Colorado Wastewater (WW) Operator D Practice Exam

In the activated sludge process, soluble BOD is removed mainly through microbial metabolism. Microorganisms in the aerated mixed liquor take up soluble organic matter (absorption into their cells and use as a substrate) and oxidize it for energy, converting it to biomass and carbon dioxide. This biological oxidation is what lowers the BOD in the treated water.

Filtration would only separate solids and doesn’t address dissolved organics. Dissolution isn’t a removal mechanism—if anything, it would keep substances dissolved. Precipitation would require chemical reactions to form insoluble solids, which isn’t how soluble BOD is removed in this system. Thus, absorption (uptake by microbes for metabolism) is the best description of how soluble BOD is removed.