Sewage pollution can be prevented by treating sewage before passing into water course in sewage or effluent treatment plant. The sewage treatment involves -Primary treatment, secondary treatment and tertiary treatment. Primary treatment consists of shredding, churning, floatation, screening and sedimentation. In Secondary treatment the organic matter is decomposed by microorganisms and through sewage fungus by passing through water hyacinth pond, trickling filter method or activated sludge method. Aeration helps in oxidation of sludge. Tertiary treatment is a physico-chemical treatment for removing turbidity and dissolved substances. So, the correct answer is 'Biological process'