Everything You Ever Wanted to Know About Sewers

Manhole covers are round because the cannot fall or be dropped into the sewer conduit.
Manhole covers are round because the cannot fall or be dropped into the sewer conduit. | Source

By Joan Whetzel

In the 2nd Century BC, the Romans developed the first sewer systems to help take care of their raw sewage problems. It wasn't until the 1800s that Europe and the US understood the need for sewers to improve sanitation in their major cities as a disease control measure. Today's storm and sanitary sewer systems are made from concrete, much as those built since Roman days. Concrete sewers are strong and have life expectancy far greater than any other construction material available. Whether the city you live in has separate storm and sanitary sewer systems or a combined system, they all work in the same basic way. They collect the rain, sewage, or both, and direct the water (and other detritus) into a sewage treatment plant. The treatment plants clean the water before directing it to the ocean or into the city's public water supply. The sewer system is set up under the city streets in a maze of conduits, utility corridors and steam tunnels.


Manhole covers

Manhole covers serve two purposes: to seal off the sewer system from the general public and to provide access points for sewer workers and inspectors. These access covers are quite heavy and can only be removed with special tools called manhole cover bars, which are straight rods with a bend on end (to insert into holes in the manhole cover and lift it up) and a handle on the other end.

Manhole covers are made from cast iron, which is the reason for their weight. The weight - around 250 to 300 pounds - has 2 main functions, (1) it keeps the cover in place during storms and when there's a lot of water pressure in the system following a storm, and (2) people so inclined to steal such things will be less inclined to do so if it's too heavy to carry off.


Manholecovers are mostly round, except for a few that are only used for certain very specific jobs. They come in various sizes ranging from 22 to 60 inches in diameter, and are slightly larger than the manhole they rest on. The lip of the manhole runs approximately 1 inch in diameter smaller than the cover, so that the cover won't fall in. The reason for the circular shape is that round covers cannot fall or be dropped in to the sewer conduit.

Storm and Sanitary Sewers

Most city and residential wastewater (sanitary) sewers travels down a separate sewer system to an raw sewage treatment plants. Some older cities, like New York City, have a mixed or combined sanitary-storm sewer system. This system functions quite well as long as there's no storms with a lot of rainfall. When heavy rainfall occurs, however, it overwhelms the sewer drainage systems and the treatment plants. For the cities with separate sewer systems, residential and business wastewater and industrial waste are diverted to the appropriate treatment plants, whereas the storm runoff is diverted to areas - like reservoirs - where it can be held in one place until it is safe to release the excess water into the rivers and oceans.

Both types of sewer systems generally reinforced use concrete pipes ranging in size from 1 to 12 feet in diameter, which are placed in dugout ditches and are packed in dirt to withstand the water pressure on the inside and prevent them from shifting or cracking. The Army Corp of Engineers recommends that the pipes be made of concrete mix that will last for 70 to 100 years, though many of the concrete pipes in existing sewer systems exceed this recommendation

Detecting Sewer System Problems.

Residential and business plumbing empty into sanitary sewers beneath the city streets. These sewage pipes are buried 10 feet or more underground When a sewage backup occurs within only one section of the drainage, it indicates that the problem is inside the house or business. These problems can usually be fixed with a plumbing snake. If the plumbing drainage is spread out over a wider area - several houses, business or city blocks - then the problem is a major blockage in the sewer system, which needs to be cleared by city workers.

Cleaning Sewer Lines

There are several methods for clearing sewer system blockages.

1. High pressure flushing forces sediment and other debris out with water pressure.

2. Root cutting, a procedure performed by sewer jet trucks, force high-powered water through the pipes using an hydraulic motor with an attached root cutting blade.

3. Trailer mounted bucket machines clear debris from larger sewer lines by positioning two trailers over alternating manholes, and pulling the bucket from one end of the sewer line to the other until the blockage is cleared from the entire line.

4. Vacuum trucksevacuate sediment, garbage and debris from the sewer pipes in much the same fashion as a household vacuum with a hose attachment. They are sometimes used in conjunction with the sewer jet trucks.

Repairing Damaged Sewer Lines

Repairs to damaged sewer lines begin with repairs to the larger mainlines before moving on to smaller lateral lines that connect residential and business plumbing to the main lines. When major reconstruction needs to be performed to combined storm-waste sewage pipes, the sewer systems are separated and replaced with separate pipes for the a storm sewer system and a wastewater sewer system, which reduces the amount of water flowing through the pipes and reduces the costs of water treatment. Repairs to culverts involves restoration that redirects storm runoff more efficiently and reduces erosion.

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