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A major type of water pollution is wastewater, which is the water produced by livestock operations and human activities, including human sewage from toilets and gray water from bathing and washing of clothes and dishes. In this module, we will review the effects that wastewater pollution can have on natural water bodies and then discuss the technologies that have been developed to treat wastewater and minimize its effects.

Learning Objectives

After reading this module you should be able to

Regardless of the specific contaminant, pollution can come from either point sources or nonpoint sources (FIGURE 41.1). A point source is a distinct location from which pollution is directly produced. Examples of point sources include a particular factory that pumps its waste directly into a nearby stream or a sewage treatment plant that discharges its wastewater from a pipe into the ocean. A nonpoint source is a more diffuse area that produces pollution, such as an entire farming region, a suburban community with many lawns and septic systems, or storm runoff from parking lots. The distinction between the two sources can help us control pollutant inputs to waterways. For example, if a municipality can determine one or two point sources for most of its water pollution, it can target those specific sources to reduce its pollution output. Controlling pollution from nonpoint sources is more challenging because it represents a negative externality. As we saw in Chapter 10, it is difficult to address negative externalities since they are not typically reflected in the cost of making the products that generate them.

Environmental scientists are concerned about wastewater as a pollutant for three major reasons. First, wastewater dumped into bodies of water naturally undergoes decomposition by bacteria, which creates a large demand for oxygen in the water. Second, the nutrients that are released from wastewater decomposition can make the water more fertile. Third, wastewater can carry a wide variety of disease-causing organisms.

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Oxygen Demand

When organic matter enters a body of water, microbes that are decomposers feed on it. Because these microbes require oxygen to decompose the waste, the more waste that enters the water, the more the microbes grow and the more oxygen they demand. The amount of oxygen a quantity of water uses over a period of time at a specific temperature is its biochemical oxygen demand (BOD). Lower BOD values indicate that a water body is less polluted by wastewater, and higher BOD values indicate that a water body is more polluted by wastewater. If we were to test the BOD of natural waters over a 5-day period in a liter of water, we might find a BOD of 5 to 20 mg of oxygen coming from the decomposition of leaves, twigs, and perhaps a few dead organisms. In contrast, wastewater might have a BOD of 200 mg of oxygen.

When microbial decomposition uses a large amount of oxygen in a body of water, the amount of oxygen remaining for other organisms can be very low. Low oxygen concentrations are lethal to many organisms including fish. Low oxygen can also be lethal to organisms that cannot move, such as many plants and shellfish. Some areas, known as dead zones, become so depleted of oxygen that little life survives. Dead zones can be self-perpetuating; organisms that die from lack of oxygen decompose and cause microbes to use even more oxygen.

Nutrient Release

When wastewater decomposes it releases nitrogen and phosphorus. As you may recall from Chapter 3, nitrogen and phosphorus are generally the two most important elements for limiting the abundance of producers in aquatic ecosystems. The decomposition of wastewater releases nitrogen and phosphorus, which in turn provide an abundance of fertility to a water body, a phenomenon known as eutrophication. As noted at the beginning of this chapter, the Chesapeake Bay experiences eutrophication from wastewater decomposition as well as from nutrients leached from fertilized agricultural lands during precipitation. When a body of water experiences an increase in fertility due to anthropogenic inputs of nutrients, it is called cultural eutrophication.

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Eutrophication initially causes a rapid growth of algae, known as an algal bloom. As we discussed in Chapter 3, this enormous amount of algae eventually dies and microbes then rapidly begin digesting the dead algae; the increase in microbes consumes most of the oxygen in the water (see FIGURE 7.5 on page 88). In short, the release of nutrients from wastewater initiates a chain of events that eventually leads to a lack of oxygen and the creation of dead zones once again. One of the most impressive dead zones in the world occurs where the Mississippi River dumps into the Gulf of Mexico, shown in FIGURE 41.2a. The Mississippi River receives water from 41 percent of the land of the continental United States. Each summer there is an influx of wastewater and fertilizer that causes large algal blooms followed by substantial decreases in oxygenated water and massive die-offs of fish (FIGURE 41.2b). While some dead zones arise naturally, most are caused by human activities. In 1910, 4 dead zones were known around the world. This number increased to 87 dead zones in the 1980s and more than 400 dead zones by 2008, which is the most recent estimate.

Disease-Causing Organisms

For centuries, humans have faced the challenge of keeping wastewater from contaminating drinking water. This can be difficult because throughout the world many people routinely use the same water source for drinking, bathing, washing, and disposing of sewage (FIGURE 41.3). Wastewater can carry a variety of pathogens including viruses, bacteria, and protists. Pathogens in wastewater are responsible for a number of diseases that can be contracted by humans or other organisms coming in contact with or ingesting the water. Such pathogens cause cholera, typhoid fever, various types of stomach flu, and diarrhea.

