Sewage treatment how does it work

The Romans improved on even that: After considering Rome's many accomplishments, Pliny the Elder called the sewers "the greatest accomplishment of all. The majority of human waste, though, was simply thrown into the streets; aqueduct water was used to wash the streets and sweep that waste into the drains. Because Roman sewers lacked ventilation, the only egress for sewer gas was those same drains and latrines. On the plus side, Romans also invented portable toilets, setting urns by the side of the road near the entrances to the city vendors would rent you what Schladweiler calls "a modesty cape".

Further, the 1st-century emperor Vespasian had workers collect the contents of urinals, which he then taxed and sold to fullers, tradesmen who cleaned and dyed the Romans' clothing--they had figured out that the ammonia in urine had cleaning powers.

After the fall of the empire, Romans kept throwing filth in the streets, but nobody was washing them. In Rome many sewer pipes fell into disrepair. Everywhere else people got along without them as they always had: at best using latrines unlined pits or cesspits pits lined with perforated masonry that let liquids drain away into the soil while solids piled up for eventual removal and at worst throwing their waste into the streets and leaving it there.

In the 13th century the French king Philip II paved the streets of Paris to reduce the stench, with the result that afterward the waste sat on the stones instead of percolating into the soil.

In the 14th century, one of his successors, Philip VI, ordered Parisians to sweep in front of their houses and take the refuse to a dump; crews of sanitation workers were organized to clean up whatever was left.

In a return to the technology of the Roman Empire, in Paris opened a series of drainage canals that also carried waste--the biggest was lined with masonry and called the Grand Egout, or Great Drain. By the 16th century one British royal castle had to post signs reminding people not to "foul the staircases, corridors, or closets with urine or other filth. The world changed in , when the city of Hamburg, after suffering a terrible fire, decided to lay sewer pipes while rebuilding. The new pipes vented through house drains and had a mechanism for flushing using tidewater.

The system was efficient, didn't stink, and became a worldwide model. Before the introduction of these sewers, typhoid, transmitted through water tainted by sewage, caused Immediately thereafter the Parisians began turning their 14th-century sewer system into a wonder of the world, building hundreds of miles of huge brick tunnels to carry away stormwater and everything else Parisians cared to sluice inside.

When early American cities such as Boston and Philadelphia began paving their streets with cobblestones in the 17th century, gutters--and even some underground sewers--were included among the improvements.

Private citizens built Boston's first systems, designed, like the Cloaca Maxima and the Grand Egout, to drain cellars and swamps. Bostonians soon grew weary of the constant repairs those wooden sewer lines required and undertook a sort of public-private partnership by issuing construction permits for sewers; everyone who wished to connect a drain had to share in the cost, and the contracts stipulated requirements about pavement reconstruction.

Philadelphia had a system of culverts and some underground sewers by , and New York City started putting a few sewers underground later in the century. Human waste, though, remained mostly a personal matter of cesspits and privies. Sewers really took off in , with John Snow's discovery that the London cholera epidemic was caused by sewage-tainted drinking water.

With advances in microbiology, people began to understand that human waste carried disease in the form of microbes, and increasingly they wanted to protect themselves from their sewage. What's more, the introduction of reliable water service in the 19th century and the spread of the modern flush toilet the British Public Health Act of , which required every home to have some kind of sanitary arrangement, listed "water closet" as one of the alternatives to an ash pit or privy vastly increased the amount of wastewater households generated.

Cesspits and privies that had already created offensive nuisances now produced vast, vile-smelling seepages, overwhelmed by the new volume of water. And it wasn't just toilets, either--connections draining sinks and tubs began overwhelming sewer pipes, too; in Boston tried to slow the tide, literally, by passing a law requiring a doctor's order for every bath.

As cities grew in size and density during the Industrial Revolution, they all had to build more, and better, sewers. The cholera epidemic wasn't motivation enough for London, but the "Great Stink" of , when the Thames smelled so bad that Parliament considered relocating, got the city government's attention; it built new sewers in the s and '60s to carry waste downstream from central London. Brooklyn introduced sewers in , and Chicago not long after. Boston, still largely building sewers privately, had about miles of sewers in ; by that had expanded to miles, and new houses were expected to connect to the system both for pump and washbasin waste and for the human waste now going into flush toilets instead of privies.

Every city had its own problems and its own characteristics. Some of Boston's sewers had outfalls dammed by the tide 12 hours of every 24; others, built by unscrupulous contractors in land reclamation projects like the Back Bay, sagged and lost their downhill slope, causing settling, clogs, and backups.

Sylvan Seattle had pipes made of wooden staves--and faced a tide problem so severe that at certain times of day toilets became foul geysers; eventually the city simply rebuilt itself higher than its sewer pipes.

In Chicago, the outfalls of the sewers made such a mess of Lake Michigan that during large rainstorms the plume of tainted water flowed all the way out to the intake for the water system. In response, engineers built a series of canals and reversed the flow of the Chicago River, turning it from a drainage into Lake Michigan into a flow from Lake Michigan toward the Mississippi. They also moved the intake farther out into the lake.

