The plague from rat fleas was brought about by the practice. Leonardo deVinci designed the sewers of Paris. He had a separate channel for waste and storm flow. Early farmers used outhouses for human waste, pigs and chickens for kitchen waste, and manure for fertilizer Material in water tends to flow toward sewer plants. Industrial wastes can poison the organisms used in sewer plants to remove organic wastes.
Metals are a special problem for treatment plants as lead, cadmium in Milwaukee.
Materials not readily metabolized such as PCB's, Kepone can also enter municipal sewage and cause problems. Sewer plants are sensitive to anything that will kill their organisms. The sea is also used for waste. Rats - Landfills need to be sensitive to rat problems. Garbage is covered each night and in some cases covered with plastic sheeting.