The correct option is C Flocculation
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In Primary treatment, the solid waste which is not soluble in water is separated out. This can be done through screening, which filters out the wastes that are slightly larger in size. Sedimentation is used to remove solids that settle at the bottom of the tank at the treatment plant and Flocculation in which fine particulates are clumped together so that it can be easily removed. Secondary treatment uses biological processes to treat the water. One of these processes include bioremediation, in which living organisms are added to the waste to degrade it organically or transform contaminants or to reduce them to environmentally safe levels.