Sanitary Sewage Overflows During High Precipitation Events

A popular retirement community in Southeast USA that offers numerous subdivisions and recreation areas with spectacular views of mountains, lakes, and protected woodlands. The 12,000-acre property is home to 6,500 residents and maintains eleven lakes for boating or fishing, 90 holes of golf, award-winning sports facilities, multiple swimming pools, recreation & fitness complexes and miles of paved walking paths & hiking trails.

Customer Challenge

Situated at 2000 feet above sea level, it seems unlikely that high influx and infiltration would be a problem for the retirement community. However, the original infrastructure was sized for a fraction of the current population and services, making it no longer adequate for the thriving community. This caused massive infiltration and inflow issues with their current sewage collection system. The submersible pumps they were using in the current lift stations could not keep up with the required inflow and infiltration, causing sanitary sewage overflows during high precipitation events.

These sewage spills, caused by problems in the collection system, sparked concern from the Department of Environmental Protection. After subsequent discussions, the Public Works Administration responsible for the community was given an ultimatum: invest in a permanent, longterm solution that would prevent future overflow events or face costly fines from the department.

Additionally, the long-term solution needed to be a cost-effective, sustainable solution for the community to avoid increasing homeowners association and utility fees for residents. Public Works was faced with two possible approaches for a long-term solution; upgrade the collections system or upgrade the pumping stations. The cost to investigate root causes of the system failure followed by upgrading the collections system was substantially higher than the cost to upgrade the pumping stations. However, even this approach would result in substantial investment cost and ultimately higher operating expenses.

To accommodate the intermittent increases in volume, equipment upgrades would need to be sized for peak flows. In addition to the cost of large submersible pumps, the existing 25 horsepower (HP) motors would need to be replaced with 75 HP motors. The variable frequency drives would also need to be replaced to support larger pumps and motors.

From a service and maintenance perspective, this approach brought its own issues. Sizing the pump stations equipment for infiltration events that would only account for 10% of the actual run time meant the equipment would be running well below optimum capacity and efficiency, resulting in higher operating costs. After adding up the equipment costs to upgrade larger submersible pumps, it was determined the price would be almost $30,000 more than it would cost to install sound attenuated diesel pump packages.

The Solution

After evaluating the pumping requirements of the systems, two models of sound attenuated diesel pump packages were selected to upgrade various pump stations. An SAPP66S12 diesel package was selected for the initial pump station. Following installation and preliminary testing, it was confirmed that the SAPP66S12 would provide more than adequate flow and pressure to meet the historical volumes during high infiltration periods. Three additional sound attenuated pump packages were then installed, including the second model selected for this job - an SAPP64Sl7l71 diesel package.

Controlled by LOFA CP750 panels, the pumps feature an auto start/stop function that is triggered by a level transducer. As the level of wet well rises, the pump speed increases to maintain safe levels during rain events. The control panel is wired to the main VFDs so that the main lift station does not run unnecessarily when the diesel package is running. When the level drops below a set point, the diesel package shuts off. The sound attenuated diesel pump packages would also serve as back-ups in the event of power outages and featured integrated fuel tanks sized to provide 24-hour run time. The SCADA systems would also alert the maintenance crew to let them know whenever the packages were running.

The Results

After installation, the four sound attenuated diesel pump packages provided protection during high rain events. As the level of wet well increased during high precipitation events, the pumps increased speed to maintain a safe level in wet well. The packages also offered an additional layer of protection in the case of submersible pump failure or electrical failure.

There were also additional cost and time savings realized by the community and Public Works Administration from installing the sound attenuated packages. At a savings of approximately $30K for each package, the cost for these packages was almost $100K less than the next closest alternative of upgrading the submersibles.

Additionally, keeping the number of pump models in the packages reduced the number of spare parts needed for repairs and amount of service training needed for preventative maintenance. The diesel pump packages also eliminated the need for standby generators, which reduced the costs for additional equipment and maintenance. After realizing the cost-effectiveness of this approach, the retirement community continued adding diesel packages each year to upgrade the remaining pumping stations that were critical to smooth operations. Most importantly, costly and hazardous sanitary sewage overflows are no longer a recurring problem for the community.