How to Turn Manure into Electricity
Manure is an abundant resource on farms that can be turned into a valuable energy source. Converting manure into electricity, known as anaerobic digestion, offers many benefits:
Reduce greenhouse gas emissions – Manure emits methane, a potent greenhouse gas. Capturing it reduces emissions.
Generate renewable energy – The biogas can be used to produce heat and electricity on site.
Create fertilizer – Digested manure is an excellent fertilizer for crops.
Control odors and pathogens – The process reduces unpleasant odors and kills disease-causing pathogens.
This guide will cover the key aspects of planning and operating an anaerobic digester to extract energy from manure.
Choosing an Anaerobic Digester Design
There are several types of digesters available. The main options are:
Covered Lagoon Digester
- An anaerobic lagoon with a floating, insulated cover
- Low cost and simple to build
- Lower biogas production than other designs
Complete Mix Digester
- A heated, insulated tank with mixing equipment
- More efficient digestion and increased biogas output
- Higher capital cost but well-suited for larger farms
Plug Flow Digester
- A long, heated tank with no mixing
- Manure flows through sequentially enabling high solids digestion
- More efficient than lagoons but requires pumping of manure
Key Components of an Anaerobic Digester System
A complete digester system requires various components for feedstock input, processing, and energy utilization:
- Feedstock intake – For pumping manure slurry into the digester
- Pre-treatment – To remove coarse particles and adjust solids content
- Digester/reactor – Where organic material is converted to biogas
- Gas handling – To collect, store, and scrub the biogas
- Gas use – Equipment to utilize the biogas for heating, power generation, etc.
- Effluent storage – For holding the digested manure before field application
- Controls – For monitoring and controlling temperatures, flows, gas production, etc.
Proper design is needed to ensure adequate retention time for digestion, match the system scale to farm size, and enable efficient gas capture.
Optimizing Inputs for Best Biogas Production
To maximize biogas yields, optimize the feedstock entering the digester:
- Manure – Use fresh manure for highest biogas potential. Avoid over-dilution.
- Co-digestion – Adding food waste, fats, or crops can increase biogas output.
- Solid content – Adjust to 8-12% solids for optimal digestion.
- Particle size – Break down large fibers through grinding or maceration.
- Nutrients – Balance carbon, nitrogen, and trace elements to aid digestion.
- pH – Maintaining a neutral pH helps digestion. Add buffers if needed.
- Temperature – Heat input to 95-135°F for mesophilic bacteria or 135-160°F for thermophilic.
Monitoring inputs and digester conditions enables tweaking for peak biogas generation.
Utilizing the Biogas Produced
Raw biogas from manure is roughly 60% methane and 40% carbon dioxide, with trace gases. Before use, the biogas must be:
- Collected – via piping from the digester to a gas storage tank.
- Cleaned – by removing water, hydrogen sulfide, and particulates.
- Processed – compression and possibly separation of carbon dioxide.
The cleaned, methane-rich biogas can then be used for:
- Heat production – Biogas combustion in a boiler provides heat for digester heating, pasteurization, building heating, etc.
- Electricity generation – Biogas can fuel engines, turbines or microturbines to produce electricity, often with heat recovery.
- Vehicle fuel – After upgrading to pure methane, biogas can fuel modified vehicles or generate compressed natural gas.
Making Digested Manure Work for Your Fields
The manure exiting the digester, known as effluent, retains its fertilizer value for fields. Benefits include:
- Nutrient retention – Most nitrogen, phosphorus, potassium are still available.
- Improved fertilizer – More readily available nitrogen and decreased odor.
- Enhanced soils – Increased organic matter builds better soil structure and moisture retention.
- Weed seed destruction – Digestion kills many weed seeds present in raw manure.
- Reduced pathogens – Up to 99% of pathogens and parasites are destroyed compared to raw manure.
Proper storage and timed application based on crop needs enables fully utilizing the fertilizer in digested manure. Spreading costs are reduced compared to raw manure due to lower volumes and more liquid consistency.
- With proper planning and design, manure can become an on-farm energy resource via anaerobic digestion.
- Biogas production not only provides renewable power and heat, but also reduces greenhouse gas emissions from manure storage.
- Digested manure retains its value as a fertilizer for crops.
- A manure digester requires significant upfront investment and ongoing operation costs, but can provide long-term economic and environmental returns.
Converting manure to biogas energy takes coordination of digester components, optimized inputs, biogas usage, and field application of effluent. But the benefits of on-farm energy and improved manure management merit consideration of this renewable energy strategy.