Clear water from illegally plumbed sump pumps, underdrains, and other sources posed a major problem for Dave Hofer, P.E., assistant county engineer for the Engineering Division of the New Castle (Del.) County Special Services Department.

 

A key contributor was Brandy-wine Hundred, a series of sprawling old suburban communities with 40,000 homes and just over 100,000 people. The gravity sewers there have a constructed overflow discharging to the Delaware River.

 

The overflow was built with federal funds in the 1960s when such practices were legal, but the state Department of Natural Resources and Environmental Control (DNREC) reclassified it as a combined sewer overflow (CSO) and demanded action.

 

In 2004, the DNREC and the county approved a plan to expedite and expand an existing sewer rehabilitation program. Over the past five years, extensive work in two of the county’s worst areas reduced wet-weather backups from six to 10 per year to one or two. Throughout the process, Hofer and his staff looked for solutions to give the sewer mains 100 years of service life.

 

Unique subdivision

Instead of townships or parishes, Delaware divides its counties into hundreds. Brandywine Hundred has 400 miles of 8- to 54-inch sewer pipes and three major pump stations with more than 10-mgd peak pumping capacities. The largest pump station in the hundred has a peak capacity of 70 mgd.

 

The New Castle County sewer department has 112,000 connections generating 55 mgd. It treats 2 mgd at four small plants, and the remainder goes to the treatment plant at Wilmington, Del. The stormwater conveyance system is separate.

 

As the area grew, the sewer system expanded without adequate planning. Some pipes are undersized, and others do not meet modern standards. Mains and laterals ranged from Orangeburg and vitrified clay pipe to cement asbestos, concrete, and PVC.

 

In the late 1980s, the county launched a rehabilitation program, replacing the Orangeburg pipe. A maintenance program now covers 500 miles of pipe per year. The program reduced sewage backups from 500 to fewer than 50 per year. In 2004, the county planned a three-phase program to eliminate all CSOs by December 2029. “We were to complete Phase 1, with an estimated cost of $149 million, by 2014,” says Hofer. “At the same time, we began educating homeowners that sump pumps should empty onto the ground outside their houses.”

 

Although work progressed on schedule, the pace was unsatisfactory to the EPA. In 2008, the agency, DNREC, and county agreed on a new plan. “We now must eliminate the last-constructed CSO and wet-weather-related SSOs in the Brandywine Hundred sewers by December 2018,” says Hofer. “The new deadline is quite a change for us, and so is the price. Phases 1 and 2 are just over $300 million.”

 

Brandywine Hundred encompasses one-quarter of the sewer system. Laid in the 1930s, the lines show significant age-related deterioration. About 15 percent of the homes have illegally plumbed sump pumps and underdrains.

 

“DNREC, EPA, and the county want to see this job done, so we’re all working together harmoniously,” says Hofer. The current system has no overflows until 2 inches of rain falls in 24 hours. While most communities consider that satisfactory, the county wanted better results — zero overflows.

 

“We’re designing our improvements to withstand a three-year storm that lasts 24 hours and that coincides with the average annual high groundwater table,” says Hofer. “This works out statistically to be an event that occurs once every six to 10 years. We set that return period as our performance goal, then worked backward to find out what storm and groundwater event return periods would combine to make it happen.”

 

Working smart

To achieve its objective, the county hired Malcolm Pirnie, an environmental consulting firm in Wilmington. After modeling the Brandywine Hundred sewer system, engineers determined that rehabilitation of deteriorated pipes would remove 35 percent of inflow and infiltration (I&I) during peak storm events, ending overflows. However, success depended on tightening and enforcing specifications on rehabilitation contracts.

 

“Malcolm Pirnie engineers said that most municipalities write in everything the manufacturers and vendors say belongs in the specifications, then tend not to enforce the tests prescribed by ASTM,” says Hofer. “That causes problems.”

 

For instance, CIPP liners are designed to ASTM standards, and some involve specialized tests. Hofer found that long-term strength tests were overlooked most often. “They measure how strong the liner is after aging 10,000 hours and are used to predict if the liner will last 50 years,” he says. “We design our CIPP pipe projects to last at least that long, and new pipe projects to last 100 years. The tests for new pipe are similar.”

 

The county now requires contractors or independent laboratories to do the ASTM tests. The county’s performance testing, which looks for defects that were supposedly fixed, evaluates rehabilitated pipe 18 months after it is installed. The camera crew usually finds only a few defects, but occasionally they are major.

 

For instance, one problem was traced to an improperly manufactured liner that failed. “Had we not checked it, we would have closed out the contract and been left with a failed liner that we would have had to fix,” says Hofer. “We use similar testing on new pipe, plus laser profiling to measure its allowable deformation.”

 

Performance testing also uncovered numerous CIPP defects from well-established contractors lining the mains. “We found pinhole leaks and deformations,” says Hofer. “Holding the contractors accountable was a first for them. We had one contractor who did two jobs, then told us he wasn’t returning because he could go to other cities that didn’t enforce specifications or testing and make money. And that was Malcolm Pirnie’s point. You’re not working smart if you don’t check the quality of contractual work. We’re charged a bit more now, but we’re getting what we paid for.”

