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Detail of interior with sanitary sewer equipment

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The Sanitary Sewer Topic Ideas

The sanitary sewer is a system of underground pipes that carries sewage from bathrooms, sinks, kitchens, and other plumbing components to a waste water treatment plant where it is filtered, treated and discharged.

Sanitary Sewer System The city's sanitary sewer collection system consists of pump stations and an intricate maze of pipes constructed under city streets. The system collects wastewater (such as from the bathroom, laundry facilities or kitchen sink) and transports it to the City of Midland Wastewater Treatment Plant.

Some of the pipes used in this process convey sewage by way of gravity, while others pump wastewater under pressure; these pipes are called "force mains." Midland's sanitary sewer system also collects storm water from the footing drains of many homes and structures built prior to 1988.

Once at the Wastewater Treatment Plant, wastewater is treated in a complex system according to stringent State of Michigan regulations and is then discharged into the Tittabawassee River.

Pump Stations & Gravity Sanitary Sewer Pipes The Wastewater Treatment Plant is responsible for 41 pump stations ranging in size from 20 to 5,500 gallons per minute capacity. Approximately 987,474 feet of gravity sanitary sewer pipes run throughout the city. These pipes range in size from 6 to 48 inches in diameter.

The City's sanitary sewer system is cleaned on a two-year rotation with some areas cleaned more often. Wastewater staff also video inspect sewers to evaluate their condition. In addition, smoke is blown through pipes in various sections of the city as needed to help locate any sewer line breaks or defects and identify locations where storm water unnecessarily enters the sanitary sewer system.

Sewer Laterals and Manholes

Where the water distribution system ends, the sanitary sewer system begins. Drains which carry away used drinking water are the point of origin for the sanitary sewer network. In effect, the sanitary sewer system is simply a network of pipes, manholes and pumps that function as the return system for the water distribution system. Inside drainage connects to a sewer lateral which transports wastewater away from the premises. A cleanout is an access point on the sewer lateral that allows a plumber to clear debris if the line is clogged. Sewer laterals are constructed at downward angles, or slopes, allowing wastewater to flow by way of gravity to the main. When the sewer system relies on gravity to power it, it is called a gravity-fed system. Besides being connected to the sewer lateral, the inside drainage system is also connected to vents which allow air to circulate inside the lateral and the main to which the lateral is connected. Hydrogen sulfide is produced if the bacterial breakdown of organic waste in a sewer system occurs in the absence of oxygen. Hydrogen Sulfide (H2S) is a poisonous gas with a “rotten egg” odor which quickly loses its ability to be detected by the human nose. Breathing air containing 100 ppm hydrogen sulfide is considered an immediate threat to life and health. Inhaling a concentration of 1000 ppm may result in instant death.

Vented Sewer Manhole Lid

The sewer vents atop buildings and the vented manhole covers over the sewer mains work together to make sure oxygen gets into the system so the production of hydrogen sulfide is minimized.Some sewer operators do not use vented manhole covers in order to prevent storm water from entering the sanitary sewer system at manholes. The unnecessary treatment of storm water at the wastewater treatment plant creates higher operating costs for the sewer operator.

Sewer Pipe Slope

Depending on pipe diameter, gravity-fed sewer pipes must be laid at certain downhill slopes so that the pipes are self-cleansing. These slopes are necessary for the following pipe diameters: PIPE SLOPE 6” 1.00% 8“ 0.45% 10” 0.30% 12” 0.25%

Sewer Manhole

Manholes connect sewer mains to the surface in order to enable inspection and maintenance and to permit venting of the sewer mains. Manholes are located approximately 500 feet from each other and they perform additional tasks: • Junction chambers—used to connect sewer mains of differing diameters • Drop manholes—used to connect two mains that are at different elevations • Flushing manholes—located at the end, or head, of sewer mains and used to discharge sediment from the system • Lamp holes—a small opening between two manholes for inspection of sewage flow

Lamp Hole

Lift Stations and Force Mains

Lift Station Pit

To expand the reach of a gravity-fed sewer system, lift stations are constructed to “reset” the starting point for the gravity-fed system and allow gravity to take over the process once again. A lift station works by taking in sewage from the entrance main and pumping it up a vertical pipe to the maximum height local topography will permit. At the top of the lift, the exit main moves the sewage on its downward slope again. This process can be repeated as needed while sewage makes the journey to the treatment plant.

Lift Station

For wastewater to travel uphill, the line must be pressurized. Commencing at a lift station, a pressurized sewer main is known as a force main. Once the force main reaches its highest elevation and begins to slope downward, the line becomes a gravity-fed system again.

Lift Station X-Ray View

Force mains may form a network of pipes which are separated from each other by valve junction boxes. This diagram represents a lower pressure force main system consisting of smaller diameter pipes. Additionally, a force main system may include isolation valves, air release valves and cleanouts.

“Sewer Chewer”

Sewer chewers are often installed at lift stations to reduce the size of solids which come into the station from the gravity-fed main.

Vented Lift Station

Since there are locations along the sewer system where hydrogen sulfide collects, lift stations may be vented. Hydrogen sulfide also collects in mains with flat grades and mains with sags.

Final topic: Streamflow

Streamflow is measured as an amount of water passing through a specific point over time. The units used in the United States are cubic feet per second, while in majority of other countries cubic meters per second are utilized. One cubic foot is equal to 0.028 cubic meters.