Anaerobic Digestion

Anaerobic DigestionProposed new facility at the Piscataway Wastewater Treatment Plant in Southern Prince George’s County that will convert biosolids from several WSSC wastewater treatment plants (Seneca, Damascus, Parkway, and Piscataway) into electricity. Proposed digestion and cogeneration improvements at the Piscataway wastewater treatment plant will 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”).

Decomposing biosolidsCogeneration is the simultaneous generation of electricity and heat that is normally discarded during electrical generation. Cogeneration is a much more efficient form of electric generation because no heat is wasted, as in conventional power plants.

WSSC’s Seneca, Piscataway, Damascus, and Parkway Advanced Wastewater Treatment Plants (AWWTPs) currently produce approximately 70,000 tons of biosolids per year. Biosolids are AWWTP sludge that has been treated to remove odors and disease-causing bacteria and allow beneficial reuse, typically as a soil fertilizer.

WSSC is considering anaerobic digestion with cogeneration as an alternative to the currently used chemical intensive lime stabilization process to treat the sludge. Anaerobic digestion is a biological process that converts a portion of the 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 reduce the energy consumption at the Piscataway AWWTP to zero entirely.
  • 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 (a biosolid that has had a higher standard of treatment and doesn’t require special management practices to protect the public) using a process preceding digestion called Thermal Hydrolysis, which render the sludge more readily digestible by breaking down the cellular structures of the solids thus allowing more readily and thoroughly digest the material. 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 is estimated to cost between $107 and $117 million. The baseline cost of existing lime stabilization plant upgrades that would have been necessary without the AD/ CHP project would have cost $50 million, making the net cost $57-$67 million. If WSSC can obtain public or private funding for the facility, the cost will be even less.

So far, WSSC has received $570,900 in federal funding. This provided for the feasibility study/conceptual design to determine the economic, environmental, and financial viability. It allowed WSSC to investigate potential anaerobicdigestion (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 was completed in December 2011. The results of the study were favorable and capital funding has been made available. Based on the approval of both County Councils WSSC is expected to proceed with the detailed engineering and construction of a commercial project.

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