So how do sewers work?

I know this is a bit of an odd question, but I ponder about these things sometimes. Do sewers (i.e. the domestic waste-water pipes under the road, that houses are connected to) run on gravity, or is there lots of pumping machinery under there?

The reason I ask is that I was looking at a big new housing development not far from me, in quite a hilly area, and trying to work out the logistics of managing all the waste-water and directing it off the site. It seems pretty complicated.

And what if a new housing development is built at the bottom end of a town, at a lower elevation than the main sewer? Do they have to build a whole new sewer lower down, or do they pump it up the hill, or what?

There are two basic types of urban sewers, sanitary and storm. Sanitary sewers carry the waste from houses and buildings and they normally operate on a gravity system w/ a pitch of one eighth to one quarter inch per foot. This is the optimal pitch for allowing liquid and solids to flow efficiently, a steeper pitch tends to leave the solids behind, sanitary sewage is 99% water. Sanitary sewers normally end in a sewage treatment plant.
Storm sewers use a similar pitch as the are solids present there also. Storm sewers are usually diverted to a natural waterway. In places where there isn’t sufficient elevation they usually use pumping stations to assist. Some older cities may have combination sanitary/storm sewer system and these have become a problem w/ increased population. You will find that in most areas these combination sewers are being replaced w/ modern divided systems. There’s a lot more to it, but that’s the basic idea.

Pump it.

It’s every bit as nasty as it sounds, especially since the pumping station needs to screen out anything unpumpable and dispose of it separately. You’d be amazed at the number of boards, car parts, ropes and similar large or tangly things that get into a sewer.

Related to sewers is a problem faced by developers of things like shopping malls - a big building surrounded by a big parking lot means rain is no longer able to just filter into the ground. They can’t simply dump all that rain into the sewer system, so that rain water’s collected and sent into catch basins that will gradually release the water to the ground.

Beautiful summary! I believe it’s mandatory everywhere (in the U.S. at least) for any replacement to separate sanitary and storm sewerage.

Probably well in excess of 95% of sanitary sewers are the gravity feed which A.R. Cane describes. When sewer lines run at a lower grade than what they are supposed to drain into, as in new development on low-lying land, a lift station is installed, which essentially takes the flow from the input sewers, and pumps it into an output sewer. In some cases, this calls for grinder pumps and a a force main, a narrower-diameter, partly-pressurized sewer line. The circumstances were one of these are needed might best be described by a civil engineer specializing in sanitary sewerage, but my work related to community water and sewer systems in the past has encountered several occasions when they were needed, though at this late date I couldn’t give you reasons why.

All in all it’s a dirty/nasty business but someone has to do it.
The grinder is called a comminutor and grinds/cut all waste passing the inclined grate to small bits the flow easily. The lift pumps are installed in a sump and either lift the waste to a gravity flow line OR into a force main going to a gravity flow access point. Comminutors and lift pumps are installed in pairs whenever possible to allow for continued operations when maintenance is necessary. The grate is to prevent large items from blocking the comminutors. Never know what you will find on the grate!

On a lighter note: A new sewer connection was made to serve a new building and a few months after occupancy they called to say it was stopped up. Sure enought the manhole was full and overflowing. A check of the main trunk revealed a 2 x 4 had been left in the branch line where it entered the trunk. The pipe foreman called the carpenter foreman:“Whose job is it to cut a 2 x 4 .” Reply “You know ding dong well that’s a carpenters job.” Pipe foreman “Well send one down to … I’ve got one to be cut!”

I’m an environmental engineer–my company designs and does construction oversight for sanitary sewers, sewage pumping stations, storm drains, etc.

As has been mentioned, most sewage collection systems are gravity sewers. All of the gravity sewers for a given area flow to a convenient low point, where a sewage pumping station is installed to pump the pressurized sewage up to a higher elevation manhole in an adjacent area. All of the sewage eventually ends up at a wastewater treatment plant (WWTP). The treated effluent is safe to discharge into a waterway or into the ground.

Sometimes you have individual homes that are at a lower elevation than the road, such as houses along a lakefront. Or you might have a bunch of up-and-down hills. In such a situation, it is sometimes a good solution to install low-pressure sewers. For this, each home (or group of homes) get its own little pumping station, referred to as a grinder pump. Each home’s grinder pump has a little wetwell, which when it fills up, automatically activates the grinder pump to discharge into the common low-pressure pressurized sewer line in the road. The sewage eventually discharges into a manhole that empties directly into the WWTP or a gravity sewer system.

Also, historically, sanitary sewers and storm sewers were combined, which was no problem, because both dumped into the nearest waterway. Nowadays, all sanitary sewage is treated, but storm drainage is not. So if the sewers are combined, it results in a lot of extra water being treated, at great expense. In fact, in a severe storm the wastewater treatment plant can be overwhelmed, and be forced to bypass untreated raw sewage from the treatment process directly into the receiving water body. This results in beach closures and swimming bans and is an obvious health threat, so whenever possible, old sewer collection systems are being separated as they are replaced.

To add to what’s been said already:

Sanitary sewers traditionally have been gravity lines. They tend to follow the dendritic patterns of creeks as they flow towards a sewage treatment plant. In some cases, there is no downstream line to flow into. In those cases, pump stations or “lift” stations are employed to pump the gray water into an adjoining basin which has lines to carry the flow to the treatment plant.

In our area (Jackson, Miss metro), historically each town had its own sewage lagoons. During the seventies EPA decided it was a better idea for large metro areas to have a common sewer treatment plant. All the outlying areas were tied in. Thus, gray water from my sink travels some 20 miles to the Savannah Street plant south of Jackson.

An interesting aside to robby’s comments: Since the advent of the Phase II stormwater regulations (brought on by the Clean Water Act of 1978), some municipalities have found that it is cheaper to upgrade their combined lines, and treat it all at the plant, than it is to put individual controls at each stormwater outfall.