The City of Duluth is taking a top-to-bottom approach to eliminating sanitary sewer overflows (SSOs). In fact, it has to.

 

For one thing, the Minnesota city’s steep hillside location along Lake Superior means everything flows downhill — fast. For another, Duluth is under an EPA consent decree to halt all SSOs into the lake by the end of 2016.

 

“We have seven years to clean up our act, period,” says John Center, project coordinator for the city’s SSO control initiative. “The clock started ticking last October.” In response, the city has launched a comprehensive, leading-edge program to fix the problems:

• Finding and remediating all clear-water drain connections and leaks in the hillside neighborhoods at the top of the system.

• Building overflow storage capacity at the bottom of the system along the lakeshore.

TV inspection, continuous flow monitoring, and innovative “sewershed” modeling are among the tools Duluth is using to achieve total compliance by the EPA deadline.

 

Topography lesson

Duluth’s “vertical topography” and the sheer age of its infrastructure are formidable challenges. With a population of about 86,000, the city stretches in a long, narrow band along the shoreline. Homes and businesses occupy a steep bluffside that drops some 800 feet to lake level.

 

“Our sewer lines come straight down the hill to our interceptors,” explains Center. “It’s 40 mile-per-hour water hitting a T connection.” Center has seen situations where dataloggers in manholes have simply been washed away.

 

The interceptors lead to the wastewater treatment plant owned and operated by the Western Lake Superior Sanitary District (WLSSD) and serving Duluth and several suburban communities. Duluth’s contribution is about 11 mgd. Many laterals and connecting sewers are old. Some date to the late 1800s and are 6-inch clay pipe.

 

These factors combined to create serious infiltration and inflow problems and SSOs, which the city and regulatory agencies have attempted to address for more than 30 years. The WLSSD was created in 1971 and began treating wastewater under an NPDES permit to clean up the heavily polluted St. Louis River.

 

In 1974, the district and Duluth were issued a new permit to stop SSOs. “But nothing really happened,” explains Center. Another attempt by regulators to stop overflows in 2004 ultimately led to a lawsuit and the current consent decree.

The situation was serious. Todd Carlson, water quality specialist and part of the Duluth SSO management team, points out that between 1995 and 2004, the city recorded 250 overflows amounting to 47 million gallons. “We averaged nearly 30 overflows a year, a lot of them right downtown near the hotels and major businesses,” he says.

 

Getting serious

Says Center, “We don’t need any more sewage flowing down Superior St. (the main drag).” Duluth’s commitment to improvement is equally as serious. Sitting around the conference room table in the utility’s headquarters building on Garfield Ave., Center and his team are passionate as they describe the SSO elimination program.

 

To begin with, the team has followed a watershed approach to water management and has defined the Duluth system as a sewershed consisting of 30 basins. Then sub-basins are identified within each basin, and the most critical are targeted for action first.

 

Steve Lipinski, manager of utility operations, is a strong advocate of sewershed modeling, explaining that it breaks down a problem like SSOs into manageable tasks while maintaining a holistic view of the system. “With our unique topography, the sewershed modeling approach makes a lot of sense,” he says. “In some cities, overflows are the result of plugged lines or oil and grease, but here it’s infiltration and inflow. We have more than 15,000 drains and lines connected to the system, and a lot of them are leaking.”

 

Once the critical sub-basins are identified, Duluth uses a methodical, continuous program of flow monitoring and TV inspection to identify and eliminate the upstream sources of I&I. Flow monitoring is critical, not only to identify problem areas, but to show progress as the system is tightened up.

 

The data is vital to the accurate design of the overflow storage facilities. “Flow monitoring enables us to target where we need to remove water,” says Carlson. “It helps us build the road map. It’s essential that we establish an accurate dry-weather flow base.”

 

As Center puts it, “It’s expensive to build these storage tanks, but we need adequate capacity. So, we really have to get our numbers right.”

 

For that reason, flow monitoring runs 24/7/365, using 30 flow measurement stations monitored by a dedicated two-person crew. “We have one full-time monitoring team, working closely with me,” explains Carlson. “This team is familiar with the territory. They know the history of each site and when to move the flowmeters.”

 

Duluth uses ISCO 4150 series flow loggers and 2100 series flow modules. The team likes the older 4150 units because the crews are familiar with them. “They’re simple, easy to use, and they work,” says Carlson.

