How to Turn Cow Manure into Sustainable Electricity

How to Turn Cow Manure into Sustainable Electricity

Introduction

Turning cow manure into electricity may seem far-fetched, but it is actually a proven method of generating renewable energy. With methane digesters, the organic matter in manure can be converted into biogas, which contains energy-rich methane that can then be used to generate electricity.

Converting cow manure into power brings several key benefits:

  • It provides a sustainable energy source, reducing reliance on fossil fuels
  • It reduces greenhouse gas emissions from manure storage
  • It creates an additional revenue stream for farms by selling electricity
  • The process generates a nutrient-rich fertilizer as a byproduct

How Anaerobic Digestion Works

The technology behind turning manure into energy is called anaerobic digestion. Here’s an overview of the multi-step process:

Collection of Manure

The first step is to collect fresh manure from barns and pens on a daily basis. scrapers, vacuums, or flushing systems can be used to gather the manure and transport it to the methane digester. Generally, the more liquid-based the manure is, the easier it will be to process.

Anaerobic Digester

The manure is deposited into an airtight tank called an anaerobic digester. Inside the tank, naturally occurring microorganisms break down the organic matter in the absence of oxygen through a process called anaerobic digestion.

This produces biogas, which is primarily made up of methane (CH4) and carbon dioxide (CO2). The biogas also contains trace amounts of hydrogen sulfide (H2S) and other gases.

Methane Production

Methane is the primary component of biogas that makes it valuable as an energy source. Most manure biogas contains 55-65% methane, with the remainder being CO2 and other gases. The methane is produced when the microorganisms consume and digest components like sugars, starches, proteins, and fats in the manure.

Biogas Collection

The biogas produced in the digester rises to the top of the tank, where it can be continuously collected in pipes or flexible covers. From here, it is transported to a generator to be converted into electricity. Sometimes, biogas may be refined and concentrated into pure methane first.

Electricity Generation

The methane-rich biogas can be used to fuel a generator that converts the gas into electricity. This electricity can then be used to power equipment and facilities on the farm. Any excess electricity can be sold to the local utility grid. The electricity produced is 100% renewable energy.

Digestate Byproduct

After the anaerobic digestion process, the remaining manure byproduct is called digestate. The digestate has lower odor and pathogens compared to raw manure. It also still contains many key nutrients that can be used as a biofertilizer to support crop growth when applied onto fields.

Methane Digester System Designs

There are a few main types of anaerobic digester designs used for processing cow manure:

Covered Lagoon Digester

This low-cost digester consists of a large earthen lagoon sealed with a flexible cover, which traps the biogas. The watery manure mix is heated by the sun. These have low upfront costs but less biogas output in cold weather.

Plug-Flow Digester

In this system, manure flows as a continuous stream through a long, heated tank. The tank is usually made of concrete. The constant inflow pushes older material out the far end. It has a higher capital cost but produces more methane year-round.

Complete Mix Digester

With complete mix digesters, the manure is agitated to maintain a uniform consistency throughout the tank. This maximizes biogas production but requires more moving parts and energy inputs.

Attached-Growth Digester

This emerging design uses plastic media inside the tank to provide surface area for microbial growth and digestion. The increased concentration of microbes improves the breakdown of manure.

In determining digester type, factors like cost, climate, energy needs, feedstock, and labor requirements must be balanced. Most systems also have additional components like pumps, piping, gas storage, and flares.

Benefits of Manure Digestion Systems

Installing a methane digester brings several key benefits:

  • Renewable energy production – Methane digesters provide reliable on-farm electricity or heat from a sustainable source.

  • Reduced odor and pathogens – Digestion slashes nuisance odors from manure by up to 90% while eliminating disease-causing organisms.

  • Extra income – Money can be saved or earned from avoided energy costs or selling excess electricity to the grid under net metering laws. The biogas may also be converted to pipeline-grade methane for additional revenue.

  • Greenhouse gas reduction – Collecting methane from manure prevents its release into the atmosphere, where it would contribute 25 times more strongly to global warming than CO2.

  • Improved fertilizer – Digestate retains most nutrients for fields and has lower odor and weed seeds than raw manure.

Overall, manure digesters allow farms to manage waste more sustainably while gaining an additional income source and improving their public image.

Challenges of Adoption

Though digesters have clear benefits, some key barriers exist to their widespread adoption:

  • High upfront capital costs – Building a digester system can cost from hundreds of thousands to several million dollars based on size and type, presenting a hurdle for smaller farms. Grants and incentives can offset some costs.

  • Expertise required – Digesters require significant expertise to size properly, integrate with farm operations, and maintain. Lack of qualified staff can lead to underperformance or failure.

  • Consistent feedstock – Digesters require a steady daily volume of manure to maintain biogas output. Lack of manure during parts of the year decreases energy production.

  • Power purchase agreements – Securing a fair contract to sell excess electricity to the grid can be difficult in some areas.

  • System maintenance – Digesters have many moving parts and monitoring requirements, along with periodic maintenance needs. Lack of upkeep can lower gas yields.

However, the technology is improving and adoption is accelerating as more farms experience successes. Government incentives, private partnerships, and higher energy prices also improve the investment case for digesters.

Case Study: Vermont Farm’s Methane Digester Success

Pleasant Valley Farm in Richmond, Vermont operates a 900-cow dairy. In 2005, with assistance from state and federal grants, they installed a complete mix methane digester system made by GHD.

Benefits realized:

  • Offset 50% of farm’s power usage – They generate 125 kW per hour for on-farm facilities and equipment.

  • Prevent methane emissions – Digester avoids over 2,500 tons of CO2-equivalent greenhouse gases annually.

  • Fertilizer production – They process 2.5 million gallons of liquid digestate and 1,500 cubic yards of solid digestate for field application each year.

  • Revenue generation – They sell excess electricity to their utility, generating over $100,000 in annual income.

With a well-run system, their digester has been a great success, providing environmental, operational, and financial gains.

Conclusion

Though digesters require significant investment and expertise to implement, they can provide farms with an extremely valuable source of sustainable energy from an abundant waste product in cow manure. With proper planning and maintenance, methane digesters are primed to play an expanding role in energy generation and waste management on dairy and livestock operations in the future. Their benefits are too substantial for farms and communities to ignore.