 

The county reduced I&I by lining with CIPP or chemical grouting. Laterals are lined as far into homeowners’ properties as possible. “We could remove the equivalent flow from the mains, but it’s cheaper to line part of the laterals,” says Hofer. “It has reduced I&I a lot.”

 

Longer life

The rehabilitation program includes upgrading 30- to 40-year-old interceptors that had failed prematurely. “We didn’t believe it was proper to replace them with materials having equally short design lives,” says Hofer. “We want new interceptors to last 100 years, and we are researching those products.”

 

One long-lived material is vitrified clay pipe (VCP). “The Romans used it, and their lines are still operational,” says Hofer. “When municipalities first installed VCP, however, it didn’t have good bedding or joint material.”

 

Today’s gaskets provide tight joint seals. Although many engineering and inspection firms state the manufacturer’s specifications for the pipe bedding and haunching, municipalities rarely check them.

 

“Our project inspectors and engineers are down in the trenches making sure the proper depth of bedding is installed and compacted properly, and that the bedding under the haunch of the pipe is installed and compacted,” says Hofer. “They follow up with a laser profiling test on flexible pipes, ensuring that they do not exceed the manufacturer’s allowable 30-day deflection amount.”

The biggest change in specifications is requiring warranty inspections 18 months after contractors complete the projects. The challenge was how to pay the contractors so they had an incentive to return, and how to handle any problems discovered. “Depending on the problem and its severity, we may simply accept a price reduction,” says Hofer. “The policy works well for us, and we still have the option of rejecting the work.”

 

Innovative avenues

Because vitrified clay is available in sizes only up to 42 inches, the county is looking at HDPE and fiberglass-reinforced pipe for larger mains, and polymer concrete pipe for tunnels. All are impervious to corrosion, but polymer concrete is stronger. The county uses that pipe for manholes expected to have high levels of hydrogen sulfide.

 

The rehabilitation program includes upgrades to pump stations. As part of that initiative, an engineering firm designed a 45-mgd station to replace a 20-mgd station. “We’ll use the old one as a backup peak wet-weather station or reconfigure it as a 30-mgd peak station,” says Hofer. “We’re big on Plan B because Plan A doesn’t always work.”

 

To make manhole benches and channels impervious to corrosion, Hofer contacted Henkel Technologies Corrosion Engineering, a specialist in corrosion control for the chemical industry. “We’re returning to brick benches, only gluing the bricks together with Furalac Green Panel mortar,” he says. “Henkel makes acid storage tanks from Furalac, and they have a 30-year life. The furan resin-based mortar will easily stand up to anything in our sewer environment.”

 

Although more costly than standard mortar, the Furalac joint is only 1/8-inch thick instead of the typical 3/8 inch. “Once it’s in, we can forget about it,” says Hofer. “I think we’ll get our 100-year life expectancy.”

 

Homeowners, who used to be upset by sewer backups, are now the biggest supporters of the rehabilitation and maintenance program. Most overflows had been caused by roots and debris, and by pipes that became overloaded during rainstorms.

 

“Homeowners like all the information we send on projects in their neighborhoods, and they can see a big reduction in backups,” Hofer says. “Although the work isn’t completed, they acknowledge that we’ve done a tremendous job.” For his efforts, Hofer received the 2006 Collection System Award from the Water Environment Federation.

 

Phase 1 of the Brandywine Hundred project is on schedule and will conclude in December 2014. Phase 2 will meet the December 2018 deadline. The rehabilitation should survive into the 22nd Century.

 

Mother Necessity

If the Engineering Division of the New Castle County Special Services Department needs a product that does not exist, the team invents it or works with manufacturers to produce it.

 

For example, manhole covers were a major source of inflow. “We were using manhole inserts, but they were a high-maintenance product,” says Dave Hofer, P.E., assistant county engineer. “Crews kept repairing or replacing them because they were knocked out or broken, or homeowners punched holes in them to drain their driveways.”

 

Hofer envisioned a gasket in a manhole cover without pick holes. He went to manufacturers and tried some of their ideas, but they didn’t work. He then assembled an in-house design team. Their concept press-fit an O-ring gasket into a dovetail groove in the cover’s underside. A Wisconsin foundry made the prototype.

 

“The ironworkers had some ideas to refine the design, and so did we,” says Hofer. “The new covers sealed the system so tightly that tremendous slime built up inside the manholes, forcing us to reinstate one pick hole for ventilation.” More than half the 45,000 manholes in the sewer system have the cover, and neighboring municipalities are beginning to use them.

 

The county also worked with the Infra-riser division of East Jordan Iron Works Inc. to create high-density rubber composite adjustment rings that are inserted under manhole frames. When vehicles drive over the cover, the ring affords some resilience, protecting the mortar from crushing.

 

“Our smaller collection manholes have Infra-riser seals, and they work terrifically,” says Hofer. “We wanted them on our interceptor projects so that truck traffic wouldn’t beat the expensive polymer concrete manholes to death. However, the company didn’t make one that fit a 40-inch opening with a 36-inch cover.” Hofer approached the manufacturer, who then made the mold, and Hofer got his seals.