Gravity separators are used to remove which from wastewater?

• A. Suspended Solids
• B. Free Oil
• C. Dissolved Salts
• D. Bacteria

Answer: (B)

Gravity separators remove immiscible liquids from water by using density differences. Oil is lighter than water, so in a quiet, long residence time environment the oil droplets or film rises to the surface and can be skimmed off. This is ideal for wastewater that contains free oil or grease, separating it as a distinct oil layer before discharge. Dissolved salts stay dissolved and require other treatment methods, while bacteria need disinfection or biological treatment rather than separation by gravity. Materials like suspended solids are typically removed by clarifiers designed specifically for solids, not by gravity oil–water separators. So the function is best described as removing free oil from wastewater.