Equipment malfunction and human error also lead to raw-sewage discharges. At treatment plants, sewage might undergo only minimal treatment or be subjected to an array of purification steps. The various levels are known as primary , secondary , and tertiary treatment. In primary treatment, sewage is screened to remove large particles, then sent to settling chambers. Sludge is raked from the bottom of settling chambers for disposal, and the liquid may be aerated before chlorination and release.
Chlorine is added to kill bacteria.
The problem is that it is nondiscriminating, killing both the disease-causing bacteria and those that would help break down the sewage after its release. In secondary treatment, the liquid is aerated and sometimes planted with fresh bacteria. It might also be run through a filter system and sprinkled on a bed of coal or gravel over a broad area conducive to bacterial growth and oxidation. It's usually chlorinated before release. Sludge is removed from settling tanks and treated.