In activated sludge wastewater treatment, secondary clarifiers are designed to prevent TSS carryover to the effluent and concentrate biological solids (MLSS) for return to the aeration basin. A balance must be maintained by giving solids time to settle and compact, yet not so much time that the microbes in the bed become anaerobic. What we mean by anaerobic clarifier beds is that all dissolved oxygen is depleted. If nitrate/nitrite is present from ammonia oxidation, the microbes first turn to nitrate creating small bubbles and floating solids. If there is no nitrate, the microbes will start fermentative respiration and sulfate reduction. Both of these can be a problem for odors and creating what we call septic sludge.
Most clarifiers are designed for between 2 - 4 hours hydraulic residence time, solids are recycled at a rate to prevent long solids residence times. A rule-of-thumb is to keep bed depths at 2 feet. With most clarifiers, a 2 foot bed recycles sufficiently concentrated MLSS while also preventing anaerobic conditions in the clarifier. Systems with bulking or mechanical problems often start to have trouble with 2 foot bed depths. Another problem with running deep clarifier beds is the potential for rake problems, usually torque on the motor.
Good process monitoring habits for secondary clarifiers:
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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