When I speak to people about biogas I am often asked:

“How much biogas will it take to run my 35 HP diesel tractor?” (or)

“How much gas will I need to heat about a liter of water?”

To begin the second part of this blog series, I will present a chart from the book: Biogas Plants: Designs and Details of Simple Biogas Plants by Ludwig Sasse © 1984. Let us consider the following chart in order to judge how much gas we will need to power certain applications.

(If the above picture is too difficult to read, please download the attached Jpeg at the bottom of this page)

To run a 35 HP diesel tractor for one hour we will need approximately 14.7 m³ of biogas.

(35 bhp) x (1 hour) x (420 liters of biogas per hour) = 14,700 liters or 14.7 m³

Remember from Part One that 1 kg of cattle manure produces 40 liters of biogas in 80 days at 78.8°-82.4°F. If we use these parameters, one would need approximately 183.75 kg of cattle manure to be retained for 80 days to make enough biogas for one hour of tractor usage.

Assuming that one gets this system rolling so that it can make one hour of biogas a day for the tractor, then you would need a container that could hold 29.4 m³

(187.75 kg manure +187.75 l of water)= 367.5 liters

(367.5 liters)(80 day retention time)= 29,400 l or 29.4 m³

If 1 m³ is 35.31Ft³ then (35.31 Ft³) (29.4 m³) =1039 Ft³

A container with the dimension of 3 feet deep by 12 feet wide and 30 feet long would be large enough to hold this much feedstock and water for 80 days. This style of anaerobic digester would most resemble a covered lagoon.

If a farmer’s intent is to power heavy machinery, then the system will need a suitably sized gasholder and sophisticated gas cleaning system to scrub the biogas of corrosive hydrogen sulfide and non-combustible carbon dioxide. But wait!! Before you completely dismiss biogas because it is not likely to be the source of power for our transportation system let’s first consider other potential applications.

China and India have significant experience utilizing biogas for domestic use and they typically build small scale plants to meet the needs of one or multiple families. These domestic necessities include boiling water, cooking, and light. I think it is safe to say that in the United States, some of us take these critical domestic necessities for granted. However, if we have a clearer understanding of biogas then we might be able to secure these basic domestic needs for our friends and family where biogas proves feasible.

To boil 5 liters of water for 35 minutes requires 165 liters of biogas. According to the table above a family of five uses 850-2500 liters of biogas a day (avg. 1675liters a day).

Using the same parameters above, the 42 kg of feedstock would have to be retained for 80 days.

(42 kg manure + 42 l of water)= 84 liters

(84 liters)(80 day retention time)= 6,720 l or 6.72 m³

If 1 m³ is 35.31Ft³ then (35.31 Ft³) (6.72 m³) =238 Ft³

The volume of a cylindrical tank is given by the formula:

V= Л x r² x h

V= (3.14) (4) ² (5) =251.2 Ft³

Therefore, a cylinder with the dimension of 5 feet tall by 8 feet wide would have a large enough volume to hold this much feedstock and water for 80 days. While this size tank and a suitable gasholder is not exactly small, it seems like a more realistic digester than the one planned to run the tractor. Biogas used in the domestic context can burn decently with a large amount of carbon dioxide. Corrosive hydrogen sulfide can be removed by pushing the gas through a tube filled with steel wool.

I hope this two part series has been useful for those who want to make estimates related to scaling a biogas digester to a specific use. Although there are other complications related to using biogas as a transportation fuel, I intended to show that based on large amount of material related to size of digester that biogas is not a suitable fuel source of transportation fuel on most farms that do not have a massive animal operation. However, biogas need not be waved off as a useless pursuit if one scales a system to meet domestic needs. Biogas has been termed appropriate technology not for its application as a transportation fuel but as an energy source to meet critical domestic necessities like cooking and lighting.

Introduction:
In the following two blogs I will write about the main parameters influencing biogas production, how much biogas to expect from 1 kg of substrate (both plant-based material and cattle manure), and how much biogas is used in various applications. In order to illustrate my points I will be using tables and data from the book: Biogas Plants: Designs and Details of Simple Biogas Plants by Ludwig Sasse © 1984. I intend to use Metric system for my calculations. Click here to open a new window that contains the necessary Imperial conversions from Metric.

