What is sludge bulking?
Sludge bulking is a severe problem that can plague activated sludge wastewater treatment plants (WWTP). It’s been estimated that 60% of activated sludge WWTP in the USA experience continuous or intermittent sludge bulking. This is concerning as sludge bulking results in poor settling in the secondary clarifier which can cause solids washouts. The impact of these events include poor effluent quality, reduced treatment capacity due to reduced solids inventory and poor dewatering performance.
There are two types of sludge bulking: filamentous and viscous. Filamentous bulking is caused by the proliferation of filamentous organisms. Filamentous organisms grow as long strands which, when over abundant, results in a looser sludge that does not compact. Viscous bulking is caused by the biomass producing excessive quantities of extracellular polysaccharides (EPS). EPS acts like a sponge, retaining water and reducing sludge compaction. Sludge bulking can be caused by various problems including low dissolved oxygen, low food to microorganism ratio, nutrient deficiencies and toxicity.
How to identify sludge bulking
Sludge bulking is traditionally identified using a combination of microscopy and measurement of sludge volume index (SVI). SVI is calculated as the ratio of the total suspended solids concentration to the volume of sludge after 30 minutes of settling. While both these methods are adequate at identifying bulking, they are time consuming and, due to the accuracy of the tests, might not identify a bulking event until it is well underway. With these methods, corrective action can only happen after the bulking event occurs, instead of identifying the problem before it becomes an issue.
Adenosine Triphosphate (ATP)
Adenosine Triphosphate (ATP) quantifies small changes in sludge bulking, allowing you to take corrective action early. During sludge bulking ATP tests the mixed liquor is passed through a screen and ATP is measured on the retained sludge and the original sample. When bulking events occur, more sludge is retained on the screen. Results are typically represented as the fraction of retained ATP to total ATP.
The tests can be done on-site in less than 10 minutes — saving you time and money. A bulking event can take days, or potentially weeks, to evolve. By diagnosing it sooner, it can be resolved by determining and correcting the root cause. This decreases the likelihood that more severe actions, such as chlorinating the return activate sludge, will need to take place. ATP has been shown to predict SVI 10-15 days earlier than traditional methods, providing a significant lead time to implement preventative measures.