TREATMENT OF POLLUTANTS (Cont)
We require two levels of sewage treatment.
Primary sewage treatment simply filters out unwanted items such as sticks, stones, garbage, and other debris that arrive at the treatment plant and allows time for the solid materials to settle out.
Secondary treatment uses aeration and aerobic, or oxygen-using, bacteria to break down organic wastes. The water is then treated with chlorine to kill bacteria and discharged into adjacent rivers and streams.
Treatment plants remove approximately
90 percent of the organic waste and suspended solids,
less than 70 percent of the toxic metals and synthetic organic chemicals,
50 percent of the nitrogen in the form of nitrates, and
30 percent of the phosphorus in the form of phosphates.
CONCERN: This remaining discharge is still high in nutrients and is not pure water entering the surface water. More advanced treatment systems are available, but they are rarely used due to their high cost. The remaining sludge is sent to a landfill as waste or applied to the land as a soil additive.