- 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.
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.
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:
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.
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:
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:
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:
(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.
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