With the growth of the sewer rehabilitation industry, more municipalities are choosing the trenchless process to repair their aging infrastructure, allowing them to stay compliant and avoid unpermitted discharges. During the sewer rehab process, sewer flow cannot be shut down without causing major environmental issues. Therefore, to implement the trenchless sewer rehab, temporary bypasses are necessary to allow uninterrupted sewer flow to be routed around the work area.
Not surprisingly, bypass pumping adds another level of complexity when designing a sewer rehab project — and with that complexity comes risk. There is substantial risk to the environment and the community if a bypass is designed incorrectly, along with a financial burden for delays or unanticipated change orders.
That’s why it’s imperative when a rehab project is in the design phase to focus closely on the bypass design, not just the method of sewer rehab. Considering some critical details can help.
Look at Scope and Flow
The first step to deciding the proper bypass design is to determine if a bypass is even necessary. Are there diversions available or parallel sewer lines? Or will flow-through plugs work? If not, take flow into consideration.
Flow sets the design parameters needed to be able to perform a bypass — essentially, it is a baseline for the bypass suppliers and contractors to design the bypass pumping system.
Stating the flow requirements in bid documents accomplishes three key matters:
- It reduces the bid price disparity of bypass subcontract numbers.
- It reduces the chances of an under-designed system that could lead to catastrophic failure.
- It reduces the chances of paying more for an over-designed system.
If the flows to be bypassed are known, engineers or system owners can incorporate the requirements into the specs for the project. It’s important to provide not only average daily flows, but also the anticipated peak wet weather flows. Doing so provides a baseline for bidders to design the bypass.
If the flows are unknown, it is advisable to conduct a flow study to ensure an accurate flow requirement is incorporated into the project bid specifications.
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When possible, provide an accurate sewer map to gain an overview of the collection system, including size and location of manholes and piping, lift stations, and wastewater treatment plants in the rehab area. It is especially important to provide not just the information for the rehab section, but also the sewer information upstream and downstream of the section being rehabilitated. This information can add value to the bidders by offering possible alternative routes for the bypass system. This may reduce the length of discharge piping or reduce the impact on the community. If accurate maps are not provided, it can result in extra access costs for the bypass.
When it comes to managing risk in sewer bypass design, it’s important to protect the work environment and community, as well as look at what drives the cost for the bypass.
Understand Access Points
To reduce the chance of accruing unknown costs or potential change orders, it is important to determine access points to the work environment ahead of time. Often, the sewer bypass will be outside the scope of the sewer rehab. For example, the bypass design may require diverting flow upstream from the sewer line rehab segment, and pumping downstream of the rehabilitation segment. In this case, it’s necessary to consider the access upstream, as well as downstream — along with any potential obstacles for the discharge piping along the route.
These obstacles could include right of way access, sewer easements, or roads that may require trenching to lay discharge piping through that area. Additionally, pipe laydown areas need to be considered for pipe storage during the bypass build and breakdown. Also consider suction and discharge access. For some bypass systems, it may be necessary to remove manhole cones or add additional manholes for suction access to accomplish the design flow. For some projects where the sewer line is greater than 20 feet, benching the pumps may be required to access the flow and reduce suction lift, thereby increasing the bypass hydraulic efficiency.
Bypass access is a critical component to a rehabilitation project design.
Additionally, look for ways to reduce the impact on the community as part of the bypass design. Ascertaining sewer easement access upfront to keep the community informed will reduce project delays and limit additional project cost for the bypass or rehab access. Keep in mind it may make more sense to extend the bypass beyond the sewer easements to avoid heavily congested areas or difficult property owners, or for easier routing of bypass discharge with less restoration. This foresight can help reduce the bypass cost.
Reducing risk in a bypass design has a monetary value associated with it that needs to be balanced for any sewer rehabilitation project. However, there is no one-size-fits all when it comes to building in the contingencies that impact cost. Still contingencies are essential to ensuring the system has been designed with the correct flows to handle large weather events or other occurrences that could lead to failures.
It’s important to determine the level of redundancy for both the bypass pumps and piping. For example, will the bypass pumps be designed for 100% redundancy? If so, the cost of the bypass will increase, but the risk of issues will decrease. On bypass systems for sewer rehab of pipe 24 inches and below, the cost of building in 100% redundancy is easier to balance with the benefits it offers.
On larger bypass projects, bypass pumps can be designed for a lower percentage of redundancy. The rule of thumb is one standby unit per sewer bypass system for the largest pump being used. This reduces costs but increases risks. Therefore, a balance can be struck by increasing the pumping redundancy, where it makes sense, to 25% or 50%. However, with that, the space required to implement the increased redundancy must be taken into consideration. Increased redundancy equals more space required for implementation.
When implementing bypass design for large diameter pipe rehab projects, it is often necessary to account for multiple bypass discharge lines to convey the required flow. Pipe redundancy is something to consider for large diameter rehab projects since the risk is greater if something catastrophic happens. Having a backup discharge line to convey the flow in an emergency allows for uninterrupted bypass operation while the damaged pipe is being repaired. This reduces risk and downtime. However, there is an increased bypass cost.
When it comes to managing risk in sewer bypass design, it’s important to protect the work environment and community, as well as look at what drives the cost for the bypass. The flow, access, and redundancy all factor in, but so do the distance of the bypass, mobilization to the work site, and the duration of the project. Along with upfront bypass design planning, a collaboration between the engineers, municipalities, and contractors will reduce risk, lessen the impact on the community, and increase project value, ultimately leading to a successful rehab project.
Ladd Gould is the strategic customer manager-pump solutions at Sunbelt Rentals