You wouldn't know it by looking, but the Paris Utility District in the Village of South Paris, Maine, has a brand new wastewater treatment plant.

From the outside, you see the same brick and mortar of an old industrial wastewater treatment plant that was shut down for nearly 20 years. Walk inside and you're in for a surprise. "The plant is now all state-of-the-art," says utilities manager Steve Arnold. "We even converted everything to all-new LED lighting."

What used to be a manually run mechanical plant now has full SCADA control. Grit that used to be shoveled by hand is automatically removed. Oil heat is a thing of the past — heating and cooling are provided by an innovative system that uses plant effluent (see sidebar).

It's no miracle

South Paris didn't find a wastewater treatment fairy. The district spent several years figuring out how to reuse what it had to build a treatment system for today's needs.

The old combined sewer and stormwater treatment plant was built in 1975 as two separate units: one for industrial wastewater and one for municipal sewage and stormwater. Then the industries left the community, and the industrial half of the plant was shut down in 1985.

What was left operating was quickly becoming outdated. "We had a metal grit channel in the headworks with a cantilever bucket elevator with scoop shovels that dumped debris on a belt that emptied into a container," recalls Arnold. Grit was shoveled into five-gallon buckets. "If we had a storm, what was usually one or two buckets would become 50, and we had to carry them up two flights of stairs," Arnold says.

In 2004, the district called on the engineering firm Woodard & Curran of Portland to design a new plant. The result is a project that won an Engineering Excellence Award from the Maine chapter of the American Council of Engineering Companies. "They were only using one-sixth of the original secondary system," says Paul Rodriguez, senior project engineer with Woodard & Curran. "Over two years, we looked at what they had and evaluated several options."

Right-sizing

The original wastewater treatment plant had three influent lines, one from a tannery, one from a cannery, and one for municipal wastewater and stormwater. Flows averaged 1 to 2 mgd, versus a design flow of 3 mgd.

"Our drinking water plant had three wells running 24/7 pumping 1,200 to 1,400 gpm, so we always had water coming in," says Arnold. "As those industries closed, we used less and less of the plant."

The municipal wastewater and stormwater continued to be treated in aeration basins with mixers. There were six cells of 239,000 gallons each, but eventually the plant was only using one, and it was usually at half capacity. The plant was oversized, much of the equipment was nearing the end of its life, and regulations were getting stricter. "We had a great big dinosaur, and a duplication of everything," says Arnold.

After considering the options, the district and Woodard & Curran decided the best option was to use the industrial portion of the plant for a new activated sludge secondary treatment plant. Its design flow is sized at the current NPDES permitted flow of 0.65 mgd to meet today's needs of 1,000 service connections in a community of 3,500. Because it treats combined flow, its peak flow design is 1.5 mgd. Average flow is 0.35 mgd.

"The upgrades were separated into phases that focused on every aspect of the wastewater collection and treatment facilities, including the headworks, biological treatment, disinfection, sludge dewatering, pumping stations, and a new control system that also monitors all the pump stations in the collection system," says Rodriguez.

Getting to work

Work began with a new headworks in the existing building, installed from 2005 to 2007. A bypass channel was jackhammered into the floor so that the new equipment from Headworks could be installed without shutting down the plant.

The new secondary treatment system went online in September 2010. "Our TSS used to be in the range of 11 to 22 mg/L," says Arnold. "We're down to around 2 to 5. We have reported a couple of months of 100 percent removal, and our inspector says we can't do that! BODs are in single numbers and used to be 15 to 22 mg/L. The process is tremendously efficient, and the levels of treatment we're getting are fantastic."

Arnold, who during the 1990s was a compliance inspector for the Maine Department of Environmental Protection assigned to South Paris, observes, "They always met their permit limits and did a good job with what they had. But it's like night and day. We've acquired the tools we need to do the job easier and more efficiently by deploying the latest in technology."

Arnold calls the $10 million investment in the new plant not an upgrade or refurbishment but a "reinvention," with the idea of reusing as much as possible to reduce costs and construction work. Rodriguez adds, "They're doing a fantastic job running the plant. It's beyond what anyone could have expected going into this.

Arnold's team includes chief operator Paul Lowe (wastewater and drinking water), wastewater operators Wayne Kennedy (master electrician), Roja Hawkins and Barry Lambert, part-time operator Barry Morse, lab manager Louise Grant, and office manager Penny Lowe.

