-load plant, and the number of species of bacteria was also smaller than in the low-load plant, where thermophilic bacteria and fungi were typically present. Tests carried out under controlled laboratory conditions showed that changes in conditions in the aeration section (temperature, pH or addition of a chemical) had little effect on the removal of AOX but a considerable effect on the removal of low molar mass compounds such as chlorinated phenols. An anaerobic/facultative lagoon before or after the activated sludge process did not improve the removals. Removing colloidal material from biologically treated effluent had a great influence on AOX.

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