Combined Sewer System: How Does This System Operate?

Combined sewer systems gather domestic, commercial, and stormwater runoff in one pipe. These systems continue to collect and transport raw sewage and stormwater in the same pipe.

Combined sewer systems aim to transport this mixture of untreated sewage and stormwater to a wastewater treatment plant for processing.

Dry weather and light rain pose less of a threat to combined sewers since the volume of water may be kept within the system and discharged to wastewater treatment plants.

However, the amount of water in the combined sewer system increases during heavy rainfall events or snowmelts.

Let's examine the combined sewer system in further detail, including its operation and advantages and disadvantages. The combined sewer overflow will also be discussed.

What Are Combined Sewer Systems?

A combined sewer uses a network of pipes, tunnels, and pump stations to discharge sewage and urban runoff to a treatment plant or disposal site.

The storm, roof, and sanitary sewer connections to a single mainline often go to a water treatment facility.

These systems are meant to discharge extra water to nearby lakes, rivers, or surrounding water bodies during wet weather events (such as heavy rainfall or melting snow).

Dams are used to provide basic passive filtration to accomplish this. These incidents are called combined sewer overflows (CSOs).

They are untreated discharges from the wastewater conveyance infrastructure of a municipality, just like sanitary sewer overflows (SSOs) and storm water discharges.

Combined sewer systems are durable and suffer little harm from combined sewer overflows and surcharges, thanks to their design.

It may have several detrimental environmental effects, including introducing heavy metals, pollutants, and chemicals into the aquatic environment.

Unfortunately, these drain systems must be regularly maintained to avoid the accumulation of debris, industrial waste, or other contaminants, regardless of how the sewer line is configured.

If not, sewage backups or surges of sewage backflow into nearby water bodies may hurt local wildlife and human health.

What Is the Main Function of Combined Sewer Systems?

Rainwater runoff, domestic sewage, debris, and industrial wastewater are all collected in the same pipe by combined sewer systems, which are sewers.

Most combined sewer systems send their wastewater to a plant, which is processed before being released into a body of water.

Uncontaminated storm water dilutes sewage, but rainwater runoff can dissolve or suspend almost anything it comes into contact with on roofs, streets, and storage yards.

Landscape irrigation, dewatering during construction, and washing sidewalks and buildings can also discharge into combined sewers during dry weather.

What Is a Combined Sewer Overflow?

The term "CSO" (combined sewer overflow) refers to when combined sewer systems are overburdened by too much stormwater and overflow into nearby streams and rivers.

These overflows are called combined sewer overflows (CSOs), including poisonous substances, debris, untreated human and industrial waste, and stormwater.

This water has not yet been treated by a wastewater facility or has received only primary treatment.

By the turn of the 20th century, combined systems were present in most centralized wastewater management cities.

They pose a major water pollution concern in American cities with combined sewer systems. CSO discharges can be compared to "urban wet weather" discharges.

CSOs have led to poor water quality, dangerous swimming conditions, beach closures, and limits on shellfish harvesting to stop the spread of human infections and disease.

Although few overflow occurrences were anticipated, combined sewer systems were never intended to accommodate all water during heavy rainfall and wet weather flows.

However, as urbanization and heavy rainfall events have increased, CSO events have become more frequent.

Communities use innovative strategies to achieve upgrading stormwater basins, decoupling impervious surfaces, and enhancing stormwater infiltration using green infrastructure.

In many places, low-impact development design is employed to reduce runoff from new construction by increasing the amount of stormwater that soaks into the ground.

Communities must create long-term plans for dealing with water quality issues brought on by CSOs and a strategy for dealing with water quantity issues.

NOTE: To restore or maintain the chemical, physical, and biological integrity of the nation's water by preventing the discharge of pollutants, including CSOs, and major federal regulations, such as the Water Pollution Control Act of 1948 and the Clean Water Act of 1972 and its subsequent amendments, were put into place.

What Steps May Be Taken to Stop Combined Sewer Overflows (CSOs)?

There are three general methods for getting rid of CSOs:

1) Sewer Separation

Construct a second network of pipes to keep stormwater and sewage apart.

Pipes won't overflow after heavy rains anymore, thanks to the removal of sanitary sewer overflows.

Building a new plumbing infrastructure is difficult and expensive, particularly in heavily populated urban regions.

2) Boost Storage Capacity

During periods of heavy rain, store extra mixed wastewater and gradually release it to the wastewater facility.

Rainwater and combined wastewater can be stored in underground tanks and larger pipes like interceptors and tunnels.

3) Treatment

Transport the combined wastewater and stormwater to a facility for treatment before releasing it into an aquatic environment.

The water that receives treatment will grow if additional water is discharged to the municipal wastewater treatment plant or a satellite treatment facility.

Enlarging the facility to handle the increased flow could be necessary.

NOTE: In April 1994, the US. The Environmental Protection Agency (EPA) created the CSO Control Policy, the national framework for CSO control, through the National Pollutant Discharge Elimination System permits program. According to the US Environmental Protection Agency, combined sewer overflows are a top priority for roughly 860 communities across the US.

How Does a Combined Sewer System Work?

During CSO occurrences, when combined sewage and wet weather flows surpass the capacity of the sewage treatment plant, sewers can result in a major water pollution concern.

When there is a high amount of surface runoff, the strain on specific tributary branches of the sewer system may result in a backup where domestic sewage flows out.

Relief points allow discharges of untreated human and industrial waste to flow into rivers, nearby streams, and other local water bodies when combined sewer systems encounter these.

Such occurrences usually have bad environmental and lifestyle effects, such as beach closings, contaminated seafood that is hazardous to eat, and contaminated drinking water sources.

Some methods used to mitigate combined sewer overflows are as follows:

• CSO storage
• Increasing wastewater treatment capacity
• Retention basins
• Screening
• Disinfection facilities
• Lowering stormwater runoff
• Rain gardens
• Rain barrels
• Roof gardens
• Real-time decision support systems

Gravity sewer designs are used less often when building new sewer systems.

Modern sewer designs construct these sewers to eliminate sanitary sewer overflows, while many older cities and towns still use combined sewer systems that were previously built.

What Are the Advantages of Combined Sewer Systems?

The combined sewer system has the advantages listed below:

1. Because sewers are vast, cleaning them is simple.
2. The expense of maintenance is lower.
3. By diluting the sewage, the stormwater weakens it.
4. It's simple to reach the self-cleaning velocity.
5. This system only needs one sewer set. It then becomes cost-effective.

What Are the Disadvantages of Combined Sewer Systems?

The drawbacks of a combined sewer system are as follows:

1. The increased workload of the sewage treatment plant.
2. There is unnecessary pollution in the storm water.
3. The sewers have a huge diameter.
4. This system is economically unviable when pumping is necessary to lift sanitary sewage.
5. The combined sewer may overflow during heavy rainfall, which could cause problems for the populace.

Conclusion

Whatever type of sewer system is used, one thing always needs to be maintained and monitored to keep it clean and working properly.

Pressure in the sewer lines increases when buildup and obstructions in the sewer system prohibit the water from flowing as it should.

A combined sewer system has the benefit of typically treating both stormwater and industrial wastewater to meet water quality standards before discharging them into a water body.

Regardless of the system type, appropriate maintenance and cleaning are most important.

Depending on the particular application, various sewer cleaning machines can easily maintain storm drains, catch basins, and sewer lines.