Making the old new

Screening is now done with a Mahr Bar screen with a Vulcan wash press that automatically bags the waste material for incineration. Inclined screw conveyors (WSG & Solutions) automatically remove grit from an optimized grit tank that includes fiberglass baffles, new diffusers, and a pair of three-lobe positive-displacement blowers (Aerzen) for aeration.

"What used to be a carbonation/stabilization tank serves as our new chlorine contact chamber," says Arnold. Old equalization tanks became the new aeration tanks. The old primary clarifiers for the industrial wastewater system are the new secondary clarifiers, and the former upflow clarifiers became the new biosolids storage tanks. What used to be the pump building now houses the new treatment equipment.

The old influent pumps are now stormwater pumps. When the forward-flow pumps get overwhelmed with stormwater, the excess water flows over a channel wall and is pumped to one of the new storage tanks, made from reused parts of the old facility. Repurposing tanks around the plant saved millions in construction costs.

Two old aeration tanks and several secondary clarifiers were cleaned and stripped of all components and now provide 1.73 million gallons of stormwater storage capacity. After rain events, the contents of the storage tanks are reintroduced through the headworks for complete treatment.

While the old plant experienced two to four combined sewer overflows per year, there has been only one since the new plant went online. That was in June 2012, when 9 inches of rain fell in 40 hours on a weekend. "We had taken 3- or 4-inch rainfalls and weren't sure what it would take to fill the tanks and cause an overflow event," Rodriguez says.

To fund the project, the utility issued a 40-year bond for $5.4 million and used grants for the rest — just over $1 million from a Maine Department of Economic and Community Development block grant, and more than $3 million from a U.S. Department of Agriculture Rural Development block grant. All seven pump stations were also upgraded and hooked into the new SCADA system.

New treatment process

"Operators had to be trained by every vendor on every piece of equipment," says Arnold. "We set time aside and did training for three weeks straight — 45 minutes here, a couple of hours there. I would say everyone had more than 80 hours of training. And we're still learning. We spent hours on the SCADA system, and we had to train on the new effluent thermal heating system SCADA as well."

The biological treatment is done with a plug flow reactor with an anoxic selector zone. "Anoxic swing zones con­trol nitrification/denitrification for alkalinity recovery and reduce the nitrate loading to the secondary clarifiers," says Rodriguez. "The system de-couples mixing and aeration and includes state-of-the-art controls to minimize power consumption. Enhanced secondary treatment is done through chemical precipitation using metal salts, polymer and magnesium hydroxide."

High-efficiency equipment was used throughout, including Allen-Bradley soft-start variable frequency drives (Rockwell Automation) and pumps from Watson-Marlow and Flygt.

After screening and primary treatment, wastewater flows to aeration basins and clarification, then to ferric chloride treatment for phosphorus and chlorine disinfection (May through September) before discharge to the Little Androscoggin River.

"We have a lot more equipment online," notes Arnold. "We expect a 1 or 2 percent decline in electricity use." Even if the plant's $85,000 electricity budget remains the same, the new equipment provides better reliability.

Alarm response

The old plant had standard alarm systems for functions like flow measurements and level indications, but it was all operated manually. Now it is all monitored and tracked by a FactoryTalk View SCADA system from Rockwell Automation with Allen-Bradley PLCs. All the data is available on laptop computers from remote locations. "From home, we can see exactly what's going on at the plant," says Arnold.

Before the rebuilding project, all alarms sounded at the sheriff's department. A dispatcher then paged the on-call operator, who had to make a phone call to confirm the alarm. "You had to drive to the plant and look at the alarm board," says Arnold. "If it was at a pump station, you would then have to drive out there. Many times, by the time you got there, the alarm had cleared, but you still had to check it." Then it was back to the plant to reset the alarm panel and call the sheriff's department.

"Now alarms automatically call an on-call cellphone, and an automated voice tells you exactly what and where the alarm is," says Arnold. "That in itself is worth its weight in gold." There are fewer high-water alarms because the new system can handle stormwater surges automatically.

"In the past, we had to come down to the plant and make sure everything was all right," says Arnold. Because of the automation and updated equipment, the number of alarms has dropped to one-tenth the previous level.

It took some time, and a lot of study and planning, but the Paris Utility District expects long years of reliable performance from its new treatment plant.

Editor's Note: Paul Rodriguez, the senior project engineer quoted in this story, is no longer with Woodard & Curran. His replacement on the Paris project is project engineer Rob Polys.