How Do I Choose The Right Biogas Digester Size?

“I’ve been living on a small farm in Gloucestershire, UK, and have just started exploring biogas for our energy needs. We primarily have animal waste from our livestock and a decent amount of kitchen scraps, but I’m not entirely sure what size digester I need to get the right amount of gas to meet our needs. What factors should I take into account to make an informed decision on the digester size? Any help would be appreciated!” Thanks, Mark, Gloucestershire, UK.

How Do I Choose The Right Biogas Digester Size?

Great question, Mark! Choosing the right biogas digester size is one of the most important decisions you’ll make when setting up your system. Getting it right ensures efficiency, cost savings, and sufficient gas generation for daily use. Considering your farm setup with livestock and kitchen scraps, let’s break down the key factors to help you choose the correct size digester for your biogas needs.

Why Size Matters in Biogas Digesters

The size of your biogas digester affects not just how much gas you produce but also how efficient the system is at breaking down the organic material you feed into it. Too small a digester, and you might not produce enough gas for your needs. Go too large, on the other hand, and you could end up wasting resources and spending more on equipment and maintenance than necessary.

A biogas system that’s too small might struggle to handle all the material you want to process, leading to inefficient digestion and often too little gas. Conversely, an oversized system can run below capacity, causing extra costs for the equipment, and it might not operate efficiently because there isn’t enough feedstock going in regularly.

1. Assess Your Feedstock Availability

One of the first things to consider, Mark, is the type and quantity of feedstock you have. In your case, it sounds like you have a combination of livestock manure and kitchen scraps. Different materials have different biogas yields, so understanding your total feedstock volume is crucial.

• Animal Waste: Animal manure is one of the most common feedstocks for small- to medium-sized biogas digesters, especially on farms like yours. Cows, for instance, produce a significant amount of manure, and even a single cow can provide enough waste to sustain a digester. If you have cows, chickens, or pigs, you’ll want to estimate the daily manure output in kilograms.
• Kitchen Waste: Kitchen scraps typically have a higher biogas yield compared to animal manure. They break down faster and provide richer material for methane production. Estimate the average daily volume of kitchen scraps you generate and add it to your total feedstock calculation.

To be more specific, here’s a rough guide on various biogas yields per kilogram of dry weight:

This table should give you a rough idea of how different materials contribute to biogas production. For example, if you’re generating a combination of 20 kg of cattle manure and 5 kg of kitchen waste each day, your potential daily biogas production could be roughly 7 to 10 cubic meters.

2. Determine Your Biogas Demand

The next step is calculating how much biogas your household or farm needs. Different appliances use varying amounts of biogas, and it’s essential to have a clear picture of your daily gas demand.

Common uses of biogas include cooking, heating water, and running generators for electricity. For instance:

• Cooking: One cubic meter of biogas can provide about 2 hours of cooking on a standard biogas stove.
• Water Heating: Around 0.5 cubic meters of biogas can heat 100 liters of water.
• Electricity Generation: One cubic meter of biogas can produce approximately 1.5 to 2 kWh of electricity using a biogas generator.

Let’s assume you’re using biogas for cooking and water heating on your farm. If you’re cooking 3 meals a day and heating 100 liters of water, you may need around 4 to 5 cubic meters of biogas daily. Having this baseline consumption figure is vital when choosing your digester size, because you want to produce at least the amount of gas you need daily, to avoid relying on alternative energy sources.

3. Matching Your Digester Size to Your Feedstock and Demand

Now that you have an idea of how much feedstock you can provide and how much gas you need, Mark, you can figure out the right digester size. Digesters are sized typically based on their volume in cubic meters (m³), representing the total amount of material they can hold.

A typical home biogas digester produces between 0.3 and 0.6 cubic meters of gas per cubic meter of digester volume, depending on the inputs and the digester’s efficiency. Most people find success with digesters that are between 2 to 6 cubic meters for small- to medium-scale setups, especially on a farm with both manure and kitchen waste.

To calculate the digester size, take these steps:

• Estimate your feedstock. Let’s say you have around 25 kg of feedstock from animal waste and kitchen scraps each day.
• Choose your gas production estimate from the table (for mixed waste: around 0.3 to 0.4 m³ per kg of feedstock).
• Multiply: 25 kg * 0.4 m³ = 10 cubic meters of gas production per day.
• If you need 5 m³ of gas daily, a digester volume of around 5 to 7 cubic meters could be ideal, giving you enough capacity to consistently meet demand, with some buffer for variations in feedstock input.

4. Retention Time and Digester Efficiency

Another aspect to think about is the retention time, which is the amount of time the feedstock needs to stay in the digester before it’s fully broken down into biogas. On average, retention time is about 20 to 30 days, depending on the digester type, feedstock material, and ambient temperature.

If your digester is too small, materials may not stay in the system long enough to fully break down, leading to reduced biogas output and potential maintenance issues like blockages or odor. If you’re in a cooler environment, like Gloucestershire, retention times may need to be longer as the digestion process slows down in colder temperatures. In that case, Mark, you might want to aim for a slightly larger digester to give more time for the materials to decompose.

5. Climate Considerations

Since you live in the UK, weather can play a significant role in biogas production. Biogas generation is a microbial process, and microbes thrive best in warmer temperatures. In northern climates, digesters can slow down or become less efficient during the colder months.

To counteract this, consider insulation options or think about building the digester underground where the earth’s natural temperature can help keep things warmer. In colder climates, some people also opt for slightly larger digesters to account for the reduced production during the winter months. This way, you won’t run out of gas when you need it most.

Troubleshooting Common Issues with Digester Sizing

Too Little Gas Production

If you’ve already got a digester but find it’s not producing enough, the first thing to check is whether you’re feeding it enough material. Revisit your feedstock estimation and confirm you’re adding the right amount. You may also want to ensure the retention time isn’t too short and that the digester is not overloaded, as this can reduce gas yields. Don’t forget to monitor factors like pH levels or temperature – all of these can affect your yields.

Overflow or Underutilization

Overflow typically happens when the digester is too small to handle all the waste being added daily. The result can be leaks, poor gas production, and an inefficient system. On the flip side, underutilization occurs when a digester is too large, leading to wasted volume since there’s not enough biogas being generated to justify the size. Larger digesters also come with extra upfront and maintenance costs that could be avoided with the right sizing.

Final Thoughts…

Mark, it sounds like you’re in a great spot to get your biogas setup working efficiently. Remember that choosing the right digester size relies heavily on accurately understanding your feedstock volume and your household or farm’s energy needs. A digester that corresponds with your gas needs and climate can make a world of difference.

Thank you for submitting your question, and I’m confident you’ll make a great decision with the information you’ve got now. Best of luck with your biogas adventures, and don’t hesitate to reach out if you face any more challenges!