Wet well pump-down control is the logic that decides when a lift station's pumps start, when they stop, and which pump runs, so the wet well fills and drains within a safe band without the pumps short-cycling or the well overflowing. It can be as simple as float switches turning pumps on and off between two levels or as refined as a variable frequency drive holding the level steady, and it always has to share the work fairly across the pumps. This page explains fill-and-draw and continuous control, lead-lag-standby alternation, and self-cleaning drawdown in SCADA.
Wet Well Pump-Down Control in one line: Wet well pump-down control is the pump start, stop, and alternation logic that keeps a lift station's wet well level within a safe range. In fill-and-draw control, pumps start at a high setpoint and stop at a low one; in continuous control, a variable frequency drive modulates pump speed to hold a steady level. Lead-lag-standby logic rotates duty across pumps, and SCADA manages the setpoints, alternation, and self-cleaning drawdown.
A lift station collects incoming sewage or stormwater in a wet well and pumps it onward, and its control problem is simply to empty the well fast enough to keep up with the inflow without ever letting it overflow or running the pumps dry. The oldest and most common approach is fill-and-draw, on-off control. The well is allowed to fill to a high start setpoint, at which a pump turns on and pumps the level down to a low stop setpoint, at which it turns off, and the cycle repeats. The start and stop levels are set by floats, pressure or ultrasonic level sensors, or level switches.
The gap between the start and stop levels matters because it sets how often the pumps cycle. If the two setpoints are too close together, the pump starts and stops frequently, and this short-cycling is hard on motors and starters and can trip thermal protection, so the draw-down volume between start and stop is sized to give the pump a reasonable minimum run time. Fill-and-draw is robust and simple, needs no variable speed drive, and is entirely adequate for the many stations where a steady discharge flow is not required.
Continuous level control takes a different tack using a variable frequency drive. Instead of fully emptying and refilling the well, the drive modulates the pump speed to match the outflow to the inflow, holding the level at or near a single target. As inflow rises the drive speeds the pump up, and as inflow falls it slows down, so the level stays roughly constant and the pump runs continuously rather than cycling. This gives a smooth, steady discharge flow, which is valuable where the downstream process or a receiving plant prefers even flow, and it avoids the mechanical stress of frequent starts, at the cost of a drive and more control complexity.
Most lift stations have more than one pump, both to handle high inflows and to provide redundancy, and the control logic has to decide which pump does what. The lead pump is the one that starts first and does the routine pumping. A lag pump starts only when the lead cannot keep up and the level keeps rising past a second, higher start setpoint, adding capacity for peak flows or wet weather. A standby pump provides backup if a duty pump fails. In a two-pump station one pump is typically lead and the other lag and standby combined.
Alternation is what keeps the pumps wearing evenly. If the same pump were always the lead, it would accumulate all the run hours and wear out while the other sat idle and potentially seized. So the control alternates the lead assignment, swapping which pump starts first on each cycle or on a schedule, so run time and starts are shared roughly equally across the pumps. This both extends the life of the station and keeps the standby pump exercised, since a backup pump that never runs is not a reliable backup.
The alternation and staging logic also has to handle faults gracefully. If the lead pump fails to start or trips, the control must promote another pump to lead so pumping continues, and it must raise the appropriate alarms. High-high level, indicating the pumps are losing ground against inflow, calls all available pumps into service and warns of an impending overflow, while a low-low level protects the pumps from running dry. This layered logic, lead, lag, standby, alternation, and level-based staging, is the core of what a lift station controller does.
Wet wells accumulate grease, scum, and settled solids, and a station that always stops pumping at the same mid-level leaves a floating grease mat and a settled layer undisturbed, which builds up over time into odor, blockage, and level-sensor fouling problems. A self-cleaning, or pump-down, cycle addresses this by periodically drawing the well down much lower than normal, sometimes to the point of drawing the pumps in a controlled snore, so the higher velocity and the falling level scour the accumulated grease and solids from the walls and floor and carry them out. SCADA schedules this deeper drawdown at intervals so the well cleans itself rather than requiring manual cleaning.
A SCADA-controlled lift station holds all of this logic and makes it visible and adjustable. The start, stop, lag, and alarm setpoints are entered and tuned in software rather than by physically moving floats, the alternation is managed automatically, run times and start counts are logged per pump for maintenance, and the self-cleaning cycle is scheduled and recorded. The continuous level, pump status, flow, and motor currents are trended, so an operator can see a pump that is running longer than it used to, a sign of wear or a partially clogged impeller, before it fails outright.
Because lift stations are usually unstaffed and scattered across a collection system, remote monitoring is where a cloud SCADA platform such as Merobix earns its place. It carries wet well level, pump status, alternation, and alarms to any browser, so a high-high level, a failed pump, or a loss of power at a remote station reaches an operator immediately rather than being discovered after an overflow. Trending run times and starts across all stations lets a small utility spot the pump heading for failure and schedule the maintenance, and the logged setpoints and cycles support the reporting a collection system has to keep, turning a network of isolated stations into one manageable picture.
Fill-and-draw control turns pumps fully on at a high level and off at a low level, so the well cycles between two setpoints and the pumps run intermittently. Continuous control uses a variable frequency drive to modulate pump speed and hold the level near a single target, matching outflow to inflow so the pump runs steadily and delivers an even discharge flow. Fill-and-draw is simpler and needs no drive; continuous control gives smoother flow and fewer starts at the cost of more complexity.
The lead pump starts first and does the routine pumping, the lag pump starts only when the lead cannot keep up and the level keeps rising, adding capacity for peak flows, and the standby pump is backup if a duty pump fails. Control logic alternates which pump is lead so run hours and starts are shared evenly, extending pump life and keeping the backup exercised. In a two-pump station one pump is typically lead while the other serves as lag and standby.
Wet wells accumulate a floating grease and scum mat and a settled solids layer that a normal pumping cycle, always stopping at the same mid-level, never disturbs, leading to odor, blockages, and fouled level sensors. A self-cleaning cycle periodically draws the well down much lower than usual so the higher velocity and falling level scour the grease and solids off the walls and floor and carry them out. SCADA schedules and records these deeper drawdowns so the well cleans itself instead of needing manual cleaning.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.