Sludge bulking is a severe problem that can plague activated sludge wastewater treatment plants (WWTP). It has 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.
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. This results in corrective actions being taken after the bulking event becomes a significant issue.
ATP is an alternative measure that can quantify small changes in sludge bulking allowing preventative actions to occur earlier. 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 quickly (<10 minutes), resulting in a time savings relative to traditional parameters. A bulking event can take days, or potentially weeks, to evolve. If a bulking event can be diagnosed 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, need to take place. ATP has been shown to predict SVI by 10-15 days, providing a significant lead time to implement preventative measures.