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.

We all know the best way to prevent grease accumulation in collection systems is to "keep grease out". Even with well maintained grease traps and public education, collection systems tend to suffer from grease accumulation in problem areas such as bends, bellies, slower flow zones, and lift-stations.

As I biological treatment person, I was involved in multiple field tests that used biological maintenance options. Of course some worked better than others, this post will detail some of the things that we found when going from lab to the field.

As an insoluble organic, grease (usually long chain insoluble fatty acids) causes problems in the following ways:

• Sticks to pipe walls which restricts flows. In severe cases, this can cause SSO.
• Harmful to lift station equipment and anaerobic zones under grease film produce H2S
• In wastewater plant the FOG is slow to degrade and can cause floating sludge/scum
• Can promote hydrophobic microbe growth - the most seen is Nocardia

So what did we learn from numerous field tests

​Grease traps
Grease traps are not checked enough - often restaurant managers barely know they exist. Inspecting the trap can lead to timely pumping. Biological additives, when used in a maintenance program, can reduce BOD5 and FOG entering the colletion system. The best biological products in testing were metered liquid formulations containing fatty-acid degrading cultures. Side benefits to stress to restaurant managers include - fewer mystery odors in parking lot and reducing emergency plumber call outs for drain line clogs. 

Gravity mains
We looked at manual dry product dosing and adding liquid cultures with a metering pump. The liquid by virtue of more frequen dosing fequency was more effective. Grease accumulation rates decreased and the grease that accumulated was partially degraded and easily jetted from pipe walls. Camera guys noted the grease was "fluffy" versus the normal hard grease seen in problem lines. Keys for success in treating gravity mains included dosing upstream of the problem. It is best to do a survey and find sources of the grease and dose at a point where grease degrading biofilm can be established upstream of the problematic site. This is where experience comes into play and we can use tools to find the best dosing point.
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Lift Stations
Having worked with time-release blocks, manual dry products, and metered liquid blends; I have found that the best product depends on lift-station size, amount of grease, and access issues. Often if we treat grease traps and gravity mains upstream, the lift-station grease is controlled. This is because bacteria are able to grow on pipe surfaces forming a grease degrading biofilm. This biofilm contains aerobic and microaerophilic organisms that are the same ones seen in healthy wastewater treatment plants. So by treating collection systems, you are moving biological wastewater treatment upstream. We have used this to lower loading to WWTP that are undersized for increased influent loadings while waiting on new treatment plant construction.

Here are a few take-home messages from the trials:

• Grease degrading microbes come in many forms - it is not just Nocardia. You can favor specific ones to prevent Noardia foams on aeration basins by doing upstream  pretreatment.
• It is best to keep grease out of a system via education. However some still enters the collection system. Treating upstream is most efficient. 
• Treatment is not as simple as ordering something out of a catalog and hoping for the best. A collection system treatment program should involve investigation, selecting proper product for the location, and finding proper addition points. 
• Collection system programs behave by natural laws. There are no bug steroids, super bugs, or anything that has a mystery. We know that controlling the environment, promoting the right organisms, and seletively modifying microbial populations can achieve treatment goals. 

I often hear that ammonia removal across a system is low based only on influent and effluent ammonia numbers. I often ask what is influent and hear that the system does not run TKN. TKN is more difficult to run than ammonia, but is very useful for systems with significant influent organic nitrogen. 

For dicussion purposes, TKN = Ammonia + Total Organic Nitrogen. It does not include nitrite or nitrate. 

Influent TKN takes into account the generation of ammonia from biologial waste degradation by heterotrophic organisms. While some of the nitrogen is used in cell division and other metabolic processes, excess is converted biologically into ammonia. It is this ammonia that is used by your nitrifiers (ammonia oxidizing bacteria & nitrite oxidizing bacteria). 

So when you are looking at ammonia removal effiency across a system, you should look at influent TKN as a better indicator of efficiency than using influent ammonia. Now in some systems influent TKN is 90% ammonia. In that case, influent ammonia is fine. If your TKN:Ammonia ratio is consistent, you can just run influent ammonia. Others with inconsistent organic nitrogen loads should run TKN daily.

If I ask people about their problems with low temperatures, the most common problem is maintaining ammonia oxidation (AKA Nitrification) during winter months. So why can we see decent BOD/COD removal with low temperatures and not ammonia/nitrite oxidation to nitrate? 

It all has to do with AOB & NOB having slow growth rates under all conditions that become more exagerated under low temperature conditions. Secondly, genera that oxidize ammonia/nitrite are mesophilic organisms that don't do well at temperatures under 10 - 12 Deg C.

Nitrifier Growth Rates
Ammonia Oxidizing Bacteria (AOB) - often a mix of Nitrosomonas, Nitrospira (the COMAMMOX subset), Nitrosococcus - are the slowest growing of the chemotrophic nitrifiers. That means the initial conversioni of ammonia into nitrite is usually the rate limiting step. The AOB growth under ideal conditions (D.O. > 2 at cell level, pH 7.2 - 8.2, sufficient alkalinity, and no inhibition) maxes out at 10 - 12 hours versus 30 - 60 minutes for many heterotrophic (BOD removing) organisms. AOB ideal growth happens between 25 - 30 deg C.  As you decrease temperature below 20 Deg C, time required for cell replication increases and if operating at a low MCRT, you may see the AOB population "washout".

More on Temperature Impact on Nitrifier Population
Nitrifiers growth slows as you drop below 20 deg C and is so slow that by 10 Deg C that we consider the cultures dormant. Unlike the heterotrophic populations that can adjust to lower temperatures with low range mesophiles and some psychrophiles that thrive at temperatures between 3 - 15 deg C. The increase in lower temperature heterotrophic organisms is the reason that you can keep BOD removal even with low temperature wastewater. In fact, I have worked with cultures that grow well at 5 deg C. 

Ways to keep nitrification at low temperatures
Now that you know why nitrification is a problem at lower temperatures, what can you do to keep ammonia from reaching the effluent? Here are some suggestions:

• Increase MCRT to avoid washout
• Keep pH, Alkalinity, and D.O. at ideal levels 
• Try to keep inhibitory compounds amines, sulfides, phenol, solvents, and other inhibitory compounds out of the biological unit as much as possible
• Moving from surface aeration to diffusers can help "insulate" and reduce temperature drop across the biological unit