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Sewers are underground pipes that take away dirty water and human waste from homes, offices and many other places. The sewers lead to treatment plants that remove the harmful and unwanted parts, returning clean water to the environment. Things like chemicals, bacteria, and ultraviolet light help clean the sewage. Many rules and regulations exist for how clean the final discharge (effluent) must be. These rules can be at the national, state, and regional level. Cleaning crews use special trucks and tools to help keep the sewers clear of grease, tree roots, and other blockages. More rural areas may have septic systems where sewers do not yet exist.