Bulking sludge, plainly stated, is when the volume occupied by settled sludge in the clarifier is greater than usual and is the result of limited compaction/consolidation of sludge, often times leading to greater solids loss from the clarifier than would otherwise be considered normal. Settling of mixed liquor suspended solids (MLSS), most often referred to as sludge, is a four stage process that includes: flocculation, beginning of blanket, settling and compaction. Many of the bulking sludge issues in the clarifier can be traced back to excessive MLSS in the treatment system, increased hydraulic loads to the system leading to excessive accumulation in the clarifier, excessive flow rates leading to expansion of the blanket or a reduction in the return activated sludge (RAS) pumping rate. Some process conditions, however, are not so much dependent on the volume of solids or flow. Rather, they are the result of the type and relative population of biology present. In many cases the predominance of filaments plays a large role in reduction in compaction. Many operators think that filaments don’t exist in the plant at all when the plant is operating properly and settling issues don’t exist. To the contrary, while the presence of filaments does not adversely affect the treatment process in relatively small numbers, they can create settling and compaction issues if their numbers become too great.
In the case of excess MLSS, the obvious corrective action is limiting solids accumulation in the secondary clarifiers. To this end the corrective action depends on the cause of the excess accumulations. Is bulking due to the MLSS concentrations in the aeration tank and/or clarifier? If due to solids in the clarifier, increasing the RAS rate may be enough to remedy the bulking issue. If solids concentrations in the aeration tanks and accumulations in the clarifiers are issues then wasting solids from the treatment system is necessary. To calculate the volume of waste activated sludge (WAS) to remove, the operator needs to know the target and existing MLSS concentrations along with the WAS concentration.
Calculation:
Existing MLSS mg/L – Target MLSS mg/L = MLSS to be removed mg/L
MLSS to be removed mg/L X Aeration Volume (MG) X 8.34= MLSS to be removed Pounds
MLSS to be removed Pounds / WAS mg/L / 8.34 X 1,000,000 = WAS to be removed Gallons
Example:
Existing MLSS (3,236 mg/L) – Target MLSS (2,750 mg/L) = MLSS to be removed (486 mg/L)
MLSS to be removed (486 mg/L) X Aeration Volume (0.435 MG) X 8.34 = MLSS to be removed (1,763 pounds)
MLSS to be removed (1,763 pounds) / WAS (8,500 mg/L) / 8.34 X 1,000,000 = (24,872) Gallons
Example:
Existing MLSS (4,250 mg/L) – Target MLSS (3,500 mg/L) = MLSS to be removed (750 mg/L)
MLSS to be removed (750 mg/L) X Aeration Volume (0.435 MG) X 8.34 = MLSS to be removed (2,721 pounds)
MLSS to be removed (2,721 pounds) / WAS (8,500 mg/L) / 8.34 X 1,000,000 = (38,383) Gallons