Parameters of Biogas Production:
Biogas production depends on the amount and nature of the fermentation slurry, temperature, type of digester, and the retention time. Gas production is encouraged by high, uniform temperatures (e.g., 33°C), long retention times (e.g., 100 days), and through mixing of slurry. In contrast, gas production is adversely affected by low and fluctuating temperatures (15°C-25°C) (59°F-77°F), short retention times (e.g., 30 days) and poor mixing (Sasse, 18-19). To put this into perspective consider the following example:

--1 kg of cattle dung yields only 15 liters of biogas in a retention time of 30 days at a digester temperature of 20°C (68°F). If the retention time is increased to 100 days and the digester temperature to 33°C (91.5°F), 1 kg of cattle dung gives 54 liters of biogas.

As you can see, the amount of biogas created from 1 kg of feedstock depends on the temperature of the slurry and the retention time inside the digester. A lot of data on biogas production per unit of manure or material is derived in a laboratory setting. In the lab, the material is retained for only 30-35 days. This short retention time often considered the optimal time period to store the substrate before new substrate should be added to replace it. This is because up to 50% of the available methane is produced in the first four or five weeks. The remaining methane is available at a much slower rate as the materials are kept longer inside the digester.

This consideration matters when deciding whether to create a continuous or a batch system. When operating a continuous feed system the operator often wants to extract the greatest amount of biogas in the least amount of time. Furthermore, the operator needs to make more room in the system for daily arising of manure. When material is fed in a batch, all the material is loaded and the operator decides how to digest the material. Plant substrates are typically digested in batches because when introduced to the digester they do not mix well. In addition, plant matter has the potential to yield more gas when stored longer. Finally, one has to expend energy to collect plant materials and to process them into a form in which they can be digested. If we are going to go through the effort to create a substrate we need to try to get as much energy out of it as possible to make the process more feasible.

Substrate Information and Biogas Production:
Cattle manure has a typical methane content of 65%. Leaves (carbon) are 58% methane and fresh grass (nitrogen) is 70% (Sasse 12). The average of these two materials is 64%. It is important to note that successful anaerobic decomposition of plant-based substrates have been achieved, however there is considerably less data for these materials than with animal manure. Since the combination of two plant materials has fairly close methane content to cattle manure I feel that it is safe to use the following chart to set production expectations:

Therefore, 1 kg of cattle manure yields 40 liters of biogas at 26°C-28°C (79°F-82.5°F) when retained for 80 days (11 weeks, 3 days). If we start a 200 kg batch of material, keep a an average temperature of26°C-28°C (79°F-82.5°F) and retain it in the digester for 80 days then we can expect 8000 liters (8 cubic meters) of biogas.

This table can help us make general predictions related to biogas production from the cattle and plant-based feedstock described above. Keeping this information in mind I want to transition to the next blog where we can begin to scale a biogas plant to meet a specific gas use. Since we can make estimates related to how much gas can be expected from a given mass of feedstock, we can predict how much gas is produced from an already existing biogas plant. This will help us select the appropriate utilization of the gas, (i.e. cooking or engine fuel), based on daily biogas production.

It is great to announce that Laguna Farm has joined the Energy Farm Network. Located in Sebastopol, California, Laguna Farm is a CSA that supports over 400 members. Farmer Scott Matheson has over 20 years experience in Community “Shared” Agriculture and maintains a keen interest in alternative energy. Laguna Farm gets a majority of its power from two large-scale power generation units-a 15.3 kW solar array and a 15 kW generator that runs on biodiesel.

Scott is very interested in alternative energy research and is currently working with Post Carbon Institute to plan and construct a biogas digester suitable for processing on-farm materials. The project is in the early stages of development and the initial goal is to generate methane from a substrate of “green chop”. The biogas feedstock will be mostly plant-based and will originate from 3 acres of land that is not intended for growing vegetables. Scott will already be harvesting some plant matter and mixing it with manure to make quality compost to feed back into the farm land; and a portion of this material will be fed into an anaerobic digester.

This system is exciting because we are planning on using waste heat from the diesel generator to help heat the digesters. Sebastopol has a mild climate and the addition of a heating system will help remedy one of the biggest obstacles in biogas generation, namely temperature. Much research has been done on biogas generation from manure-based substrates; however, we will be attempting to create biogas from primarily plant material. This style of feedstock has its own challenges and opportunities and much of the data collected on these types of systems is new. For instance, one advantage to using plant material is that it contains more methane per unit weight than manure based feedstock. On the other hand, plant-based systems are a challenge because they are often difficult to agitate and the material optimally needs to be processed into small pieces that mix well in water. We will share our system designs and project details as this new relationship evolves through the spring and summer.

