Imagine being blindfolded 100 feet away from the edge of a cliff and told you must walk as close to the edge of the cliff as you can without going over. This is the analogy Engineering Manager Michael Stuer employs to describe the past efforts of the Lowell Regional Wastewater Utility (LRWWU) to control combined sewer overflows by storing the wet-weather flow within the interceptor pipes.

“That’s what we did,” Stuer says. “We didn’t know where the cliff’s edge was, so we didn’t push it.”

That was about 10 years ago. Things are a little different today. LRWWU installed a SCADA system to provide its treatment facility staff with the “eyes and ears” to remotely operate gates at combined sewer overflow (CSO) diversion stations in order to maximize the capture of wet-weather flow through in-line storage.

A key component of that system was the gate control actuators. LRWWU needed a safe, reliable and instantaneous method to operate its many flow control and CSO diversion gates at the satellite facilities. They had to be able to respond swiftly to changing flow conditions. After installing several different types of actuators, LRWWU adopted the use of REXA actuators to meet the challenging conditions.

The LRWWU system

LRWWU owns and operates a wastewater treatment facility and more than 200 miles of sewer lines serving five Massachusetts communities along the Merrimack and Concord rivers. The plant treats an average flow of 25 mgd but has a peak wet-weather flow capacity of 110 mgd.

During wet-weather conditions, flows in excess of the secondary treatment capacity (62 mgd) bypass the secondary treatment system, recombine with secondary effluent, and are then disinfected before being discharged to the Merrimack River.

LRWWU also operates a combined sewer system – wastewater and stormwater are conveyed to the treatment plant using the same interceptor pipeline network. During rain storms, depending on the size, duration and timing of the event, stormwater can rapidly inundate the combined sewer system, driving flows in the interceptors above treatment capacity. The interceptor pipes, ranging from 36 to 120 inches in diameter, surcharge and trigger CSO discharges in the local waterways.

The LRWWU combined sewer system employs nine CSO diversion stations to help relieve the interceptor system of excess flow. These structures contain sluice gates used to control flow within the interceptor. To protect against high-flow damage, the diversion structures are also equipped with CSO diversion sluice gates that are used to release the excess flow into the river when levels rise above the capacity.

There are a total of about 20 gates that are used to manage flows in the collection system during storm events. Until recently, control of these gates was a big problem.

Operating challenges

The flow control and diversion sluice gates were originally controlled by traditional hydraulic oil actuators. The power of hydraulic oil makes this type of actuator a good fit for applications with large force requirements, but hydraulic actuators require routine maintenance, including oil/fluid and filter changes. Over time, they tend to break down or leak, especially if maintenance isn’t completed on a regular schedule.

This was the situation in Lowell: The actuators broke down to a point where they could no longer be reliably automated. In addition, oil lines began to leak more frequently and became problematic, especially for the lines outside the station. This placed extra strain on the LRWWU staff and reduced the reliability of the gates during rain storms.

The challenges of maintaining hydraulic oil actuators were compounded by an unreliable control system, leaving LRWWU in a precarious position when it rained. Based on the reliability of the gate actuators and the control system, treatment facility staff felt it was necessary to physically be at each station to observe, firsthand, that the gates opened and closed when actuated.

“We used to drive out to our [CSO] stations ahead of the storm to manually open and close gates based on the storm forecast,” Stuer recalls. “After a high-flow event, we would drive back out to manually move the gates back to where they were before [the storm].”

Operation of the CSO station gates was guesswork. Workers would manually open and close gates where they thought flows were building to dangerous levels. Because of drive time associated with each manual operation, gates were often just left open for the entire storm because it was too time consuming to open and shut gates to match precipitation conditions, which could start and stop during any storm. In addition, it was not uncommon for the Merrimack River to rise above the elevation of the diversion structure outfalls, which resulted in river water entering the combined sewer network when diversion gates were left open for too long.

Remote operation

In 1997, LRWWU indeed suffered from this manual operation when one of the diversion stations had a catastrophic gate failure during a wet-weather event. An 84-inch-diameter interceptor pipe upstream of the station was fractured and a geyser of sewage spewed from the interceptor, causing severe damage in the yard of a private residence. The critical interceptor pipe required substantial repairs, as did the yard.

This event helped emphasize the significant drawbacks of manual operation. As a result, the LRWWU adopted a renewed focus to update its system with 21st century technology. The challenge was to incorporate technology that would allow LRWWU to operate its system remotely from the treatment facility. It was essential to be able to handle wet-weather flows with a high level of confidence, without operators traveling to each station.

