Most activated sludge systems have two things in common...
Insufficient mixing which leads to poor solids suspension, poor processing, and potential violations and
Excessive energy use & cost trying to accomplish item #1.
There are other challenges too of course. Odors need to be kept below the "neighbor noticing" threshold. Issues with nitrification / denitrification can easily become large permitting problems. And let's throw in chlorination / dechlorination and THM removal in there for good measure.
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