15.3 kW Solar Array

The Building with the Set of Stairs is Home to the 15 kW Generator that Runs on Biodiesel. (Possible biogas digester heat source.)

Farmer Scott with an Electrolizer Unit He is Researching. (It is inteded to improve gas mileage.)

The Area Where the Truck Sits is a Potential Spot for the Biogas Digester. (In front of the truck is area where green chop can be harvested.)

Biogas is created when organic material is fermented in anaerobic conditions. Specialized bacteria thrive in anaerobic (without oxygen) environments, consuming organic acids and respirating combustible methane. The bacterial activity inside a biogas digester is very similar to the processes occurring in a compost pile. The bacteria break down carbon and require a certain amount of nitrogen to sustain their bodies. Thus, a biogas digester needs to maintain a specific balance between carbon and nitrogen. This balance is expressed in a carbon to nitrogen ratio (C/N). The ideal C/N for a pile of aerobic compost is 25-35. Similarly, the optimal carbon to nitrogen ratio in which organic mater is decomposed inside a biogas digester is 25-35.

Because this C/N is similar to aerobic compost, many people have assumed that the finished product of biogas digestion is a natural fertilizer. They have suggested that the effluent of a digester can be diluted with water an added directly to the soil or to plants. However, research by Dr. Elaine Ingham, a soil biology expert puts this practice into question. She has studied the microbial life of the soil in her laboratories in Corvallis, Oregon at both the Sustainable Studies Institute and Oregon State University. She states in her Compost Tea Brewing Manual 5^th ed. that teas formed in anaerobic conditions contain high levels of natural alcohols that poison and overwhelm other soil microbes, thereby destroying a portion of the life in the soil and harming plants. At present, only aerobic (oxygenated) compost tea can be guaranteed to actually benefit and stimulate life in the soil.

Given this research we must be careful with the ways in which we choose to use the effluent slurry left over after the biogas digestion process. Although there are some harmful alcohols, the slurry may contain a C/N of 30 and be full of water soluble nutrients. This is indeed useful. We need not throw it away as waste, and instead we might build a separate compost pile and add the effluent to it in order to create a healthier environment for soil organisms. The aerobic composting process may do away with some of the natural alcohols and add a diversity of aerobic microbes to the effluent. This extra step may be a way to make good on the promise of biogas—the creation of combustible energy and a high quality fertilizer.

It has been a week since the initiation of the probiotic fertilizer and the batch-style biogas system at the Local Energy Farm Demonstration Project located at UBC. At present, certain aspects of daily farm upkeep rely on the work of dedicated volunteers. I considered this in the creation of the biogas digesters and attempted to make a system that was as easy to maintain as possible for the volunteer workforce.

At minimum, the biogas digesters need to be agitated once a day. In my perfect world, agitation would occur three times—once in the morning and twice during the heat of the day. Remember, agitation breaks up the hard layer of scum that tends to form on the surface of plant based substrates and it mixes the plant matter in order to allow bacteria to come into contact with new material to digest. Agitation should take about thirty seconds for each digester. A volunteer cycles the handle clockwise for about 5-10 revolutions and then counter clockwise for another 5-10 cranks. Simple!-Finished and on to the next farm task!

The probiotic fertilizer needs less frequent agitation (once a week instead of daily). The “airtight” lid is taken off the brew, a wooden oar is inserted into the mix, and the contents are mixed and churned for a minute or two. Usually this makes a lot of foam as carbon dioxide is released from the mixture. After mixing the lid is re-secured and awaits the next week.

As you consider the infrastructure and process you are going to develop on your farm try to make it use as less energy as possible for up keep and maintenance. To invest a little extra thought and energy in the planning and design phase can allow you to have multiple initiatives underway, which, once started, can continue without a lot of extra physical input.

I have been using a template presented by the United Nations Department of Food and Agriculture as a general guide for the creation of the biogas system at the Energy Farm at UBC. This booklet suggested using rubber tire inner tubes as the gas capture system, a suggestion that I eventually chose against at the Energy Farm.

