Saprophytic micro organisms decompose is wage into harmless inorganic compounds. In the process, pathogens are also killed as they are out competed by the saprophytes. However, this natural cycle takes a long time, and is only effective in sparsely populated areas.
In many cities and big towns especially in developed countries, sewage is sent to sewage treatment plants. Here, the sewage is separated into liquid and solid matter or sludge. These are acted on by
• aerobic bacteria (whose activities are speeded up by aeration) ; and
• anaerobic bacteria.
The final harmless liquid is emptied into water bodies, while the sludge is used as fertilizer.
