F/M ratio & operating wastewater treatment systems

Food to microorganism ration has long been used to operate activated sludge sytems. I will start out saying calculated F/M should not be the only operational parameter you shoud evaluate. F/M has good and bad points that should be considered. As with all wastewater control information, you should use F/M with other data points and knowledge of your treatment system. 

F/M background

• Food in F/M is usually BOD5 in lbs or kg per day. The 5 day lag in data can be a problem so many systems use either COD or TOC as a substitute. If you want to use either COD or TOC, you need to find the average ratio of either to BOD5 to compare your F/M to other systems or design parameters.
• M is often referred to as microorganisms - again in lbs or kg. I favor calling M - the mass as it includes living microbes, dead microbes, EPS, adsorbed organics, and particulates in a matrix. Typically we use MLVSS as the proxy for M, but some systems use MLSS which requires less time and equipment. Just note which test you are using! 

If you think about F/M from an ecological perspective, you can see many underlying problems with using this as a control parameter. First, loading (F) can be many things and some compounds are more readily biodegraded than others. It also does not consider nutrient removal in the F. Secondly, the M ignores the biomass viability and all solids are assumed to be microbes.

In addition to F/M consider the following parameters that you probably already run

• MCRT (mean cell residence time)
• Microscopic exam 
• SV30/SVI
• Clarifier bed depth
• Oxygen Uptake Rates (OUR & SOUR)
• D.O. in aeration basin
• ATP 
• Molecular testing (qPCR and MCA) - a less frequent test but very powerful as it looks directly at the microbes functioning in the system.
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In the end, don't obsess with a single F/M data point. Relate F/M to other tests and make managerial decisions based on all pertinent data. Use your judgement and knowledge of our system.