Water naturally flows downward with gravity and your sewer system is built to take advantage of that. These pipes are interconnected all throughout your home and connect to one larger pipe called the sewer main, or sewer lateral.
This main flows downward, away from your home, to the municipal sewer main line. Waste water from every home and business collects and flows through their nearby municipal sewer main.
Home DIY. How Sewer and Septic Systems Work. Each time you flush the toilet or you wash something down the sink's drain, you create sewage also known in polite society as wastewater. One question that many people might ask is, "Why not simply dump this wastewater onto the ground outside the house, or into a nearby stream?
If you release wastewater directly into the environment, things get very smelly very fast. It contains harmful bacteria. Human waste naturally contains coliform bacteria for example, E. Once water becomes infected with these bacteria, it becomes a health hazard. It contains suspended solids and chemicals that affect the environment. For example: Wastewater contains nitrogen and phosphates that, being fertilizers, encourage the growth of algae.
Excessive algae growth can block sunlight and foul the water. Wastewater contains organic material that bacteria in the environment will start decomposing.
When they do, these bacteria consume oxygen in the water. The resulting lack of oxygen kills fish. The suspended solids in wastewater make the water look murky and can affect the ability of many fish to breathe and see. The increased algae, reduced oxygen and murkiness destroy the ability of a stream or lake to support wildlife, and all of the fish, frogs and other life forms quickly die.
No one wants to live in an place that stinks, is full of deadly bacteria and cannot support aquatic life. That's why communities build wastewater treatment plants and enforce laws against the release of raw sewage into the environment.
Private Treatment: The Septic Tank In rural areas where houses are spaced so far apart that a sewer system would be too expensive to install, people install their own, private sewage treatment plants.
These are called septic tanks. The tank looks something like this in cross-section: In this picture, you can see three layers. Wastewater comes into the septic tank from the sewer pipes in the house, as shown here: A septic tank naturally produces gases caused by bacteria breaking down the organic material in the wastewater , and these gases don't smell good. The following diagram shows an overhead view of a house, septic tank, distribution box and drain field: A typical drain field pipe is 4 inches 10 centimeters in diameter and is buried in a trench that is 4 to 6 feet about 1.
The gravel fills the bottom 2 to 3 feet of the trench and dirt covers the gravel, like this: The water is slowly absorbed and filtered by the ground in the drain field. Urban Wastewater Systems In urban and suburban areas where people are packed closer together and where there is a lot more wastewater to treat, the community will construct a sewer system that collects wastewater and takes it to a wastewater treatment facility.
Here's what each stage does: The first stage, known as primary treatment , does the same thing a septic tank does. It allows the solids to settle out of the water and the scum to rise. The system then collects the solids for disposal either in a landfill or an incinerator. In every sewer main, there are periodical vertical pipes that meet the ground with manholes.
These access points are spaced depending on the local code and needed just in case a problem arises in the sewer main. Say your toddler has a nasty habit of flushing toys. If they do this enough and the toys make it into the main, then there might eventually be a major clog that city workers need to remove.
Sewer mains will flow into progressively larger pipes as they accumulate more and more sewage, until eventually, they reach a wastewater treatment plant. Usually, these plants are in low-lying areas to make engineers jobs of designing sewer systems much easier. However, all of this gravity-fed sewer system talk is the ideal scenario.
What happens if your house is below the elevation of the wastewater treatment plant or if it needs to travel over a hill? The answer is obvious: you need to pressurize it.
This is done using grinder pumps or lift stations to pump the sewage over hills. These stations collect sewage from lower elevations and utilize pumps to push it up to the needed height where it can gravity flow from there on out. These pumps are specially designed to handle the high amount of solids like toilet paper and flushable wipes. We leave the sludge and return to the water. Primary treatment is mechanical--settling; secondary treatment is biochemical--activated sludge digestion.
Tertiary treatment combines mechanical and photochemical: The water flows to a dozen deep sand filters, which remove the remaining fine particles.
The final hurdle the water clears is ultraviolet treatment. We walk over to a small, low concrete building, in front of which Lynch pulls up a flat piece of aluminum to reveal a 4- or 5-foot-wide channel of water flowing smoothly past banks of green-glowing UV lamps.
They look much like fluorescent lights, though they have a self-wiping mechanism to keep them clean. Sterilizes 'em. Once fish are swimming in your treated sewage, you've got to feel like you're getting the job done. Out behind the UV building we finally reach the finished product: the effluent, flowing out in strong wide arcs into a canal from a dozen UV channels.
The water seems to have a greenish tinge, but when Lynch fills a little water bottle and holds it up it looks utterly clear. Then we drive down toward the end of the line, where the effluent enters the Neuse River. There, in a tiny green glade at the bottom of the hill, the river bends into a cove, near the bank of which a few surface bubbles give away the presence of the underground pipe.
