Environmental Problems, Their Causes, and Sustainability 1

where bacteria break down remaining wastes. After amonth or so, the water is released into the artificialmarshes, where plants and bacteria carry out furtherfiltration and cleansing. Then the purified water flowsinto the Humboldt Bay with its abundant marine life.The marshes and ponds also serve as an AudubonSociety bird sanctuary and provide habitats for thousandsof otters, seabirds, and marine animals. The towncelebrates its natural sewage treatment system with anannual “Flush with Pride” festival. However, somecities do not have the land available for this approach.Mark Nelson has developed a small, low-tech,and inexpensive artificial wetland system to treat rawsewage from hotels, restaurants, and homes in developingcountries (Figure 22-18). This wastewater gardensystem removes 99.9% of fecal coliform bacteria andmore than 80% of the nitrates and phosphates from incomingsewage that in most developing countries isoften dumped untreated into the ocean or into shallowholes in the ground. The water flowing out of suchsystems can be used to irrigate gardens or fields or toflush toilets and thus help save water.Genetic engineering may also get into the act. Theguts of some insects are resistant to pesticides. Researchershave isolated the gene that provides this resistance,and they have transferred it to easily culturedbacterial species. They envision passing contaminatedwater through a large vessel or bioreactor containingthe genetically modified bacteria that consume thepesticides. Stay tuned about developments in thispromising area of frontier science. This might be an interestingcareer choice.Currently about 1.7 billion people do not haveaccess to adequate sanitation. And the world’s populationis projected to add 2.9 billion more peoplebetween 2004 and 2050—an average of 172,000 newpeople per day needing access to safe drinking waterand adequate sanitation. Without greatly increased investmentin conventional and unconventional sewagetreatment systems, the number of people with inadequatesanitation could reach 3 billion by 2050. Dealingwith this important challenge will take scientific andengineering ingenuity and lots of money.How Successful Has the United States Beenin Reducing Water Pollution? Good and BadNewsWater pollution laws have significantly improvedwater quality in many U.S. streams and lakes, butthere is a long way to go.Great news. According to the EPA, the Clean Water Actof 1972 led to a number of improvements in U.S. waterquality. Between 1992 and 2002, the number of Americansserved by community water systems that metfederal health standards increased from 79% to 94%.(1) Raw sewage drains bygravity into the first pooland flows through a longperforated PVC pipe intoa bed of limestone gravel.(3) Wastewater flows throughanother perforated pipeinto a second pool, wherethe same process is repeated.SewageWetland typeplantsWetland typeplantsTreatedwaterFirst concrete pool(2) Microbes in the limestone gravelbreak down the sewage intochemicals that can be absorbedby the plant roots, and the gravelabsorbs phosphorus.45 centimeterlayer of limestonegravel coated withdecomposing bacteriaSecond concrete pool(4) Treated water flowing from thesecond pool is nearly free ofbacteria and plant nutrients.Treated water can be recycledfor irrigation and flushing toilets.Figure 22-18 Solutions: wastewater garden. Hotels, restaurants, and homes in developing countriescan use this small gravity-fed artificial wetland system to treat sewage. It uses only 1.9–3.8 square meters(20–30 square feet) of space per person.http://biology.brookscole.com/miller14513