Environmental Problems, Their Causes, and Sustainability 1

are used for drinking water, bathing, and washingclothes.How Can We Reduce Water Pollution fromPoint Sources? The Technological ApproachSeptic tanks and various levels of sewagetreatment can reduce point-source waterpollution.In rural and suburban areas with suitable soils, sewagefrom each house can be discharged into a septic tank(Figure 22-15). About one-fourth of all homes in theUnited States are served by septic tanks.In U.S. urban areas, most waterborne wastes fromhomes, businesses, factories, and storm runoff flowthrough a network of sewer pipes to wastewater orsewage treatment plants. Some cities have a separate networkof pipes for carrying runoff of storm water fromstreets and parking lots. But 1,200 U.S. cities have combinedthe sewer lines for these two systems because itis cheaper.Bad news. Heavy rains or too many users hookedup to the system can cause combined sewer lines tooverflow and discharge untreated (raw) sewage directlyinto surface waters. According to the EPA, atleast 40,000 such overflows occur each year in theUnited States. The EPA estimated that each year1.8–3.5 million people get sick from swimming in waterscontaminated by sewage overflows. The EPA estimatesthat it would cost $10 billion a year for a decadeGravel orcrushedstoneHouseholdwastewaterDrainfieldSeptic tank withmanhole (for cleanout)Nonperforated pipeDistribution box (optional)Vent pipePerforated pipeFigure 22-15 Solutions: septic tank system used for disposalof domestic sewage and wastewater in rural and suburban areas.This system separates solids from liquids, digests organicmatter and large solids and discharges the liquid wastes in anetwork of buried pipes with holes over a large drainage or absorptionfield. As these wastes drain from the pipes and percolatedownward, the soil filters out some potential pollutants, andsoil bacteria decompose biodegradable materials. To be effective,septic tank systems must be properly installed in soils withadequate drainage, not placed too close together or too nearwell sites, and pumped out when the settling tank becomes full.to install dual systems, add capacity, and repair the nation’s$2 trillion aging sewer network. To help protectpublic health, environmentalists want Congress tochange the Clean Water Act to require the EPA to monitorsewer leaks and overflows and report them topublic health authorities.Raw sewage reaching a treatment plant typicallyundergoes one or both of two levels of wastewatertreatment. One is primary sewage treatment. It is aphysical process that uses screens and a grit tank to removelarge floating objects and solids such as sandand rock, and a settling tank that allows suspendedsolids to settle out as sludge (Figure 22-16). By itself,primary treatment removes about 60% of the suspendedsolids and 30–40% of the oxygen-demandingorganic wastes from sewage but removes no phosphates,nitrates, salts, radioisotopes, or pesticides.A second level is called secondary sewage treatment.It is a biological process in which aerobic bacteriaremove up to 90% of dissolved and biodegradable,oxygen-demanding organic wastes. This is done bytrickling wastewater through beds of gravel coveredwith films of aerobic bacteria or by passing it throughan aeration tank where air is pumped through a slurryof aerobic bacteria before the wastewater is sent to asecond settling tank.A combination of primary and secondary treatment(Figure 22-16) removes 95–97% of the suspendedsolids and oxygen-demanding organic wastes, 70% ofmost toxic metal compounds and nonpersistent syntheticorganic chemicals, 70% of the phosphorus(mostly as phosphates), 50% of the nitrogen (mostly asnitrates), and 5% of dissolved salts. But this process removesonly a tiny fraction of long-lived radioactiveisotopes and persistent organic substances such assome pesticides.Because of the Clean Water Act, most U.S. citieshave combined primary and secondary sewage treatmentplants. According to the EPA, however, at leasttwo-thirds of these plants have at times violated waterpollution regulations. Also, 500 cities have failed tomeet federal standards for sewage treatment plants,and 34 East Coast cities simply screen out large floatingobjects from their sewage before discharging it intocoastal waters.A third level of cleanup is advanced or tertiarysewage treatment. It is a series of specialized chemicaland physical processes that remove specific pollutantsleft in the water after primary and secondarytreatment. Its most widespread use is to removephosphates and nitrates from wastewater before it isdischarged into surface waters to help reduce nutrientoverload. Advanced treatment is expensive and isused to treat only 5% of the wastewater in the UnitedStates.Before discharge, water from primary, secondary,or advanced treatment undergoes bleaching to remove510 CHAPTER 22 Water Pollution