Trenchless rehabilitation projects are by nature executed on systems that are already in use. But for a repair to be executed safely and effectively, the pipeline must be taken out of service for the duration of the work.
This adds some complexity to trenchless rehabilitation projects. An alternative routing for the product flow must be installed in order to create a window for the repair to be executed.
Bypass pumping is one method of establishing an alternative route while repairs are carried out. It is often used in sewer repairs to prevent sewerage from backing up into homes while work is executed on existing infrastructure. (Read Is Trenchless Rehabilitation Right for Your Home?)
The method involves setting up the bypass piping route and using pumps to move the product through the bypass and back into the main system.
Why is Bypass Pumping Important?
It is well known that the underground piping networks in the United States are aging and in need of significant rehabilitation. A Deloitte report quotes that the restoration of underground pipes is expected to cost $1 trillion USD over the next 25 years.
The volume and pace of trenchless rehabilitation projects to take care of this deteriorating infrastructure will be significant. These projects can only be executed with minimum impact to communities and cities by using methods like bypass pumping. (Read The Essential Pros and Cons of Trenchless Sewer Rehabilitation.)
How is Bypass Pumping Installed?
The first step in bypass pumping is to identify the points on the existing line where the bypass will begin and end. A number of factors should be taken into account when making this decision:
- Are the start and end points easily accessible?
- What is the bypass routing that has the least impact on the community? (It may not always be the shortest route.)
- Is there sufficient space for pumps and equipment to remain in place for the duration of the project?
The second step in the process is to size the system for the maximum potential flow while the bypass is operational. This involves evaluating flow rates taking into account peak conditions as well as rainfall events that may increase the flow temporarily.
It also requires an assessment of the piping depth, piping capacity, length of the bypass and all other factors that could affect the flow of product.
The results of the sizing exercise are a specification for the number and capacity of bypass pumps as well as the pipe size for the bypass line. It is important to note that a safety factor must be built into the solution and spare pumps are essential to avoid a system shutdown.
For more complex systems, more than one bypass may be necessary to clear a sewer line for repair. Each branch of the system that connects to the affected sewer must be identified and bypassed to ensure that no product enters the line during repair.
Once the system is designed and installed, it must be tested to ensure it delivers according to the design expectations, and that the pipe and couplings maintain their integrity at test pressures above the normal operating conditions of the bypass.
During the phase where a bypass is operational, the main sewer line is plugged at each end to avoid the flow of sewerage into the pipe where repairs are being executed.
Where Can Bypass Pumping Be Used?
Bypass pumping is an effective method for trenchless repairs for small scale and large-scale systems.
At the upper end of the scale, some projects have implemented systems as with a capacity as high as 156 MGD (million gallons per day). In this particular project, 18 pumps were required to manage the sewer flow including rain events.
Problems with Bypass Pumping
One of the hazards of interfering with an existing system to install a temporary solution is the potential for spills and environmental incidents.
In one incident two of three bypass pumps installed failed leading to an overflow of sewerage from the mains line into a local river. The spill was actually first noticed by a member of the community who notified the authorities.
Any incident like this involves damage to the environment, negative impacts on the community as well as negative publicity. It highlights the importance of redundancy so that backup pumps have sufficient capacity to handle a failure of the main pump. It also highlights the needs for constant monitoring of temporary systems so that incidents can be caught early before they impact the community.
In another incident, the bypass piping itself failed causing a flow of mixed sewerage and industrial waste into a stormwater drain and then into a river. Piping failures can be caused by external damage.
Because bypass piping has above ground sections and can also cross roads, there is potential for piping to be struck, driven over or even vandalized.
Alternatives to Bypass Pumping
For very small projects it may be possible to use temporary storage to contain the sewerage flow while the repair is executed. Care must be taken when adopting this approach to make sure that a delay in repair doesn’t result in running out of storage capacity and a spill.
For other projects, where bypass pumping is not feasible, it may be necessary to lay a new pipe alongside the old one. Once the project is completed flow can be switched from the old system to the new.
Conclusion
Bypass pumping is a method to keep sewerage lines free of sewerage so that trenchless repairs can be executed without interrupting the users. It is a method that is widely used with much success but requires some care.
Sizing of bypass pipes and pumps are critical to ensure they can cope with peak flows and rainfall events. These temporary systems must also be carefully monitored as spills can easily migrate to environmentally sensitive areas.
The vast need for trenchless rehabilitation of the aging underground sewer network implies that bypass pumping is here to stay for some time.