Lynch says if you didn't know where the pipe was you could follow shad upstream to find it: "The water's warm and highly oxygenated," so the fish love it. Lynch says the river is the natural place for the water anyhow. Every drop of Raleigh water comes from the Neuse watershed and ought to return there.
Waste on the ground would be dispersed by rain and the water would be filtered through the earth, entering the groundwater system clean; matter that made it to the stream would become waterlogged and sink. That's exactly where the process came from. We've just concentrated it," Lynch says. What the river does is exactly what a sewage plant does, and until around World War I most people didn't really think plants were needed at all.
It was understood that watercourses were to some degree self-cleaning, that "the solution to pollution is dilution. Chicagoans felt great about their famous flow-reversal of the Chicago River, sending their sewage into the Mississippi instead of into their own water supply of Lake Michigan. Predictably, the people of St. Louis were not as delighted. Their expression of dissatisfaction eventually landed them in front of the Supreme Court.
Raleigh itself was sued in the late s by downstream neighbor Smithfield. Smithfield won, and in Raleigh's first sewage treatment plant opened, treating 12 million gallons per day.
The current plant replaced it in ; it's expanded since, and now it's expanding again. Raleigh's sewage treatment will be limited not by space, but by nitrogen.
In the state of North Carolina set limits on nitrogen based on totals for that year. Raleigh's plant released 1. In it released , pounds, its lowest total ever. Still, the state-mandated limit will keep dropping, and Raleigh's capacity to treat its sewage will probably finally reach its limit in pounds of nitrogen rather than in millions of gallons per day.
Modern sewage treatment is a boon, of course, but it creates a new problem. If you treat sewage so effectively that by the time water leaves the plant it's almost drinkable, then what do you do with all the sludge you cleaned out of it? People have been fertilizing with their sewage since the ancient Athenians, but modern Americans seem to mistrust the process. As recently as , when water-starved San Diego considered a water-saving measure to reintroduce effluent from its sewage plant into its reservoir--from which, of course, the city's water would still be treated before use--the Union-Tribune opined, "your golden retriever may drink water out of the toilet with no ill effects.
But that doesn't mean humans should do the same. Two years later San Diego's mayor claimed the reuse measure was a waste of money and vetoed it. The city council overturned his veto, but it's clear: Even in the parched Southwest, people are frightened of wastewater reuse. In , when Raleigh introduced a plan to save water by reusing effluent from the Neuse plant--by piping it only to large-scale users, such as universities, business campuses, and country clubs, and only for irrigation--some politicians resisted.
They claimed to object to "subsidizing" large-scale users through lower rates for the reuse water. Fortunately, enough people remembered the drought; the system was built, using pipes foresightedly laid almost a decade earlier.
Given that people feel that resistant to highly treated water, what on earth to do with all this sludge? New York used to dump its sludge far out in the ocean, a practice that didn't represent a huge improvement over dumping raw sewage, until that was outlawed in , after which the city started shipping sludge by train to a Texas landfill. Plenty of cities still landfill their sludge. Raleigh chooses to follow the example of Milwaukee: More than 90 percent of its sludge is beneficially reused as biosolids nationwide about 50 percent of sludge is reused.
Some of Raleigh's muck is thickened on gravity belt presses and applied directly to fields as Class B biosolids according to the EPA, this use requires a buffer between fields and the public, and they can't be used on crops for human consumption. Some of this was overapplied to the Raleigh farmland surrounding the plant in recent years; a series of poor practices resulted in groundwater contamination near the plant, so the practice was stopped, though the plant currently has the highest certification level of the National Biosolids Partnership the NBP is a nonprofit comprising three national groups: Two represent the sewage treatment industry, which may not be the most objective source for biosolids practice assessment, and the third is the Environmental Protection Agency, which probably is.
Some primary sludge is dewatered and sold to a composting company, which treats it and sells it to agricultural products companies as safer, more highly treated Class A biosolids. The rest is mixed with lime, which raises the temperature high enough to pasteurize it, and sold to farms, institutions, and the public as Raleigh Plus, Raleigh's own version of Milorganite. Maybe I'll use some on my lawn. Nobody has any statistics on how it affects gophers.
By permission of Rodale, Inc. Scott Huler was born in in Cleveland and raised in that city's eastern suburbs. He graduated from Washington University in ; he was made a member of Phi Beta Kappa because of the breadth of his studies, and that breadth has been a signature of his writing work.
He was the founding and managing editor of the Nashville City Paper. His most recent book, On the Grid, was his sixth. Follow Scott Huler on Twitter.
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