With billions of dollars in federal stimulus money available to rehabilitate aging sewer systems, municipalities will face issues in trying to keep roads and businesses open during the construction. It’s important to recognize the challenges of working around existing buildings, roads, and other structures.

Typically, in rehabilitating sewers, the flow cannot simply be diverted. It must be bypassed — temporarily pumped around the pipe being repaired or replaced. This requires pumps capable of moving trash-laden wastewater.

The pumps are deployed up-stream from the faulty pipe, and a temporary pipeline is installed from the pumps to a downstream section of the sewer. Temporary plugs set in place between these points allow the pipe to be removed or repaired. Depending on the number of homes or businesses the line serves, the bypass may range from a few gallons to several thousands of gallons per minute.

While this process sounds simple, advice from an experienced bypass company may be a good idea because of the many factors that must be considered. These contractors fully understand pumps, pipe and fittings and have experienced field personnel on staff to ensure proper installation and performance.

Before going to work, a contractor needs to know peak flow, pipe size, pipe depths, the duration of the work, access issues, and any other issues related to a particular project. This information facilitates proper pump and equipment selection.

Peak flow

The peak flow is the highest flow on the pipeline throughout the day. It usually occurs in the early morning and in the evening, related to people’s work schedules. Peaks also can occur during national events, such as halftime during the Super Bowl.

The contractor must know the peak flow to size the bypass pumping system properly. If the peak flow is not known or is inaccurate, serious problems can occur, including costly sewage spills. Once started, some by-passes cannot be stopped, so every measure must be taken to prevent a spill from occurring, including additional pumps if necessary.

Pipe depth

The depth of the pipe and the acceptable sewage level in it helps determine the pump selection. If the sewer is deeper than about 25 feet, submersible pumps must be used. In this case, the trash pump is lowered into the pipe opening and connected to the temporary discharge pipeline. If the sewer line is shallower than 25 feet, above-ground non-clog or trash pumps may be employed. In this case, a suction pipe or hose is lowered into the line, leaving the pump above ground.

Suction lifts greater than 25 feet are not really achievable, since the pump’s capacity will be heavily affected by the greater depth. A pump curve may look attractive for flow capacities, but when the pump’s net positive suction head requirement (NPSHr) is considered, many pumps are unusable where there is significant suction lift, as on bypasses from manholes.

(NPSHr refers to the head pressure that causes the wastewater to flow into the pump. An experienced bypass contractor will always take this into consideration when sizing a pump, rather than simply using a standard pump curve with its flow-versus-head capacities. Any-one who fails to consider the NPSHr is likely to see a sewage spill when the pump does not keep up with the flow.)

Sewer pipe size

If the peak flow is not known, then knowing the pipe size along with the downslope gradient of the pipeline, helps in estimating the maximum flow. While actual flow measurements are desirable, they are not always available, and experienced contractors can help in those instances.

Duration of work

Some bypasses can last as little as a few minutes; others may go on for months or years. Some bypasses can be shut off at the end of the day, but most need to run 24/7. Projects lasting less than a day are less intense and problematic than those requiring sustained, around-the-clock operation.

Usually, bypasses that last less than a day, or can be shut off at day‘s end, require minimal equipment or manpower. Generally, a properly trained and equipped pump expert can oversee the pumping and react if there are any problems.

On long-duration and around-the-clock bypasses, the contractor must consider additional personnel to maintain the system, and manage alarms, backup pumps and fittings, 24-hour emergency contacts, city noise ordinances, traffic and access control, and many other items.

Access issues

On occasion, a bypass occurs in a remote area where there is ample space for installation. More often, especially in the case of infrastructure rehabilitation, bypasses occur in public right-of-way, and daily activities must be disrupted as little as possible.

The proper equipment must be installed safely, within minimal space requirements. Installations in roads or near buildings limit access for both installing and operating the equipment. For bypasses in streets, traffic may have to be diverted or a section of the road closed. In some cases, the suction or discharge pipe must be routed to and from the pumps and may need to be buried to keep a road or buildings open. Other items like curtains for sound attenuation and aesthetics may be required.

