Question

Which of the following components is/are NOT removed in the secondary treatment of sewage?

• A. Suspended settleable organic solids
• B. Colloids including dissolved organic matter
• C. Pathogens
• D. Fat and grease

Hint:

Secondary treatment is the biological stage; it targets dissolved/colloidal organic matter, not settleable solids, floatables, or pathogens.

Answer 1

The Correct Option is A, C, D

Picture wastewater moving through a treatment plant step by step, and ask at which step each of these four components actually gets taken out.

1. Suspended settleable organic solids: these are heavy enough to sink under gravity in a reasonable time, so they are removed in the primary sedimentation tank, the very first major treatment step, well before biological (secondary) treatment even begins. So secondary treatment is not the stage responsible for removing them.
2. Colloids including dissolved organic matter: these particles are too small and too light to settle by gravity, so primary treatment leaves them behind. It is exactly this dissolved and colloidal organic matter that bacteria in the secondary biological reactor (activated sludge tank or trickling filter) feed on and convert into sludge and carbon dioxide. This is the defining job of secondary treatment, so this component is removed at this stage.
3. Pathogens: disease-causing organisms are not the specific target of biological treatment; some die off, but the numbers that survive are still unsafe. A dedicated disinfection step (chlorination, ozonation, or UV) after secondary treatment is what reliably removes them. So secondary treatment is not the stage that removes pathogens.
4. Fat and grease: being lighter than water, fats and oils float, and they are skimmed off in the primary treatment stage (or even earlier, in a grease trap), not treated biologically. So secondary treatment does not remove this component either.

Only the dissolved and colloidal organic matter (option B) is the component secondary treatment is built to remove. Everything else, the settleable solids, the pathogens, and the fat and grease, is dealt with at a different stage of the treatment train.

Let's summarize:

• Primary treatment removes settleable solids and floatables (fat, grease) by physical settling and skimming.
• Secondary treatment removes dissolved and colloidal organic matter biologically.
• Disinfection (a separate step) removes pathogens.

So the components NOT removed in secondary treatment are suspended settleable organic solids, pathogens, and fat and grease, options (A), (C), and (D).