December 2010, Vol. 22, No.12

Plant Profile

Village of Essex Junction (Vt.)

PlantProfile_Pic

Location: Village of Essex Junction, Vt.
Startup date: 1964
Service population: 30,000 estimated
Number of employees: 6
Design flow: 12,500 m3/d (3.3 mgd)
Average daily flow: 7600 m3/d (2.0 mgd)
Peak flow: 17,800 m3/d (4.7 mgd)
Annual operating cost: $1.5 million

The operators in Essex Junction, Vt., run their wastewater treatment plant like a business. They work hard to justify the plant’s budgets and facility needs, seek improvements and upgrades that make treatment more effective and less costly, and take advantage of opportunities to collaborate with other towns and organizations.

The Village of Essex Junction Wastewater Treatment Plant is a 12,500-m3/d (3.3-mgd) conventional activated sludge plant that achieves seasonal nitrification and year-round chemical phosphorus removal. Its processes include flow equalization, primary settling, biological aeration, secondary clarification, effluent filtration, and chlorination/dechlorination.

The plant’s solids are thickened and then anaerobically digested. The digested solids are dewatered by mobile high-solids centrifuges and land-applied or landfilled. The biogas generated by the digester is put to work.

Power pioneer

About 8 years ago, the plant became the first small facility in New England to install a methane-fueled microturbine combined heat and power (CHP) system. Today, the plant consistently and reliably generates 35% to 40% of the power it needs to operate. The CHP system is unique in that it can blend methane and natural gas as needed for the turbines or for the digester process. The heat from the energy-generating turbines’ exhaust is also captured and put to use heating the anaerobic digester. The system provides all of the necessary heat for the digester.

Essex Junction enhances methane production by adding other wastestreams to its anaerobic digester. The plant adds in fats, oils, and grease; emulsified oils from an alternative-fuel end user; and difficult-to-manage liquid waste from commercial facilities.

As a side benefit, these additions of tough-to-handle wastes seem to have enhanced the plant’s anaerobic digestion. The facility has been managing these difficult wastes by direct digestion successfully for more than 10 years. With recent process control modifications, final biosolids production is down despite increases in load and flow to the facility.

Energy management is a focus for the plant because it can be used as a cost-cutting and process control tool. The CHP installation allows for full use of the digester’s methane, with greater than 80% recovery of the energy input to the system.

Ongoing upgrades

During the past decade, other projects throughout the plant have optimized treatment and increased capacity. In 1999, for example, Essex Junction installed a double-basin flow equalization tank and the associated pump station. This installation allowed for peak flow control both on the plant site and from remote pumping stations.

The equalization tank also enabled operators to control peak flows through the plant’s secondary clarifiers and effluent filters. Managing flows through these units during peak events is the operators’ greatest challenge, according to Jim Jutras, water quality superintendent.

At the time the facility was constructed, design standards for nitrifying activated sludge processes were relatively new. By today’s updated standards, the plant’s secondary clarifiers are undersized. Essex Junction currently operates at or near its secondary clarifier solids loading and hydraulic capacity.

Maintaining control of effluent solids during peak flow conditions sometimes requires operator intervention. Staff members successfully use various process control methods, such as switching the process flow scheme to contact stabilization, or diverting a small portion of flow to empty and available process tanks, or controlling flow equalization output to anticipate flow surges, to avoid solids loss during peak flows and successfully maintain effluent compliance.

Now, the plant has reached a flow where an upgrade is the best solution. Essex Junction is in the design stages for an upgrade to its secondary train to enhance solids-settling performance and nutrient removal, as well as add redundancy to the system.

An already completed change that added process capacity was the installation of gravity belt thickeners (GBTs) in 2004. Adding GBTs prior to anaerobic digestion provided 760,000 L/d (200,000 gal/d) of freed capacity for the plant. Prior to the GBT installation, waste activated sludge was cothickened in the primary clarifiers. By modifying the in-process cothickening of solids to an off-line process, the state regulatory agency allowed for the increased capacity. This installation also reduced hydraulic loading on the anaerobic digester, resulting in increased digestion performance and capacity.

People power

The Essex Junction staff members also seek out ways to enhance the business of treating wastewater that have little to do with process control. For example, each year, the plant tries to host one student intern from a local college or university. The student and the plant work to find a mutually beneficial project. The student studies a topic that meets the needs for his or her field of study, and the plant gets a chance to evaluate operations that might otherwise go unquestioned.

Most recently, a business student from Saint Michael’s College (Colchester, Vt.) reviewed the financial performance of the plant’s CHP installation after 7 years of operation. He verified that the system met its initial financial objectives and paid for itself in 7 years.

Essex Junction also found a way to increase purchasing power for itself and its neighbors. The village formed a regional chemical purchasing consortium with 12 other water and wastewater treatment plants spread over eight communities. The consortium enables all of the members to participate in the annual quotation process for bulk chemicals together. This ensures an economy of scale for each member; smaller facilities benefit from the volume purchasing power, and larger facilities benefit by consolidated purchase volume with price savings and stability.

Recognition

Earlier this year, U.S. Environmental Protection Agency (EPA) Region 1 Administrator Curt Spalding presented the plant with a 2009 EPA Region 1 Wastewater Treatment Plant Excellence Award.

This awards program recognizes the employees of publicly owned wastewater treatment plants for their commitment to improving water quality with outstanding plant operations and maintenance and ongoing operator training. This program has found that, particularly with the smaller facilities, conscientious operators and staff continue to perform exceptionally well with limited resources, according to EPA.

“The professionals operating these wastewater treatment plants, as well as the municipalities and the state environmental agencies that support them, are essential to keeping our environment healthy,” Spalding said at the award ceremony. “I am proud to give them the credit they deserve.”

 

Sewer maintenance and repair

The operators at the Village of Essex Junction (Vt.) Wastewater Treatment Plant also lend their talents to operating and maintaining the collection system that delivers flow to the plant. In addition to maintaining and operating the village’s sewers, the operators also perform the control and alarming duties for major pumping stations in each of the three towns the plant serves.

They recently finished a 5-year project to identify and complete needed collection system repairs. In 2005, the village paid off a sewer bond and decided to reinvest the money that had gone to those payments to assess the village’s infrastructure and maintenance needs.

Operators recorded and rated the condition of the community’s sewer assets using digital video and Pipeline Assessment and Certification Program ratings developed by the National Association of Sewer Service Companies (Owings Mills, Md.).

During the past year, the village leveraged its savings with available stimulus money to make the needed repairs. They used cured-in-place pipeline repairs to fix the pipeline defects found in the complete collection system evaluation. In fact, they were able to perform all repairs identified for near-term repair. The final result is that all sewer lines in the collection system now have a rating of 3 or better, which means they should require no repairs for 10 to 20 years or longer

 

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