The U.N.’s suggestion is that tire inner tubes are simple to repair, easy to acquire, and can suitably store biogas for later use. While I find this idea appealing in situations of scarcity and as a means of reusing rubber that might be simply thrown out, I feel that taking a little more time to build a drum style gasholder is better overall choice for this biogas system.

The basic design of the dome style gasholder is as follows:

1.Invert a drum so that the holes on the lid face toward the ground

2.Remove or cut off the top of the drum

3.Insert a PVC pipe into one of the holes in the lid so that it stands vertically inside the inverted drum

4.Fill the main drum with water to about 3 inches below the top of the pipe and place another drum inside (inverted so that can collect the gas)

This “floating dome” style of gas collection captures a large amount of gas that is distributed using only one outlet. Moreover, all biogas digesters can have their gas routed to this single gas collecting dome. As the gas collects under the drum, the water acts as a seal so no gas escapes. Weight is put on the top of the second drum in order to determine the pressure of the system when connected to a stove or appliance. As the drum is pushed closer to the water more pressure is created and gas flows out through the PVC pipe toward the appliance.

I inverted a 50 gallon plastic drum, cut the top off, filled it up with water and placed a 20 gallon plastic garbage can to form the gasholder. I liked the fact that I could use plastic in this part of the system because it ensures the long life of the gasholder. While the inside of digesters Ludwig and David had to be painted with a protective paint, the plastic drums need less preparation and are guaranteed not to corrode from the gas.

Gasholder of Prototype Biogas System

The biogas construction project has been completed! On Tuesday I was able to connect the digesters to the gasholder, buffer the substrate, and add the seed cultures.

I found that buffering the substrate required much less alkaline material than I expected to use. It took 40grams of baking soda to buffer the clover/leaf batch inside digester David, while digester Ludwig used 35grams. This was enough to change the acidic solution (pH 5) to a more basic solution between pH 7-8. I checked the pH with litmus paper multiple times during the day and the pH of the clover/leaf substrate remained a constant 7-8. Farm manager, Mark Bomford, noted that the leafs inside the substrate have an excellent buffering capacity and will be able to absorb a lot of the baking soda. So, while the substrate seemed adequately buffered at 7-8 on the day that I added the seed cultures, it might become more acidic after the leaves absorb the baking soda water. I am nervous about his suggestion as we are now left to monitor how the buffering plays out.

Four different anaerobic seed cultures were used to inoculate the digesters. I used a culture of lama and sheep manure, a culture of lama and clover manure, a mixture of sheep and clover, and a mix of only clover and leaves. These cultures were maintained under protection inside a glass greenhouse for over five weeks. During that time I had observed gas production on the surface of all cultures using the sheep and lama dung and felt that those cultures offered a large population of methane forming bacteria. However, the mixture of only leafs and clover did not seem as active and I did not expect it to be a suitable seed. As I added the seed cultures to the digesters I checked the pH of each one. As I expected, the cultures using lama dung were defiantly basic at a pH of 7-8. The leaf and clover mix was very acidic at pH 5.

The acidic clover leaf culture lends us insight into the necessity of brewing appropriate bacteria cultures for seeding. Anaerobic bacteria can and will grow inside plant only mixtures, however, even in optimal growth conditions, it might take months. That is why a source of dung is useful when creating live cultures in a shorter period of time. I think it is reasonable to suggest that the plant only seed was stuck in the acid forming phase, where bacteria are breaking down the plant material into fatty acids. Without a suitable amount of methane forming bacteria to gobble up those acids the process gets stuck and the solution remains acidic. Literature suggests that this process will correct itself eventually because the bacteria that form the fatty acids will begin to drown in their own toxins, slowing their reproduction and allowing methane forming bacteria to make headway against the overabundance of the acids.

After the seed cultures were added to the digesters I put the lids on and sealed them closed. I stuck plumbers putty under the lip of the lids so that I could assure a gas tight seal and then bolted the lids down. Agitation of the material was not hampered by the lid or the temperature sensors and the entire system seemed to be functioning as expected.

