Primary treatment of sewage mainly targets the removal of which class of contaminants before biological secondary treatment?

Introduction / Context:
Primary treatment prepares wastewater for biological processes by reducing settleable and floatable load. Clarifying its principal objective helps in sizing sedimentation tanks and anticipating downstream loads.

Given Data / Assumptions:

• Municipal sewage with typical solids spectrum.
• Conventional preliminary units (screens, grit chambers) are upstream.
• Primary settling tanks follow preliminary treatment.

Concept / Approach:

Primary treatment comprises sedimentation to remove settleable suspended solids (a large fraction of which are organic) and scum skimming. While oil/grease and grit are addressed too, the main quantitative removal is of suspended solids by gravity settling, cutting BOD by about 25–35% prior to secondary treatment.

Step-by-Step Solution:

Verification / Alternative check:

Mass balances show primary sedimentation removes significant TSS and a portion of BOD; dedicated API separators are used when oil/grease is the main concern.

Why Other Options Are Wrong:

(b), (c), and (d) each cover only a subset; primary treatment is broader but quantitatively centered on suspended solids removal via settling.

Common Pitfalls:

Equating primary treatment solely with screening or grit removal; overlooking scum removal as ancillary to sedimentation.

Final Answer:

Large suspended organic solids by sedimentation