To prevent sludge bulking from occurring, the following items should be carefully controlled in an activated sludge plant:
1. Maintain the correct F:M ratio. Carefully review plant records to determine what F:M ratio produces the best quality effluent and long term settling stability. Keep track of influent solids loadings and maintain the desired level of solids in the aeration basin through careful regulation of sludge wasting rates.
2. Prevent low D.O. levels from developing in the aeration basin. The concentration of D.O. in the aeration basin can be determined quickly and accurately using a calibrated field D.O. meter. This check should be performed on activated sludge at least once a week. There is no reason for persistently low D.O. concentrations to exist during normal conditions, provided the aeration system is adequate. However, peak flows and slugs of high-strength waste will cause the aeration basin D.O. levels to sag. Remember that the level of D.O. that must be maintained in the system in order to fully metabolize the waste load is a function of the F:M (explained earlier in this text). As a general rule of thumb, try to maintain at least 1.0 mg/L.
3. Stop grease from entering the aeration basin. This is particularly important when dealing with filaments such as Microthrix parvicella and Nocardia. Controlling grease discharges to sewers, using tight mesh barscreens or fine screens and employing primary clarifiers are the best methods for reducing grease in the aeration basin.
4. Employ anoxic zones or an anoxic cycle in the treatment train. Anoxic zones create a location where the soluble BOD in the influent is taken up very rapidly by the desirable bacteria in the mixed liquor. Many filamentous type bacteria have a hard time competing in this environment. In effect, the anoxic zone “selects” against their growth. For this reason, anoxic zones are sometimes called “selectors”. A side benefit is the controlled denitrification that can be obtained in an anoxic zone.
5. Take care not to recycle filaments back into the process by digester supernating or filtrate recycling.
Click the link to see a table of Activated Sludge Problems: Activated Sludge Problems