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Worldwide, the major waterborne diseases are cholera and hepatitis. Cholera, which claims thousands of lives annually in developing countries (FIGURE 41.4), is not common in the United States. However, hepatitis A is appearing more frequently in the United States, usually originating in restaurants that lack adequate sanitation practices. The bacterium Cryptosporidium has caused a number of outbreaks of gastrointestinal illness in this country. Large-scale disease outbreaks from municipal water systems are relatively rare in the United States, but they do occasionally occur. They are relatively common in many regions of the developing world.

The World Health Organization estimates that 1.1 billion people, nearly one-sixth of the world’s population, do not have access to sufficient supplies of safe drinking water. In addition, half of the 3.1 million annual deaths from diarrheal diseases and malaria could be prevented if all people had safe drinking water, proper sanitation, and proper hygiene. Approximately 42 percent of the world’s population lacks access to proper sanitation and over half of these people live in China and India. In sub-Saharan Africa, only 36 percent of the population has access to proper sanitation.

Because of the risk that pathogens pose, we need the ability to easily test for the presence of pathogens in our drinking water. It is not feasible to test for all of the many different pathogens that can exist in drinking water. Instead, scientists have settled on using an indicator species, an organism that indicates whether or not disease-causing pathogens are likely to be present. The best indicators for water that is potentially harmful are fecal coliform bacteria, a group of generally harmless microorganisms that live in the intestines of human beings and other animals. One of the most common species of fecal coliform bacteria is Escherichia coli, abbreviated E. coli. Most strains of E. coli live naturally in humans and are not harmful, although there are strains that can be deadly to people who are very young, very old, or possess weak immune systems. Given that E. coli is commonly found in human intestines, detecting E. coli in a body of water is a reliable indicator that human waste has entered the water. This does not necessarily mean that the water is harmful to drink, but the presence of E. coli does indicate an increased risk that wastewater pathogens are in the water.

Public water supplies, such as drinking water sources and swimming pools, are routinely tested for E. coli. Homeowners with a single-family well might test their water less frequently or not at all. Public health authorities recommend declaring water unsuitable for human consumption if any bacteria are present. For safe swimming and fishing, the acceptable level of E. coli is higher; for example, swimming at a public beach or in a river is considered safe as long as the fecal coliform bacteria levels are less than 500 to 10,000 colonies per 100 megaliters of water. A pool, beach, or campground with contaminated water likely would be posted with a sign such as “The Department of Health has closed this water supply because of the presence of fecal coliform bacteria.”

Because proper sanitation is so important, we need ways of treating wastewater to reduce the risk of waterborne pathogens. Humans have devised a number of ways to handle wastewater. The various solutions all have the same basic approach: Bacteria are used to break down the organic matter into carbon dioxide and inorganic compounds, which include nitrogen and phosphorus, and the harmful pathogens are outcompeted by nonharmful pathogens. In this section we will look at the two most widespread systems for treating human sewage: septic systems and sewage treatment plants. We will also describe manure lagoons, which are used to treat waste from large livestock operations.

Septic Systems

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In rural areas, houses often treat their own sewage with a septic system, which is a relatively small and simple sewage treatment system that is made up of a septic tank and a leach field. Septic systems are often used for homes in rural areas. As shown in FIGURE 41.5, the septic tank is a large container that receives wastewater from the house. Having a capacity of 1,900 to 4,700 L (500–1,250 gallons), the septic tank is buried underground adjacent to the house. Wastewater from the house flows into the tank at one end and leaves the tank at the other end. After the tank has been operating for some time, three layers develop. Anything that floats will rise to the top of the tank and form a scum layer. Anything heavier than water, including many pathogens, will sink to the bottom of the tank and form the sludge layer, which we define as the solid waste material from wastewater. In the middle is a fairly clear water layer called septage. The septage contains large quantities of bacteria and may also contain some pathogens and inorganic nutrients such as nitrogen and phosphorus.

Through gravity, septage moves out of the septic tank into several underground pipes laid out across a lawn below the surface. The combination of pipes and lawn makes up the leach field. The pipes contain small perforations so the water can slowly seep out and spread across the leach field. The septage that seeps out of the pipes is slowly absorbed and filtered by the surrounding soil. The harmful pathogens in the septage can be outcompeted by other microorganisms in the septic tank and therefore diminish in abundance, or can be degraded by soil microorganisms after the septage seeps out into the leach field. The organic matter found in the septage is broken down into carbon dioxide and inorganic nutrients. Eventually, the water and nutrients seeping out into the leach field are taken up by plants or enter a nearby stream or aquifer.

There are significant environmental advantages to septic systems. Because most septic systems rely on gravity—water from the house flows downhill to the septic tank, and water from the septic tank flows downhill to the leach field—no electricity is needed to run a septic system. However, sludge from the septic tank must be pumped out periodically (every 5 to 10 years) and taken to a sewage treatment plant.

