Don't make your aeration basin an aerobic digester Handing and disposing of secondary biological solids can be difficult, expensive, and at times limited by equipment. Many of us are taught that by running a longer sludge age, the cell yield will decrease and make fewer solids requiring wasting. Therefore, I have seen a tendency to move toward long sludge ages which goes with very low F/M and high levels of MLSS. You may ask - "What can go wrong with very long sludge ages?" Let me list my top observations:
- Increased utility costs keeping excess solids aerobic
- More polymer use in secondary clarifiers as pin floc forms
- Non-filamentous bulking - Thauera & Zooglea species can blossom under low F/M
- Nocardia - if you have long chain fatty acids or grease - slow growing Nocardia can enter the picture
- Problems with dewatering secondary solids - EPS entrained water can require more polymer to dewater solids
- Increased chance of metals toxicity - remember metals can build up in the floc which can inhibit nitrification
- Lower populations of k-rate (fast growing) organisms can result in low activity if a spill or shock loading event occurs. Remember microbes are different & MLSS is composed of many organisms with different ecological niches
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