How to Build a Backyard Biogas Digester
Building a backyard biogas digester can provide a sustainable source of energy and fertilizer right in your own yard. Biogas digesters use organic waste to produce methane gas that can be used for cooking or generating electricity. The leftover waste material is an excellent organic fertilizer packed with nutrients for your garden.
Constructing a simple, low-cost digester is a fairly straightforward weekend project. With some basic materials and tools, you can build an anaerobic digester that will convert your food scraps and yard waste into renewable biogas and nutritious compost. In this comprehensive guide, I will walk you through the entire process of designing, building, and operating your own backyard biogas plant.
Choosing a Digester Design
The first step is deciding what type of biogas digester best suits your needs and budget. There are three main digester designs to choose from:
Floating Drum Digester
This simple digester consists of a cinder block or brick chamber covered by a floating metal drum. As gas is produced, the drum rises to store the biogas. Easy to build with basic materials, this is one of the most common small-scale digester designs.
Fixed Dome Digester
A dome-shaped chamber is built above ground and sealed, with an outlet pipe for the gas. No moving parts to wear out makes this a low-maintenance option. The concrete dome requires some masonry skills to construct.
An inflatable plastic or fabric bag sealed at the top and weighted at the bottom forms the digester chamber. Very inexpensive to build but less durable than other designs. Well-suited for warm climates.
Consider your construction skills, climate, and budget when deciding which design will work best for your needs. The floating drum digester is easiest for beginners to tackle.
Sizing Your Digester
Determine the digester size based on your available organic waste material and energy needs. Most backyard digesters range from 50-1000 gallons. To estimate your volume:
Gather 1 week's worth of all organic material (food scraps, manure, yard waste, etc)
Multiply this by 4-8 weeks to determine the minimum digester volume needed
Make sure to size up for future expansion. It's better to have excess capacity than limit your inputs. Plan for at least a 500 gallon digester for a household system.
Now it's time to start building. Here are step-by-step instructions to build a simple floating drum backyard biogas digester:
- 55 gallon metal drum
- Cinder blocks or bricks
- Shovel, trowel, mixing tools
- PVC pipes, valves, fittings
- Gas stove or appliance hookup
Clear a level site - Dig a pit the size of your planned digester chamber. Remove any sharp objects.
Lay cinder block base - Arrange blocks and mortar in a circular or square shape. Make sure surface is flat and level.
Install drum - Position cleaned drum in center of base. Check it is level and stable.
Attach fittings - Install inlet, outlet, and gas pipes through the drum wall using waterproof seals.
Build block chamber - Stack blocks around drum, leaving 1 ft clearance below drum rim. Mortar blocks together.
Install cover slab - Place a flat concrete slab on top of the block walls to seal the chamber.
Connect gas line - Run PVC gas line to stove or appliance, with valve and pressure gauge.
Seed with slurry - Mix manure and water into a slurry. Pour into digester chamber through inlet pipe.
Load organics daily - Start feeding the digester daily with food scraps, crop waste, manure, etc.
Monitor temperature and pressure - Check digester temperature stays ~95-100°F. Gas production will begin in 1-2 weeks.
Use biogas - Once pressure gauge shows gas production, open gas valve for use. Need ~0.5 psi to run stove.
Remove digestate - Regularly drain fermented slurry from outlet pipe to use as liquid fertilizer.
With simple operation and maintenance your home biogas digester will produce renewable cooking fuel, electricity, heat, and organic fertilizer for years to come!
Maximizing Biogas Production
To optimize the biogas output of your digester:
- Maintain internal temperature of 90-100°F
- Use an insulated chamber in cold climates
- Shred and mix inputs to increase surface area
- Achieve 6-8 week retention time
- Keep pH around 6.5-8.0
- Add crop residues high in carbohydrates
- Mix in animal manure for bacterial inoculant
Monitor factors like gas pressure, pH, and retention time to identify any issues reducing your biogas production.
While biogas systems are generally safe, proper precautions are needed:
- Methane gas is flammable - avoid sparks and open flames
- Do not operate appliances inside your home
- Check for gas leaks regularly with soapy water
- Wear gloves and wash hands after handling digestate
- Do not exceed 25% animal manure in digester inputs
Proper venting and gas monitoring equipment is crucial for safe operation. Always follow local building codes and permit requirements.
Frequently Asked Questions
What can I feed my biogas digester?
- Food scraps and waste
- Crop residues and garden trimmings
- Animal manure and bedding
- Sewage sludge and organic wastewater
Avoid meat, bones, dairy, oil, and processed food.
How much gas will it produce?
Expect 0.5-1 ft3 of biogas per ft3 of digester volume daily. So a 500 gallon digester may yield 250-500 ft3 per day.
What maintenance is required?
- Daily feedstock loading
- Draining fertilizer digestate
- Checking temperature and gas pressure
- Monitoring pH
- Cleaning the digester annually
Routine maintenance keeps your digester running efficiently.
Can the spent waste be used as fertilizer?
Yes, the digested organic matter is an excellent nitrogen-rich fertilizer for your garden and landscaping.
How long will a backyard digester last?
A well-built digester can produce biogas for 20 years or longer with proper maintenance and operation.
Constructing your own backyard biogas digester is a fun project that provides renewable energy and organic fertilizer. This comprehensive guide covers key design considerations, construction steps, operational guidelines, safety tips, and frequently asked questions about running a home biogas system. With basic DIY skills and materials, anyone can build and operate their own small-scale biogas plant. Not only will a home digester reduce your energy bills and environmental impact, but it will also provide a fascinating learning experience in circular sustainability.