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Anaerobic Digester Mechanical Systems: Optimizing Performance

Thursday | January 6, 2022
All News

Anaerobic digesters (AD) convert biodegradable organic materials into biogas and produce a residual effluent (digestate) that can be put to beneficial use. This requires combining a chemical process with a mechanical system. The mechanical system primarily moves materials through the chemical process using pumps, pipes, mixers, blowers and compressors. It's important to keep this complex system in good working order.

Digester tanks

Digestate pretreatment might involve mechanical solids separation equipment such as screw presses or vibrating screens. Digester tanks are made of concrete or steel. Regularly check for leaks, cracks or other evidence of structural problems. Monitor for the accumulation of digester bottoms. Regularly check the surface of heating coils for any slurry caking. If heating coil pressure builds up, check heat exchanger inlet strainers for fouling.

Pumps

Three types of pumps are used in AD systems — centrifugal, chopper and positive displacement. Centrifugal pumps use an impeller to fling low solids influent through the process. If a cutter is added to the centrifugal pump inlet, it becomes a chopper pump that grinds down the solids into more uniform particle sizes before pumping them. A positive displacement pump uses rotary lobes or a piston to move small slugs of material through the system. It's a good idea to have at least one redundant pump.

Pipes

Repairing pipe leaks and keeping them from plugging is critical. Use a soapy water solution to check for leaks at pipe joints, fittings, valves and equipment connections. All valves should be kept serviceable. Keep an eye out for corrosion.

The AD tank operates under pressure, and pipe blockage increases pressure. An automatic pressure relief valve (PRV) should be installed to relieve overpressure. The PRV should be connected to an igniting flare to burn any released biogas. Downstream operations may require installing a vacuum relief valve to prevent an excess vacuum from occurring.

Biogas is naturally saturated with water vapor. Piping should be fitted with one or more self-dumping condensate traps to remove any condensed water.

Mixers

Mixing is important to maintain uniformity in the physical properties of the influent and to achieve proper chemical characteristics. It primarily prevents the accumulation of settled solids. Submersible mixers are typically mounted to rails or lifts attached to the tank sidewall. External mount mixers are located horizontally outside the tank on sidewalls or vertically on digester covers. Jet mixers continuously remove liquid and return it at high pressure through a series of nozzles using a separate pump.

Regularly check rotary mixers for leaking through the mounting seal, and keep the gearbox lubricated. Monitor the level of scale or cake on the blades and note missing or loose blades.

Blowers and compressors

Biogas is primarily composed of methane and carbon dioxide. It may also contain small amounts of water vapor, hydrogen sulphide, and ammonia. For onsite use, blowers or compressors are used to increase the biogas pressure. Keep an eye on specified flow rates, inlet and outlet pressures, and temperatures. Compressor oil systems require special attention to oil levels and replacement intervals. Hydrogen sulphide is typically removed via adsorption into solid media, by a biological scrubbing system, or by a chemical scrubbing system.

Consult the U.S. Environmental Protection Agency's Anaerobic Digester/Biogas System Operator Guidebook for more detailed information including details on the post-processing of biogas into renewable natural gas.

Dedicated to exceeding member's expectations for safe, reliable, efficient service while being a good citizen in our communities.

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1800 West Washington Street 

P.O. Box 440

Mount Pleasant, IA 52641

To REPORT AN OUTAGE CALL

319-385-1577 | 866-242-4232

Access Energy Cooperative is an equal opportunity provider, employer and lender. To file a complaint of discrimination write USDA, Office of Civil Rights, 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call 800-795-3275 (voice) or 202-720-6382 (TDD).

©2022 Access Energy Cooperative. Safe, reliable, efficient service and a good citizen in our communities.

  • Home
  • My Cooperative
    • About AEC
      • About Us
      • Our Staff
      • Board of Directors
        • How to Run for the Board of Directors
          • Nomination Process Begins for 2021 Board Election
      • Monthly Magazine
      • Service Area
      • Annual Meeting
        • Election of Directors
      • Cooperative Principles
      • Where Does Your Power Come From?
      • History of Co-ops
      • About Touchstone Energy
      • Employment Opportunities
    • Policies
      • Articles of Incorporation
      • Bylaws
      • E-Business Privacy Policy
      • Identity Theft Protection
      • Privacy Policy
      • Rights and Remedies
      • Statement of Nondiscrimination
    • Member Info
      • Dividends
        • Treasure Hunt
      • What does it mean to be a member?
  • Your Account
    • Payment Options
      • Bank Draft
      • Budget Billing
      • Credit/Debit Card Payments
      • Payment Drop Box
    • Billing
      • Your Meter
      • Questions About Your Bill
      • Understanding Your Bill
    • IA Energy Assist Program (LIHEAP)
    • Moving/Address Changes
    • Rates
    • Dividends
  • Residential & Business
    • Residential
      • Residential Rebates
      • Rates
      • Products Available
      • Financing
      • Energy Audits
      • Energy Saving Ideas
    • Business
      • Business Rebates
      • Rates - Commercial & Industrial
      • Power Quality
      • Services
        • Business Energy Audits
        • Thermal Imaging
        • Questline
        • Business Energy Advisor
        • BizConnections
        • Energy Saving Ideas
  • Safety
    • General
      • Blinking Lights
      • Restoring Power
      • Safety Tips
      • Maintenance Programs
      • Safety Equipment
      • Safety Quiz
      • Frequently Asked Questions
    • Safety Policies and Procedures
      • Grain Bin Clearance
      • Tree Trimming
      • Call Before You Dig!
  • Community
    • Legislation
      • Current Legislative Issues
      • National Legislative Issues
      • Contact Your Legislators
    • Economic Development
      • Economic Development Partners
      • Revolving Loan Fund
    • News
      • Coop News
  • Co-op Programs
    • Programs for Youth
      • Access Energy Cooperative Scholarship Program
      • Summer Work and Internships
      • Youth Tour Contest
      • Kids Zone
    • Tools For Schools
      • Teachers Zone
      • Get Charged!
      • Safety Demonstrations
    • General
      • Photo Contest
      • RECare
      • Co-op Connections
      • Together We Save
      • Lineworker Scholarship Application
    • International Assistance
  • Going Green
    • General
      • A Balanced Fuel Mix Promotes Power Reliability
      • Electric Vehicles
      • Energy Saving Ideas
      • Programs to Help You Save Energy
      • Access to Green Energy Program
      • Environmental Partners
      • Rebates and Low Interest Loans
      • Solar Energy Generation
    • Wind Energy Info
      • Questions to Ask Vendors
      • Small Wind Systems
      • Tariff Requirements
      • Ten Steps to Wind Systems
      • Wind FAQs
  • Outage Center