Cow manure has long been seen as a waste product, but with rising energy costs and climate change concerns, many are now looking at cow manure as an abundant source of renewable energy. As a dairy farmer, I have access to a lot of cow manure each day. I realized that instead of just disposing of it, I could use anaerobic digestion to produce biogas and create electricity and heat from this free and renewable fuel source.
How Anaerobic Digestion Works to Produce Biogas
Anaerobic digestion is a natural process in which microorganisms break down organic matter like cow manure in an oxygen-free environment. This produces biogas, which is comprised primarily of methane and carbon dioxide.
The anaerobic digestion process involves three main steps:
Hydrolysis – Complex organic polymers like carbohydrates are broken down into simple sugars, amino acids, and fatty acids.
Acidogenesis – The products of hydrolysis are further broken down by acidogenic bacteria into simple organic acids like acetic, propionic, and butyric acids.
Methanogenesis – Methane-producing archaea convert the organic acids into methane gas and carbon dioxide.
The entire anaerobic digestion process takes several weeks, during which the organic material is converted to biogas containing approximately 60-70% methane.
Key Benefits of Using Cow Manure for Biogas Production
Using cow manure for biogas production through anaerobic digestion provides many benefits:
Renewable energy source – Biogas can be used to generate electricity and heat in a sustainable way.
Reduced greenhouse gas emissions – Capturing methane from manure prevents its release into the atmosphere as a potent greenhouse gas.
Odor reduction – Anaerobic digestion significantly reduces odors from untreated manure.
Soil enhancement – The nutrient-rich digester effluent can be used as a fertilizer.
Additional revenue stream – Biogas can be used on-site or sold to produce an income from waste.
How to Set Up an Anaerobic Digester on Your Dairy Farm
Setting up an anaerobic digestion system on your dairy farm involves several key steps:
Determine Digester Type
The main types of digesters suitable for dairy farms are:
Covered lagoon digester – A large covered lagoon holding manure with flexible cover to collect biogas. Simple and low-cost, but limited gas production.
Complete mix digester – Manure is mixed with heat and recycled effluent. More complex and costly but with higher gas yields.
Plug flow digester – Manure moves through a heated, underground concrete tank. Higher capital cost but efficient gas production.
I chose a complete mix digester for the high biogas production potential.
Size the Digester Properly
The digester needs to be sized based on the number of cattle on your farm and the amount of manure produced. Common sizing factors are:
- Lagoon digester – 0.06 m3 per kg of volatile solids
- Complete mix – 0.04 m3 per kg of volatile solids
- Plug flow – 0.034 m3 per kg of volatile solids
With 150 head of cattle, I needed a 600 m3 complete mix digester.
Maintain Proper Temperature
Anaerobic bacteria operate best between 30-40°C. The digester contents need to be heated using the biogas or external heat sources.
I use a biogas boiler to recycle heat and maintain 38°C.
Pretreatment like solids separation and dilution improves digester performance.
- Solids separation increases retention time and biogas yield.
- Dilution with water speeds up hydrolysis.
I added a solids separator and mix manure 1:1 with water before pumping to the digester.
Choose End Use for Biogas
Biogas can be used in many ways:
- Generate electricity using generators or microturbines
- Fire boilers to produce heat for farm, greenhouse, or district heating
- Upgrade to biomethane for use as vehicle fuel or injection into natural gas grid
I installed a combined heat and power unit to produce 80 kW of electricity and 150 kW of heat.
Implementing an anaerobic digester to produce biogas from cow manure has allowed me to generate renewable energy and additional income from waste. With proper planning and system sizing, dairy farmers can tap into the potential of cow manure as a sustainable fuel source. While the upfront investment can be significant, the long-term benefits make anaerobic digestion of cow manure a worthwhile endeavor.