The Complex World of Sewer Networks

By Denise Sullivan
Published: December 7, 2017 | Last updated: July 5, 2023
Key Takeaways

Within the three major types of sewers is a complex network of different types of sewers used for different purposes from stormwater removal to sanitary waste removal, all of which benefit from trenchless maintenance.

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 need/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 is/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.

Maintenance and Repair Materials

No matter the type of sewer, cities must adhere to a regular maintenance schedule. Cities must ensure that their sewer systems undergo routine inspection, maintenance of limit problems and ensure they stop waterline breakages before they happen.

The first step in maintaining a working sewer system is regular inspections. To make the reviews go as quickly and smoothly as possible, workers use video cameras to inspect the line; this is a trenchless method known as pigging, which uses what the industry calls "intelligent pigs" (robotic inspection machines) with CCTV cameras mounted on them. These cameras are tied back to television monitors on the ground, which allows workers to see inside the pipe without having to dig it up. During these CCTV inspections, they look for blockages, cracks and infiltration. Workers reinspect trouble spots for additional issues on a 3 to 6-month cycle.

Flushing the system with a powerful water jet nozzle is standard maintenance in most areas. The frequency of flushing depends on the individual city's maintenance schedule. However, most municipalities perform a straight flush on all lines once every four years. (Learn more in "Trenchless Sewer Repair and Cleaning 101.")

Annually, workers address root issues, which may block the line and cause damage to the pipe. A robotic saw goes through the plumbing cutting off any infiltrates. Once finished sawing, workers flush the line and treat it with an herbicide. They also reinspect the line to note any significant damage to the pipe wall.

Despite constant reminders that grease should be collected in cans and thrown away in the trash, many residents still pour fat down the drain. Once the oil hardens, it can cause blockages and potential damage to the pipe. Many cities now inspect the pipes for grease buildup and once every 3 to 6-months, they use a grease emulsifier flush on city lines.

During routine maintenance, workers rehabilitate manholes. Most often the frame and cover are replaced or rehabbed. Additionally, workers may need to spray cementitious mortar or an epoxy liner. Any other rehabilitation depends on the manhole condition at the time of inspection. (Read on in "A Look at Structural Manhole Rehabilitation.")

Finally, any lines that show wear or cracks may need relining. Relining is a cost-effective way to repair broken lines.

The Role of Trenchless Technology

Trenchless technology allows cities to rehabilitate water lines without disrupting service or the surrounding areas. Instead of tearing up roads and walkways to access the lines underneath, only small patches of earth are disturbed to create entry and exit trenches. In many cases, workers use manholes or other already established access points to complete rehabilitation and repair projects.

The most common trenchless technology method used is pipe relining either using sliplining or cured-in-place pipe, also known as CIPP. With sliplining, the pipe inserted into the existing line is much smaller and requires grouting between the existing wall and new plumbing to prevent infiltration.

With CIPP, workers use a liner permeated with resin. This flexible liner can go further and fits tighter to the existing plumbing, thus, itrequires no grout. The liner can go further than sliplining and can accommodate bends.

In some cases, the existing sewer line is too damaged to continue using even with a liner. The existing pipe may also be too small for the area it services. In this case, pipe bursting is the better option. The pipe bursting method uses a mandrel followed by the new line. The mandrel breaks up the existing plumbing, forcing it into the soil around it and acting as bedding for the new tube. (Learn more in "Sliplining or Pipe Bursting for Pipe Repair?")

Sewer lines play an important role in keeping the populace healthy. By removing waste from our residences and businesses, residents do not have to worry about contaminants in their soil or drinking water. Proper maintenance and sewer type selection is the key to keeping the population happy and healthy.

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Written by Denise Sullivan | Technical Writer @ Trenchlesspedia

Denise Sullivan

Denise Sullivan is an accomplished freelance writer from Louisiana, with a Associate's Degree in Journalism from Eastern Oklahoma State College. She also graduated from East Central University with a Bachelor's in Biology. Denise began her writing career writing operations and maintenance manuals and software utility manuals for flight simulators. Since, she has expanded her writing to a broad spectrum of topics.

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