During the mechanical step in the sewage treatment plant I. solid waste material settle out. II. bacteria break down 85%-90% of organic matter. III. chlorine, ozone, or ultraviolet light are used.

I. solid waste material settle out

Explanation:

In the primary sedimentation stage, sewage flows through large tanks, commonly called primary sedimentation tanks. The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are usually equipped with mechanically driven scrapers.

On the other hand, bacteria break down 85%-90% of organic matter is a biological step in the sewage treatment plant. Chlorine, ozone, or ultraviolet light are used in the chemical step of the sewage treatment plant.

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