All these "solutions" merely moved the problem. As one historian said in describing Boston's covering a brook filled with sewage and routing it to the Charles River rather than directly into Boston Harbor, this "somewhat lessened the nuisance caused by it, or at least transferred it to another locality.

Lennox, Massachusetts, built the first such system in , and Memphis built one in Since then, that's what everybody has built. Raleigh laid its first sewer pipes in Fayetteville Street, Raleigh's main road, wasn't paved until , at exactly the same time the first water pipes were being laid; where water pipes go, sewer pipes soon follow. The privies of Raleigh's population of barely 10, almost certainly had not yet polluted the soil enough to foul its wells, and the new sewer pipes, running north to Crabtree Creek and south to Walnut Creek, would not have discharged more than the streams could absorb.

A stream running at about 6 cubic feet per second can absorb the waste of about 1, people, so to support 10, people the two creeks together would have had to flow at around 60 cfs. Currently, on a dry day in a dry month, they flow at about 75 cfs. Now, with 2, miles of pipes all heading roughly southeast to Raleigh's wastewater treatment plant, the sewer collection system turns out to be the only infrastructure stream that follows that natural tree pattern that I'd expected to find everywhere.

The leaves are houses, connected by 4-inch service lines to 6- or 8-inch mains that run mostly beneath streets, and then to , , or inch collectors that start out along streets but head downhill to creek basins, leading to larger and larger pipes and finally to the plant. I sat down with a friendly GIS expert to check it out. The GIS map easily showed me the path of my own wastewater: the 4-inch lateral in my yard--the same pipe that "flushable" wipe clogged--runs into an 8-inch main, which heads downhill along my street until it crosses the Pigeon House Branch, down by the pool I like to sit by.

It runs along the Pigeon House until it joins a inch PVC east of town the path is following rivers by then, not roads , and thereafter joins larger and larger pipes--some made of PVC, some of reinforced concrete, some of ductile iron.

Eventually this stream hits the dual inch reinforced concrete pipes that head directly to the sewer plant, though those sometimes separate into three or four pipes, for ease of maintenance. It's simple and, especially after the spaghetti tangle of the water lines, rather satisfying. It's much like the stormwater system, if every ravine in every drainage basin remained piped and they all came together in one place before entering the Neuse.

To find out what happens in these pipes, I talked to Raleigh's dean of pipage, sewer collection superintendent Hunter "Gene" Stanley. Combined systems manage overflows with relatively simple mechanical junctions called regulators: basically weir dams in pipes or junction boxes. A weir is nothing more than a low barrier for steering water. When flow is routine, the dam routes it through pipes to the treatment plant; during large rain events, the flow of mixed stormwater and wastewater rises high, overtops the weirs, and flows directly through outfalls to rivers or lakes.

Such an event is called a CSO, or combined sewage overflow. New York dumps about 40 billion gallons of CSOs into its rivers and harbors every year. But before you draw too much comfort from Raleigh's system having to convey only sewage the plant treats about 45 million gallons per day that are generated by the , or so customers connected to the system; it's rated for 60 million gallons, and it's being expanded to 75 , consider this: The increase in flow caused by nothing more than rainfall and street flow coming in through manhole vents in low-lying areas can nearly double the flow to the treatment plant.

Though catching and correcting the breaks and overflows are an unavoidable part of his job, Stanley stays focused on preventive maintenance. Stanley grew up in rural North Carolina and has called his preventive maintenance management "an ol' country boy work system"--he copies pages from the map book of his system and gives them to his crews. When the crew has flushed and inspected every line on the map, it comes back. The department logs its maintenance in feet per day, and it likes to reach , feet per month if it can, meaning that every pipe in the system gets a look-see once every few years.

GIS keeps the maps updated, of course, but Stanley's system has been working since they were using nothing more than blueprints and as-built surveys; finding that what's an 8-inch pipe on the map is really a 6-inch is just part of keeping on top of things.

That's why you carry different-size saw blades in your truck. Stanley says a sewer is a simple thing: The pipe needs to drop about half a foot per feet of length, a slope of 0. Bigger pipes inches or larger--can slope even less. But they all must flow downhill, powered by gravity, which is why sewer pipes so commonly crisscross the stormwater drainages: Raleigh Public Utilities Department director Dale Crisp calls all the sewers that run in a particular drainage a "sewershed," which for a while became my favorite new word.

Of course, if wastewater pipes followed only natural gullies, the mains would eventually have to parallel the river, and for many reasons, from aesthetics to the catastrophic results of a spill, nobody wants that. The system generally moves downhill, but pipes sometimes need to cross rises. So the city has more than lift stations, where the contents of pipes are pumped to join other flows or where wastewater from lowlying areas collects in sumps.

When the water gets high enough, it trips a float valve and a pump clicks on and lifts it up a hill--kind of like your toilet, only this float valve starts the flush instead of stopping it. I visited one lift station, a by foot rectangle of electrical boxes that look like a central air-conditioning system behind chain-link fences between two houses, controlling an underground sump; even when it's pumping, if you were more than 10 feet away you wouldn't hear it.

