Which of the following are colloidal material in domestic waste?

The correct answer is Option (2) → Fecal matter and bacteria

Domestic sewage contains suspended solids such as sand, silt and clay, colloidal material such as faecal matter, bacteria, cloth and paper fibre, and dissolved materials such as nitrate, ammonia, phosphate and sodium.

Option (1) Suspended clay and silt — Incorrect. Clay and silt are suspended solids in sewage, not colloidal material. They settle out in primary treatment.

Option (2) Fecal matter and bacteria — Correct. Domestic sewage contains suspended solids (sand, silt, clay), colloidal material and dissolved materials. The colloidal material is faecal matter, bacteria, cloth and paper fibres; clay and silt are suspended solids, and nitrate, ammonia, phosphate and sodium are dissolved.

Option (3) Dissolved ammonia and calcium — Incorrect. Ammonia and calcium are dissolved materials, like nitrate, phosphate and sodium. They are not colloidal.

Option (4) Fecal matter and clay — Incorrect. Faecal matter is colloidal, but clay is a suspended solid. So this pair mixes two different kinds of material.