Research Anaerobic Digestion

Proposed digestion and cogeneration improvements at Seneca and Piscataway wastewater treatment plants may reduce energy use, carbon footprint, and cost

Understanding the Issue

Anaerobic Digestion is the natural decomposition of organic materials by microbes that thrive in an oxygen free environment (An–Aerobic = “Without-Oxygen”)

Cogeneration is the beneficial utilization of heat that is normally discarded during electrical generation.

WSSC’s Seneca and Piscataway Advanced Wastewater Treatment Plants (AWWTPs) currently produce approximately 30,000 tons of biosolids per year.  Biosolids are AWWTP sludge that has been treated to allow beneficial reuse as a soil fertilizer. 

WSSC is considering anaerobic digestion with cogeneration as an alternative to the chemical intensive lime stabilization process currently used to treat the sludge.  Anaerobic digestion is a biological process much like human digestion that converts waste sludge to methane gas.  This methane is in turn used to power clean-burning engines, producing electricity and heat needed for the digestion process.

What are the Benefits of the Upgrades?

• Provide a Source of Renewable/Green Power.  WSSC’s AWWTPs are large energy consumers.  These upgrades could cut the energy consumption at these AWWTPs in half. 
• Reduce the Amount of Biosolids Produced.  Anaerobic digestion would further cut biosolids production in half and eliminate chemical lime purchases, currently required for biosolids treatment.
• Improve the Quality of Biosolids Produced.  Lime stabilized biosolids are prone to odors and are currently distributed as a Class-B product.  Class-B biosolids rely on a combination of treatment and best management practices to protect public health.  It is possible to produce Class-A biosolids using higher temperature digestion; these superior biosolids do not require product-handling safeguards to protect the public from pathogens.  In addition, anaerobic digesters can also convert restaurant grease waste to additional power – providing an environmentally sound reuse opportunity for a nuisance waste that currently clogs County sewers and causes overflows.
• Significant Reduction in WSSC’s Carbon Footprint.  Generating heat and electricity using the methane produced by the process is carbon neutral. The upgrades will dramatically reduce the electricity purchased and the truckloads of biosolids leaving each plant while eliminating current lime purchases.  All of these improvements reduce the carbon footprint of each AWWTP, roughly cutting the overall carbon-dioxide-equivalent emissions in half.

What’s the Plan?

The overall project for both AWWTPs is estimated to cost between $30 and $40 million.  So far,   WSSC has received $570,900 in federal funding.  This will provide for the feasibility study/conceptual design to determine the economic, environmental, and financial viability.  It will allow WSSC to  investigate potential anaerobic digestion (AD)/combined heat and power (CHP) technology alternatives to optimize gas and electricity production and residual solids volume reduction, and recommend conceptual design/process for commercial application at each AWWTP.

The study/conceptual design phase is projected to be completed by the summer of 2011.  At that time, if the results of the study are favorable and funding is made available, WSSC will proceed with the detailed engineering and construction of a commercial/demonstration scale project.