Sewer systems are a necessary part of cities around the globe. They transport contaminated wastewater away from populated areas to treatment facilities to treat and decontaminate the water before being returned to the ecosystem. The lines also help to carry excess rainwater away, helping to reduce the likelihood of flooding.

Sewer Systems

There are three main types of sewer systems: separate, combined and partially separate systems. Each has its unique factors. The type of sewer used is dependent on the area of installation and demand of the city.

-Separate Sewer Systems

Separate sewer systems use two lines, one for sanitary waste and the other for stormwater. In this system, the closed-circuit transports only sanitary waste. Stormwater, on the other hand, diverts to drain into a local river, lake or return to the ocean. With a separate sewer system, treatment plants receive smaller quantities of water for cleaning before dumping. However, since the sanitary line is independent of rainwater, the flow may not reach a self-cleaning velocity. As a result, cities may need to install pumping stations.

-Combined Sewer Systems

Combined sewer systems have only one line which services both sanitary and stormwater waste. A closed circuit contains all sewage. Some municipalities opt for this type of wastewater removal to make it easier to run pipes to residential areas. A combined system is best in regions that receive adequate rainfall as rains help the sewer achieve a self-cleaning velocity. Areas with minimal rain must employ pumping stations throughout the city, which increases the cost and negates any cost benefits of using the system. Regions that see excessive rain may have sewage spilling in public areas due to overflow. This spillage may be a health hazard for citizens. Water treatment plants must decontaminate all water before dumping.

-Partial Sewer Systems

A partial sewer system is an amalgamation of a separate and combined sewer system. This method allows for a small portion of stormwater to enter the sanitary water line to aid flushing. With this system, water runoff from roofs combines with drainage from kitchens and bathrooms. A partial system is more economical for many cities, as it requires minimum pumping and a reasonable sewer size. In comparison to a separate sewer system, partial systems do have a higher flow and treatment plants.

Sewer Types

Within each sewer system there are several different types of sewers that work together. Some systems may have each component while others only employ a handful.

– Aerial Sewers

An aerial sewer is unburied and supported on bents or pedestals. Typically, aerial lines are sanitary sewers. These pipes are often found over canals or waterways such as creeks or small rivers. Aerial sewers are also built in areas where underground passing is not viable and where other trenchless methods cannot facilitate the installation of these pipes underground.

-Branch Sewers

Branch sewers are the connecting lines that run from homes and businesses to the primary sewer system. Segregating sewer systems with branch lines helps workers rehabilitate and locate blockages quicker. These lines tend to be larger in capacity and diameter than the systems into which they empty.

-Collector Sewers

Collector sewers carry wastewater from a source to an interceptor sewer line. The Interceptor line then takes the wastewater to a treatment facility.

-Critical Sewers

A critical sewer is a sewer that has to meet specific criteria in order to be considered “critical.” Its classification includes the location and depth in which the system will be installed, or even the construction material of the system. Typically found in a strategic area, these large diameter pipes are a crucial part of a municipalities sewage system.

-Sanitary Sewer

Sanitary sewers connect to sinks, toilets, washers and any other plumbing component a home or business may have. The waste collected from these lines goes to treatment plants for filtering and treatment before discharge.

-Storm Sewer

Storm sewers collect and carry rainfall runoff. Unlike sanitary sewers, these systems are not designed to handle hazardous waste. In a separate sewer system, storm drainage does not go to treatment plants before discharge.

-Trunk Sewer

Often referred to as the primary sewer, trunk sewers are large lines that move waste from smaller tributaries to the treatment plants. Many municipalities use these as part of a tiered system that includes branch pipes, main lines and then the trunk lines. The trunk lines often flow into an interceptor before going to the treatment plant.

-Gravity Sewer

A gravity sewer uses different elevations to move waste through the lines. The name implies that gravity does the work instead of the use of vacuums or pumps to force the wastewater through the pipes. Most sewer systems are gravity sewers. However, they are often used in conjunction with forced mains as the plumbing connects to the treatment plant.

The Most Complex Sewer Systems in the World

Some of the world’s sewer systems are true engineering marvels, designed to handle massive amounts of water, waste, and flood risks. Here are a few of the most complex:

-Tokyo’s Underground Flood Tunnels

Tokyo’s G-Cans Project is a massive underground flood control system, featuring tunnels and reservoirs that divert floodwaters to prevent citywide flooding. It can handle millions of gallons of water and includes a huge underground hall for temporary water storage.

-Paris’s Historic Sewers

Paris’s sewer system, built in the 19th century, spans over 2,100 kilometers and is an engineering masterpiece. Modernized over the years, it’s also a popular tourist attraction, showcasing the city’s rich history and advanced civil engineering.

-London’s Thames Tideway Tunnel

London’s sewer system, dating back to Victorian times, is being updated with the Thames Tideway Tunnel. This 25-kilometer-long tunnel will prevent sewage overflows into the River Thames, addressing issues in an aging system.

-New York City’s Combined Sewer System

New York’s system manages both stormwater and sewage in a combined network of over 13,000 kilometers of pipes. Efforts to separate stormwater and sewage aim to improve efficiency and reduce pollution.

-Beijing’s Modernized Sewage System

Beijing has modernized its sewer system with real-time monitoring and wastewater reuse strategies, ensuring a more efficient and sustainable approach to waste management.

Conclusion

Sewer systems are vital for urban sanitation and environmental protection. Understanding the various types, including separate, combined, and semi-separate systems, helps cities manage wastewater efficiently. Innovations in global sewer engineering, such as advanced flood control tunnels and modernized infrastructure, highlight the importance of adapting to urban challenges. Continuous improvement and investment in these systems are crucial for sustainable urban living and public health.

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