(a) Sewage, if untreated, and discharged directly intorivers it will lead to severe pollution of the waters with organic matter and pathogenic bacteria protozoa and many other disease causing organisms. This water will cause outbreaks of water borne diseases upon use.
(b) Anaerobic bacteria, digest the aerobic bacteria and the fungi in the sludge and the remaining organic matter in anaerobic sludge digestion. Bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide during this digestion. These gases (biogas) is inflammable and can be used as source of energy.