Bulking in activated sludge plants treating paper mill wastewaters

As part of a larger project to examine the causes of bulking in activated sludge plants treating wastewaters from paper mills, two laboratory-scale activated sludge plants were run in parallel. This was to evaluate the impact of variable environmental and operational parameters on the sludge ecology, and in particular, on the filamentous bacteria in the sludge. A survey of paper mills in the UK showed that there was a significant difference between the activated sludge plants at the paper mills using virgin fibre and those using recycled fibre. An examination of samples from activated sludge plants at the paper mills showed that there did not appear to be any significant correlation between filament abundance and the settlement characteristics as measured by stirred specific volume index (SSVI). The surface charge carried by the sludge particles was also measured and it was found that this parameter was better related to the SSVI. Similar results were found for samples from the laboratory plants. Seven different sets of operational conditions were applied to the laboratory-scale plants. These were aimed at simulating the conditions noted for the full-scale plants during the mill survey. The effects of low dissolved oxygen and low organic loading rates were examined. The effect of inorganic sulphur compounds and volatile fatty acids was studied. A change in raw material from virgin fibre to recycled fibre (corrugated paper) caused an immediate and continuous deterioration in settlement. Divalent cations, calcium and magnesium, were found to be successful in controlling settlement in the final clarifier.