I've been looking for ways to lower my energy bill and reduce my environmental impact. As a dog owner, I realized that my pet's poop could be a surprising source of renewable energy. Dog poop contains methane, which can be converted into electricity and heat using a home biogas digester.
What is a Biogas Digester?
A biogas digester is an anaerobic system that breaks down organic matter like food scraps and animal waste. Through the process of anaerobic digestion, bacteria transform the organic material into biogas - mainly comprised of methane and carbon dioxide.
The biogas can then be used to generate electricity and heat. The solid matter leftover after digestion is an excellent organic fertilizer that I can use in my garden.
Benefits of a Home Biogas Digester
Installing a small biogas digester in my backyard has many advantages:
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It allows me to generate renewable energy from my dog's poop and other organic waste. This lowers my electricity bill and reduces my carbon footprint.
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It reduces the amount of waste going to landfills. Almost anything that was once alive can go into the digester.
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The nutrient-rich fertilizer leftover from the process helps my garden grow.
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Digesters reduce greenhouse gas emissions. Methane from landfills is 25 times more potent than carbon dioxide!
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I can use the biogas for cooking, heating water, or powering appliances. The heat generated can warm my house or greenhouse.
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My digester requires little maintenance. It passively converts waste into energy without needing my constant attention.
Types of Small Digesters
There are several types of digesters available for home use:
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Floating drum digesters are the most common small-scale system. Gas pressure pushes up a floating drum that stores the biogas.
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Fixed dome digesters have an immovable enclosed dome where the gas is stored. The pressure has to rise high enough to force gas through the exit pipe.
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Plug flow digesters are long, heated containers where waste flows through slowly. This type is efficient but requires more temperature control.
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Garage-style digesters are designed for colder climates. They can be placed in a greenhouse or buried underground to maintain temperature.
For my backyard digester, I plan to build a simple floating drum system. This low-cost, low-tech design seems appropriate to start out with as a beginner.
Choosing a Location
When selecting a location for my biogas digester, there are several factors I need to consider:
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It should be situated close enough to my house so that piping the gas is convenient.
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The site needs ample space for the size digester I want to build.
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I'll need access to water to mix the feedstock for the digester.
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It works best placed in full or partial sun and sheltered from cold winds. This helps keep the interior warm.
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Good drainage and ventilation are essential.
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Turning the effluent into my garden should be easy.
After surveying my property, the south-facing area below my deck seems like the ideal spot.
Sizing the Digester
Properly sizing the digester volume is crucial for optimum biogas production. Some key calculations include:
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Estimating the amount of feedstock (poop & organic waste) available per day.
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Figuring the hydraulic retention time - how many days the feedstock will reside in the digester. This is typically 15-40 days.
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Considering the loading rate - the amount of feedstock that can be processed daily per cubic meter of digester capacity.
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Accounting for gas storage in the digester headspace.
For my single dog, I'll likely start with a digester around 200 gallons (750 liters). This should provide enough capacity as I get the system up and running. I can always expand it later if needed.
Construction and Installation
Now that I've chosen a design and sized my digester, it's time to build it. Construction involves:
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Assembling the tank from plastic barrels, concrete, fiberglass or metal drums. Sealing it properly is vital.
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Attaching the inlet and outlet pipes, gas valve, and slurry overflow.
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Adding an access hatch or removable lid for cleaning and maintenance.
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Building the gas drum with valves to store and release the biogas.
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Installing the heating system if needed. This could be as simple as running warm water through pipes in the tank.
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Insulating the digester and/or embedding it underground helps maintain interior temperature.
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Waterproofing the digester and piping is essential. I'll use sealant on all joints.
With the right parts and watertight construction, I'm confident I can build an effective backyard biogas system. Proper feeding and maintenance will keep it running smoothly.
Feedstocks for the Digester
The digester requires a slurry of organic material and water to start producing biogas. Some feedstocks I plan to use include:
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Dog poop - My dog's waste will be the primary feeding material.
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Food scraps - Vegetable peels, outdated produce, coffee grounds, etc.
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Yard waste - Grass clippings, leaves, mulch and wood chips.
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Newspaper/cardboard - Soiled paper and cardboard break down well.
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Livestock manure - If I can get manure from a local farm, it adds great nutrients.
I'll shred or blend these items with water to create a liquid slurry. The proper carbon-to-nitrogen ratio is 25-30:1. Monitoring this balance and pH optimizes methane production.
Collecting and Storing the Biogas
As digestion occurs, biogas containing methane, carbon dioxide and trace gases will accumulate in the dome of the digester. From there it can be:
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Fed directly into a biogas stove or boiler via pipe.
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Stored in an inflatable gasbag or floating tank.
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Compressed and collected in bottles or cylinders.
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Burned in a generator to produce electricity immediately.
For now I plan to compressor and bottle the gas for occasional generator use. Once I produce more biogas, I'll consider powering appliances or heating.
Using the Effluent Fertilizer
After digestion, I'm left with a nutrient-dense slurry of effluent that I can apply as fertilizer:
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The effluent is low odor and pathogen free.
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It contains lots of nitrogen, phosphorus and potassium for plants.
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The effluent is an excellent fertilizer for my vegetable garden, fruit trees and lawn.
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It's free of weed seeds and chemical additives.
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As a liquid, the fertilizer infiltrates quickly with no runoff issues.
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The fertilizer can be safely composted if desired.
This organic approach to fertilizing my yard helps eliminate waste while improving my soil.
Safety Precautions
While biogas systems are generally safe, I do need to take some precautions:
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Methane gas is highly flammable, so sparks and open flames are prohibited near the digester and piping.
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The digester tank and pipes need durable seals and regular leak checks.
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I'll wear gloves and goggles when handling feedstocks to avoid pathogen exposure.
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Equipment like compressors and generators must be kept far from the house and protected from weather.
With some common sense preparations, I'm ready to harness my dog's waste for home energy! I'm excited to fire up my backyard biogas digester.