LRWWU initiated a plan to deploy programmable logic controllers (PLCs) at each satellite CSO diversion station to provide the instrumentation necessary for remote control. The network of PLCs was connected to the plant through radio communications. The next step was deciding on the right gate actuator to pair with the PLCs and radios to complete the control system.

The belief was that upgrading the flow control gates in the existing infrastructure with electric actuators would help the LRWWU prevent another event like the one in 1997. However, at that point, “We were not even trying to minimize CSO [discharges]. We were focused on protecting against surcharges [like the one in 1997],” Stuer recalls.

Initially, the new system worked. LRWWU was able to operate its flow control and diversion gates remotely from the treatment facility via SCADA. Staff started gaining confidence that they could control the system reliably during storm events and avoid any public or private property damage. But times changed. The wet-weather operating policy that originally focused on minimizing upstream system surcharging had to be refocused to meet changing state and federal requirements to minimize CSO discharges.

Evolving to safe storage

LRWWU modified its gate operations to enable in-line storage through the use of large-diameter (72- to 120-inch) interceptor pipes, storing wet-weather flows until after a storm subsided. The stored flow would be conveyed to the plant after the storm, minimizing CSO discharges.

LRWWU attempted to use electrically actuated flow control gates to store the flow in the pipe upstream of each gate, but soon realized that the electric gate actuators had limitations. To avoid motor burnout, LRWWU initially established 25 percent increments for gate adjustments. However, the gate opening and closing speeds didn’t match the highly variable flow conditions during storm events. Smaller increments were used but overall this approach resulted in constant “hunting” by the gate control system.

“We realized that unless we could modulate those gates, we weren’t able to fully utilize storage. And electric gates couldn’t do it. They couldn’t modulate continuously,” Stuer says.

Around this time, LRWWU became aware of an existing technology that was being introduced into the wastewater market – a REXA actuator. It is essentially an electric actuator that employs the power of hydraulics as a gearbox. Unlike the hydraulic oil actuators, the REXA actuators require no oil or fluid changes, contain no filters and require zero periodic maintenance. However, similar to the hydraulic oil systems, the REXA actuators have continuous modulation capabilities.

“The [old] hydraulic actuators were expensive to put in and expensive to maintain, and we couldn’t rely on them for remote operation,” Stuer notes. “These new REXA actuators were modestly priced and gave us continuous modulating duty, so they were the best of both worlds.”

The first REXA actuators were installed in Lowell in early 2006. Since then, only one actuator has experienced any downtime resulting in a service disruption. The actuator, 7 years old at the time, was an older design that did not employ a pressure limiting option standard in current REXA designs.

On a collections system sluice gate, it is common for objects to obstruct the movement of collections system gates. The newer REXA actuators combat this problem with pressure limitation of their hydraulics, whereby each actuator will sense an object impeding gate movement. Under this scenario, the actuator will enact a self-imposed stall condition and send an alarm relay to the SCADA system to notify the end-user of the gate obstruction, so the object can be removed.

The older REXA design operated for seven years without this pressure limiting protection until 2013, when service conditions finally caught up with it. The actuator was down for less than a week, during which time REXA completely upgraded the old design with the presently standard pressure limiting capabilities.

The new REXA gate actuators provided highly flexible, continuous modulation of the control gates, allowing the LRWWU to safely raise operating setpoints at each CSO diversion station to minimize CSO discharges. Meanwhile, LRWWU had real-time control of the system to avoid any excessive system surcharging that, in the past, caused public and private property damage.

“Gate modulating was the culminating strategy for safe storage,” Stuer says.

Safe storage is a success

LRWWU’s adoption of the REXA gate actuators as its standard operator for the modulation of its flow control and CSO diversion sluice gates at the nine CSO stations has been a huge success. “I don’t want to put it all on gate modulation, but this feature is a big part of our reduced CSO discharges and safe operation of the interceptor system,” recalls Stuer.

The ability to both change setpoints and reduce increments has allowed the LRWWU to safely optimize the in-line storage of its wet-weather flow while minimizing its untreated CSO discharges. Since the REXA gate actuators have been installed in 2006, LRWWU has seen its annual CSO discharge volumes decrease by more than 80 percent, from an average of almost 700 million gallons to last year’s total of 125 million gallons.