Tips to Install a Methane Digester

A methane digester — also known as an anaerobic digester — can be a great source of biofuel and fertilizer.
By Cliff Williams
November 2013

"Urban Crofting," from author Cliff Williams, urges the reader to seek a sustainable lifestyle with modern, simple methods.
Cover Courtesy Outskirts Press
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Urban Crofting (Outskirts Press, 2013) is based on the lifestyle and practices of the Essenes, an ancient people that lived a sustainable existence that would be envied by many today. Cliff Williams incorporates this knowledge into a new personal, family-sized agricultural way of life that can be achieved even in an urban neighborhood. In this excerpt from the chapter "Methane Digesters," Williams gives readers valuable tips necessary to make the most of their sustainable biofuel and fertilizer generating set-ups.

You can purchase this book from the MOTHER EARTH NEWS store: Urban Crofting.

Introduction to Methane Digesters

My introduction to methane digesters became the inception of urban crofting, with methane digesters as a source of biofuel and fertilizer. Methane digesters have the potential of making a huge impact on your health. Besides the obvious biofuel aspect, methane digesters can help restore the micro-nutrients in our diet that are missing in the food chain today.

Where Urban Crofting is concerned, our new circle of life will be greatly dependent on creating a source of food for our plants. To be in balance, nutrient-rich sustenance has to flow from plants to us, and back to the plants.

Let’s re-visit the food chain that was intended to be. Mankind’s sustenance comes from plants, and plants and their sustenance comes, in part, from humans — mainly from our wastes. Simple enough on the surface, but let’s look deeper. The plants that provide nutrition for us “had” the ability to provide complete nutrition for us. All the carbohydrates, proteins, minerals, vitamins, Prana (life), macro and micro nutrients, enzymes, beneficial bacteria, etc...everything our bodies needed was in vegetables, grains and fruits. Not only do plants have the ability to draw everything they need for a perfect life from the soil, they also assimilate everything that we need. Many micronutrients needed by humans are drawn up by plants even though they themselves have no need for them. Wow! Again, even though they don’t need all those micronutrients, they bring them up for our benefit. That is only if they are in the soil to begin with. This is an incredible feat that shouldn’t be overlooked.

We still need to go a little deeper into this before you have a clear picture of where we’ve gone wrong. The wrong is the fact that many of the micronutrients are gone from our food chain. Plants draw up the nutrients we need from the soil, and we return the nutrients we need to the soil with our bodily eliminations of our urine and excrement. “Matter is neither created nor destroyed, it is transferred”. The root system of plants brings up the nutrients, and they are used in the biological function and growth of the plant. The part of the plant that is of benefit to us, the plant matter, is created by the plants from soil matter. Our bodies break down the plant matter and use the ingredients for our biological function and growth. Our bodies continually grow new cells to replace dead cells, using that plant matter. The waste and dead cells are returned to the soil, where they are broken down for the next plant cycle by the topsoil. Matter is neither created nor destroyed, it is changed from one substance to another in a continuous loop, as part of the circle of life.

This is a simplified overview of this circle of life, as there is an incredible amount of information on this subject. The fields of science including agronomy, biology, nutrition and health disciplines are all rolled into this. These sciences are advancing rapidly, and concepts on health and nutrition seem to change almost daily — especially where nutrition is concerned.

What’s wrong with this picture is the fact that many of the micronutrients and macronutrients that are the building blocks of life are now missing from the circle of life. So where did those nutrients go? How did they get out of the life cycle? “Oh crap” is the answer. Our excrement is the answer.

Mankind slowly entered the industrial revolution and started moving off the farms and into cities. Farmers brought the crops from the farmland to the cities, but didn’t bring back the excrement. I can’t say that I blame them, at least where “taking crap” is concerned! Cities usually formed along rivers and the excrement was, and still is to some extent today, being dumped into the rivers and carried to the sea. The more civilized we became, the more we emptied the nutrients from our circle of life. If you live in a rural area, your septic system is designed to inject the “waste” under the top soil or to contain it in an evaporative mound. The nutrients have been removed from the soil and returned to the bottom of the ocean, or the earth, where it is unavailable.

We’re in a tough spot, and I don’t see any way to fix the system as it exists. I do however see a way to come back to that perfect lifestyle on a small scale, and that is where the methane digesters come in. Urban Crofting is about re-creating your own “farm”.

In the Urban Crofting model we are creating a new circle of life that incorporates some necessary aspects of the original circle of life. We want to build our soil with as many of the ingredients of life that we can, and then keep those nutrients going around and around in our circle. The methane digesters are the perfect tool for building the soil and keeping those nutrients in our circle, as there are now some other consequences of the nutrient-deficient existence we’ve created. The biggest hurdle is the consequences of disease and sickness that we’ve brought upon ourselves. Our excrement is not only stinky, but now is filled with organisms and pathogens that can kill humans. It’s kind of a crappy predicament we’ve created, but there’s a way out of it.

One of the most natural healthy acts should be to reintroduce our excrement into the soil but now it is toxic. Fortunately one of the benefits of methane digesters is that it breaks down those organisms almost completely. However, almost isn’t good enough. It may be once we distance ourselves from those lifestyles that promote toxicity this won’t be a concern, but for now we have to have some sort of remediation. The current methods available are heat treatment or chemical treatment and the last thing we want to do is introduce a bunch of chemical poisons into our perfect circle of life.

