You may not think about sewage very often, but it's a part of everyday life for all humans and all communities. Sewage refers to the liquid waste produced by people when they use the toilet, take showers or baths, or wash clothes, dishes, or even pets. The average American produces 80-100 gallons of wastewater every day – that's almost two full bathtubs!
Anything that goes down a drain is classified as sewage. In some places, sewage also includes storm water runoff from roofs and streets, or liquid waste from factories or farms. Sewage is mostly water, but also contains solids and microbes that can be harmful.
After you drain the bathtub or flush the toilet, have you ever wondered what happens to that water? Sewage, also referred to as wastewater, is usually carried away from houses or buildings by a system of pipes. Then where does it go? Let's find out.
Sewage Through History
Because human bodies have always produced urine and feces (pee and poop), human waste has always been a part of life. When early humans lived as hunter-gatherers, it wasn't much of a problem. But when large numbers of people began living together in settlements and cities, what to do with all that sewage became a challenge.
The Indus people lived 4,000 years ago in the region now called Pakistan. They had a system of indoor toilets and street drains. They poured water down the drains which pushed waste into pipes that led to a river or a cesspool, a pit dug into the ground to hold waste and garbage. Public baths and toilets were also found in ancient Greece and Rome, and the Romans created a complex underground sewage system to carry wastewater away from the city. However, during the Middle Ages these early sanitation efforts were abandoned. People in cities just threw the waste into the streets where it mixed with rainwater in open ditches. Some people kept cesspools in their yards, which often overflowed. Can you imagine the smell that filled the air? In fact, in London in 1858, the stench was so bad that the Parliament (government) had to leave the city, an event known as The Great Stink.
The problem was more serious than just the terrible smell. Sewage can contain harmful bacteria that can make people very sick. Sometimes people got their daily water from the same place that sewage was dumped. In European cities, drinking water contaminated by sewage caused epidemics of typhoid and cholera, serious diseases that resulted in many deaths. Even when wastewater was carried out of the city, it went directly into nearby rivers and lakes, making local water sources unsafe for human use. The link between sewage and disease wasn't really understood until the mid-1800's. It was then that cities began to implement sewage treatment.
Sewage treatment is the process of dealing with sewage so it does not cause harm to people or waterways. In the United States, the first sewage systems were built in New York and Chicago in 1855. Worldwide, these systems have saved millions of lives since they were built in the 19th and 20th centuries. Perhaps no factor is more useful in the control of disease than the science of sewage disposal.
When you flush the toilet in your house, wastewater goes down the drain into a pipe that carries it to a sewer under the street. The word “sewer” comes from a Latin word that means “to carry away water,” and that's exactly what it does. Sewers are large underground pipes that carry away dirty water and human waste from all the nearby homes. Sewers are built so that the wastewater flows downhill by gravity, and smaller sewer pipes join up with larger pipes on their way. The pipes from houses are about 6-12 inches wide, while the main city sewers are about 3 to 5 feet wide. Sewers no longer dump water directly into rivers or the ocean. Today, they carry wastewater to sewage treatment plants that remove the harmful parts and clean the water so it can be returned to natural bodies of water.
There are three types of sewer systems: sanitary sewers which carry wastewater from our homes into treatment plants, storm sewers which carry rainwater from roads and roofs into lakes and rivers, and combined sewers which are a single pipe system for both wastewater and storm water.
When urban sewer systems were first built, most were combined systems. Newer sewage-collection systems separate surface water from wastewater so that treatment plants do not become overloaded during storms. Sewers are inspected and repaired by workers who get into the sewers through a manhole, an opening in the street protected by a heavy cover.
Nature has very effective ways of cleaning small amounts of impurities from water as it flows in a stream or river. Modern sewage treatment plants are designed to do the same thing, but in less time and at greater volume. Today, more than 16,000 water treatment plants provide wastewater collection, treatment, and disposal in the United States. These facilities process about 34 billion gallons of wastewater every day!
Wastewater goes through many stages at the treatment plant. When it reaches the facility, the raw sewage is called influent. In the first step, the influent passes through bar screens, designed to separate out large items such as sticks, rags, jewelry, plastics or trash. If you've ever heard of a toddler who flushed a toy down the toilet, you'll know why bar screens are needed! The next step is called the grit chamber, a large settling tank where solids like sand and gravel settle to the bottom of the basin. Then the liquid flows on to the primary clarifier. Here, the wastewater is slowed down in a sedimentation tank so that heavier organic matter falls to the bottom. A rake skims off the fats, oils, grease and soaps that float to the top. These materials are commonly called “scum,” while the organic matter at the bottom is called “sludge.” The scum and sludge are removed from the water. These steps are known as Primary Treatment, which uses physical processes such as screening, settling and skimming to separate solids from the water.
In Secondary Treatment, biological processes are used. This takes place in an aeration basin where helpful microorganisms, such as bacteria and protozoans, digest the remaining organic waste dissolved in the water. Oxygen is bubbled into the water for these tiny “bugs” that are kept in just the right conditions to grow and thrive. These microbes are the “work force” of a wastewater plant.
At this point, the water goes to the secondary clarifier. Some of the sludge from the bottom is sent to the digester, where it is further processed and broken down by bacteria. In the digester tanks, methane gas is given off which may be turned into energy. Some treatment plants get all the energy they need to run their operations in this way. The dried, treated biosolids may be used for fertilizer to improve farm soils.
