22-6 Preventing and Reducing Surface Water Pollution

How Can We Reduce Surface Water Pollution from Nonpoint Sources? Emphasize Prevention

There are a number of ways to reduce nonpoint water pollution, most of it from agriculture. Farmers can reduce soil erosion, especially by keeping cropland covered with vegetation and by reforesting critical watersheds. They can also reduce the amount of fertilizer running off into surface waters and leaching into aquifers by using slow-release fertilizer, using none on steeply sloped land, and planting buffer zones of vegetation between cultivated fields and nearby surface water. Applying pesticides only when needed and relying more on biological control of pests can reduce pesticide runoff. Farmers can control runoff and infiltration of manure from animal feedlots by planting buffers and locating feedlots and animal waste sites away from steeply sloped land, surface water, and flood zones

How Can We Reduce Water Pollution from Point Sources? Legal and Market Approaches

Some water experts suggest that industrialized nations such as Canada and the United States should adopt a discharge trading policy that uses market forces to reduce water pollution (as has been done with sulphur dioxide for air pollution control in the United States, p. 499). Under this program a water pollution source is allowed to pollute at a higher level than allowed in its permit by buying credits from permit holders with pollution levels below their allowed levels. Some environmentalists support discharge trading. But they warn that such a system is no better than the caps set for total pollution levels in various areas, and call for careful scrutiny of the cap levels. They also warn that discharge trading could allow pollutants to build up to dangerous levels in areas where credits are bought.

How Can We Reduce Water Pollution from Point Sources? The Technological Approach

In rural and suburban areas with suitable soils, sewage from each house can be discharged into a septic tank. About one-fourth of all homes in North America are served by septic tanks. In Canadian and U.S. urban areas, most water- borne wastes from homes, businesses, factories, and storm runoff flow through a network of sewer pipes to wastewater or sewage treatment plants. Some cities have a separate network of pipes for carrying runoff of storm water from streets and parking lots. But many cities have combined the sewer lines for these two systems because it is cheaper.

Raw sewage reaching a treatment plant typically undergoes one or both of two levels of wastewater treatment. One is primary sewage treatment. It is a physical process that uses screens and a grit tank to remove large floating objects and solids such as sand and rock, and a settling tank that allows suspended solids to settle out as sludge (Figure 22-17). By itself, primary treatment removes about 60% of the suspended solids and 30–40% of the oxygen-demanding organic 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 bacteria remove up to 90% of dissolved and biodegradable, oxygen-demanding organic wastes.

A third level of cleanup is advanced or tertiary sewage treatment. It is a series of specialized chemical and physical processes that remove specific pollutants left in the water after primary and secondary treatment. Its most widespread use is to remove phosphates and nitrates from wastewater before it is discharged into surface waters to help reduce nutrient overload.

Before discharge, water from primary, secondary, or advanced treatment undergoes bleaching to remove water colouration and disinfection to kill disease- carrying bacteria and some but not all viruses. The usual method for doing this is chlorination.

What Should We Do with Sewage Sludge? An Unsettled Problem

Sewage treatment produces a gooey sludge containing a slimy mixture of bacteria-laden solids and often- toxic chemicals and metals when sewer systems mix industrial and household waste. In North America, about 9% by weight of this sludge is placed in large circular digesters and kept warm for several weeks to allow anaerobic bacteria to decompose organic mate- rials and produce compost for use as a soil conditioner.

From an ecological standpoint, it is desirable to recycle plant nutrients in sewage sludge to the soil on land. But there are problems with using sewage sludge to fertilize crops. harmful bacteria and other pathogens and toxic chemicals are not present, sludge can fertilize land used for food crops or livestock. But removing bacteria (usually by heating), toxic metals, and organic chemicals is expensive and rarely done.

How Can We Improve Sewage Treatment? Eliminate Toxics

Environmental scientist Peter Montague calls for redesigning sewage treatment systems to prevent toxic and hazardous chemicals from reaching sewage treatment plants and thus from getting into sludge and the water discharged from such plants. He suggests several ways to do this. One is to require industries and businesses to remove toxic and hazardous wastes from water sent to municipal sewage treatment plants. Another is to encourage industries to reduce or eliminate toxic chemical use and waste. Another suggestion is to have more households, apartment buildings, and offices eliminate sewage out- puts by switching to waterless composting toilet systems.

Solutions: How Can We Treat Sewage by Working with Nature? Ecological Purification

More than 150 cities and towns in North America use natural and artificial wetlands to treat sewage as a low-tech, low-cost alternative to expensive waste treatment plants.

First, sewage goes to sedimentation tanks, where the solids settle out as sludge that is removed and processed for use as fertilizer. Next, the liquid is pumped into oxidation ponds, where bacteria break down remaining wastes. After a month or so, the water is released into the artificial marshes, where plants and bacteria carry out further filtration and cleansing. Then the purified water flows into the Humboldt Bay with its abundant marine life.

The marshes and ponds also serve as a bird sanctuary and provide habitats for thousands of otters, seabirds, and marine animals. The town celebrates its natural sewage treatment system with an annual “Flush with Pride” festival. However, large cities do not have the land available for this approach