Wastewater plant to Generate Heat, Power from Digester Gas

The Essex Junction Wastewater Treatment facility in Essex Junction, VT, has selected Northern Power Systems to engineer, build and install a $245,000 on-site power system

The Essex Junction Wastewater Treatment facility in Essex Junction, VT, has selected Northern Power Systems to engineer, build and install a $245,000 on-site power system that will burn methane gas produced by wastewater processing to generate electricity and heat for the facility. The new cogeneration system will produce over 400,000 kWh of electrical output per year, equivalent to 41% of the facility's current annual demand.

At the same time, the system will reduce the plant's CO2 emissions by over 500,000 pounds — the equivalent of eliminating 42 cars from the road per year. As part of its commitment to removing financing obstacles to such environmentally sound systems, Northern helped the Essex Junction facility obtain grant and rebate assistance for the project from various outside sources.

The Essex Junction Wastewater Treatment system will employ a new controls method — the first of its kind for use in a biogas cogeneration application — developed by Northern Power Systems for the project. The controls method will enable the facility to conduct peak shaving (reducing metered demand by boosting output kW during short, higher demand times) while monitoring the power and heat requirements of the site. The system is expected to yield a total annual electric utility savings of about $30,000.

The Essex Junction Wastewater Treatment facility processes approximately 1.7 million gallons of wastewater per day from the suburban Burlington communities of Essex Junction, Essex and Williston. The facility also runs a biogas anaerobic digester plant which produces 56,500 therms of methane gas per year. The plant flares 45% of this methane gas and burns the other 55% in a boiler to heat the digester tanks, accelerate decomposition of the solid waste, and kill off pathogens in the process biosolids. The new system will dramatically increase efficiencies by employing microturbines to burn 99.8% of the methane gas, a design that will eliminate the need to flare the previously unused portion. The electricity produced will power pumps, grinders and other process equipment.

Benefits, Savings

The Essex Junction plant will realize significant environmental benefits and energy cost savings by blending the methane with natural gas, which also serves as a back-up gas source in the event of digester maintenance or interruption of biogas supply. Modulating the mix allows the microturbines to conduct peak shaving to a greater extent than if running on methane alone, automatically increasing the output of the microturbines to meet the additional electrical needs. Blending also improves the reliability of the system by stabilizing the fuel input into the turbines.

Through cogeneration, the recovered heat from the turbines will be used to heat the digester tanks and will offset 100% of the need to run a boiler for this same purpose.

According to Dan Reicher, executive vice president of Northern Power Systems and former U.S. Assistant Secretary of Energy, the system developed with the Essex Junction facility has widespread applicability nationwide. Reicher, who led a $1.2 billion program to advance energy efficiency, renewable energy and distributed generation, said, "An EPA Clean Water Needs Survey indicates that there are some 3,300 wastewater treatment facilities similar to the Essex Junction plant in the United States. From delivering basic electricity, to cost reductions and reduced environmental impact, this type of system design delivers terrific value for anaerobic digestion wastewater treatment facilities."

The average cost per kWh produced by the system will be 2.46 cents (including annual maintenance expenses), yielding a 6.37-cent savings when compared to the current utility rate. In addition to the energy savings, peak shaving will accrue $4,500 in savings, thereby yielding a total annual electric utility savings of about $30,000.

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