Knowing the Flows

A comprehensive monitoring system helps a major sewerage agency keep tabs on its system, optimize capacity, and help drive down SSOs
Knowing the Flows

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State-of-the-art flow monitoring technology is a key to the Washington Suburban Sanitary Commission’s effort to manage and eliminate sanitary sewer overflows (SSOs), as mandated in a 2005 consent decree.

Supplied by ADS Environmental Services, the 174 flow monitors and 16 rain gauges help WSSC collect data to establish baseline flows, determine pipe capacities, and track weather-related inflow and infiltration (I&I) in its 5,400-mile collection system. WSSC serves nearly 1.9 million people in Montgomery and Prince George’s counties (Maryland), next to Washington, D.C.

“We’ve worked with ADS for over 25 years,” says Glen Diaz, sewer analysis unit coordinator. “The equipment is accurate, durable and reliable. We really like the Web-based system that makes data available in real time to anyone with an Internet connection, not just a limited group of people.”

Several new monitors and rain gauges recently installed represent an upgrade. Flow monitoring was among several requirements of the 2005 consent decree involving WSSC, the U.S. EPA, the U.S. Department of Justice, and the Maryland Department of the Environment. The agreement details a comprehensive $350 million program to mitigate and eliminate basement backups and SSOs in the collection system and at pump stations and treatment plants in the 1,000-square-mile service area.


Detailed agenda

WSSC dates to 1918, and portions of its sewer system are older. Some lines consist of vitrified clay, ductile iron, PVC, concrete and PCCP pipes. The system is prone to I&I during wet weather, and to clogging by roots and debris. Fats, oils and grease (FOG) is an especially difficult problem, as there are thousands of restaurants and other food establishments in the area.

The commission manages the system by separating it into 25 distinct sewer basins (sewersheds). “Ours is a separate sanitary sewer system consisting of mostly gravity flow,” explains Diaz. “A sewershed consists of a network of neighborhood sewers tying into collector sewers, all flowing into a common trunk line.”

In 2002, the commission began discussing the need to improve the system. Three years later, WSSC signed the consent decree and embarked on a 12-year plan to address its issues. In that same year, WSSC experienced 167 SSOs, resulting in the loss of 4.8 million gallons, largely related to severe weather. Specifically, the agreement requires WSSC to:

• Inspect and clean about a third of its sewer system.

• Conduct comprehensive sewer surveys in nine of its sewer basins, using state-of-the-art technology to collect data on flows, rainfall, and manhole and pipe conditions.

• Continue and enhance its flow monitoring program.

• Develop computer models of sewer system sections to determine present and future requirements and plan sewer line improvements.

• Rehabilitate sewers as necessary over a 10-year period.

• Enhance its FOG program through increased inspection and permitting programs.

The consent decree also requires WSSC to help protect source water quality by purchasing buffer land around its Patuxent River reservoirs, eliminating illegal drain connections to the sewer system, and improving cold-weather nitrogen removal at its Western Branch treatment facility.


On track so far

The sewer rehabilitation project is on schedule to meet the consent decree requirements, according to contracts unit coordinator Mark Behe. “At present, we’re rehabbing or replacing about 30 miles of sewer on a very aggressive schedule,” he says. “We have projects ongoing in at least 10 of our sewer basins.” Trenchless technology is the choice for rehabilitation wherever possible. Methods include grouting of joints, cured-in-place pipe (CIPP) using approved manufacturers, and pipe bursting.

WSSC is also grouting and lining manholes using approved manufacturers’ materials.

Sewer cleaning is a fundamental strategy for removing blockages from the collection system, maintaining hydraulic capacity, and so helping to prevent SSOs. Sewer cleaning is classified as:

• Proactive, focusing on sewers with no history of problems.

• Preventive, targeting sewer segments known to have blockages and SSOs.

• Emergency, performed to remediate the impacts of SSOs.

While the triggers for cleaning may be different, the methods and equipment used are similar. These include waterjetting, rodding, and bucket-machine cleaning.


New flowmeters

The ADS flowmeters are critical to the program, because the data they record and report is used to recalibrate the sewer model and to help determine the areas in need of rehabilitation, repair, or replacement.

“The data drives our SSES (Sewer System Evaluation Study) and enables us to determine which basins require the most attention,” says Andrew Fitzsimons, project manager for flow metering. “We can see what the flows are, versus what they’re designed to be, and we can anticipate problem areas. The closer our sewer flow is to our water production flow in a certain basin, the more we’re assured that our system is getting tighter.”

WSSC has 151 flowmeters permanently stationed throughout the system, measuring both diurnal and wet-weather flows. In addition, 23 billing meters measure flows into or out of the WSSC system, such as flows to the Blue Plains Wastewater Treatment Plant in Washington, D.C. Also, a number of onsite treatment facilities contribute flow to the WSSC system. Fitzsimons says the meters on these lines give WSSC accurate numbers for billing purposes.

The flow metering program also includes a number of temporary meters — portable units that can be taken into a targeted area to measure flows relating to a particular problem, such as chronic sewer backups in a specific neighborhood. Generally, says Fitzsimons, these meters stay on location for about two months.


Updating technology

WSSC continues to improve the flow monitoring system. Several meters were upgraded to the latest wireless technology to address communication limitations from telephone service. Over the last two years, meters were relocated and added to meet WSSC modeling recommendations and to enhance flow balancing capabilities. One of the latest ADS Triton meters, with surface velocity radar, was recently installed in a high-entry-risk 102-inch sewer.

Lastly, ADS has supplied 16 state-of-the-art rain gauges, positioned every five to 10 miles throughout the WSSC service area to log precipitation amounts and track I&I impacts. The gauges record about 40 inches of rainfall in a normal year, consistent with the 39.3-inch average recorded at Reagan National Airport over a 30-year period.

Fitzsimons says his group likes the new ADS flowmeters because they have built-in redundancy, and because they are designed to perform low-level data cleanup, freeing staff from doing that task manually. “This saves us several hours of extra work,” he says.

The data analysis is part of a turnkey contract ADS has with WSSC. ADS is responsible for manufacturing, delivering, installing, maintaining and calibrating its equipment in the field. ADS has also helped WSSC determine the best locations for the flow monitors, evaluating sites for flows, pipe size, grade and safety issues.

The ADS team reviews and finalizes data from each site on a weekly basis, issues maintenance work orders, and finalizes current and historical trend data for posting to IntelliServe and for further review and discussion by the WSSC project team. Both landline and wireless connections are available.

Thus far, through the combined efforts of WSSC and ADS, the flow metering system uptime percentage has been 97.2, several points better than the EPA-mandated 90 percent. “The system enables our staff to be able to concentrate on the most critical collections system maintenance,” Diaz concludes.


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