With the help of combined sewer systems, rainwater and sanitary wastewater are carried to the sewage treatment plant in the same pipe. These systems have limited capacity and in the case of heavy rainfall or snowmelt, the volume of the wastewater in the sewer system or treatment plant can exceed the maximum capacity limit. Therefore, these systems are designed in a way to discharge the excess amount of water to rivers, lakes or streams to deal with combined sewer overflows.

The combined sewer overflows can be defined as the untreated rainwater or sewage discharges when heavy rainfall or snowmelt fills up all the volume of treatment plants or municipal sewer systems and the system exceeds the maximum capacity. The authorities have to discharge this untreated sewage directly into streets, basements, parks, lakes, streams and rivers.

Combined sewer overflows can contain rainwater runoff, raw sewage from industries and homes, and chemicals that wash off street. This poisonous potion is unpleasant and very dangerous. It can contain many pathogenic microorganisms which can result in spreading disease among humans. Combined sewer overflows also contain ammonia, petroleum products, human waste, pesticides, etc. In this mix, pathogenic bacteria and viruses are the most prominent pollutants.

Each year, there are thousands of combined sewer overflows occur, according to the U.S Environmental Protection Agency (EPA). It can cause serious water quality problems when this wastewater and untreated sewage infect our water and thus, it’s a big threat to our drinking water. It can also cause damage to property by backing up basements.

With so many overflows, engineers and utilities have developed myriad tools to help curtail overflow events.

Green infrastructure

It is possible to reduce the peak flow by planting more trees and promoting green rooftop and rain gardens. Rainwater can also absorb into the ground by using permeable pavement. It will capture the rainwater where it will fall and store it for later use. This way it is possible to reduce peak flows before they enter the combined sewer system.

Increased storage capacity in the collection system

The rainwater can be stored into the storage system and after the rainstorm end, it can slowly release to the treatment plant. The storage capacity can be increased by making new storage units like storage tunnels. This will make the combined sewer system to treat more wastewater.

Sewer treatment plant expansion or increased storage at the plant

There are two ways to increase the system capacity to deal with rainwater: one is to increase the storage capacity and the other is to increase the capacity of the treatment plant to process more sewage and rainwater at the plant in a given time. It is possible to make a better system by implementing both on an existing combined sewer system.

Sewer separation

Most of the combined sewer systems are using sewer separation to control the overflow events. In sewer separation, separate pipes are used for sanitary and rainwater instead of using one pipe. In this way, rainwater can directly discharge to the body of water without mixing it with sanitary water. It is important to make a feasibility analysis before converting a combined sewer system into a separate sewer system.

Infiltration and inflow reduction

Due to cracks that appeared in pipes, water can infiltrate and damages the houses and businesses. Therefore, it is important to fix the cracks that appeared in combined sewers to reduce the amount of water that leaks from these cracks.

CSO-related bypass of the secondary treatment step at the sewer treatment plant

In the case of heavy rain, it is possible to create more capacity in the treatment plant by carrying out only the primary treatment of the mixture of rainwater and sewage and bypassing the secondary treatment. It is preferable to carry out a partial treatment of all the wastewater compared to a complete treatment for part of the wastewater and to leave certain parts without any treatment.

Overflow retention basins

It is possible to prevent the overflows by building overflow retention basins (ORB), which are storage facilities. ORBs will build at the collection system of combined sewer overflows points and the function of ORBs is to collect the necessary volume at these points. The occurrence of combined sewer overflows can efficiently control with the help of ORBs.

Screening and disinfection facilities

It is possible to treat combined sewer overflows with the help of screening and disinfection, without storing it. The sanitary trash can remove from the combined sewage with the help of screens. Sodium hypochlorite can use for disinfection and it can be mixed with the flow as they pass through the screens and remove the debris. The flow rate is adjusted in a way to provide enough time to sodium hypochlorite to kill bacteria.

Smart infrastructure

With the help of advanced technology, it is possible to implement a real-time decision support system, by using sensors, actuators, and controllers. The internet of things and cloud computing can make it possible for us to control actuated assets like movable gates in sewers and rainwater management systems, making it easy to monitor and control combined sewerage overflow events efficiently.