CASE Studies Grinder Pumps for Sewer Renewal a new best practice for solving infiltration and inflow issues

By Keith mchale, environment one Corporation (e/one)

ow pressure sewer systems Because the system is closed and using semi-positive pressurized, the grinder pump is L displacement grinder pumps capable of transporting have been employed extensively over two miles, depending to provide central sewer service on the topography and other to areas where system parameters. The pump could not be installed and unit is generally smaller than a to provide a solution for the household washing machine. renewal of failed gravity systems. Because it does not rely on A low pressure sewer system gravity flow, the system provides consists of a network of small more options and flexibility for diameter pressure pipes and siting. individual pumping units that Several manufacturers offer convey wastewater directly to grinder pumps for low pressure a treatment facility or central sewer systems with a semi- sewer. Developed more than positive specifically designed forty years ago, low pressure for household wastewater sewer systems have a large body applications in low pressure of well documented experiences sewer systems. that show dependable operation, In addition to allowing for long term reliability, and lower the passage of the ground solids life-cycle and social costs when without clogging, the geometry compared to gravity sewers. of a semi-positive displacement The primary component of a pump produces a nearly vertical low pressure sewer system is a pump curve. The flow from the grinder pump station installed pump is generally constant, at each individual property in and is somewhat independent the system. The grinder pump of the pressure in the system. station collects the wastewater, In practical terms, the pump grinds the solids into small characteristics means that, particles, and discharges it to unlike a system using centrifugal the pressurized network. pumps, when a semi-positive The model DH071 grinder pump station from E/One Sewer Systems, a popular model for single-family homes. About the Author Keith McHale, P.E., is a professional engineer with more than twenty years of experience in wastewater collection and treatment, including gravity and pressure sewer design, and infiltration and inflow (I/I) evaluations. Keith is the I/I project manager for Environment One Corporation. He holds a BS in civil engineering from the University of Massachusetts at Amherst and is a member of WEF and NEWEA. For more information about E/One products, call 518.346.6161, email [email protected], or visit www.eone.com/sewer.

16 | JANUARY 2014 www.modernpumpingtoday.com displacement pump is activated, it LOW PRESSURE FOR SEWER renewal measures provides many will deliver essentially the same flow RENEWAL economic, social, environmental, and rate, no matter where the operating Most central sewer systems installed public health benefits. Renewal of point is within the allowable range of in the past use gravity as the moving the collection system infrastructure operating pressure. force to transport wastewater to includes options to return the The choice of the semi-positive treatment facilities. Most systems, system to an acceptable level of displacement principle was made with however, cannot convey the performance. Repair, rehabilitation, input from the ASCE Staff and Steering wastewater solely by gravity. Gravity and replacement are a progression of Committee more than forty years ago sewers may not always be the best the activities employed to address and based on these irrefutable hydraulic choice of a new or rehabilitated renew failing infrastructure. Sewer fundamentals. The development of central collection system because of replacement is an extensive approach the wastewater pump began with terrain, soil type, groundwater, and to infrastructure renewal. Replacement no vested interest in any particular other factors. Excessively hilly or flat involves the complete abandonment technology, so the developers were terrain requires deep excavations of the existing sewer pipe and free to start from the “ground floor” and large pump stations. Low the installation of a new pipeline. and devise a unique class of pump, pressure sewers using semi-positive Unlike replacement with a gravity tailored in every respect to the specific displacement grinder pumps can be collection system (replace in kind) the needs of pressure sewer systems. a cost effective solution to sewer replacement of a failed gravity sewer In evaluating the equipment for renewal. with a low pressure sewer system use in pressure sewer systems, the Over time, gravity sewers fail. Root using semi-positive displacement designing engineer should begin with intrusion, pipe cracks, joint failures, grinder pumps can be viable and cost two main choices: first, select the and other deficiencies allow excessive effective option. pump characteristic (H–Q curve), groundwater and to enter Using small diameter pressurized either centrifugal or semi-positive the system. Referred to as infiltration pipe buried just below the frontline, displacement; second, select the and inflow (I/I), this excessive flow a low pressure sewer system can equipment manufacturer based on has detrimental environmental be installed with low impact additional pump characteristics, and economic effects on the excavation techniques or with including: overall operation of the wastewater collection and treatment system. • Performance features The excess flows, or essentially • Reliability and track record in the clean groundwater and rain water, “field” can quickly overload the collection • Ease of installation system capacity, resulting in sanitary • Serviceability sewer overflows (SSOs), backups of • Preventive maintenance raw sewage into basements, and high requirements operating costs and equipment wear • Real estimates of operating and due the transport of the excessive maintenance costs flows. • Initial cost Limiting the amount of I/I in • Life-cycle cost sewer systems through infrastructure