The newer 2100 modules, however, have a temperature-reading feature that enables the team to measure runoff from the spring snowmelt by temperature change of the water. “We’re proud of our data collection,” says Center. “Working with our consultant (CDM — Minneapolis office), we’re achieving about a 98 percent confidence level in our numbers.”

 

Center says the accurate data also helps the utility communicate both the problems and the successes of the program to local policy-makers. Adds, Lipinski, “Sewershed modeling allows us to constantly evaluate what’s happening in our system, and it shows how well we’re doing. Flow monitoring will forever be part of our preventive maintenance program.”

 

Drain disconnect

Through previous smoke- and dye-testing, Duluth has disconnected about 98 percent of its roof drains, but footing drains remain an issue. The goal is to identify and disconnect 630 of these drains a year. As an incentive to property owners, the city makes up­ to $2,150 available as a grant for removing the footing drain and house trap and installing a sump pump to re-route the water outside to a storm drain.

 

The money comes from a citywide sewer rate increase enacted ­specifically for the SSO program. The funding required a change in state law, which previously prohibited public funds for use on private property. Duluth led the campaign for change, and now public dollars can be spent on private-property projects that improve the overall sewer system.

 

Leaking laterals are another target of Duluth’s program to reduce flow at the source. In the beginning, the team reports, the public was convinced that wet-weather intrusion was a problem with city mains, not private lines. But the acceptance of shared responsibility came after a citizens’ I&I task force looked into the issue.

 

Duluth initiated a demonstration program showing how laterals could be slip-lined with CIPP technology instead of open cuts. And property owners have a cash incentive to correct their problems. From its surcharge fund, the city will pay 80 percent of the first $5,000 required to replace a failing lateral. The city will also pay a flat $3,000 to property owners remediating laterals upstream of a targeted sub-basin.

 

Using an Explorer mainline pan-and-tilt camera from UEMSI, the city performs extensive TV inspection on a block-by-block basis during wet weather to pinpoint clear-water inflow. The videos serve as convincing evidence to property owners whose lines need to be repaired or replaced.

 

“TV inspection has been a great tool for us,” says Carlson. Once a leaky lateral is identified, the property owner has 90 days to replace it and it must pass a city inspection. Failure to do so can result in a hefty surcharge on the owner’s sewer and water bill. But the city tries to facilitate a solution. Says Sandy Mass, project coordinator, “We meet one-on-one with the property owner and bring a list of licensed contractors to contact for the replacement.”

 

Center explains that in order to comply with the consent decree, Duluth must get 175 laterals replaced every year through 2016.

 

Bottom line

At the other end of the system — along the lakeshore — cranes and dozers are busy digging excavations for the latest in a series of overflow storage tanks. When the project is complete, Duluth will have six tanks, from 300,000 to more than 8 million gallons, producing a combined storage capacity of about 15 million gallons. After a storm event, the contents of the tanks are bled back into the sewer system.

 

“We need the tanks because of the deadline we face,” explains Center. “Once our system is tight and we reach compliance, the tanks will serve as a safety net and will also allow us to control the level of flow we deliver to the WLSSD treatment plant.”

 

Designers have blended the tanks beautifully with their surroundings. The lakeshore is the site of expensive condominium and marina developments, and landscaped parkways wind among them. So, to avoid degrading property values or scenic views, the tanks are belowground and landscaped on top. They are essentially unnoticeable. The lift stations are specially equipped to prevent solids buildup and counter odors.

 

“We’ll spend about $37 million on the storage tanks,” says Center. Federal grants, stimulus money, and city bonding are being used to cover the costs. While 2016 is still a ways off, the Duluth SSO team expresses confidence they’ll meet their objectives. In fact, they’re already seeing results.

 

Observes Carlson, “The numbers indicate we’ve already cut our average dry-weather flow to the treatment plant from 18 to 20 mgd to around 10 to 11 mgd, and our average wet-weather peak flows from more than 100 mgd to around 36 mgd.”

 

And, he reports that during 2009, the city experienced only one serious overflow. “We’re keeping clear water out of the system and preventing overflows from getting into Lake Superior,” Carlson ­says. “That’s the bottom line.”