We must provide oxygen to the microorganisms to drive BOD and ammonia removal.  Approximately 1 lb of oxygen is needed for every lb of BOD5. Approximately 4.3 lb of oxygen is needed for every lb of ammonia. To ensure enough oxygen is provided, we typically maintain a Dissolved oxygen (DO) concentration in the range 2 mg/L within the aeration basin.  This 2.0 mg/L value is the point of diminishing returns based on lab testing of microbial growth rates.  DOs higher than 2.0 will cost you more $$ and bring incrementally less benefit.  Aeration costs are commonly 50% of a WWTP’s operating budget.  So ensuring you are not over-aerating literally saves money.

There are numerous ways to controlling the amount of oxygen provided including constant airflow, timer-based airflow, DO-control, and ammonia-based control. How do you control the amount of oxygen provided?