Taxing sewer treatment

It starts with a flush.

Miles away, the flush surfaces at what Moccasin Bend Sewage Treatment Plant Superintendent Alice Cannella calls "the gates of Hell."

The gates are where all sewage treatment pipes for the greater Chattanooga area converge to enter the Moccasin Bend plant.

Three stories below ground level on a sunny day, a massive pipe gushes 65 to 70 million gallons a day of raw sewage and sewage-tainted stormwater runoff into the 50-year-old treatment plant that has been upgraded or expanded three times.

Ms. Cannella says it's the rainy days that give the 140-million-gallon-capacity plant a workout.

"A quick one-inch rain can jump it to 100 million," she said. "We could start seeing maximum flows if we had a day that sees a 2-inch rain."

In most sewage treatment systems, rain has little impact.

But Chattanooga's treatment system is one of only three in Tennessee and about a half dozen in the Southeast that includes stormwater runoff in what's known as a combined sewage overflow system.

The combined systems generally are antiquated, and most of the 772 of them in the nation are in the Northeast, according to U.S. Environmental Protection Agency data.

Jerry Stewart, director of Chattanooga's waste resources division, said only about 70 of Chattanooga's 1,250 miles of sewage lines are combined sewage and stormwater lines.

Most are in the central business district of downtown Chattanooga.

"To replace that 70 miles (with separate lines) would cost more than $400 million," he said. "I could take that $400 million and spend it on about 600 miles of sewer that was built before 1970 -- that is clay and concrete pipe. Clay and concrete pipe settles and breaks, and over half our sewer system was built before 1970."

Storing runoff

Under the gun from EPA in the 1990s, Chattanooga tried to stop the sewage-tainted overflows that largely went straight to the Tennessee River when it rained.

To slow the flow and give the treatment plant time to assimilate all the water, the city began building gigantic underground holding tanks.

Over the next decade, the city built eight cavernous holes under the ground in various locations downtown and in North Chattanooga. This year, city workers completed a ninth one.

The first, completed in 1992 for $6 million at Ross's Landing, temporarily can store 3.4 million gallons.

The largest, the size of two football fields behind Howard School of Academics and Technology on Market Street, can hold 7 million gallons -- seven Tennessee Aquariums. It cost $15 million.

Altogether, the nine giant holes cost $49.3 million. They are called combined sewer overflow treatment facilities, or CSOs.

The only "treatment" in the CSOs is the settling out of solids. In the sewage business, that's called primary treatment.

Although the new draft stormwater permit for Chattanooga doesn't apply directly to the sewage treatment plant and its CSOs, Ms. Cannella said anything that requires new development to be designed in such a way to hold the first inch of rain on-site during a 72-hour period will have a positive impact.

"That would actually help us," she said.

How wastewater is cleaned

Ms. Cannella's "gates of Hell" actually see wastewater from far more homes and industries than are in Chattanooga proper.

Also flowing into the Moccasin Bend treatment plant is sewage from Red Bank, Hamilton County, Soddy-Daisy, East Ridge, Collegedale and Lookout Mountain in Tennessee.

The plant also serves the Georgia areas of Rossville, Lookout Mountain, Walker County, parts of Wildwood, Fort Oglethorpe and Ringgold.

"We could ultimately take on Trenton and Chickamauga," Mr. Stewart said. "And we're considering taking on Signal Mountain -- subject to them tightening up (their own stormwater overflow)," he said.

"I'm not going to just let them have a drain coming here and take up my system, because they don't want to tighten up their system," he said of the mountain town that now is under a development moratorium because rain events severely overpower its separate sewage treatment plant.

Wastewater, once inside the Moccasin Bend plant, is screened by a number of filters to pull away trash that would interfere with treatment. Then the waste alternately is settled, aerated, chlorinated and dechlorinated.


"The biggest issue with this plant is E. coli (bacteria)," Mr. Stewart said.

It's a biological system, and as such can be affected by many things, Ms. Cannella said.

"Temperature can change the process," she said. "Different industrial discharges can change the process."

Operators have to continually monitor and test to be sure the process is working no matter what the complications.

Stormwater can be one of those complications.

"I have to bat 1,000 every day," Mr. Stewart said. "I cannot, according to the Clean Water Act, have any illicit overflows or discharges. And sanitary overflows are unpermitted. So you have to report that."

He said the plant itself usually is at 98 or 99 percent of meeting that requirement.

Adding other regulations to his list of responsibilities, he said, he gauges system "compliance days."

"We're somewhere around 80 percent," he said.

But he said he knows the public doesn't look at sewage treatment in the same way he does.

"The only two times people think of us is when they get their sewer bill, or when they flush and it doesn't go away," he said with a smile and shrug.

Continue reading by following these links to related stories:

Article: Plugging the water gaps

Article: Stormy challenge for water quality

Article: Runoff raises concern

Article: Cities unite to solve wastewater problems