Sewage Treatment Plants

Although household septic systems work well for rural areas in which each house has sufficient land for a leach field, they are not feasible for more developed areas with greater population densities and little open land. In developed countries, municipalities use centralized sewage treatment plants that receive the wastewater from hundreds or thousands of households via a network of underground pipes. In a traditional sewage treatment plant, wastewater is handled using a primary treatment followed by a secondary treatment.

As shown in FIGURE 41.6, the primary treatment allows the solid waste material to settle into a sludge layer. The remaining wastewater undergoes a secondary treatment that uses bacteria to break down 85 to 90 percent of the organic matter and convert it into carbon dioxide and inorganic nutrients such as nitrogen and phosphorus. The secondary treatment typically aerates the water to add oxygen; this promotes the growth of aerobic bacteria, which emit less offensive odors than anaerobic bacteria. The treated water sits for several days to allow particles to settle out. Settled particles are added to the sludge from the primary treatment. The remaining water is disinfected, using chlorine, ozone, or ultraviolet light to kill any remaining pathogens. The disinfected water is then released into a nearby river or lake, where it is once again part of the global water cycle.

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The combined sludge from the primary and secondary treatments must be taken away from the sewage treatment plant. To reduce the volume of material prior to transport and help remove many of the pathogens, sludge is typically exposed to bacteria that can digest it. After digestion, most of the water is removed from the sludge, which reduces its volume and weight. This final form of sludge can be placed into a landfill, burned, or converted into fertilizer pellets for agricultural fields, lawns, and gardens.

Sewage treatment plants are very effective at breaking down the organic matter into carbon dioxide and inorganic nutrients. Unfortunately, these inorganic nutrients can still have undesirable effects on the waterways into which they are released. Although nitrogen and phosphorus are important nutrients for increasing primary productivity, when high concentrations of these nutrients are released into bodies of water from sewage treatment plants, they fertilize the water, which can lead to large increases in the abundance of algae and aquatic plants. In response to this problem, large sewage treatment plants are now developing tertiary treatments that remove nitrogen and phosphorus from the wastewater. The ultimate goal is to release wastewater that is similar in quality to the water body that is receiving it.

Legal Sewage Dumping

Sewage treatment plants are critical to human health because they remove a great deal of harmful organic matter and associated pathogens that cause human illness. It might surprise you to know that even in the most developed countries, raw sewage can sometimes be directly pumped into rivers and lakes. Sewage treatment plants are typically built to handle wastewater from local households and industries. However, many older sewage treatment plants also receive water from stormwater drainage systems. During periods of heavy rain, the combined volume of storm water and wastewater overwhelms the capacity of the plants. When this happens, the treatment plants are allowed to bypass their normal treatment protocol and pump vast amounts of water directly into an adjacent body of water.

How big a problem is this? According to the U.S. Environmental Protection Agency, overflows of raw sewage occur approximately 40,000 times per year in the United States. In Indianapolis, for example, more than 3.8 billion liters (1 billion gallons) of raw sewage are dumped into surrounding water bodies each year during periods of high rain. Around the country, such incidents result in the contamination of drinking water, beaches, fish, and shellfish, and can lead to human illness. Between 1.8 million and 3.5 million illnesses are associated with swimming in sewage-contaminated water in the United States each year, and 500,000 illnesses are linked to drinking sewage-contaminated water. Illnesses caused by eating contaminated shellfish are estimated to cost $2.5 million to $22 million annually. The solution to this problem is straightforward though expensive. Municipalities facing sewage overflow need to modernize their sewage treatment systems to prevent the influx of storm water from overwhelming their capacity to treat human wastewater. Unfortunately, the cost of such work can be considerable.

Animal Feed Lots and Manure Lagoons

Although the impact of human waste on the environment and human health is generally understood, people are less familiar with the problem of animal waste. On a small scale, animal manure can contaminate waters when farm animals are allowed access to streams for food and water because, inevitably, they will defecate in the stream. As you may recall from Chapter 11, large-scale manure issues arise with concentrated animal feeding operations that raise thousands of cattle, hogs, and poultry. The manure from these operations contains digested food and often also contains a variety of hormones and antibiotics that farmers use to improve the growth and health of their animals.

Farms that raise thousands of animals in a single location often use a manure lagoon to dispose of the tremendous amount of manure produced (FIGURE 41.7). A manure lagoon is a large, human-made pond lined with rubber to prevent the manure from leaking into the groundwater. After bacteria has broken down the manure—the same process that occurs in sewage treatment plants—the manure can be spread onto farm fields to serve as a fertilizer. One major risk of manure lagoons is the possibility of developing a leak in the liner that would allow the waste to seep into and contaminate the underlying groundwater. Another risk is a possible overflow into adjacent water bodies. Like human wastewater, overflow of animal waste into rivers can lead to disease outbreaks in humans and wildlife. Finally, the application of manure as fertilizer can create runoff that moves into nearby water bodies. “Do the Math: Building a Manure Lagoon” shows you how to calculate the appropriate size for a manure lagoon.