If the bypass will have a long duration, ample room must be made available for routine equipment maintenance such as engine fueling and oil changes.

In higher-flow bypasses, multiple pumps may be needed, adding to the equipment footprint. Simply put, you should not assume that a bypass system can be installed or operated without proper planning and permitting from local governing bodies.

Other issues

Any specific sewage bypass operation may encounter a variety of other issues including:

• Sound-attenuated units to limit noise pollution.

• Accounting for friction losses in the temporary pipeline.

• Backup pumps as required by local jurisdictions (most likely a minimum of 100 percent backup capacity).

• Redundant pipeline.

• Contingency plan for sewage spills.

• Additional discharge pressure on the pump discharge to over-come line pressure when discharging into a force main.

• Provision for replacing or repairing faulty pumps.

Pumps and equipment

A wide variety of bypass equipment and pumps are available. Typically, piping is HDPE, steel, or PVC where allowed. Hoses need to be of rugged construction, and cities typically specify the types allowed. Valves generally must be of a type that prevents trash in the sewage from tangling on obstructions.

The pumps are the most important item in a bypass. All pumps are bound by the laws of physics. At sea level, the theoretical maximum suction lift for water (specific gravity of 1.0) of any pump under a perfect vacuum — whether the pump is self-priming, vacuum-assisted, diaphragm-assisted, or compressor/venturi-assisted — is 33.9 feet.

However, other factors limit the pump to a practical lift of substantially less — on the order of 25 to 28 feet. This is why submersible pumps are needed at deeper settings; they are not limited by suction lift and instead push water up.

For bypasses with suction lifts of less than 25 feet, an above-ground self-priming or prime-assisted pump is typically used. The pump NPSHr (not merely the flow-versus-head curve) plays a major role in the proper pump selection.

Types of pumps

A self-priming pump is one that will prime itself as long as it is full of water. These pumps are generally the easiest to use, as they consist of a pump and its driver (electric motor, gasoline engine, or diesel engine).

A suction hose or pipe is inserted into the manhole or sewer line opening, then connected on the pump’s suction side. A discharge hose or pipe is connected on the discharge side. The operator fills the pump chamber, and turns on the pump. Typically, these pumps are used where priming time (the time it takes from startup until the pump starts pumping wastewater after evacuating the air within the suction pipe or hose) is not critical. The priming time depends on suction lift, but can take several minutes.

A prime-assisted pump will prime itself under dry conditions (with or without water). In general, prime-assisted pumps will prime substantially faster than self-priming pumps. The prime-assisted pump package consists of a pump, driver, and priming device.

The priming device may be a vacuum pump, diaphragm pump, or compressor/venturi. In general, a vacuum-assisted pump has the quickest priming because of the air-handling capability, followed by the diaphragm assist, and then the compressor/venturi set-up. This is worth considering when selecting pumps for projects.

For bypasses with lifts greater than 25 feet, various types of submersible pumps must be used. The most common are electric motor-driven (pump and motor are lowered into the wastewater), and hydraulic-driven (a hydraulic power unit, hydraulic hoses, and hydraulic pump). Application of these will depend on whether an electric unit poses a safety issue. Nearly all submersible pumps require ample space within the pipeline or manhole to install the pump, as most are of substantial size. It may be necessary to modify the sewer line to allow pump installation, and that may mean removing upper manhole cones, cutting the pipe open, or installing at other access points.

Knowledge is power

When it comes to pumps and equipment, many pump distributors have general knowledge of pumps and applications, but few have experienced the actual set-up, installation and maintenance of bypass systems. As more money flows into repair and rehabilitation of decaying sewer systems, firms that do bypassing routinely are the best suited to perform the work or supply equipment and materials. As is true in any business, experience is often the best qualification.

Jerry Soto is bypass estimation manager with Griffin Dewatering Corp., a dewatering service company based in Houston, Texas. He can be reached at 713/671-8000 or griffin@griffindewatering.com.