The treatment of industrial wastewater is an important measure to prohibit eutrophication and intoxication of lakes and oceans. Major industries often have location-basedwastewater treatment and to avoid undesired effluents it is essential to make it robust.The integrated pulp- and papermill Braviken is experiencing a malfunction in their wastewater treatment plant. The wastewater treatment at Braviken is divided into multiplestages where one of them is biological treatment by an activated sludge process. Thisstage is modeled as a Low Sludge Production (LSP) where an aerated selector helps tofaciliate the microbial growth of the wastewater. The microorganisms digest wastewaterpollutants in this step only to be consumed by micro-animals in the next step, the activesludge basin. During recent winters, Braviken have experienced a problematic sludge bulking in the selector. During the most problematic episodes, manual removal of the foamhave been needed with an excavator. The aim of this study have been to explain why thefoaming occurs and present an action proposal to prevent the bulking.To answer the questions at issue, previously measured monitoring data from the foamingepisodes have been analysed. A principal component analysis was performed, using monitoring data from each problematic winter and compared to the non-bulking referencewinter of 2017-2018. Additional sampling was collected and analysed to investigate theconnection between process management and its influence of the physical characteristicsof the wastewater.When the investigation had been accomplished it was concluded that the waste waterbulking was caused by a low nitrogen and phosphorus concentration combined with a highorganic carbon influent. This caused a reduction in the metabolism of the microorganisms,resulting in a increased colloidal surface charge and high extracellular polymeric substance(EPS)-content. The reason for the problem occuring during winters can be explained bya generally low water temperature in the selector and a physical difference in the woodcompared to summer wood. When performing microbial studies of the wastewater itoccured to be a presence of filamentous bacteria in both the selector and the previousleveling basin. This presence was concluded to be a result of reduced aeration and anincreased hydraulic retention time during the winter 2020-2021.To avoid future problematic foaming in the activated sludge process, an increased dosage ofnitrogen and phosphorus is recommended. The result show that a increased concentrationof phosphorus and nitrogen in relation to influent COD had a reducing effect of thecolloidal surface charge in the wastewater.