Tertiary sewage treatment is used mainly to remove which of the following…

Concept

Primary and secondary sewage treatment mainly remove settleable solids and biodegradable organic load. Tertiary treatment is the polishing stage used when residual contaminants must be reduced beyond secondary-treatment levels.

A major purpose of tertiary or advanced treatment is nutrient removal. Nitrogen is reduced through processes such as nitrification-denitrification, while phosphorus is reduced through chemical precipitation or enhanced biological removal.

Application

Classify each listed substance by the treatment objective it represents:

• A. Nitrogenous compounds are residual nutrients whose discharge can promote eutrophication; advanced treatment includes dedicated nitrogen-removal processes.

• B. Phosphorus compounds are residual nutrients; tertiary chemical or biological processes are used to reduce them.

• C. Humic materials are refractory natural organic matter. Specialized advanced processes may reduce them, but they are not the principal nutrient pair targeted in the standard stage-wise classification used here.

• D. Organic waste denotes the broad biodegradable load measured through parameters such as BOD; its main removal occurs during primary settling and secondary biological treatment.

Contrast and cross-check

• A and D only mixes one nutrient class with the broad biodegradable organic load.

• B and C only mixes one nutrient class with refractory natural organic matter.

• C and D only contains organic-matter categories rather than the two nutrient classes.

• A and B only contains both nutrient classes addressed by advanced nutrient removal.

Therefore, the best-supported answer among the offered choices is A and B only.