Level Headed

Pressure transducer level sensors help a Massachusetts wastewater utility safely optimize storage capacity in its interceptor system
Level Headed

Interested in Flow Control?

Get Flow Control articles, news and videos right in your inbox! Sign up now.

Flow Control + Get Alerts

The Lowell (Mass.) Regional Wastewater Utility sewer separation program and remote gate control system upgrades had significantly reduced combined sewer overflows (CSOs) to local waterways.

To further reduce CSOs, operators at the 32 mgd Duck Island Wastewater Treatment Facility wanted to make full use of the 48- to 120-inch reinforced concrete interceptor system for storage of wet-weather flows.

Because the pipes are fairly flat, ultrasonic level sensors in diversion stations throughout the interceptor system enabled operators to extrapolate levels for upstream pipe segments. But without a true understanding of levels in all interceptor segments, the utility lacked confidence when establishing set points for the automatic gate control in remote diversion stations. “In the past, we erred on the conservative side to protect the public and their property,” says Mike Stuer, utility engineering manager.

To address the lack of understanding, the utility undertook an interceptor level monitoring program with engineering consultants Woodard & Curran and CDM. The project team developed a plan to install level sensors in critical locations that did not have level monitoring. The key was to “see” levels in all reaches of the interceptor system during heavy rainfalls. The team selected FlowAlert level sensors from ADS Environmental Services.

“We liked these sensors because they focused on level measurement,” says Stuer. “The critical component was the IntelliServe real-time software that connects to an ADS flow monitoring website, enabling operators to see what was happening as it happened.”

The utility purchased 10 level sensors, and ADS installed and maintained them for one year. During the monitoring period, the sensors verified that a 1-million-gallon storage pipe was underutilized. More important, the level monitoring program provided a better understanding of the interceptor system, allowing operators to optimize its storage capacity without fear of damaging property.


Bad day in Lowell

Built in 1980, more than eight miles of interceptor pipes run along both banks of the Merrimack and Concord rivers, conveying flow to the Duck Island facility. During heavy rains, diversion stations throughout the interceptor system discharge into the river, relieving the system of excess flows. Stormwater and infiltration create combined flows of more than 100 mgd at the treatment plant.

The motivation for interceptor level monitoring was a catastrophic blowout in an old brick interceptor in 1998. “We blew the crown right off the pipe and flooded a neighborhood because we had to drive to the stations and manually open the diversion gates,” says Stuer. “Over the past 10 years, we’ve been installing ultrasonic level sensors tied to the SCADA system at Duck Island, and now we control the gates from the plant.”

The blowout greatly increased the utility’s sensitivity about interceptor storage. That required the project team to make a strong case for safe storage. “One of the most interesting things about interceptor level monitoring was assuring management that what we were doing was safe — that we weren’t going to have another catastrophe,” says Stuer. After consideration, Mark Young, utility executive director, agreed to support the project.


Picking spots

Aaron Fox, utility engineer supervisor, identified the optimal locations for the level sensors and managed the project’s technical aspects. Brandon Kelly, utility staff engineer, coordinated with Mike Armes, ADS project manager, to install the units, which required no external power, cabling, or other infrastructure.

The sensors use a pressure transducer attached to a pipe invert to sense the head of the water column above it. Two float switches provide multilevel, date- and time-stamped depth notification. An optional depth sensor records depth readings for depth trending and flow calculation.

The wireless communications platform instantly sends low, high, and high-high flow condition reports via emails and text messages without special software. The advanced circuitry minimizes power usage and extends battery life.

“Although the suggestion was made to measure the flow rate in the interceptor, we decided to avoid measuring velocity for fear of complicating things,” says Stuer. “All we wanted to know was the level.”

Kelly worked with ADS to calibrate the sensors every six weeks throughout the program. “We monitored any segment where we weren’t sure about the levels or wherever we had information gaps between the diversion stations,” says Stuer.


Tangible benefits

Stuer and his engineers developed charts that tracked 17 level trends in the diversion stations and FlowAlert locations, then correlated the levels with gate positions, flows at the plant, precipitation, and activity in the interceptor system.

“We went to the plant managers and graphically demonstrated that we had unused storage capacity,” says Stuer. “Consequently, we increased the set points on every diversion gate to utilize that capacity.

“For example, we increased the gate-open set point from 9 to 9.2 feet on a 120-inch pipe. It may not sound like much, but when it is two-tenths of a foot over several miles, that’s a substantial volume.” Raising the set point in one 96-inch pipe by 4 feet enabled the most dramatic capacity increase.

The sensors also identified 3,000 feet of 96-inch pipe that carried very little flow even during prolonged downpours. “About one-half mile from the plant, the elevation in the North Bank interceptor drops seven feet,” says Stuer. “It’s like a waterfall inside the pipe. Wet-weather flows during heavy rains surcharge the lower segment, while the upper segment is almost empty. We’re going to install flow-control gates at that location and gain one million gallons of additional storage.”

The utility removed the FlowAlert sensors after the study and now relies solely on the ultrasonic sensors in the diversion stations to monitor and adjust the system. The ADS units, however, are not in retirement.

“The Duck Island plant is using a 30-year-old hydraulic grade line,” says Stuer. “To determine the facility’s actual wet-weather treatment capacity, we installed six ADS sensors throughout the treatment process. They are enabling us to better understand our system and to become more confident in our decisions.”


Comments on this site are submitted by users and are not endorsed by nor do they reflect the views or opinions of COLE Publishing, Inc. Comments are moderated before being posted.