环境概论第讲水污染教程.ppt

APES: Notes 20 * Roughly half the people in the US obtain drinking water from groundwater. Also used for irrigation and industry. Most common pollutants are pesticides, fertilizers, and organic compounds. Come from landfills, underground storage tanks, leaching. Once groundwater is polluted, it does not readily cleanse itself by natural processes. It is hard to measure and clean groundwater pollution b/c of access and groundwater moves very slowly so contaminants do not readily disperse. Relatively recent concern. Used to think groundwater was protected by rock and soil above it. APES: Notes 20 * APES: Notes 20 * APES: Notes 20 * APES: Notes 20 * APES: Notes 20 * Turbid water is treated with a chemical coagulate (aluminum sulfate) that causes the suspended particles to clump and settle out. Then filtered through sand to remove remaining suspended materials as well as many microorganisms. Disinfected to kill any remaining disease-causing organisms. Most commonly use chlorine. Can also use ozone or UV radiation. APES: Notes 20 * APES: Notes 20 * APES: Notes 20 * Chlorine byproducts, formed when chlorine reacts with organic matter in treated wastewater, are tentatively linked to several kinds of cancer (rectal, pancreatic, and bladder), an increased risk of miscarriages, and possibly rare birth defects. Chlorine still used in US, but at lower levels. Chloramine (chlorine and ammonia) is an alternative that does not have harmful byproducts, but has been shown to increase lead in water. Fluoride has been added to drinking water since 1940s to prevent tooth decay. Most believe fluorine is safe. Studies have failed to link it to adverse health effects. APES: Notes 20 * Primary treatment—removes suspended particles by mechanical processes like screening and gravitational settling. Solid material that settles out is called primary sludge. Wastewater treatment facilities for about 11% of the US population have primary treatment only. Secondary treatment