Every waste treatment system has some bottleneck that limits treatment capacity. Many systems can operate in the short term above design capacity. However, eventually you will encounter problems controlling operations. At this point, you can supply additional oxygen, use polymers to increase floc density & settling rates, and even use activated carbon or chemical oxidation to adsorb excess organics. In today's post, I want to focus on the most common bottlenecks in biological wastewater treatment systems. Make sure you know where you have bottlenecks to prioritize capital upgrades, or develop operational plans that minimize the bottleneck impact. Here are the most common bottlenecks:

• Secondary clarifiers
  Hydraulic overloading is an obvious problem. But,  you should also note that solids flux rates are also a limiting factor. If solids do not settle and compact, the bed depth control may be lost and clarifier fails to work properly. This is why we often do maintenance evaluations on pumps, rakes, and weirs.
• Aeration (dissolved oxygen)
  Oxygen transfer is also a common problem. As fine bubble diffusers become less efficient as they foul, oxygen transfer efficiency can be insufficient for maintaining oxygen residuals in the floc. 
  
• Mixing 
  This is more of a problem in lagoon based systems, but mixing is another possible bottleneck. Improper mixing can result in short-circuiting and failure to keep MLSS suspended in the water column.
• Waste solids handling
  Wasting can be limited by digester capacity, press or centrifuge functioning, or some other solids handling issue. When the bottleneck is wasting, the system often has "old slduge" and the associated fines/turbidity seen as deflocculation occurs.
  
• Primary treatment
  Here we can have insufficient primary clarifiers, oil water separators, DAF units, and even EQ tanks in need of sludge removal. In all cases,  you have a potential for solids, oils, or high COD waste to enter the biological unit.
  
  
  

A total microbial census of MLSS using advanced 16s rRNA sequencing is interesting, but what is the practical application of this big pile of data. As with most new testing, I often get the "what does this mean" question. So, I will highlight a few ways we are using the data.

• Track populations of Ammonia Oxidizing Bacteria (AOB) & Nitrite Oxidizing bacteria (NOB). We all know that the AOB & NOB cultures are both slow growing and susceptible to upset. Genetic testing can monitor these populations and give early warning of population increase or decline.
• Find your true position on the growth curve - MCA data gives information on microbial diversity and if k-rate or r-rate strategists are dominating the biomass. This allows for great information that looks at the microbial populations rather than calculations such as F/M or MCRT.
• Systems with bulking tendencies - either filamentous or non-filamentous - have used MCA testing to monitor both filament and viscous bulking form microbes. The test helps guide operators before bulking effects effluent quality.
• Rapid information about changes in biomass makeup due to operational changes. Changing influent makeup, MCRT, aeration, or any other major control creates a change in biomass. MCA testing gives information about what is changing and how it will impact effluent quality.

The list above is how we are currently using MCA test data. By running time series tests on individual systems and comparing with other similar systems, we are able to show what is the normal biomass makeup. From there we can use data analysis an modeling for finding ways to optimize treatment and make sure the WWTP is running at peat efficiency.

While toxic shock loadings are usually found in industrial wastewater treatment plants, municipal facilities with combined industrial flows can experience loss of viable biomass from a toxic or quasi-toxic shock loading. Common causes of toxicity include:

• Phenol
• Cyanide
• Tall oil or pine oil
• d-limonene
• Solvents and surfactants in high concentrations
• Sulfides

No matter what the source of toxicity, the sequence of events follows a pattern:

1. High concentrations of the problem compounds inhibit biological activity of sensitive organisms - often this is slow growing microbes such as AOB & NOB (nitrifiers). In effect, a microbial kill.
2. Respiration rates slow, turbidity increases, deflocculation can occur, and indicator protozoa decline. 
3. Organisms that are both more resistant to the compounds, utilize the compounds for energy, or fast growing (k-rate strategists) begin log phase growth. DOUR increases.
4. As the toxic compounds are diluted and biologically degraded, the microbial population begins to drift back to pre-shock populations.

Understanding the toxic shock pathway with respect to microbial populations helps operators mount an appropriate response. Key steps include:

• Being vigilant to spills or shock loadings through routine monitoring
• Determine sources and attempt to keep toxic compounds out of the system in high concentrations
• If needed, use bioaugmentation to boost the k-rate strategists to move quickly from lag phase to log phase growth (shorten time for population adjustment)
• If high strength waste is stored in an EQ tank, begin bleeding in slowly to avoid toxic levels (remember a poison is a factor of its concentration).
• Always monitor the recovery and when you are "bleeding in" a high strength waste stream

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.