Bacteria form biofilms (in wastewater we call this floc) for many reasons. Among the most important is to protect the individual cells from the surrounding environment and capture/concentrate nutrients necessary for growth. Montanta State University actually has a group focusing on biofilms - http://www.biofilm.montana.edu/

The researchers at Montana State have created a great infographic on why biofilms are important that I want to share. It is a very good read.

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. 

When we use the term "bugs" for our wastewater microbial poulation, we are simplifying what is actually a community of thousands of microbial genera. The organisms form functional groups that ensure complete waste treatment. An important part of this action is the formation of Extracellular Polymeric Substances (EPS). The EPS is the glue that holds cells in a biofilm or floc. EPS is composed of polysaccharides, proteins, enzymes, and DNA that form a matrix containing bacteria, particulates, and stored organic compounds. Even filamentous bacteria are needed for "best" quality floc. As long as the filmanets are functioning in their macrostructure role and inside the floc, the flilaments are beneficial. So maintaining EPS at proper composition and levels is key to separating the biomass from treated effluent even if you use clarifiers, DAF, MBR, or other solids separation system. Now for a few common reasons why secondary polyer demand can increase:

• Filamentous bulking - bulking is where filaments bridge floc and become the dominant microbial form. If you see filaments at a 5 or 6 (Scale 0 - 6), then you should go through the identification of the filament, associated cause, and take corrective measures. This is where someone with filamentous bulking expertise can help you with the best control options.
  
  
• Non-filamentous or Zoogleal bulking - this is a problem with the natural slime or biological polymers produced by microorganisms in the MLSS. It is much more common than people realize. Like filamentous bulking, you need to indentify the cause and work to get the excess EPS out of the system by wasting. We are now having success monitoring for the organisms responsible of non-filamentous bulking using qPCR and increasing wasting rates before bulking is observable in the MLSS.
  
  
• Old Sludge (Low F/M) If the problem is "fines" or turbidity - you can look at "old sludge" which is very low F/M where microbes start to degrade the biopolymers holding floc together. Unless your influent organics concentration will soon increase, you should waste to maintain a proper F/M ratio. Once you are in the ideal F/M zone for your system, it should start to clear itself up.

Keeping stable Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) populations in wastewater can be a challenge. Some factors that can result in population instablity include:

• Low or high temperatures
• High organic loading - 80% of BOD5 typically needs removal before nitrification starts
• Low D.O.
• Sludge age is less than needed to support populations of slower growing AOB/NOB
• Inhibitory compounds (many things can slow or totally inhibit AOB/NOB growth)

Over the past several weeks, I have been watching Aster Bio's lab run a very intesting test for examining waste streams for chronic toxicity or inhibition. Each reactor was run as a sequencing batch reactor (SBR) and we monitored outlet ammonia, nitrite, and nitrate. We also diretly measured the AOB/NOB populations in the MLSS using qPCR testing. The goal was to determine the impact on AOB/NOB populations of both the individual waste stream with varying dilutions.

What I have learned or seen proof of working during the testing

• AOB cultures are more sensitive to changes in waste makeup & inhibition than NOB
• Changes in influent makeup often cause a drop in AOB/NOB numbers. But without significant inhibition, the AOB/NOB populations will naturally increase - this time lag can be reduced by adding AOB/NOB culture slurry.
• Inhibition is rarely just one single chemical in wastewater. Several inhibitory compounds can combine to inhibit growth.
• Various pre-treatment and in-biological unit options exist to reduce the toxicity and promote AOB/NOB growth. Optimizing the growth environment for nitrifiers is important including removing BOD5, degrading inhibitory compounds (phenol, biocides, surfactants), and keeping environmental factors in the optimal range.