There could be multiple ...
Published by Manuel Moreau, Kemira - Director, Marketing
There could be multiple reasons for sludge bulking. The fact that your problem is starting at the start of colder period, would suggest the presence of filamentous bacteria such as microthrix parvicella or nocardia. It is recommended to do a microbiological identification. Once identified, there are specific treatments to reliably solve and prevent reoccurence of bulking.