wastewater treatment

Microturbines and Hot water generator with digester in background

The Market

Wastewater treatment facilities (WWTF) that have, or are planning to install, anaerobic digesters are an excellent fit for combined heat and power applications. Because the digester produces “free” fuel, these applications, unlike others which must purchase fuel from a utility, are buffered against fuel costs. Once captured and appropriately cleaned, the biogas can be used to produce electricity using a variety of technologies including gas turbines, microturbines, fuel cells, and reciprocating engines. This electricity can be used to displace electricity purchased from the grid, to shave peak loads, and to potentially provide energy security at the facility. In addition, waste heat produced from these devices can be recycled back into the overall process for maintaining digester temperatures and/or for other processes in the plant such as drying. As a result, the following benefits can be realized with an effective CHP installation:

  1. Produces power at a cost below retail electricity.
  2. Displaces purchased fuels for thermal needs.
  3. Qualifies as a renewable fuel for green power programs.
  4. Enhances power reliability for the plant.
  5. Offers an opportunity to reduce greenhouse gas and other air emissions.

In California, 23 sites produce 38 MW of electricity. CHP for WWTP has not penetrated Nevada and Hawaii markets. It is important to note that many systems already use anaerobic digestion gas for boiler heating and so therefore care must be taken to insure that not too much ADG is diverted to CHP.

Technical Fit

Biogas is a by-product of the anaerobic decomposition of sludge at wastewater treatment facilities. As a rule of thumb, biogas flowing at a rate of 300 standard cubic feet per minute (scfm) can produce approximately 800 kilowatts (kW) of electricity along with more than 2.5 million British thermal units per hour (MMBtu/hr) of thermal energy1. With a capacity of 4.5M gallons/day, a typical WWFT can fuel a 100kW electric generator.
Depending on the type of digestion process, the amount of thermal energy required will vary. Mesophilic processes require operation in the 85° to 100°F temperature range, whereas thermophilic requires temperatures in the 120° to 135°F range. As a result, the thermal energy required must be carefully established.
In California, the electricity generated can be used to offset relatively high priced electricity from the grid and also to displace relatively high priced natural gas that would be otherwise needed to provide heat to the facility. Each wastewater treatment facility considering CHP will need to perform its own site-specific feasibility analysis to determine potential biogas generation rates, methods to compress, clean, and dry the biogas before combustion, and the potential cost benefits of generating onsite heat and electricity.

In many ADG applications, the gas produces is used to offset natural gas use in boilers.  As a result, care must be taken to insure total heating requirements are still met if some of this ADG is diverted to a DG/CHP system.

Strategic Issues

The use of gas from anaerobic digestion at wastewater treatment facilities has emerged as a possible opportunity to claim credits or garner additional funding for projects.  Similar benefits have arisen for capture and use of landfill gas, but generally, landfills do not have a major opportunity for use of waste heat whereas WWTF do.

Where to turn for additional information

A number of resources are available to assist in the process of evaluating the potential for CHP at your facility. Many of these are available on this website

Case Studies

Case studies can be an effective manner by which to evaluate how your facility might benefit from CHP. They can also provide information on lessons learned and steps that can be taken to maximize the success of your application. The PRAC has produced project profiles for a typical facility in California (Chiquita Water Reclamation Plant (Rancho Santa Margarita, CA)—120 kW microturbine CHP system):

Other case studies can be found at the California Energy Commission:

Midwest CHP Application Center website:

and at the CHP Partnership

Feasibility Studies/Project Assistance

A number of options are available for low cost or free feasibility assessments to help you understand how CHP might benefit your facility. The PRAC offers such assessments and more information can be found at:

Additional Information

Additional information can be found on the California Energy Commission website: http://energy.ca.gov/distgen/
and the Combined Heat and Power Partnership site: http://www.epa.gov/CHP/markets/wastewater.html

There are a number of technical societies whose membership deals with water and waste water treatment issues:

Information on how digester generated fuel is considered relative to renewable status is available. Using this fuel is considered to be eligible for renewable application benefits including low interest loans and higher rebates compared to non-renewable fuel systems:

California has adopted aggressive emissions standards for small scale generation devices. While regulations are in place for natural gas fired units, pending amendments may expand these regulations to include digester derived fuels as well. More information can be found at the CARB website:

_____________________________________________________

1 Opportunities for and Benefits of Combined Heat and Power at Wastewater Treatment Facilities, US EPA Combined Heat and Power Partnership Report, April 2007