Aeration treatment systems will generally produce a discharge water quality adequate for most situations. A byproduct of modern wastewater treatment is called sludge. Sludge is trash and excess and dead bacteria. Generally, the longer the retention time, the smaller the volume of sludge produced per person. Sludge can be used to improve soil quality on farmland.
Some large cities compost, dry, sterilize and bag the sludge for sale to landscaping companies.
Wastewater treatment will not reduce pathogens to acceptable levels for drinking water sources or lakes and rivers with recreational uses. Chlorine in sufficient quantity will kill the pathogens but can pollute the lake or river. Ultraviolet light systems will disinfect wastewater if the wastewater is very clear.