Vacuum treatment changes characteristics of activated sludge

Vacuum treatment of activated sludge results in sludge degassing. The aim of the study was to determine what happens with sludge flocs and bacterial cells during degassing process and to answer why the sludge settles differently after degassing. Sludge settling properties before and after degassing of activated sludge were taken under consideration. It was found that during vacuum degassing 25–100% of flocs larger than 500 µm were broken up into smaller ones. The floc area decreased by 19 ± 19%. The most important changes in the floc parameters were floc area and fractal dimension. Floc fractal dimension increased by 5.8% in summertime and decreased by up to 5.4% in the rest of the seasons. About 20% of the bacterial cells were completely destroyed. The reduction in the floating sludge resulting from degassing was 83–100%. The effects of extracellular substance release, change of sludge volume index and strength of activated sludge were dependent on the season. The release of loosely bound extracellular polymers ranged from 20 to 78%, while release of tightly bound polymers was stable and of app. 85 mg organic carbon/g suspended solids. The research results show the potential of vacuum to disintegrate not only activated sludge flocs but also completely dead or weak cells. This causes the enrichment of active cells in the activated sludge. The shear stress originating from degassing causes release of extracellular polymeric substances that may serve not only as external carbon source but also as a flocculant for the renewal of vacuum-disintegrated flocs.