What Is a Biological Sewage Treatment System and How Does It Work?

What Is a Biological Sewage Treatment System and How Does It Work?

While most sewage is treated by local municipalities, it is done so in various ways. In the United States, these local municipal facilities must follow both federal and local regulations in terms of the purity requirements for the treated effluent, and although they are treating roughly the same types of human waste, their methods for treating these wastes and to what degree are what varies.

Despite the different ways municipal facilities might treat their local community’s waste, biological sewage treatment is often implemented at some point of the process—but what is a biological sewage treatment system and how does it work?

Below is an overall explanation of these systems for those that are new to the subject or have limited knowledge. This general introduction will explain some of the more basic concepts.

What is a biological sewage treatment system?

Sewage, or human-generated wastes, is typically treated by publicly owned treatment works (or POTWs), which are all either city, county, or regional facilities. Depending on how the facilities are funded and built will dictate how they are operated, but in general, these systems process these wastes, separate out all the solids, and treat what’s left making manageable byproducts. Biological sewage treatment systems are a small part of a larger process that treat human wastes by using bacteria and other microbial accomplices to break them down into other byproducts such as sludge.

How does a biological sewage treatment system work?

For the most part, POTWs run their biological sewage treatment systems aerobically (which means they use bacteria that require oxygen to break down the wastes) using a process called “activated sludge.”

Simply speaking, the activated sludge process is a secondary treatment method that occurs after untreated wastewater is collected from throughout the city, travels through all the sewers, and enters pump stations and transfer stations that funnel wastes to the treatment plants. Here, it enters a series of pretreatments of primarily solids removal (screening, clarifying, grit removal, etc.) before it comes to the reactor basin, which is where the biological portion of the sewage treatment, or activated sludge process, takes place.

In the reactor basin (this is where all the bacteria and microorganisms are housed, fed, and aerated with supplied oxygen), the water flows through and is treated with the microorganisms. The treated water then flows through another clarifier where the biosolids (all the solids made after the biological work is complete) are separated out while the microorganisms are retained. In this clarifier, they settle to the bottom of a cone-shaped base and are either returned back to the basin as activated sludge or wasted as solids whereby they are dewatered and taken to an anaerobic digestor or farm.  

In short, sewage activated sludge is process that combines the untreated wastewater with bacteria allowing the bacteria to grow on all the constituents in the wastewater and break them down while retaining the bacteria by balancing your returned activated sludge with your wasted activated sludge.

You’re basically:

  • growing the organisms in a suspension and retaining them
  • mixing the wastewater with the biomass
  • aerating this “mixed liquor” so the bacteria can get to work
  • settling out the mixed liquor suspended solids (MLSS)
  • sending return activated sludge (RAS) to the reactor basin
  • sending waste activated sludge (WAS) to be dewatered and treated accordingly

These systems manage the concentration of bacteria in the reactor basin by how much of the activated sludge becomes RAS or is wasted as WAS. These levels keep the number of suspended solids (which act as a catalyst) at a fixed point designated by the plant operators by essentially controlling how much food the microorganisms get.

In short, they’re managing MLSS (which is the bacterial concentration in the reactor basin) by how many clarified solids are returned and how many clarified solids are wasted.

Biological sewage treatment system variations

If you are able to wrap your head around the initial concept described above, you now understand about 80% of how biological sewage treatment systems work. The rest of this article will highlight different variations of this process, which are most often extra rounds of treatment and will vary based on the contaminants present and effluent regulations.

For example, sewage wastewaters have varying amounts of have BOD, ammonia, and nitrates, and biological sewage treatment can address them all. The system, in these cases, will utilize the same basic principal design of the aerobic clarifier with RAS and WAS sludge, but now you’ll be introducing anoxic bacteria and, along with it, different flow patterns.

The sewage will often have proteins from RNA, DNA, and nucleic acids, which come from all kinds of nitrogen organic compounds. Carbon oxidation occurs after the carbon oxidizing bacteria break down these compounds, and this process creates ammonia. Because there is ammonia now, the sewage treatment is going to include biological nitrification to remove the ammonia, which gives you nitrates. And after all the ammonia is converted to nitrates, bacteria—either anaerobic or anoxic—reduce all the nitrate and create nitrogen gas, which dissipates into the atmosphere.

In short, the organic nitrogen compounds are removed through the combined process of nitrification and denitrification. Here, you won’t just have RAS, but now you’ll have an internal recycle from the aerobic reactor basin back to the anoxic, where the nitrate gets reduced to nitrogen gas, so the same concept but with different configurations.

Sometimes this cycle needs to be repeated (anoxic, aerobic, anoxic, aerobic) before it heads to the clarifier. Again, this all depends on the level of contaminates in the sewage and the governing regulations on effluent purity.

In general, these systems should be on the easier side of operation. It might, however, get a little difficult when the bacteria in the secondary clarifier do not settle. The whole process really depends upon the bacteria settling in the clarifier, because if they don’t settle you can’t RAS and control MLSS and you’ll see your system’s performance start to drop.

Indication that you might have flocculation and settling issues is when you start to see filamentous bacteria, which floats and forms microbial mats that hang around on the surface instead of settling to the bottom pf the clarifier where they can be collected. Managing these gets a bit into the more complicated sewage treatment systems topic, such as regulating residence times, oxygen levels, mixing, pH, nutrient levels, etc., but generally speaking, you should know that flocculation and settling is important to the process and operators will balance all these variables to ensure that happens.

Can SAMCO help?

SAMCO has over 40 years’ experience custom-designing and manufacturing biological wastewater treatment systems, so please feel free to reach out to us with your questions. For more information or to get in touch, contact us here. You can also visit our website to set up a call with an engineer or request a quote. We can walk you through the steps for developing the proper solution and realistic costs for your biological wastewater treatment system needs.

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