Introduction
Building a manure-powered methane digester can provide numerous benefits for a farm, homestead, or small community. Methane digesters allow you to convert manure and other organic wastes into usable biogas for heat and electricity generation. I built my own small-scale methane digester to generate biogas on my homestead, and it has been an invaluable asset. In this article, I will walk you through the entire process of constructing and operating a basic methane digester system from start to finish.
Selecting a Digester Design
The first step is deciding what type of digester design you want to build. There are three main types of digesters:
Continuous Flow Digesters
These are the simplest and most common designs. Manure and waste are constantly added to the digester, displacing earlier material. Continuous flow digesters provide a steady supply of biogas. However, they require daily maintenance.
Batch Digesters
In a batch digester, new material is only added after the previous batch has fully processed. This results in periodic biogas production. Batch digesters require less daily maintenance but more effort when loading new batches.
Plug Flow Digesters
Plug flow digesters function similar to batch digesters, except new material is added to one end of the digester as processed material flows out the other end. This provides semi-continuous biogas production with less maintenance than continuous flow.
For small home-scale digesters, continuous flow designs tend to be the most practical option. Larger community or farm-scale digesters often utilize plug flow instead.
Sizing Your Digester
Properly sizing your digester is crucial. Undersizing will limit biogas output, while oversizing increases costs. Follow these steps to calculate digester size:
- Estimate daily manure input - For example, 2 dairy cows = 50 gallons per day
- Determine retention time - This is the time material stays in the digester. Aim for 15-30 days.
- Calculate tank size - Daily manure input x retention time = minimum tank size. For the example above with a 20 day retention time, 50 gallons x 20 days = 1000 gallon minimum tank size.
Size digesters conservatively to account for expansions or fluctuations in manure supply.
Choosing a Digester Vessel
The digester vessel holds the manure slurry during digestion. This vessel must be:
- Leak-proof - It must safely contain the corrosive digester slurry.
- Sealed - The vessel must have an air-tight seal to contain biogas pressure.
- Accessible - Consider openings for loading material and maintaining the digester.
On my homestead, I used a 1000 gallon polyethylene tank designed for water storage. However, other options include:
- Concrete tanks or pits
- Old bulk tanks from farms
- Heavy flexible bladders
- Fibreglass tanks
Digester Heating Systems
Methane digesters operate best between 90-100°F. Some form of heating is required to maintain these temperatures for optimum biogas production, especially in cooler climates. Heating options include:
- Solar thermal panels - Energy from the sun heats water that circulates through the digester. This is a renewable method, but requires solar access.
- Biogas burners - Using some biogas production to heat the digester. This is self-powering but consumes biogas.
- Electric heaters - Offer precise temperature control, but require grid power.
I installed two 4x8 foot solar thermal panels on my digester to provide heating using free solar energy.
Mixing the Digester Contents
Frequently mixing the slurry allows better contact between the bacteria and organic material. This improves biogas yields. Mixing methods include:
- Pumps - Recirculate digester contents with a slurry pump.
- Mixing paddles - Mechanical paddles mounted inside the vessel provide agitation.
- Gas injection - Bubbling biogas back through the slurry mixes the contents.
- Manual mixing - For small digesters, manually stirring with a rod periodically can suffice.
I mounted a mixing paddle connected to a small 1/8th horsepower motor to automatically churn my small digester's contents for 5 minutes hourly.
Plumbing the Digester
Proper plumbing allows loading and offloading the digester, recycling mixing liquid, capturing biogas, and heating. Key plumbing components include:
- Inlet pipe - Accepts fresh manure slurry. Install a valve to control intake.
- Outlet pipe - Removes spent material. Valve it to control retention time.
- Mixing recirculation - Pump liquid from outlet back to inlet for mixing.
- Heating system plumbing - Allows heating devices to transfer heat to digester contents.
- Biogas outlet - Safely vents captured biogas from the digester headspace.
Use piping sized for adequate flows and resistant to corrosion and heat. I used 2 inch PVC piping for my small digester's plumbing.
Monitoring Your Digester
Monitoring some key parameters allows optimizing your digester's performance:
- Temperature - Track digester temperature daily to control heating.
- Biogas Production - Measure biogas volume and methane concentration. Increase production indicates better digestion.
- pH - Test pH weekly. Ideal is 6.5-8. Adjust as needed by adding alkalizing agents or acid.
- Retention time - Periodically calculate retention time based on influent/effluent flows. Adjust to maintain ideal retention time.
I monitor temperature and biogas daily and pH weekly. This gives me excellent insight into my digester's health.
Using the Biogas
The biogas produced by digesters is mostly methane along with carbon dioxide and trace gases. Methane is the valuable fuel gas. Biogas has many useful applications:
- Heating - Biogas can be combusted in modified furnaces, boilers, and heaters.
- Electricity generation - Biogas can fuel engines linked to generators to produce electricity.
- Vehicle fuel - With cleaning, biogas can fuel modified natural gas vehicles.
- Cooking - Biogas can supplement other fuels for stoves and ovens.
I run my biogas through a basic biogas burner to help heat my workshop and affiliated buildings. During warmer months, I use the surplus gas to operate a small biogas generator for electricity.
Conclusion
Constructing and operating a compact manure-powered methane digester provides numerous benefits. Digesters allow producing renewable biogas fuel from waste products and require fairly minimal maintenance. This article has provided an overview of key considerations, components, and steps involved in successfully building a functional digester system scaled to home or small farm use. With adequate planning and preparation, anyone can build their own digester to enjoy the benefits of waste-to-energy.