The sludge, now activated with additional millions of bacteria and other tiny organisms, can be used again by returning it to the aeration tank for mixing with new sewage and ample amounts of air. From the aeration tank, the sewage flows to another sedimentation tank to remove excess bacteria. As with trickling, the final step is generally the addition of chlorine to the effluent. If done properly, chlorination will kill more than 99 percent of the harmful bacteria in an effluent. Some municipalities are now manufacturing chlorine solution on site to avoid the necessity of transporting and storing large amounts of chlorine, sometimes in a gaseous form.
Many States now require the removal of excess chlorine before discharge to surface waters.
Alternatives to chlorine disinfection, such as ultraviolet light or ozone, are also being used in situations where chlorine in sewage effluents can be harmful to fish and other aquatic life. Other Treatment Options New pollution problems have placed additional burdens on wastewater treatment systems. Today's pollutants may be more difficult to remove from water. Increased demands on the water supply only aggravate the problem. These challenges are being met through better and more complete methods of removing pollutants at treatment plants, or through prevention of pollution at the source.