Page 433 - Environment: The Science Behind the Stories

The Great Lakes of Canada and the United States repre- solids and oils separate from water. The clarified water proceeds
sent an encouraging success story in fighting water pollution. downhill to a drain field of perforated pipes laid horizontally in
In the 1970s these lakes, which hold 18% of the world’s surface gravel-filled trenches underground. Microbes decompose pol-
fresh water, were badly polluted with wastewater, fertilizers, lutants in the wastewater these pipes emit. Periodically, solid
and toxic chemicals. Algal blooms fouled beaches, and Lake waste from the septic tank is pumped out and taken to a landfill.
Erie was pronounced “dead.” Today, efforts of the Canadian In more densely populated areas, municipal sewer sys-
and U.S. governments have paid off. According to Environ- tems carry wastewater from homes and businesses to central-
ment Canada, releases of seven toxic chemicals are down by ized treatment locations. There, pollutants are removed by
71%, municipal phosphorus has decreased by 80%, and chlo- physical, chemical, and biological means (Figure 15.26). At
rinated pollutants from paper mills are down by 82%. Levels a treatment facility, primary treatment, the physical removal
of PCBs and DDE are down by 78% and 91%, respectively. of contaminants in settling tanks or clarifiers, removes about
Bird populations are rebounding, and Lake Erie is now home to 60% of suspended solids. Wastewater then proceeds to
the world’s largest walleye fishery. The Great Lakes’ troubles secondary treatment, in which water is stirred and aerated so
are by no means over—sediment pollution is still heavy, algal that aerobic bacteria degrade organic pollutants. Roughly 90%
blooms still plague Lake Erie, and fish are not always safe to of suspended solids may be removed after secondary treat-
eat. However, the progress so far shows how conditions can ment. Finally, the clarified water is treated with chlorine, and
improve when citizens push their governments to take action. sometimes ultraviolet light, to kill bacteria. Most often, the
Enforcement of the Clean Water Act, however, has become treated water, called effluent, is piped into rivers or the ocean
more difficult due to several recent Supreme Court decisions that following primary and secondary treatment. However, many
exempted certain waters from EPA oversight. These court deci- municipalities are recycling “reclaimed” water for lawns and
sions suggested that waterways that are entirely within a single golf courses, for irrigation, or for industrial purposes such as
state, streams that run dry during certain times of year, and lakes cooling water in power plants.
that do not connect to larger waterways are not “navigable” (capa- As water is purified throughout the treatment process,
ble of being navigated by ships) and therefore do not fall under the solid material removed is termed sludge. Sludge is sent to
the Clean Water Act’s directive to limit discharges “into the navi- digesting vats, where microorganisms decompose much of the
gable waters” of the United States. Internal EPA analyses indi- matter. The result, a wet solution of “biosolids,” is then dried
cate that these decisions could put up to 45% of major polluters and either disposed of in a landfill, incinerated, or used as ferti-
outside the jurisdiction of the law and expose up to 117 million lizer on cropland. Methane-rich gas created by the decomposi-
Americans to higher levels of pollution in their drinking water. tion process is sometimes burned to generate electricity, helping
Efforts to change the Act’s wording to encompass all waters in to offset the cost of treatment. Each year about 6 million dry
the United States was, as of 2013, stalled in the U.S. Congress. tons of sludge are generated in the United States.

We treat our drinking water WeIgHINg tHe ISSUeS

Technological advances as well as government regulation SLUdge oN tHe FaRM It is estimated that up to half the
have improved our control of pollution. The treatment of biosolids from sewage sludge produced each year is used
drinking water is a widespread and successful practice in as fertilizer on farmland. This practice makes productive use
developed nations today. Before being sent to your tap, water of the sludge, increases crop output, and conserves landfill
from a reservoir or aquifer is treated with chemicals to remove space, but many people have voiced concern over accumu-
particulate matter; passed through filters of sand, gravel, and lation of toxic metals, proliferation of dangerous pathogens,
charcoal; and/or disinfected with small amounts of an agent and odors. Do you feel that this practice represents an effi-
such as chlorine. The U.S. EPA sets standards for over 90 cient use of resources or an unnecessary risk? What further
drinking water contaminants, which local governments and information would you want to know to inform your decision?
private water suppliers are obligated to meet.

We treat our wastewater Constructed wetlands can
aid treatment
Wastewater treatment is also now a mainstream practice.
Wastewater includes water that carries sewage; water from Long before people built the first wastewater treatment
showers, sinks, washing machines, and dishwashers; water plants, natural wetlands were filtering and purifying water.
used in manufacturing or industrial cleaning processes; and Recognizing this, engineers have begun manipulating wet-
storm water runoff. Natural systems can process moderate lands and even constructing new wetlands to employ them
amounts of wastewater, but the large and concentrated amounts as tools to cleanse wastewater. Generally in this approach,
that our densely populated areas generate can harm ecosystems wastewater that has gone through primary or secondary treat-
and pose health threats. Thus, attempts are now widely made ment at a conventional facility is pumped into the wetland,
to treat wastewater before it is released into the environment. where microbes living amid the algae and aquatic plants
In rural areas, septic systems are the most popular method decompose the remaining pollutants. Water cleansed in the
of wastewater disposal. In a septic system, wastewater runs wetland can then be released into waterways or allowed to
432 from the house to an underground septic tank, inside which percolate underground.

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