SEWER: Spinning a Success Story

Innovative vortex separators are part of a comprehensive CSO abatement program that pays big environmental dividends for the City of Saco, Maine.

By using a little Yankee ingenuity, the City of Saco, on Maine’s southern coast, has made significant pro-gress toward eliminating combined sewer overflows (CSOs).

A $10.8 million, 20-year plan, finalized in 1994-95, incorporated a series of milestone projects including sewer separation, flow slipping, and primary treatment of combined sewage at Saco’s 4.2 mgd (design) wastewater treatment plant. The city began implementing the plan in 1996 and to date has completed 98 percent of the projects.

“We’re very pleased with the progress we’ve made toward meeting the objectives of our CSO Abatement Master Plan,” says Howard Carter, deputy director of public works. Previously, the city had CSOs, as well as many older homes with roof and basement drains still connected to the sanitary sewer system. As a result, wet-weather flows often overwhelmed the treatment plant, leading to discharge of untreated stormwater and sewage into the Saco River, and threatening the area’s water quality and sensitive shellfish beds.

Initially tackling the easiest projects, the city implemented BMPs (best management practices) to help contain trash and paper products being discharged directly to the receiving waters. After getting a handle on trash and debris, the next step was to convert the existing sewer system into storm sewers only, while installing new sanitary sewer lines. The old sewer system was constructed of brick, some more than 100 years old, making the transition quite challenging.

The hard work has paid off though, and the community’s Atlantic Ocean beaches and pristine Saco River continue to provide healthy recreation for citizens and visitors alike.

Different approach

During construction and imple-mentation of the plan, the city had to make a decision about the conversion of the two main downtown sewer lines. The staff decided not to tear them up, as that would disrupt business and utilities and create traffic nightmares.

Instead, they decided to tie the influent pipes leading from these lines directly into the wastewater treatment plant by way of a larger, single 24-inch polyethylene line. Upstream of the plant, the flow in this line passes through a new diversion structure that regulates wet-weather flows with self-activating, passive flow-control valves.

These units are 22 1/2-inch and 16-inch Type C Reg-U-Flo valves, supplied by Hydro Inter-national Inc. of Portland, Maine. By harnessing the energy in the flow, the valves operate with no outside power source and use no moving parts.

The valves allow a maximum of 5.2 mgd of the combined flow to pass through to the headworks at the treatment plant, and then divert any excess flow to a Storm King Advanced Hydrodynamic Vortex Separator (HDVS), also supplied by Hydro International.

The HDVS, 22 feet in diameter, combines floatables capture, high-rate sedimentation, mixing, and disinfection in a single compact vessel. An integral self-cleaning Swirl-Cleanse screen and Grit King grit collector capture floatables and grit and essentially provide preliminary and primary treatment of the excess storm flow. The HDVS unit also provides high-rate chemical disinfection, followed by dechlorination of the flow before discharge to the river.

The system achieves disinfection in a minimum contact time of only eight minutes, contributing to the compact size of the system. The HDVS has a peak design flow of 5.6 mgd, although it can effectively handle hydraulic flows higher than that.

Like the flow control valves, the HDVS has no moving parts, relying instead on the energy of the flow. It also has a series of baffles to produce the high-rate rotary flow through the unit. The vortex separator has achieved high levels of pollutant removal. “We’ve been very impressed; it’s a great tool to have in our CSO toolbox,” Carter says.

A SCADA system made up of Allen-Bradley PLCs (programmable logic controller) and Rockwell Automation software ties all elements of the system together, including the HDVS, the wastewater treatment plant, and satellite pumping stations. The system triggers automatic sampling of the stormwater influent and effluent flows. It also controls the chemical pumps associated with the system.

Help from history

The design of the Saco storm-water control system was not easy because storm flows are infrequent and unpredictable. “Treating storm-water isn’t a finite thing,” says Christopher J. Osterrieder, senior engineer with DeLuca-Hoffman Associates Inc. of South Portland, Maine, the engineering firm that worked on the Saco sewer project. “We had to be careful to only send enough water so that we can perform within the vortex separator’s rated range.”

To gauge the storm system needs, the city analyzed precipitation in the Saco River from the 1980s and ’90s, as well as a recent four-year period. Based on these findings, city officials felt confident they could estimate future storm events and design an adequate system. To fine-tune the system and achieve optimum operability, they used simulated flows of 4 to 20 mgd to help determine how the system would ultimately be run.

Maintenance of the HDVS is minimal. After storms, the dewatering pumps transfer all solids from the separator back to the treatment plant for disposal. The maintenance crew simply performs a quick washdown of the separator tank to prepare for the next storm.

“It’s very simple, and that’s a big thing for us,” says Carter. “We have a staff of eight and work one shift.” The staff operates 29 pump stations, handles system billing, and runs the treatment plant, so simplicity and efficiency are a must.

In addition to the staff that maintains the treatment plant and pump stations, two city employees perform routine maintenance of the collection system and CSO overflow structures. They use a 2100 Series Vactor sewer cleaner truck from Vactor Manufacturing.

Recording results

To help fund the CSO program, the city charges impact fees. Each new project pays a one-time CSO impact fee of $2,001, adjusted yearly according to the Consumer Price Index. There is also a one-time sewer impact fee of $2,700, based on estimated new-home usage of 185 gpd.

In short, explains Carter, any use that contributes new flow to the wastewater system is subject to impact fees. “The fees allow the city to recover a fair portion of the infrastructure improvements made to accommodate new uses,” he says. “Without the impact fees, existing tax and sewer rate payers would be asked to pick up the cost of providing additional improvements for new development.”

Since it went online in November 2006, Saco’s stormwater control system has been highly |successful. “We’ve had some large storms, and the system still performed very well,” says Carter. “We’re getting consistent results from varying degrees of storms.”

The system received a serious test in April 2007, when huge storms along the Maine Coast created wet-weather overflow conditions for five consecutive days. The system responded well, even though hit with instantaneous peak flows above 4 mgd. Results collected for all overflow events from startup of the system through February 2008, show:

• Average influent TSS of 135.2 mg/l and average BOD of 97.2 mg/l.

• Average effluent TSS of 46.4 mg/l and average BOD of 29.8 mg/l.

That’s a TSS removal rate of more than 69 percent and BOD removal rate of nearly 66 percent. In the end, that means less waste material entering the community’s sensitive water resources — and a cleaner environment and higher quality of life for all.



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