Activated Sludge

Imagine you are on a conveyor belt. The conveyor belt is circling around a box of pizza. Every time you get close enough, you take a slice of pizza. If the conveyor belt moves too fast, you can’t grab the pizza. But if it’s too slow, you’ll starve and the pizza won’t be nice and hot anymore and it might even start to grow mold. This is how the activated sludge process was explained to me in my first Environmental Engineering lecture many moons ago.

activated sludge
A simplification of the activated sludge process.

The activated sludge process is biological. I blogged about algae-eating fish that can be used to clean water in a previous post, but this time I’m talking about using bacteria to remove unwanted nutrients. This method is used primarily in wastewater treatment because the bacteria required for the process are already present in wastewater.

An aeration tank image by Water Technology.

In order for the natural aerobic bacteria to consume the nutrients in the water, they must be encouraged with ample oxygen so they can breathe while they eat. An aeration tank provides oxygen for the aerobic bacteria. Bacteria will feed and cluster together in what we call flocs. Flocs are preferable because they settle to the bottom of the basin leaving clean water on top. However, too much aeration or mixing can break the flocs apart.

Schematic from Lenntech.

At the bottom of the settling tank, or clarifier, there is a return valve that acts as the conveyor belt for the bacteria. The valve circulates the sludge back to the beginning of the process. A small waste valve attached to the return pipes can be opened to remove bacteria when there starts to be too much. The waste rate serves as the control of how much bacteria there are and how quickly the water is sent through the system. Waste sludge can be centrifuged to extract any extra water and then may be disposed of.

Sewage treatment process from eSchooltoday.

Activated sludge is just one step in the wastewater treatment process. Before reaching this stage, wastewater has to pass through a screen and grit remover. There are primary and secondary clarifiers that settle out large particles, and a final filter and disinfection stage. All in all, wastewater treatment with activated sludge is an important process because it removes the nitrogen and phosphorus in our water that can really damage ecosystems by creating eutrophication and algal blooms.

Algal blooms can be toxic. Image from NASA Earth Observatory.

I hope you learned something today! If you take away anything, it should be that there are beneficial uses for bacteria. You can also see this in the aquarium nitrogen cycle, where fish owners use beneficial bacteria to convert nitrite to nitrate. Like my drawing? Comment below. And thanks for reading as always, Jess ❤

More on the activated sludge process: The International Water Association

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