Operator concept
What is activated sludge?
Activated sludge is a wastewater treatment process that uses microorganisms and added air to break down the waste dissolved in sewage. The microbes eat the waste, clump together into floc, and settle out in a clarifier, leaving cleaner water behind. It is the workhorse of secondary treatment at most municipal plants.
How the process works
Wastewater enters an aeration basin, where blowers push in air and a thick population of microorganisms goes to work eating the dissolved waste. From there the mix flows to a clarifier, a quiet tank where the microbes settle to the bottom as a blanket of sludge while the clear water above moves on. Some of that settled sludge is pumped back to the front of the basin as return activated sludge, so there are always enough hungry microbes for the incoming flow. The extra, called waste activated sludge, is removed and sent to solids handling.
Why operators care about it
Activated sludge is a living process, so a good operator manages the microbes like livestock: enough air, the right amount of return sludge, and a healthy balance between food and organisms. Get it right and the effluent is clean and clear. Let the air drop or the balance slip and the sludge stops settling, so the plant loses solids over the clarifier weir. That is why the process shows up all over the Class D wastewater exam.
Common questions
- What is activated sludge in wastewater treatment?
- A process that uses microorganisms and added air to break down dissolved waste in wastewater. The microbes clump into floc and settle out, leaving cleaner water.
- How does the activated sludge process work?
- Air and microbes mix with the wastewater in an aeration basin, where the microbes eat the waste. The mix flows to a clarifier where the floc settles. Some settled sludge is returned to keep the microbe population going, and the excess is removed.
- What is return activated sludge (RAS)?
- The settled microbes sent from the clarifier back to the aeration basin to keep enough active organisms working on the incoming waste.
- Why is it called activated sludge?
- The settled solids are full of living, active microorganisms, so returning them seeds fresh wastewater with hungry microbes and keeps the process going.