Complex sentence: Secondary treatment employs biological oxidation to remove nearly all suspended solids and other impurities, And, tertiary, or third level, of treatment which effectively removes bacteria, viruses and virtually all suspended solids.
Split into simpler sentences: Secondary treatment employs biological oxidation to remove nearly all suspended solids and other impurities. Tertiary, or third level, of treatment which effectively removes bacteria, viruses and virtually all suspended solids.