Selective excess sludge removal from activated sludge tanks of municipal wastewater treatment plants
2020 – 2023
Bundesministerium für Landwirtschaft, Regionen und Tourismus
The settling properties of activated sludge are a significant factor in the design and capacity of activated sludge systems. Excellent thickening properties of the sludge allow for an operation with higher suspended solids in the activated sludge tank and at the same time reduce the necessary volume of the clarifier tanks. Therefore, additional treatment capacity is gained at existing plants, avoiding the need for constructive extensions. As part of the project “SelektAra” selective discharge from continuous flow activated sludge plants will be studied and tested as a method for excess sludge removal. Up to now, this method has not been implemented in Austria nor has it been the subject of specific international scientific research. Since the activated sludge is not homogenous and contains sludge flocks of different densities a selective sludge discharge will tend to remove flocks of poorly settable biomass. Therefore, an enrichment of better settable biomass with a lower sludge index is expected. Another operational advantage of the selective excess sludge removal is the simplified determination of the exact sludge age, the knowledge of which is crucial for an energetically optimized plant operation. Previous studies about selective excess sludge removal from SBR-reactors showed a positive effect on biological phosphorus elimination. A disadvantage of the selective excess sludge removal from activated sludge tanks is the larger volume of sludge and the related higher hydraulic load on the thickener. Since dewatering polymers are dosed in regard to the dry matter content of the sludge and not the overall flow, this disadvantage is subordinate at modern plants with mechanical thickening of excess sludge. The trials of the selective excess sludge removal regarding the alteration of the sludge index will be performed on two full scale municipal wastewater treatment plants in Austria. Additionally, a detailed analysis of the microbiological properties and the composition of the sludge will take place in the laboratory. In the course of the project operational strategies for the selective excess sludge removal from activated sludge tanks will be evaluated and the permanent effects to the plant operation will be presented under a holistic consideration. The results will be made available directly to the operators of the wastewater treatment plants, the main focus being operational simplification and an energetic and cost optimized plant operation.