Prototype Biogas Digesters (David and Ludwig)

Gasholder for Digesters 

Today I was able to complete the probiotic compost tea I learned about while in Ecuador. The brew was made of clover, sawdust, yeast, molasses, humus (finished compost), rock phosphate, urine, garlic, and comfrey. I decided to throw the garlic and comfrey in to act as a natural insect repellent. This batch will be stirred once a week and will be stored next to the biogas digesters in the hoop-style greenhouse at the UBC Energy Farm. It should be ready in 45-90 days depending on winter temperatures-my guess is around a safe 75 days. The tea can be used as a foliar spray at a 1:10 ratio or as a soil improver at a 50/50 mix with water. I expect the batch to yield around 100 liters of concentrate. If you want the exact recipie email me at --> chrishansen@postcarbon.org .

The biogas project is near completion, and the lids are ready to be tightened down. I stuffed 19.5kg of mildly composted material into digester David and 10kg into Ludwig. Then I topped each digester off with water until there was about 10cm of space from the lip of the drum. The agitation system works fine but can be a little tough when digester David gets a pile of leaves pressed between the lid of the drum and the agitation arm. If I could make an improvement it would be to make the handles of the system a little bigger as to allow the user to get more torque and therefore make it a little easier to bring the arm around. However, perfection aside, the agitation system does not leak and works fine-I count that as a success.

I have some litmus papers that I have been using to check the pH of the water and substrate mixture. It is very acidic right now. On the color sheet between 4 and 5. That means that I have a long way to go to buffer the system to a pH of 7-8 to make the mixture hospitable for the methane forming bacteria that I plan to seed the digesters with tomorrow. My idea is that the substrate will seep into the water overnight and allow a substantial amount of acids to form. Then, tomorrow, I will use lime or baking soda to buffer the solution. I am leaning towards baking soda right now; however, there is plenty of lime at the farm.

For a little over a day, the feedstock materials have been undergoing aerobic decomposition in hopes of breaking down the plant matter before feeding the digesters. One of the digesters (a.k.a. David) will be fed a charge of 9Kg clover and 11Kg of dry maple leaves. Once David is filled with water the substrate will have a total solids concentration of about 11 %. The other digester (a.k.a. Ludwig) will be fed a mix of 4.5Kg of clover and 5.5Kg of dry maple and alder leaves. At about 6% solids this is a less dense concentration of material.

When these prototype batch-style digesters I decided to make two subtly different machines. Ludwig’s agitation system differs from David’s in one way—the depth of the agitation shaft. While David’s agitator is situated 6 inches below the top of the lid, Ludwig’s is positioned at 8 inches below the top. What this difference amounts to is as follows:

Ludwig’s system will be able to disrupt scum formation and will be able to mix the slurry from a deeper position. The intended idea was that the deeper agitation position would churn the substrate and allow the bacteria to come in contact with new bits of material to eat. In contrast, David’s agitation system is position higher on the drum. It too will be able to disrupt the scum formation and it has the advantage of allowing more material to be put into the digester. Given suitable conditions, more material is more biogas. Hence, I have decided on a smaller solids concentration in digester Ludwig because it might be too hard to turn the agitation handle through such a dense mixture. David, on the other hand, can handle the denser material and will break up the scum, allowing the digestion process to sort itself out below. In subsequent tests of the machines more material will be added to Ludwig to really get a test of what is the limit of the deeper agitation system as it attempts to churn dense plant matter.

We are moving forward with the biogas system at the Vancouver Energy Farm. The paint will be fully dry by tomorrow and we could technically start the digestion process. However, I will wait at couple of more days to let the clover and leaf feedstock undergo a little more aerobic decomposition. I picked up all the "plumbing" fittings at Vancouver Irrigation. These folks were quite helpful and allowed me to purchase the pipe-fittings at a wonderfully discounted rate--Thank You.

Potions of the pipe-fittings are intended to be a scrubbing chamber for hydrogen sulfide gas. Hydrogen sulfide smells like rotten eggs and is corrosive to the metal or brass in any system that you feed the gas to. However, the hydrogen sulfide gas can be scrubbed by corroding some other type of metal- in this case steel wool. The steel wool is placed inside an 18-inch long chamber. As gas pressure builds up in the digester the gas is forced through the steel wool on its way to the gasholder. Below are pictures that show the process of making the scrubbing system and how it is built into the digester lid.

Unrolled Steel Wool

Feeding Steel Wool Into Scrubbing Chamber

Biogas Digester with Scrubbing Chamber Extending from the Lid