Sludge bulking

In treatment of sewage one process used is the activated sludge process in which air is passed through a mixture of sewage and old sludge to allow the micro-organisms to break down the organic components of the sewage. Sludge is continually drawn off as new sewage enters the tank and this sludge must then be settled so that the supernatant (the remaining liquid) can be separated to pass on to further stages of treatment.

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 bypassed, recycle ratio can be increased, lime or soda can be added to the reactor or the re-aeration rate increased.

See also

• Anaerobic digestion
• Membrane fouling

Further reading

• R.J. Foot and M.S. Robinson (2003). "Activated sludge bulking and foaming: microbes and myths". In Duncan Mara and N. J. Horan (ed.). Handbook of water and wastewater microbiology. Academic Press. ISBN 9780124701007.
• Orris E. Albertson (1992). "Control of Bulking and Foaming Organisms". In Clifford W. Randall and James Lang Barnard (ed.). Design and retrofit of wastewater treatment plants for biological nutrient removal. CRC Press. ISBN 9780877629221.
• Jiri Wanner (1994). Activated Sludge Bulking and Foaming Control. CRC Press. ISBN 9781566761215.