How to Convert Manure Into Methane for Small Farms
Converting manure into methane gas, also known as biogas, can be a great way for small farms to manage waste and generate renewable energy. Methane digesters provide environmental and economic benefits by reducing greenhouse gas emissions, odor, and the volume of waste that needs to be disposed of. The methane produced can be used to generate electricity and heat or upgraded to natural gas standards for use as vehicle fuel. In this article, I will discuss the entire process of implementing a small-scale methane digester system on a small farm.
What is Methane Digestion?
Methane digestion is a natural process carried out by microorganisms that break down organic material like manure in an oxygen-free environment. This produces biogas, which is comprised mainly of methane and carbon dioxide. The decomposition happens inside an enclosed tank called an anaerobic digester. There are different digester designs, but they all provide ideal conditions for methane-producing bacteria to thrive.
Some key benefits of methane digestion include:
- Renewable energy production – The methane can be used to generate electricity and heat.
- Odor reduction – Anaerobic digestion significantly reduces odors from manure.
- Reduced waste volume – It decreases the amount of waste that needs to be handled by up to 85%.
- Nutrient recycling – The solid material leftover after digestion, known as digestate, is high in nutrients and can be used as fertilizer.
- Greenhouse gas reduction – Capturing methane prevents its release into the atmosphere. Methane is 25 times more potent than CO2 as a greenhouse gas.
Is My Farm Suitable?
Before investing time and money into a methane digester system, you need to assess if your farm is a good candidate. Here are some key factors to consider:
- Manure supply – You need a steady supply of manure waste to continually feed the digester. Most small farm digesters require at least 50-100 cows or 250-500 pigs.
- Space – Plan for the space needed for the digester, gas handling equipment, and digestate storage. This can range from 500 sq ft for small systems to a half acre or more for larger operations.
- Electricity access – If you want to generate your own electricity from the methane, you need access to electrical lines to connect to.
- Biogas use – Determine how you will use the methane – for electricity, heating, or potentially cleaned up to natural gas standards as vehicle fuel. This affects the gas handling equipment needed.
- Regulations – Check state and local laws, permits required, and utility interconnection policies if selling power.
Doing a full assessment upfront ensures your farm is well-suited before making a big investment.
Digester Designs and Components
There are a variety of digester system designs to choose from, which use different materials and configurations but follow the same basic process. The major components needed are:
This sealed, oxygen-free tank provides the environment for anaerobic digestion. Some main options are:
- Concrete tank – Most expensive option, but very durable.
- Steel tank – Corrosion can be an issue over time. Tank may need coating or lining.
- Plastic tank – Typically made of HDPE plastic. Cheaper than concrete or steel.
Piping and Pumping Systems
Pipes are needed to transfer manure into and out of the tank. Pumps move the waste through the pipes into the digester and transfer out the digestate. Valves control flow rates.
Gas Handling System
This system cleans and collects the biogas from the digester and either flares excess gas or sends it to an engine or boiler to be used for energy. Key components are gas wells, pipes, blowers, and scrubbers to remove impurities.
Generator or Boiler
These systems convert the methane into usable electricity or heat. Microturbines, fuel cells, reciprocating engines, or boilers are options.
Monitoring and Control System
Sensors, gauges, switches, and other instrumentation is needed to monitor conditions like temperature and pressure inside the digester and alert operators if parameters are out of optimal range.
Sizing Your Digester
Properly sizing your digester system is crucial for optimal performance. The main factors are:
- Number of animals – More daily manure input allows larger digester volume.
- Manure characteristics – Water content and biodegradable volatile solids determine gas production.
- Temperature – Heating the digester improves methane output in colder climates.
- Hydraulic retention time – The number of days waste stays in the digester; typically 15-30 days.
- Organic loading rate – The biological conversion capacity based on the manure input.
Work closely with an engineer throughout the planning and design process to ensure the system is sized correctly. Oversizing digesters leads to inefficiencies while undersizing causes instability.
Digester Operation and Maintenance
Operating and maintaining your digester properly is essential to maximize biogas production, manage costs, and prevent problems. Here are some key operation and maintenance tips:
- Monitor parameters daily – Check temperature, pH, gas production, and other factors to ensure stable digestion.
- Maintain consistent loading – Stick to regular manure feeding schedules and amounts. Drastic changes can upset bacterial processes.
- Remove grit buildup – Regularly remove grit and sediment that can clog pipes or damage pumps.
- Inspect equipment – Regularly check pumps, generators, blowers, etc. and replace worn parts.
- Clean out digestate – Remove stabilized digestate as needed to make room for fresh manure.
- Test digestate quality – Sample the nutrient content to determine best practices for field application.
- Keep records – Track operating parameters, maintenance, production amounts and usage.
Proper training for staff on digester operation best practices is highly recommended.
Costs and Financial Incentives
Installing a methane digester system requires significant upfront capital investment. Costs vary considerably based on factors like size, design, and equipment needs. However, there are also financial incentives available:
- Grants – Government and utility grants help offset installation costs. The NRCS and EPA offer farm digester grant programs.
- Tax credits – There is a federal investment tax credit of 26% for methane energy projects.
- Renewable energy credits – If generating electricity, income can be made from selling renewable energy credits.
- Electric sales – Revenue can be generated from selling excess electricity to the utility grid.
- Tip fees – Some farms accept off-farm food wastes for an additional feedstock revenue stream.
Work closely with financial and technical experts to fully assess project costs and funding options. The long term energy savings and environmental benefits make digesters a smart investment.
Implementing a methane digestion system requires research, planning, investment, and management. But it can be worth the effort for small farms to capture a valuable renewable energy resource from manure waste streams while managing odors and generating a nutritious fertilizer. Carefully assess if your farm is suited for anaerobic digestion and work with qualified experts to design, size, and operate your digester properly. With some upfront work, manure-based methane can provide sustainable energy and economic returns for years to come.