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Posted by on Aug 24, 2017 in Tell Me Why |

How Does Sewage Plant Work?

How Does Sewage Plant Work?

A sewage plant works by purifying waste material and recycling it for a useful purpose. Waste material from lavatories, sinks, baths and industry is known as sewage. It arrives at the sewage plant through a system of pipes and drains. At the plant the sewage is filtered to remove grit and allowed to stand for sometime in a settling tank so that the more solid matter sinks to the bottom and is removed as sludge by revolving scrapers.

The more liquid sewage is brought into close contact with oxygen, bacteria and other micro-organisms by being fed into aeration tanks may be “activated sludge plants” where air is introduced into the liquid by bubbles or with a stirring device, or they may be “percolating filters” with beds made of pieces of coke, clinker or stone. The liquid passes continually over these beds and air has free access to it.

The bacteria and other organisms used in the process are known as “aerobic” because they need free oxygen to carry out their work of oxidizing the large, potentially dangerous, molecules of the sewage into smaller, harmless ones. After further settling, the effluent, or purified water is fed back into nearby rivers.

Sludge is often “digested” by being heated to about 30 degree Celsius (86 degree Fahrenheit) and then acted upon by anaerobic bacteria which do not need free oxygen to do their work. Methane gas, CG 4, is produced, which can be used as fuel. The sludge remaining is drained or dried, and its unpleasant smell is removed. It can then be used as a soil conditioner or fertilizer.


Nearly half the state’s population sends sewage to treatment plants. Over 5,500 businesses and industries contribute wastewater as well. In addition, nearly half of the total flow in sewers is from rainy-weather street runoff and from below-ground cracks and faulty connections that allow groundwater into the system.


Sewage travels through three different sets of pipes. Water that is used in a home or industry is flushed through a building’s pipes until it reaches local sewers which are owned and operated by city and town sewer departments. These 5,100 miles of local sewers transport the wastewater into 227 miles of interceptor sewers. The interceptor sewers, ranging from 8 inches to 11 feet in diameter, carry the region’s wastewater to treatment plants. Though most of the wastewater flows by gravity some low-lying areas require pumping.
how does sewage treatment work


Any sewage treatment plant provides preliminary primary and secondary treatment to its wastewater flows. The treatment process is as follows:

Collection and Pumping 
Sewage is piped from communities to several headworks where bricks, logs and other large objects are screened out. Pumps draw the sewage through deep-rock tunnels.

Preliminary Treatment 
Mud and sand settle in a tank called a grit chamber. Later, this material, known as grit and screenings, is taken to a landfill for environmentally safe disposal.

Primary Treatment
The sewage then flows to primary settling tanks where up to 60% of the solids in the waste stream settle out as a mixture of sludge and water. This primary treatment removes very few toxic chemicals.

Secondary Treatment 
In the secondary treatment plant oxygen is added to the wastewater to speed up the growth of micro-organisms. These microbes then consume the wastes and settle to the bottom of the secondary settling tanks. After secondary treatment, 80-90% of human waste and other solids have been removed. A significant proportion of toxic chemicals are also removed by this process.


The remaining wastewater is disinfected before it is discharged to the receiving waters. This stream of treated wastewater, known as effluent, travels through a 9.5-mile Outfall Tunnel bored through solid rock more than 250 feet below the ocean floor. The tunnel’s last mile and a quarter include 55 separate release points known as “diffusers.” By extending to an area with water depths up to 120 feet, this outfall provides a much higher rate of mixing and/or dilution than is possible with present discharges into the shallow waters.

Sludge from primary and secondary treatment is processed further in sludge digesters, where it is mixed and heated to reduce its volume and kill disease-causing bacteria. It is then transported through the Inter-Island Tunnel to the pelletizing plant, where it is dewatered, heat-dried and converted to a pellet fertilizer for use in agriculture, forestry and land reclamation.

Content for this question contributed by Michael Hunter, resident of Greensburg, Decatur County, Indiana, USA