New lift station a key element of compliance
The city of Hot Springs, Ark., is undergoing a comprehensive upgrade to its municipal wastewater infrastructure. The program focuses on 16 projects referenced in a rate study that could total more than $69 million.
Anchored by three lakes in the state’s scenic Ouachita Mountains, the city’s wastewater system serves approximately 23,000 customers in this popular vacation, health spa and retirement community. The collection system serves the area within the city limits and outlying unincorporated segments of Garland County and requires maintenance of more than 600 miles of gravity and force mains.
Bobby Harris, the city’s utility field operations manager, returned to the area of his childhood and joined the utility’s management after serving 20 years in the U.S. Marine Corps (USMC), with an emphasis in the field of aviation maintenance. His immediate priority was to develop and implement a proactive maintenance program, focusing on the system’s 76 major lift stations. Several of these stations were named in an agreement between the city and the Arkansas Department of Environmental Quality summarized in the aforementioned rate study.
“From now on, one crew will be assigned exclusively to preventive maintenance procedures,” Harris said. The crew will be applying the disciplined procedures Harris learned in the USMC to the job of utility maintenance. Harris is implementing a proactive approach to maintenance within the system, rather than a “run it till it breaks” philosophy. His goal is to find the root cause of problems and correct them early on.
The initial upgrades in the retrofit and replacement program are intended to correct capacity constraints along gravity mains, pressure mains and bottleneck lift stations that become overloaded with inflow and infiltration (I&I) surges. Other issues include the age of some equipment and the network of damaged manholes along the gravity lines. Some of these sections of line date back 50-plus years and receive flows that exceed their original design. At the Fairwood Lift Station, I&I, along with clogging of the existing pumps, have caused the station to overflow repeatedly, along with a manhole just upstream of the lift station.
“Designed for 4,050 gal per minute (gpm), the Fairwood Lift Station is one of the largest of 10 stations being totally revamped,” said Jerry Williams, P.E., principal for Engineers Inc., the local engineering consultant. “This station serves a growth area beyond the formal city limits in the northwest section of the utility’s service area. Originally configured as an underground wet-pit/dry-pit facility with two 125-hp pumps, the replacement is now redesigned as a wet pit submersible station that wraps around the original station’s wetwell footprint so that the facility could continue to operate during the construction period for the new station.”
The original facility had experienced multiple overflows in the past, particularly during heavy rainfall events or when the pumps would clog because of disposable products such as duster heads, cleaning cloths and disinfecting wipes. These increasingly popular “disposable household products” are not rated flushable. They have presented challenges for some sewer pumps because they can build up on the leading edge of the impeller and become entangled within the vanes. This reduces pump efficiency and leads to a complete stoppage, as the material binds up the impeller so that it cannot rotate.
The replacement Fairwood Lift Station, completed in December 2013, was built to coincide with the 20,000-ft replacement of an existing 15-in. force main with a 24-in. force main that will serve as the primary transmission main for several larger pump stations. Portions of the original 15-in. gravity line to the lift station were discovered to be half-full of gravel during construction of the larger, overdue replacement line. The existing force main also had breakage problems due to inferior pipe material.
The replacement lift station will be equipped with Xylem Flygt Model NP-3231/765 “N” pumps. The 335-hp submersible pumps will be operated via variable-speed drives and are designed to deliver 4,050 gpm each. The technology incorporates patented clog-resistant impellers, unlike the existing station pumps. The Hot Springs utility has experienced success with the “N” pump design, as it has them installed elsewhere in its system.
The “N” pump technology handles debris-laden wastewater flows more efficiently than traditional chopper-style pumps due to the combination of its self-cleaning, semi-open, back-swept impeller and the relief groove incorporated in the pump volute. The horizontally positioned impeller vanes deliver quality hydraulic performance and energy efficiency. The pump simultaneously maximizes energy usage by self-cleaning the stringy material carried in the wastewater flow away from the leading edge of the pump impeller.
The new installation will utilize the Xylem MultiTrode MultiSmart station controller, is capable of maintaining both real-time control and protection along with historical and communications options. The installed MAS 711 pump monitoring system—which records pump starts and stops, pump vibration, station power quality and energy usage, pump bearing and seal temperatures—has the ability to calculate and record the daily inflow and outflow of the station in gallons per day. All of this information will be available to the city to help it better operate the pumps in the station and to provide troubleshooting information for the preventive maintenance group.
The electrical controls for the station are housed in a standalone control building that incorporates the latest Arc-Safe electrical component design that complies with National Electric Code- and Occupational Heath & Safety Administration-mandated standards for Arc-Flash safety. This is a step forward in providing personal safety for Hot Springs technicians, as they do occasional maintenance on the control components.
The potential environmental impact of future separate sewer overflows at the Fairwood facility mandated a backup system that would continue operating during a power outage. Williams and Harris preferred an alternative to the typical diesel-powered electrical generator for emergency outages. They also wanted greater flexibility when the submersible pumps were taken offline for any reason.
A total solution for the station’s backup, whether electrical or mechanical in character, resided in the Xylem Godwin Dry-Prime backup pump unit. The unit going into the Fairwood station is powered by a 425-hp natural gas engine. The HL250M DBS pump will operate on an aboveground foundation adjacent to the wet well. The heavy-duty pump was suited for the backup mission because of its self-priming feature and capability to handle the high discharge pressure and suction lift conditions at the new Fairwood station, which underwent some dimensional redesign to accommodate the increased flow requirements.
“I have nothing against electrical generators, but the availability of a natural gas connection on site will eliminate the risks of aging diesel fuel gelling in the generator fuel tank or the potential for fuel spills that could reach the nearby creek, plus the opportunity to reduce the city’s costs when operating during an outage of the submersibles,” Harris said. “In addition to the foreseen benefits, using natural gas to power the Godwin pump should give us a lot more flexibility. It has been sized with enough capacity to assume the pump station’s flows whenever the submersibles are offline.”
The benefits of utilizing the natural gas-powered pumping unit in lieu of a diesel-powered electrical generator were introduced to Harris and Williams by Sherman Eoff, president of Jack Tyler Eng. of Arkansas Inc., the local representative for Xylem in the state. After Harris and Williams’ researched and compared other offerings, the decision was made to utilize a complete pump station backup instead of just an electrical backup for this project.
It was only one of many changes that should resolve the former problems at the Fairwood facility.