Grinder pumps are typically installed outside of each home on the pressure sewer system. Only the access lid and alarm panel are visible to the homeowner.

www.modernpumpingtoday.com JANUARY 2014 | 17 CASE Studies trenchless directional drilling methods. through flexible sewer pipe, Additionally, a low pressure system are not required to change the eliminates essentially all excessive direction or alignment of the mainline flows over the entire service life of the sewers. This eliminates the need new system. Repair and rehabilitation to maintain and rehabilitate these methods used in sewer renewal can ancillary structures. achieve limited I/I reduction. Because Low pressure sewer systems used the wastewater in low pressure sewer to replace failed gravity sewers can systems is pumped under pressure provide a complete sewer renewal with

Gravity sewer main in need of repair.

fewer disturbances to the community and have minimal environmental impact. Based on more than forty years of project comparisons, typical capital costs for low pressure sewer systems repeatedly range from 40 to 80 percent lower than gravity systems. This low capital cost advantage is enhanced by the lower social costs including less business disruption, traffic disruption, damage to adjacent surfaces, structures, and utilities, and noise and dust pollution, low pressure sewers provide.

CASE STUDY: ELWOOD, KANSAS Elwood, Kansas, sits in the northeast corner of the state, just across the Missouri River from the city of St. Joseph, Missouri. It’s a residential and light commercial town built on alluvial sand soils deposited over thousands of years. The gravity sewer system was barely functional because of the groundwater infiltration in the deeply dug system, 15 to 20 feet (4.6 to 6 meters) in some places. Nine lift stations kept groundwater out of the system. Manholes and sections of main were constantly sinking. The town suffered a devastating flood in 1993. Following an unusually large snowpack over the winter, wet weather continued throughout the spring, saturating soil and elevating rivers and streams well into the summer. Storms dumped heavy rainfall across Kansas—amounts achieving 75- to 300-year frequencies produced record flooding on the upper Mississippi and lower Missouri rivers. Levees protecting Elwood were breached, and the entire town was washed out. The soft, sandy soil became a porous, shifting hydrography that destroyed many structural foundations and the gravity sewer system. The Federal Emergency Management Agency (FEMA) helped Elwood dig

18 | JANUARY 2014 www.modernpumpingtoday.com out and patch up its severely around the clock, a breakdown of compromised sewer. With a single station had a catastrophic funding from FEMA, the entire effect, as the entire system then system was videotaped and backed up. shown to be nearly beyond Due to this constant operation, repair. At this point, plans Elwood’s lift stations wore out were laid to rebuild with a quickly. Between the electricity more sensible solution for this to run the lift stations and the vulnerable area. repairs, the old gravity system Instead of installing another was literally sending money gravity sewer system, the city down the drain. With the pressure decided to use LPS. Other towns sewer system, the grinder nearby had used the system with pumps run for shorter duration great success. The flat topography and the closed system doesn’t and soil type further made the allow groundwater infiltration, case against a gravity system. The eliminating collapses of new sewer system was installed roadways, mains, or manholes. in two phases. The first phase, installed in 2004, consisted of CONCLUSION 245 grinder pump stations. The As existing infrastructure in second phase, completed in the United States ages, many 2008, added another 139 grinder communities are replacing their pump stations. gravity sewer systems with low The gravity system had required pressure systems. A grinder Grinder pump station installed outside of a home. extreme depths to maintain a pump- driven LPS is a viable and downgrade. This, in turn, created cost-effective alternative to I/I groundwater inflow problems that accompanied the system and was a challenges. The right pump and a well- led to mains collapsing, manholes constant strain on the Public Works designed system mean elimination of I/I sinking and sections of roadways Department budget. With nine lift resulting in lower operating costs, and sinking. A constant repair requirement stations pumping groundwater inflow a satisfied and secure customer base. ■

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