Learn more about how centeralized waste mater treatment plants work.
Sewer systems have many steps and a coordination between the city and its residence is required in order for it to be efficient. Unlike septic tanks for example, a sewer system is made out of piping that connects every resident in a city together. Most of these pipes are made out of cast iron and are generally very heavy pipes. They are generally dug and placed underneath roadways which costs a lot of money and manpower to maintain. All of the pipes drain the wastewater from each individual home into a municipal sewer main. Similar to a septic tank, sewer systems rely on gravitational force to move the sewage through the water pipes and then down into the sewer main. The sewer main is the collection part of the system where sewage (both solid and liquid) builds up. It is then sent through larger pipes that connect the sewer mains to the wastewater treatment plant. Vertical pipes are then placed leading up to roadways and covered up by manhole covers. This enables relatively easy maintenance to occur. Sewage lift systems are also needed frequently when gravitational force does not work for moving the sewage like when there is a hill or a positive grade in the terrain. Many of these systems are need in order to make the entire system run effectively which costs additional capital and manpower to build, operate, and maintain.
Finally, when the wastewater enters the treatment plant through pipes, it is time to clean the water and make it reusable. The first step in this process is called “Coagulation and Flocculation”. In this step, positively charged chemicals are added to the waste, causing neutralization of the negatively charged dissolved particles in the water. After this occurs, the problem particles coagulate and form larger particles called “floc”. The next step is Sedimentation which is when these large particles that were formed sink to the bottom of the water because of their weight. After this, filtration occurs. Filtration is when the water is moved through sand, gravel, and often charcoal in order to remove the harmful particles. Finally, is the disinfection step where the water is then treated with chemicals such as chlorine in order to kill any remaining microorganisms. After this step, the water is ready to be redistributed and awaits pumping in large tanks. Following this step the treated waste water is then dumped into clean water sources in several different points along the pipe line to ensure the water is properly diluted. This dilution process keeps any bacteria that may have made it through the process to be centralized in one area where it could be used by the population making them sick. It is assumed that if the bacteria is injected in small quantities it will not have a negative effect on peoples health. Sewage treatment plants are known to kill upwards of 99 percent of the disease causing bacteria in the waste water. However there are many chemicals that go into doing so.
http://www.mwra.state.ma.us/03sewer/html/sewhow.htm