Examensarbete/Master´s thesis proposal

Foaming problems in activated sludge plants with low solids retention times.


Foaming is a periodic problem in many wastewater treatment plants, especially in activated sludge tanks. Foam formation is caused by the presence of surfactants, which may be present in the wastewater or produced by bacteria, and filamentous hydrophobic bacteria that stabilize the foam. The activated sludge plant for carbon removal at Sjölunda Wastewater Treatment Plant (WWTP) in Malmö, Sweden, is periodically suffering of heavy foaming. The aerobic solids retention time (SRT) is low (<2 days), which makes it a high-loaded activated sludge (HLAS) plant without nitrification. Recently, automatic SRT-controlled excess sludge withdrawal was implemented for the HLAS at Sjölunda. This Master´s thesis study is a part of the evaluation of this strategy as well as to further refine and optimise this operation. In the beginning of the next decade, Sjölunda WWTP has to increase its treatment capacity to fulfil new treatment standards. Optimising the HLAS operation will help to increase the plant´s carbon removal capacity without deteriorating the effluent quality and affecting other important treatment processes at Sjölunda WWTP.


The aim of this Master’s thesis is to determine when and why foaming is occurring in the full-scale activated sludge plant at Sjölunda WWTP. We will also test the foaming potential of sludge from a few other treatment plants. The effect on foaming due to operational parameters such as SRT, aeration and feeding strategies can be investigated. Example of suggested hypotheses are:

• Low fluctuations of SRT decreases foam formation
• Very low SRT increases foam formation
• Low DO increases foam formation.


The assessment of foaming ability of the sludge from different activated sludge lines at Sjölunda WWTP will be performed with a foaming apparatus similar to that described in Stratton et al. (2002). Also visual quantification of the foaming in full-scale, microscopic characterisation of the sludge and foam, as well as analysing operational parameters will be included in the work.


Oskar Modin, associate professor at Chalmers University of Technology, and David Gustavsson, R&D engineer at VA SYD/Sweden Water Research.


Autumn 2016


Send an e-mail with a few words about you, your educational background and why you are interested in this Master´s thesis project to david.gustavsson@vasyd.se.