Raw Sewage Overflow Storm water systems in urban areas are sometimes combined with sanitary sewer systems en route to sewage treatment plants. Excessive storm water can cause this joint system to overflow. In this event, excess flow will be directed into waterways untreated, resulting in sewage contamination. Forty-six percent of the population is served by separate systems. Urban runoff is usually collected by a separate storm sewer system and discharged directly into waterways.
Combined systems are cheaper, but the potential to harm health is higher.
Some systems have diversions to accommodate heavy flow. Septic tanks are enclosures that store and process wastes where no sewer system exists, such as in rural areas or on boats. Treatment of waste in septic tanks occurs by bacterial decomposition. The resulting material is called sludge. Large portions of the population are still served by septic systems as opposed to public waste treatment facilities.