Flexible, economical PLCs provide automatic failover, backup pump control, and remote lift station monitoring.

The Moccasin Bend Wastewater Treatment Plant (WWTP) is part of the Interceptor Sewer System, which has served Chattanooga, Tennessee, and its surrounding metropolitan areas since it was built in 1952. The WWTP includes pumping stations as well as liquid and solids handling.

Our treatment plant receives an average flow of around 50 million gallons per day (gpd), and has a peak treatment capacity of 220 million gpd. We also operate the associated sewage collection system and remote sewage lift stations.

For the most part, gravity conveys area wastewater from its origins to our treatment facility, but deep-pipe areas require lift stations or pumping stations to lift the wastewater to shallower pipes.

We remotely monitor every sewer pumping station and lift station to ensure normal operation. If something fails, we receive an alarm at our central control room at the treatment plant. Other remotely monitored information includes pump status (running or stopped), number of pump starts and stops, and daily pump running hours.

We recently installed new controls at one pump station site. The new control system monitors and controls wet-well levels, and provides status and alarms over a Modbus connection. The new system uses two DirectLOGIC DL06 PLCs and an OptiMate HMI sold by AutomationDirect.

We designed an automatic failover system for this pump station site. One of the PLCs serves as the master; the other serves as the client, or as a redundant controller in case of a failure of the master unit.

We also recently redesigned the Remote Telemetry Units (RTUs) that operate and monitor our remote sewage lift stations. For the new RTUs, we also use DL06 PLCs from AutomationDirect.

Upgrading the Pump Station Control Unit

The pump station site that we upgraded with the automatic failover system consists of two constant-speed pumps started across-the-line. The pump station control unit monitors and controls the level of water in the storage or wet well. The controller starts and stops the pumps according to a user-defined level. It also provides high and low level alarms, indicates whether the pump controls are in “Hand” mode or “Auto” mode, and indicates if an overload relay has tripped.

The pump station control unit is programmed to alternate which pump starts first to distribute pump motor run time evenly. It’s also programmed to start the second pump if the first pump can’t provide enough effluent flow to keep the wet well within an acceptable range.

The operator enters level control parameters via the HMI connected to the master PLC. A Modbus connection sends these parameters to the client (or slave) PLC. If the master unit fails, pump control is transferred to the client PLC through a series of relays.

Pump station operation continues without interruption, but the operator now needs to move the HMI cable from the master to the client to provide operator interface to the client PLC. Attaching the HMI to the client PLC provides a means for the operator to change operating parameters until the issue with the master unit is resolved.

We designed the automatic failover to operate in case of PLC failure, which would normally result in an after-hours call to an electrical technician. The failover function allows the station to function normally until repairs can be made during regular work hours.

An output on the master PLC is held energized by an “always-on” bit in the PLC. If the master PLC stops operating, the output de-energizes a series of relays. The appropriate relay contacts direct the outputs of each PLC to the appropriate control relay to provide failover operation.

The two main control PLCs share operational parameters, basically level settings. The master PLC also retrieves power quality data from a power quality analysis meter through a Modbus connection. The PLC then sends all appropriate data to a data logging unit, also connected via Modbus. Establishing the Modbus communications between the non-PLC devices was the most challenging part of this project.

In this particular pump station, a series of dry contacts provide alarms and pump status indications to an RTU. One pair of contacts is used for each function. The RTU, in turn, sends this status and alarm information to the treatment plant central control room via a licensed radio system.

Upgrading the RTUs

We’ve considered replacing our RTU systems for several years. We chose the AutomationDirect PLCs as the heart of our new RTUs because we could obtain what we needed quickly and for a lower cost than from other suppliers. Because we could have hardware in our hands in a matter of days after the order, we were able to develop these systems quickly. Also, the PLCs we chose provided us with additional pump control options.

The new RTUs operate and monitor our remote sewage lift stations through a cellular connection. At the central control room, an OPC server connection provides information to a PC-based HMI. The connection between the control room and each RTU is via a cellular network.

When we installed the first of the new RTUs, we found it extremely convenient that those of us authorized to work with the PLC logic could access the unit remotely. The operator in charge of monitoring the stations has called me many times in the evening or on weekends asking me to take a look at the site.

I can connect to the PLC from my home, and see how the site is performing. I can assess the conditions and advise whether the situation is “normal” or needs attention from maintenance or operations. I can even determine whether an issue had arisen here at the plant with the PC, based on what I could see from the PLC.

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There is a communication issue with the RTUs that we’re working to resolve. The software driver residing on the PC can handle some delay between the data-request message and the response to that message. However, the communication delays we’re experiencing over the cellular-based Internet connection have forced us to look for alternate connection methods.

Our local power distributor is implementing a fiber-optic-based Internet service, which we feel holds the key to resolving this issue. We are currently evaluating sites to find good candidates for a pilot project using this higher speed connection.

We’ve installed 16 RTUs so far, and many of these units have been retrofitted into enclosures that used to contain older radio-based RTU equipment.

System Upgrades Worth the Effort

The new pump station control unit has proved to be more reliable than the original control system. We don’t have specific records of how many times the operations staff was dispatched to the site in the past, or how many times electrical maintenance was needed after hours. However, I have noticed that the frequency of issues at the site has decreased dramatically.

We attribute the drop in after-hours calls to the increased reliability of the new control unit and redundant PLC. Although we’ve thoroughly tested it, we haven’t yet had a situation that required the use of the automatic failover function. If it saves us only once, it was worth the effort.

Purchase price was one of the main considerations for the RTU system PLCs. The hardware is modular, so we don’t have to buy more than we need. We’ve installed 16 systems so far, and we calculate that our choice in PLCs is saving us around $200 per site. When you consider that we have more than 80 remote sites, the savings can add up quickly.

By Jeremy Poarch, P.E.
Moccasin Bend Wastewater Treatment Plant

Disclaimer: AutomationDirect does not guarantee the products described in this article are suitable for your particular application, nor do we assume any responsibility for your product/system design, installation, or operation.

Originally Published: Dec 1, 2011