By: Mike Baumgartner, ARWA Wastewater Technician

Dissolved oxygen or D.O. is a term we use in the wastewater business quite often. It is one of the big three parameters we control in order to have a successful process. The other two are control of organics (food) and population of microorganisms. Let’s discuss D.O. and how it is utilized.

It is a measure of the amount of free oxygen present in wastewater or mixed liquor expressed in mg/l. Remember, mg/l is the same unit as ppm. Because the treatment process is considered an aerobic condition, our goal is to maintain a metabolic process that produces energy for growth and reproduction. It is important to recognize that after the microorganisms consume their demand for oxygen there is a residual left over. That is the number you read on the meter. It can be expressed as: Supply – Demand = Residual.

Why is this important? We all have read that the D.O. in an aeration basin should run from 1.0 to 3.0 mg/l. Let me suggest to you that anything over that is a WASTE of energy and money. You will get no more benefit. The 1.0 to 3.0 mg/l should be maintained because it is the “bugs” driving force in performance. If you are having issues nitrifying and the ammonia levels are rising – it is most likely a D.O. problem. Remember, nitrifying bacteria need oxygen to work so feed them the driving force – D.O.

First appeared in Waterline, Spring 2015

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