Underground and out of sight, wastewater collection systems are often taken for granted. In their 2013 Report Card for America?s Infrastructure, the American Society of Civil Engineers (ASCE) gave the nation?s wastewater infrastructure a grade of D. The grade indicates that the wastewater infrastructure is in poor to fair condition and generally considered to be below acceptable standards. It indicates that many components of wastewater collection and treatment systems are approaching the end of their expected useful service life and are not meeting the objective of protecting public health and the environment. The ASCE estimates a $298 billion need over the next 20 years to address sewer system overflows, combined sewer overflows and other pipe-related issues.
Gravity sewers are the most commonly used central sewer/collection system. Because gravity provides the moving force to transport the wastewater, excessively hilly or flat terrain requires deep excavations and large pump stations, driving up construction, operation and maintenance costs.
Standard design practice for gravity sewers is to provide hydraulic capacity with an allowance for infiltration and inflow (I/I). Infiltration and inflow is non-wastewater flow that enters a sewer system from groundwater and stormwater. Groundwater infiltration seeps into sewer pipes and manholes through defective pipes, joint failures, faulty connections, and other deficiencies in the system. Inflow is stormwater that flows more rapidly and directly into the sewer system through a variety of sources including roof drains, foundation drains, stormwater cross-connections and defective manhole covers.?
Over time, the integrity of traditional gravity sewer systems diminishes. Cracks, joint failure, and other deficiencies develop.? Under conditions of high groundwater or during wet weather events, defects in the deteriorating pipe allow excessive I/I to enter the system. The extraneous flows can quickly overload the collection system capacity, resulting in sanitary sewer overflows (SSOs), backups of raw sewage into basements, and high operating costs and equipment wear due to the transport and treatment of the excessive flows.
Renewal: Repair, Rehabilitate, Replace
Recognition of problems associated with sewer renewal in populated areas, sensitivity to resource allocation, and the desire to achieve the highest level of I/I removal all drive utilities to take a full Triple Bottom Line (TBL) approach. A TBL analysis gives utility managers a powerful tool for evaluating sewer renewal and considering the full range of economic, social and environmental costs. The analysis provides an integrated assessment of the alternatives rather than addressing sewer renewal goals with a quick, low-cost approach that, historically, has not met the overall intent of the efforts.
Sewer repairs are generally used for localized areas of failure or deterioration. The objective is to restore the sewer to an acceptable operating condition. Repairs can consist of grouting for point repairs, replacing or lining short segments of the deteriorated pipe segments, pipe sleeves and joint repairs. Repair techniques do not extend the life of the original sewer pipe and are often temporary fixes until a more comprehensive rehabilitation or replacement program can be implemented.
Sewer rehabilitation aims to restore the structural integrity of pipe, extend the operational service life, and in many cases, recapture the hydraulic capacity of the sewer pipe. Sewer rehabilitation techniques, such as sliplining, cured-in-place pipe (CIPP) and other close-fit liners utilize the existing pipe as a host pipe. Unlike repairs that are made on identified pipe and joint failures, rehabilitation techniques typically renew entire pipe segments, manhole-to-manhole.
Sewer replacement is the most extensive approach to infrastructure renewal.? Replacement involves the complete abandonment of the existing sewer pipe and the installation of a new pipeline. The new sewer line can be installed with conventional cut-and-cover excavation techniques or with trenchless methods such as pipe bursting.
Sewer Renewal Effectiveness
A crucial component in the allocation of limited resources to address degraded infrastructure is the determination of the effectiveness of renewal for the underground asset. Numerous studies and reports have well documented that the typical approach for repair and rehabilitation of mainline sewers has not achieved the anticipated decrease of I/I from the system. In many cases, the resources expended have not resulted in significant I/I reductions. While there may not be any single reason for sewer renewal projects falling short of the expectations, one notable possibility is the manner in which early efforts were performed.?
Many I/I reduction projects focus on identifying and addressing ?highly rated? defects. While I/I through these defects is reduced, they often yield unintended consequences such as groundwater migration and ?potential I/I.? Once the mainline sewer is repaired or rehabilitated, the adjacent groundwater level could rise. The groundwater, now prevented from entering the previously identified defect, could migrate to previously unidentified defects in other parts of the mainline sewer, manholes or service laterals. Sewer replacement utilizing low pressure sewer, applied in a comprehensive manner in a smaller area, could be more successful in eliminating I/I than a more limited repair and rehabilitation program in a larger area.
Many municipalities and sewer utilities are incorporating solutions that optimize both economic and social costs as well as provide notable environmental benefits. One of those solutions is low pressure sewer.
Low Pressure Sewers
A low pressure sewer (LPS) system consists of a network of small diameter pressure pipes and individual pumping units that convey wastewater directly to a treatment facility or central sewer.? Developed more than 40 years ago, LPS systems have a large body of well documented experiences that show dependable operation, long term reliability and lower life-cycle and social costs when compared to gravity sewers.
The primary component of a low pressure sewer system is a grinder pump station installed at each individual property in the system. The pump is generally smaller than a household washing machine and is installed underground in the yard or basement of each connected property. An access lid is provided for service. The grinder pump station collects the wastewater, grinds the solids into small particles and discharges it to the pressurized pipe network. Because the system is closed and pressurized, the grinder pump is capable of transporting sewage over two miles, depending on the topography and other system parameters.
LPS systems use 2- to 4-in. flexible mains that are installed just below the frost line and follow the contour of the land. This benefit eliminates (or greatly reduces) the need for large lift stations and provides a dramatic construction cost savings when compared to gravity sewer. Based on more than 40 years of project comparisons, typical capital costs for LPS systems repeatedly range from 40 to 80 percent lower than gravity systems.
Traditionally, LPS systems have been used to provide central sewer service to areas where gravity sewer could not be installed, or the cost to do so was prohibitive. Now, because of many of the advantages they offer, LPS systems are gaining acceptance as a viable and cost effective alternative for the renewal of failed gravity collection sewers and to address problems associated with wet weather events. ?
Low Pressure Sewers for Sustainable Infrastructure Renewal
Manholes are not required to change the direction or alignment of the mainline sewer because the wastewater in the LPS system is pumped under pressure through flexible mains. This eliminates the need to maintain and rehabilitate these ancillary structures. While large open cut excavation has been the standard practice for gravity sewer replacement, trenchless technologies, such as horizontal directional drilling, allow low pressure sewers to be installed, providing a complete sewer renewal with less disturbance to the community and minimal environmental impact. Using small-diameter pressurized pipe buried just below the frost line allows for low impact excavation or trenchless technologies to be used during construction. Low pressure sewers also yield a lower social cost including less business disruption, traffic disruption, damage to adjacent surfaces, structures and utilities, as well as noise and dust pollution.
There is a great need for sewer system upgrades. Limited resources mandate that all options need to be weighed before embarking on a sewer system renewal plan. Replacing a gravity sewer system with low pressure sewer can be a viable and cost-effective solution to I/I problems.
Keith McHale is infiltration and inflow project manager for Environment One Corp.