The station has a backup pump and a generator to power it, plus a little antenna to send information back and forth to the supervisory control and data acquisition SCADA system at the treatment plant; that's plenty of equipment, but just the same, if you weren't looking for it you wouldn't know it was there. A much larger station sits on the trunk line, giving a lift to pretty much all of Raleigh's waste on its way to the plant.

It's underneath a highway on-ramp, and though some people suggested I could find it by following my nose, it didn't smell when I went out to visit it.

Stanley hands over a laudatory profile of Raleigh's sewer maintenance department in a recent issue of Municipal Sewer and Water magazine, then hands me off to Robert Smith, a sewer monitoring supervisor and asks him to show me around.

First things first: We walk the yard, checking out trucks. Sewer guys basically do three things: They perform maintenance, they respond to crises, and they "TV" pipes, sending tiny little vehicles with cameras on them up the pipes to check both their condition as part of general maintenance and whether the crews who claim to have recently maintained them have actually done so.

Smith shows off the department's various trucks. Rodder trucks have a spool of linked rods, a sort of long chain that the workers feed into a manhole and then rotate, just like someone cleaning roots or a clog out of your drain at home. Some rodders have cutting blades or spiral grabbing implements to clear roots or debris.

Flushing trucks carry enormous water tanks to feed high-pressure hoses with spinning heads on the end: Workers feed the hose into the system, usually past the next manhole, and then turn on a pump. Water pressure starts the head spinning, spraying water at thousands of pounds of pressure per square inch back toward the truck as the truck pulls back the hose, scouring the pipes along the way.

Standard now is the combination truck, which carries tanks of water for flushing and a garbage-truck-size tank for postflush water, which the truck vacuums up with a huge tube that hangs from a derrick over the cab like an elephant's trunk. The driver eventually empties that tank onto a pad in the parking area, Smith explains; water drains off into the sewer system and the cleaned-out debris--tampons, bricks, gravel, roots, supposedly flushable materials--gets loaded into a dump truck once a week and sent to the landfill.

Smith marshals those vacuum trucks when Raleigh has a sewage overflow, too. Another truck he calls a blockbuster has a water hammer--a pipe that uses water to rhythmically pound and break up large blockages. Finally, he shows me a sort of souped-up golf cart that provides access to the many parts of the system that, because they follow ravines rather than roads, are not easily reached by regular trucks. But we're standing in a parking lot while people are out in the field, rodding sewers.

Our first stop is a highway off-ramp, where two flush trucks and a pickup are parked behind orange cones. Several men wearing hard hats, green mesh vests, and rubber-palmed gloves manage a hose coming off a spool on the back of one of the trucks and running to a manhole 20 feet down a steep ravine. A hundred yards away, two guys stand at another manhole looking out for the spinning head of the water jet, which Smith says is called a Warthog. Once it's past, the guys still at the truck turn on the jet and the spool to start reeling it back in.

Over the roar of the truck engine Smith explains that on the way out the head sprays as a sort of presoak; "on the way back, it's like a broom. It looks like you sprayed foam on that pipe. Where the vacuum trucks can't reach a manhole, the crew flushes debris downstream to one the truck can reach. Once the treatment process has been completed and the wastewater has been treated as thoroughly as possible, it can be discharged into the environment.

This is another key area where sewage treatment plants differ from sewage treatment plants. Whereas you must discharge effluent from a septic tank into a soakaway for further treatment in the ground, subject to an Environment Agency Consent to Discharge, you can discharge your effluent into local water sources straight from your treatment plant. This is because of the vastly improved effluent quality that the treatment process produces. The first thought for anyone planning a new development should be getting connected to mains sewers.

They are typically the most cost-effective and reliable method of dealing with your wastewater. In some scenarios, the distance from the nearest sewer or the layout of the land can make it impossible to have your property serviced by a mains sewer. The operation of a sewage treatment plant means that you can have one installed almost anywhere, as long as you have an electrical connection.

The purpose of a sewage treatment plant is to treat the wastewater as thoroughly as practically possible — and, even though such plants can often deal with more waste than a septic tank, they will still need emptying from time to time. As you can see, the biggest disadvantage of having a sewage treatment plant is that you are relying heavily on maintenance from a professional company. To learn more about the installation or maintenance of a sewage treatment system on your site, give us a call today on Back to top of page Skip to content.

Register Login Contact us. Top of main content Home About us Responsibility Education and learning The sewage treatment process. The sewage treatment process. Did you know? Taking the wastewater away Whenever you flush the toilet or empty the sink, the wastewater goes down the drain and into a pipe, which takes it to a larger sewer pipe under the road. Screening the wastewater First, we remove large objects that may block or damage equipment or pollute our rivers. Carrying out primary treatment Wastewater still contains organic solid matter — otherwise known as human waste.

Carrying out final treatment We pass the treated wastewater through a final settlement tank, where the useful bacteria sink to the bottom. The clean water then passes over a wall near the top of the tank. Generating power We treat the sludge we collect at the start of the process so that we can put it to good use. We do this in several different ways: 1. This heats the sludge up to high temperatures, encouraging the bacteria inside to break down the waste. This creates biogas that we can then burn to create heat, which in turn creates electricity.