Urban agglomerations, transport, and industrial and agricultural activities pollute sea, underground and freshwater with their air and solid waste, but primarily their liquid waste. Urban wastewater is carried by sewers and constitutes the bulk of a city’s liquid waste. It contains an abundance of microorganisms, organic compounds, certain inorganic salts, mainly salts of ammonium, nitrogen and phosphorus as well as small quantities of other compounds. Marine transport may pollute ecosystems with petroleum products. As a rule, liquid waste from large or small industrial units contain pollutants similar to those in urban wastewater, but sometimes it also contains toxic organic compounds, various metals such as lead and mercury, or radioactive elements. Some industrial wastes damage the aquatic environment because of their high temperature. Agricultural runoff pollutes the aquatic environment mainly with inorganic salts from fertilizers, as well as toxic substances from pesticides. Urban activities also pollute rainwater, which when it runs off city streets is loaded with a wide variety of pollutants such as lead and cadmium, generally in small concentrations. All types of liquid wastes end up in receiving water, which may be on the surface or underground.