Cell yield is the amount of biological solids per unit of loading. Loading can be described as specific chemicals (ex. glucose or ammonia) but often we use BOD5 or COD as a proxy given the mixed nature of wastewater influent.  Cell yield is higher when the energy source (food) provides more energy for cell division. 

For domestic wastewater BOD5, we usually use 0.5 - 0.6 grams biomass per gram BOD5 in the influent as the yield (Y). In this case biomass includes living cells, extra cellular polymers (EPS), and all substances in the floc (MLSS). As handling and disposing of biological solids is expensive, often we would like to lower yield or amount of wasted biosolids.

Potential ways to decrease solids yield:

• Increase sludge age (Lower F/M) - by increasing biological solids in the system, you are moving along the growth curve to endogenous respiration or decline phase growth. With very low F/M, cells consume adsorbed organics and extracellular materials. This is exactly what happens in an aerobic digester. Costs for running in extended aeration mode include more energy requirements for maintaining D.O. and clarifier solids capacity to handle the increased solids flux (more solids to clarifier).
• Fixed film or MBBR addition - The addition of biofilm technology to an activated sludge system works by increasing biological solids. The film part keeps the biological solids out of the secondary clarifier. You still need to budget for more oxygen requirements from added biomass in the aeration basin.
• Bioaugmentation - You often read about adding enzymes, micronutrients, or 'bugs' to a system and seeing reduced cell yield. This works in higher F/M conditions where adding microbes helps move along the growth curve. It also helps when influent components, such as FOG, create conditions where water and insoluble compounds build up in the MLSS resulting in larger volumes of solids (bacteria yield is actually the same but you have less water and EPS in the wasted solids). 
  

Biological solids production is a given in a wastewater system. You can reduce solids by operating at decline/endogenous phase growth, but this requires energy for D.O. , maintaining solids in suspension (or providing a MBBR/fixed film support), and having secondary clarifiers capable of handling the biological solids  without carryover. No matter what strategy you use to minimize biological solids yield, you must eventually waste solids. What we call old sludge contains more particulates, dead cells, and inorganics than target MLSS. When you start consuming EPS that binds foc, the fine particles we call pin floc start to increase effluent turbidity and TSS. 

The daily microscopic exam gives you information on multiple parameters that effect your wastewater treatment system. Consider that a quick look under even a basic microscope gives you information on organic load (BOD5), dissolved oxygen, toxicity, settling potential (floc formation), and early warning of bulking. All this with a quick test, no specialty test reagents, or complex equipment. While we all talk about stains and phase contrast microscopes, even a lower end light microscope gives your great information as long as you look at the biomass daily. If you only have a simple light microscope, note the following and you will be well on the way to having a great test for detecting potential changes and problems. Here is what to note:

1. Floc - even at 10x objective (100x magnification on most microscopes), you can take note of floc size, density, and amount of non-biological particles in the MLSS. Also not clarity of water between floc - this indicates if you have smaller free bacteria or particles in solution.
2. Filaments - while not as obvious without phase contrast, filaments become visible when bulking occurs. You can also make out the "fingers" of non-filamentous bulking also called Zoogleal bulking. If you have ongoing filament problems and no budget for phase contrast, invest in a Gram Stain kit. The Gram Stain helps improve the view for about $50 for many tests.
3. Protozoa & other indicator organisms - these are the easily seen and interesting organisms to monitor with the microscope. Protozoa are unicellular organisms that "graze" on bacteria and other things in the water. Unlike bacteria, we can see protozoa type and activity with any microscope. Download a chart as a guide to evaluating protozoa and seeing how these guys relate to F/M, sludge age, and biomass "health". I have one here if you don't have one: https://www.biologicalwasteexpert.com/useful-information.htm

Microscopic exams are most useful when run daily. If we only look when problems happen, you do not know what the MLSS looks like under good operating conditions. Also knowing early signs of problems, helps alert you to changing conditions and you can prevent bigger problems. No matter how old or simple,. dust off the microscope and use it.

(If your microscope is in bad shape, there are cleaning/repair shops that can make it like new for much less than the cost of buying a new microscope. (I use Land, Sea and Sky here in Houston for regular cleaning and maintenance on my microscopes. If you are not local, you can arrange to ship for cleaning/maintenance.)

Do not overlook the power of simple observation in running a wastewater treatment plant. Changes in color, odor, or foam all tell you that something has changed. Having received several questions on aeration basin foam, I figured it was time to make a table on causes and cures for foam. While not an exhaustive list, these are the foams that I see most often.

Anytime you operate under anaerobic conditions, you are subject to growth of unwanted sulfur reducing bacteria that produce hydrogen sulfide. The SRB organisms use sulfate and elemental sulfur as an electron acceptor for cellular respiration.  In a digester with no free oxygen, nitrate, or other more favorable energy yielding electron acceptor, the SRB organisms can obtain more energy than the desirable acid forming (fermentative) bacteria that produce short chain organic acids and hydrogen used by the methanogens. Having looked at studies of anaerobic digesters, there are ways to optimize the environment for the "good" acid forming bacteria and reduce conditions favoring SRB.  Here are the options that don't involve extensive capital or chemical expense.

• Remove highest SO4 containing streams from the digester
• Increase influent pH - SRB do not thrive above 8.0 pH (literature gives 8.5 as a good target)
• Maintain proper digester temperature
• Add micronutrients if you suspect an influent lacking trace metals - Cu, Fe, Ni, Zn, Co, Mn, Mg - remember some of these can be toxic if too high - so check your influent and effluent residuals.

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