Primary Treatment removes about 60 percent of suspended solids from wastewater, and Secondary Treatment removes more than 90 percent. In larger facilities, the water then goes through Tertiary Treatment, an additional stage where chemical processes remove nutrients and water is filtered through a substance such as sand or coal. The final step in all treatment plants is disinfection, where the water is treated with chlorine to kill any harmful bacteria or viruses that may be present in the treated water. Some wastewater plants use ultraviolet light to disinfect the water.
Now the treated water is known as effluent and it is clean enough to be released into the environment. The process of cleaning wastewater uses some of the same techniques that nature uses to clean smaller quantities of water in rivers and streams: sedimentation, aeration, and filtration. Depending on the type of sewage treatment plant, the entire process takes between 12 and 36 hours.
There are strict rules about how clean the wastewater needs to be before it is returned to a river, lake or ocean. Inspectors test the water for impurities throughout each day to make sure it meets those standards. Effluent isn't drinking water, but reclaimed wastewater can be used in landscaping, power plants, irrigation, firefighting, and much more.
Septic systems are wastewater treatment systems that collect and treat sewage at the same site where it is generated, rather than moving it to a treatment plant. Often septic systems are used in rural areas where sewers do not exist. In the United States, about 20% of homes use septic systems to treat their wastewater.
Septic systems usually have two main parts, a septic tank and a soil absorption field or drain field, connected by underground pipes. Wastewater from the house runs through a pipe to the septic tank, which is buried nearby. The septic tank holds the wastewater long enough to partially digest organic matter. The solids settle to the bottom (sludge) and the fats, oils and grease float to the top (scum.) The effluent in the middle flows to the drain field. There, pipes slowly release it into the soil, where it is filtered and where microbes provide further wastewater treatment. The sludge left in the septic tank must be cleaned out every few years.
Follow wastewater through a household septic system in this animated, interactive model.
Challenges in Managing Sewage
Not so long ago, raw sewage was piped directly into rivers or bays. The water was polluted and unsafe. It's important that people continue to make sure that sewage is handled properly in their community. If sewage gets into rivers or lakes, it can cause serious problems.
Besides smelling bad, sewage contains harmful bacteria that can make people sick, and chemicals that pollute the environment. No one want to go swimming, fishing or boating in sewage-filled waters. Also, our rivers, wetlands, marshes, beaches and lakes provide important habitat for wildlife, fish and birds. They depend on clean water for their survival. If sewage gets into ocean water, beaches may be shut down and shellfish may become unsafe for people to eat.
When there is sewage in local waterways, bacteria in the water will use dissolved oxygen in the water to break down the sewage, just as they do at the treatment plant. If there is too much sewage, decaying organic matter can use up so much oxygen that there isn't enough left for the fish and other water-dwelling life. In addition, sewage can cause algal blooms due to excessive nutrients in wastewater, which also consume the water's oxygen and block sunlight.
How does sewage get into rivers, if wastewater plants are cleaning the water? In the United States, between 23,000 and 75,000 sewage spills happen each year. These spills can happen when pipes or sewage equipment are not carefully maintained. In many cities, the sewer system was built many years ago for a much smaller population than is currently using the system. Older pipes can develop cracks, and during times of heavy rain, the amount of water passing through can be more than the system can handle. Upgrading and replacing sewer systems in order to prevent sewage spills is an ongoing challenge in many cities.
Another sewage-related challenge is that over one billion people around the world still do not have access to adequate sewage treatment. Millions of tons of human waste never go through any kind of treatment. In many places, sewage is still flushed directly into streams and rivers, contaminating the water and making people sick. For some cities, large-scale wastewater treatment plants are too expensive or take up too much space. In those areas, different kinds of sewage treatment need to be developed. Composting toilets that work without water, purification systems that attach to hand pumps, and sewage-cleaning tropical plants that filter wastewater are some of the alternative methods of keeping harmful microbes out of the water supply.
How You Can Help
Sewage treatment is a very important part of caring for our planet's water supply. We all need access to clean water to stay healthy. There are things you and your family can do every day to help maintain your city's sewage system and keep our water supply safe.
One of the most important steps you can take is to not flush anything down the toilet other than human waste (pee and poop) and toilet paper. Remember that toilets are not trash cans! Thousands of sewer blockages happen each year because of people putting the wrong things down the drain. Paper towels, napkins, baby wipes, cotton swabs and dental floss do not dissolve the way toilet paper does. If undissolved material gets caught on a bend or root within a pipe, it can start a clog that could cause a sewer backup in your home or neighborhood, or it can cause pumps to stop working.
You can also make sure that fats, oils and grease do not go down the drains in your house. Floating chunks of grease can clump together into large masses that cause blockages in the sewer system. A huge clump found in a London sewer was as big as a car! You can help by scraping food from your plate into the trash before placing it in the dishwasher, and by reminding your family that oils or fats should not go down the drain. In addition, toxic products such as pesticides, paints, medicines and chemicals should not be poured down drains.
In the future, there will be a continuing need for waterwater engineers and scientists who can develop innovative ways to deal with sewage and make sure our water is clean and safe. Maybe you will be one of these problem-solvers one day!