Bacteria and amoebae in the gut and bowels are totally anaerobic. Oxygen kills them. A key aspect of the treatment is not just to remove solids, but to "run the sewage through oxygenated water". It is this which is the most important part of treatment after solids are removed. The use of chemicals is only done exceptionally. Dumping sewage into the sea is disgusting, but has been done at times when the treatment plants are not adequate.