Converting manure into methane gas, also known as biogas, is a sustainable way to generate renewable energy and deal with agricultural waste. Methane can be used as a fuel for heating, electricity generation, and even transportation. The process of anaerobic digestion breaks down organic matter like manure into biogas and nutrient-rich digestate, which can be used as fertilizer.
In this comprehensive guide, I will walk through all the key steps involved in building a manure digester to produce methane on a small or large scale.
Overview of Anaerobic Digestion Process
Anaerobic digestion utilizes microbiological processes to break down organic material in an oxygen-free environment. Here are the key stages:
Hydrolysis – Complex organic matter like carbohydrates, fats, and proteins are broken down into simpler soluble organic compounds like sugars by hydrolytic bacteria.
Acidogenesis – The soluble organic compounds are further broken down into organic acids, alcohols, hydrogen, and carbon dioxide by acidogenic bacteria.
Acetogenesis – The organic acids and alcohols are converted into acetic acid as well as carbon dioxide and hydrogen by acetogenic bacteria.
Methanogenesis – Finally, methanogenic archaea convert the resulting products into biogas which contains approximately 60% methane and 40% carbon dioxide.
The overall chemical reaction can be summarized as:
Organic matter + H2O → CH4 + CO2 + NH3 + Trace Gases
Key Components of a Manure Digester
Constructing a manure digester system requires various components that work together to create optimal conditions for anaerobic digestion.
Feedstock Storage and Handling
- A covered manure storage pit collects and contains the animal waste feedstock prior to digestion.
- A mixing pit with choppers, augers or pumps prepares the feedstock for optimal digestion.
This sealed, oxygen-free chamber provides ideal conditions for the anaerobic digestion process. Key types of digesters:
- Covered lagoon – Least expensive but low biogas production.
- Complete mix – Manure is regularly agitated to optimize bacterial activity.
- Plug flow – Manure moves through a long heated chamber. More efficient than complete mix.
- Fixed film/solid state – Manure passes over fixed media to allow attached bacterial growth.
- A gas collection system gathers the methane biogas.
- A desulfurization system removes harmful hydrogen sulfide from the gas.
- A filtration system cleans the gas.
- Compression and storage pressurizes the gas for storage and use.
- Separation equipment – Separates liquid effluent from solid digestate fibres.
- Digestate storage – Safely stores separated effluent and solid digestate.
- Land application equipment – Spreads digestate on fields as a biofertilizer.
Combined Heat and Power (CHP) System
- An engine or turbine generates electricity by burning the methane.
- Excess engine heat is captured and used to maintain digester temperature.
Sizing and Design Considerations
Properly designing a manure digester requires evaluating key factors:
- Manure production – Determine amount of daily feedstock from number of animals.
- Manure characteristics – Analyze manure composition, solid content, and consistency.
- Digester sizing – Size digester to hold minimum 15-30 day retention time to allow full digestion.
- Energy needs – Estimate heating needs and desired methane production based on energy requirements.
- Climate conditions – Account for ambient temperatures which affect system design. Colder climates need more heating.
Use these factors to select digester type, size chambers, specify pumps and piping, size gas handling and CHP equipment.
Construction and Commissioning
Digester construction requires standard earthworks, concrete work, piping, electrical, and finishing. Key steps include:
- Excavate and pour concrete digester tanks and pits.
- Install piping, pumps, heat exchangers, CHP unit.
- Monitor for leaks during hydrostatic testing.
- Inject heated water and manure slurry to pre-heat system.
- Inoculate digester with existing manure digestate to seed microbiology.
- Monitor temperature, pH, gas production to track startup digestion.
- Connect CHP unit once gas quality and quantity are adequate.
Proper startup can take 2-8 weeks before full gas production and electricity generation commences.
Operation and Maintenance
Ongoing operation and maintenance is crucial to ensure optimal performance and safety:
- Feedstock management – Collect manure daily, maintain proper solids content, chop/mix as needed before feeding to digester.
- Digester monitoring – Track temperature, pH, gas production, consistency. Make adjustments as needed.
- Equipment maintenance – Inspect, service and repair pumps, motors, CHP unit per schedule.
- Safety – Monitor methane content in air, ensure proper ventilation, wear gas detection devices when inspecting enclosed spaces.
- Effluent management – Handle and land apply digestate as biofertilizer based on nutrient management plan.
Benefits of Manure Digesters
Installing a manure digester on a farm offers many benefits:
- Renewable energy – Methane can displace fossil fuels for heating, power generation.
- Odor control – Digestion reduces unpleasant odors from manure storage.
- GHG reduction – Avoid methane emissions, displace fossil fuel use.
- Pathogen reduction – Digestion kills pathogens improving safety of effluent.
- Biofertilizer – Digestate contains more available nitrogen compared to raw manure.
- Revenue – Onsite energy use or sales of excess electricity can provide income.
With proper planning and execution, manure digesters represent a sustainable way for livestock farms to manage waste, generate energy, and improve productivity. The future is bright for this expanding technology.