Sludge bulking occurs when the sludge fails to separate out in the sedimentation tanks. The main cause of sludge bulking is the growth of filamentous bacteria.
Filamentous microorganisms grow in long strands that have much greater volume and surface area than conventional floc and are very slow to settle. Under certain growing conditions, filamentous organisms predominate. There is little robust scientific evidence that can be used to avoid sludge bulking but what there is indicates that over-loading works, having a carbohydrate rich input and having too low a recycle rate may all contribute.
To avoid sludge bulking some of the flow that enters the reactor can be by-passed, recycle ratio can be increased, lime or soda can be added to the reactor or the re-aeration rate increased.