Ideally, the flow of wastewater from collection points at residences, offices and commercial establishments to wastewater treatment plants would be completely gravity-powered. This is rarely possible because of economic or physical limitations. The most common challenge is flat terrain, which cannot easily be graded to promote wastewater flows.
As a result, lift stations must be employed to move wastewater to a higher elevation and eventually to a wastewater treatment plant (see Image 1). A typical wastewater collection system in a county or large city will have hundreds of lift stations, with smaller lift stations feeding into larger master lift stations. These, in turn, feed wastewater to the treatment plant.
Pumps are installed at each lift station to draw wastewater from the lift station’s wet well, the wastewater receiving point, and transfer it to a higher elevation. Typically, each lift station is equipped with two or more pumps, with operation alternating between the pumps to increase reliability. In high-flow situations such as heavy rain, multiple pumps can be operated simultaneously, which increases flow from the wet well. Pumps can be mounted on the floor of a wet well or in a dry pit adjacent to the well.
Monitoring and control of these lift stations—especially considering the process variables in these harsh environments—presents many challenges.
Lift Stations Controls
As shown in Figure 1, pumps are typically installed below the lowest point of the incoming wastewater feed pipe. Smaller lift stations might have a single 4-inch feed pipe, while larger stations can have multiple feed pipes, each up to 36 inches in diameter.
When the wastewater level reaches a critical point, a control system starts one or more pumps to prevent wastewater from overflowing the well, which could contaminate groundwater or local waterways. These
spills could negatively impact residents and require costly cleanup.
Wastewater often contains grease and solids, and foam is frequently produced because of turbulence in the lift station. These solids and foam can impair standard measuring techniques and instruments, causing high failure rates and requiring frequent maintenance. Conditions inside lift stations are not instrument-friendly because of high moisture levels, condensation and sewer gas.
An instrument in the pump station—either a transmitter or a switch—measures each of the parameters listed in Table 1. A transmitter continuously measures a parameter across a wide range—for example, 0 to 15 feet of wastewater level in a wet well. The transmitter then sends a signal proportional to the measured value to the control system. A switch simply measures an on-off condition such as whether a certain level has been reached by wastewater in a lift station. The switch then sends an on-off signal to the control system.
The control system in a lift station is typically a programmable logic controller (PLC), which uses inputs from the instruments along with its internal programming to control operation. The PLC often is integrated with a central supervisory control and data acquisition (SCADA) system at the wastewater plant to provide enterprise-wide connectivity and visibility.
For smaller and/or remote lift stations not monitored by a PLC/SCADA system, paperless data recorders often are installed. These recorders accept lift station signals from instruments, provide alternating pump control, and record and store relevant data.
Wet well level is the most critical measurement parameter in a lift station because level information is used to start and stop the pumps. Older lift stations often use a simple float switch to start and stop a single pump. But this technology is often insufficient because grease buildup and accumulation of debris can cause the switches to stick.
Even when working correctly, a float switch only provides a single-point measurement, as opposed to a transmitter that gives a more informative continuous indication of wet well level. Bubbler level transmitters are an older technology used for this continuous level measurement, but they require a constant air supply and use tubes that can become easily clogged.
Instead, modern lift stations use float switches for emergency backup and employ newer continuous level measurement technologies—namely hydrostatic and ultrasonic instruments.
Hydrostatic level transmitters can be safely submersed in wet wells and effectively avoid float switch and bubbler issues. Hydrostatic level transmitters measure water pressure within the wet well and infer level from changes in the head pressure.
Because it can resist abrasion, scratching and denting, a ceramic pressure-measurement cell is typically used with a hydrostatic level transmitter. Ceramic cells are sensitive enough to perform properly in the harsh environment of a wet well and provide accurate measurements even with the buildup that can occur on the cell.