How did we get here?

Lift stations are designed with “one job”: move sewage from the wet well further along in the collection system with 100% consistency. In essence, the lift station has a dead simple task with enormous reliability requirements. What emerges is the common lift station as we know it; a hole in the ground, one more pump than is needed according to flow requirements, a control system using a level sensor or float switches, and a control panel with PLC. Almost all lift stations these days look something like this and will have effectively the same control narrative. Lift station design has converged towards a point of best practices; wet wells will stay cleaner, pump will last longer, issues will be noticed quicker, and the lift station will more reliably do its “one job”.

As anyone who has interfaced with day-to-day operation of a lift station knows, the good intentions of design are rarely met with adequate tools to ensure that the station is operated according to design. The system is reduced to a single design point upon which pumps are selected, and once in place these pumps are rarely affirmed to be performing as intended. Extra, redundant relays and float switches are piled into the control panel to prevent the worst from happening, but truthfully no one knows what happens when the 7th float switch is triggered. PLC code is copied and pasted from the last project to ideally preserve reliability, but that means there might be settings or parameters that aren’t suited to the site. This is still the state of the art: design for the worst and hope for the best. But when it’s 2 AM on a Sunday, and an operator is desperately trying to get a lift station back online after both its primary and secondary means of control have failed, don’t you think we could do better than hope?

Throughout this piece, we are going to observe the ways that the Specific Energy Lift Station Guardian has given operators the tools to maintain their lift stations and feel confident that it will operate as designed.

Float Testing

In service of ensuring a lift station will do its “one job”, most regulators require that there are backup systems to prevent disruptions in operation (or at least to alert operation staff when there is a threat of an SSO). However, it is rarely the case that these systems of redundancy are verified to work. Moreover, if the backup system fails unexpectedly during normal operation, the underlying issue could lie dormant for years before a critical event that needs the backup to work exposes the hole in the control system (usually in the middle of the night). Ideally, operators would routinely see that their systems of redundancy are proven to be functional, and when proven otherwise, it is known at a convenient time when maintenance can be scheduled.

The Lift Station Guardian (LSG) is designed with a 4-tiered control system:

  1. The first tier is the local HOA. This switch, in essence, “wins” against everything else. When in Hand, the pump will run. When in Off, it won’t.
  2. When in Auto, the control is passed up to the second tier of control: a pair of backup floats. When the level hits the high float, pumps will come on and drain to low float. Think of it as a pseudo-HOA. When the high float has been triggered and the pumps are draining the wet well, the floats are in “Hand”. When the low float has turned the pumps off, the floats are in “Off”. Then, when the low float returns to the up position, the floats can be thought of as in “Auto”.
  3. The third tier of control is LSG remote HOA. This behaves just like the local HOA, except that both the local HOA and floats must give it control by both being in Auto, and the HOA is accessible via Specific Energy’s app.
  4. Finally, when the local HOA, the floats, and the remote HOA are all in Auto, Specific Energy’s intelligent level-based control system designed just for Lift Stations takes over. This is where LSG sites operate 99% of the time.

So how can we validate that these systems are working as intended to ensure the lift station performs its “one job”? Well, the level-based control is constantly being executed and proven to work as part of normal operation. The remote HOA uses the same control hooks as the level-based system, so we can confidently say that it is also continually being tested. Jumping to the physical HOA, there is no validation method that beats routine manual testing, which is common practice among lift station operators. The sticking point is the float switches. If the level-based system were to fail, this is the only automatic backup system that will keep the station operating without operator intervention, so it is critical to know that it is functional. However, there is no natural way that a lift station will test this functionality beyond hitting a critical event where the floats are needed.

The LSG includes a Float Test that does this exact validation. On a routine schedule during business hours, the LSG will allow the wet well to bypass its normal level-based control and attempt to find the high float. When the high float is located, the LSG will note its location and confirm that the pumps come on as intended. The wet well will then drain down to the low float, record its location, and confirm that the pumps disengage. When complete, the LSG will return to normal operation with the confidence that the float backup has been validated.

There are rigorous requirements built into the Float Test that the station must achieve to “pass” the test each time. For example, there are timeouts for how long the LSG can spend “searching” for each float. There is a maximum search level which indicates the point at which the LSG will stop searching for the high float if it is not found yet. This is also built in conjunction with Specific Energy text-message based alarm system, so if any issues are discovered during the Float Test, operators will be notified immediately, normal operation will resume, and maintenance can be scheduled.

For operators who use the Float Test tool on LSG, their relationship to operating lift stations has transformed. The idea that a lift station will automatically audit its own equipment is unheard of in the industry. This gives operators the feeling that their equipment is working with them as opposed to fighting them.