Heat, hmmm, we need extra heat for the greenhouse. Hey, we have some large cookers for making the ethanol mash and we’ve positioned them so the waste heat goes to the greenhouse. The methane produced by the digesters may be enough to heat treat the composted fertilizer waste from the digesters.

The only problem is the raw factor. We will annihilate all the beneficial bacteria and enzymes. 

Yes, those nutrients and minerals need to go back into the soil, and we have to re-introduce them back into our circle of life. We can put them directly into the soil in the form of fertilizers and supplements, some of which can be found at a quality nursery. We can also find quality vitamin and mineral supplements, and take them ourselves, and then let the excrement carry them back by using a methane digester. Personally, I’m going to do both.

So now we can get into a healthy discussion about methane digesters. Methane digesters can take any organic material and rapidly decompose it, returning or rather breaking down the material into components that the soil can make readily available to the plants. Not only excrement, but yard waste, food scraps, and animal waste can go into this. If it WAS alive it’s fair game.

If digesters are so good, why isn’t everyone using them? For the most part, America is the ultimate in industrialized societies, and most Americans won’t stop to pick an apple off a tree. We take our children out to a farm, like taking them to a zoo. We are too disconnected from the real circle of life to ever consider this.

Methane digesters are used to some extent in sewage treatment plants, and they are starting to show up at large dairy farms. Unfortunately, it’s slim pickings when it comes to family-sized digesters. The examples of small digesters that I’ve seen online, are for experimental gas production or treating sewage. The only small scale digesters with agricultural uses are being used in China, India and other developing nations.

Since the 1970’s, the Chinese government has been promoting small scale methane digesters, and now there are 5 million of them in use. There is a rapidly increasing amount of information on the internet, just search for “China and Methane Digester”. When I began Urban Crofting, there were no available small scale digesters I could find, so I set out to design my own. I recently noticed a site: “China Methane Digester”, Alibaba.com, that is now selling plans and components and they appear to be reasonably priced. I like what I saw, and may order through them for my next digester. MOTHER EARTH NEWS has had a couple of good articles on small scale digesters. See: How to Make a Home Methane DigesterSichuan's Home Methane Digesters, and Feedback on Methane Digesters.

Well, thus far we’ve talked about the potential of methane digesters and what is possible. Unfortunately, there is a dark side and that is Government Regulation, Government Involvement, and the fact that you will have to get permission for this project from someone that will most likely not even understand what you are asking for. It would be real easy to get pessimistic now and really go off on the Government and influence of big business — maybe later!

Seriously, there is definitely room for concern. The diseases that face mankind now are scary. Salmonella, E-coli, and the likes have killed people. And when it hits the news, it’s an industry killer also. Just recently there was the cantaloupe listeria outbreak in Colorado, and the classic example was the recent European E-coli outbreak found on German organic sprouts. 22 people died, and over 2000 were sickened, and the interesting thing was how it devastated the Spanish agricultural industry when they were wrongly accused of tainted cucumbers.

So people die from food, and you want to fertilize your garden with poop! It sounds insane, but it is what was meant to be. It is my opinion that our deviation from the natural order of things is the cause of the diseases to begin with. And if it’s not the cause, then it’s our weakened state that brought about our body’s inability to fight off those diseases. Anyhow, there are 5,000,000 Chinese people doing it without any problems.

Back to the problem at hand, and that is even though we re-create our circle of life and our bodies now have everything we need, there’s no way to know how long it will take for the strengthening to withstand a civilized disease like E-coli. The point is that even though others are using it, with no ill effects, maybe some remediation wouldn’t be such a bad thing.

For starters, I am building two systems, one for toxic human waste, and one for socially acceptable animal and vegetable waste. But it’s time now to explain how they work.

The correct term is Anaerobic Digester. This means that the digesting is done in the absence of air. There are several separate bacteria types that break down the wastes in a process that, in the end, produces methane, carbon dioxide, and a small amount of hydrogen sulfide and fertilizer. The resulting gas can be burned as is for heat, cooking or lights, or the carbon dioxide and hydrogen sulfide can be scrubbed out and the methane used as a fuel in an engine. Unfortunately, there is not enough gas to do much more than cooking and lighting.

Back to the whole concept that matter is neither created nor destroyed, the wastes are broken down, liquefied and in the end are ready for the soil to deliver to the plants. The micronutrients we added to the soil went to the plants, then to us as food. We also took in additional supplemental micronutrients and our waste went into the digester. Our micronutrients are prepared and sent back to the plants for another cycle. Eventually, as we continue supplementing, and adding more of the missing ingredients, we will restore what was meant to be.

How Methane Digesters Work

In order for methane digesters to work you will need to provide an air tight chamber for the process to occur in, air or rather oxygen must be kept out. It needs to have a way to get your waste stream into it, a way to get the methane out and finally a way to get your fertilizer out when you are done. There are two main types of digesters and they are batch digesters and continuous flow digesters. If you look in the MOTHER EARTH NEWS archives you will find a good article about a few guys that built an experimental batch digester. If you are going to pursue digesters it is worth looking into. These batch digesters are the simplest so we will start there. You won’t find many in service because they are labor intensive and don’t fit in well with most systems.

The whole digesting process has to be done at 95 degrees which creates some problems for most parts of the world. We have to provide heat to make this happen and fit these big chambers into some sort of insulated room. Remember the discussions of insulation and heat loss in designing your greenhouse, well it’s even more important here because the difference in temperature becomes even higher. We need to build a heavily insulated room for our digesters and this is another good fit for Urban Crofting.

We will design our croft so the digester room is under or adjacent to the greenhouse. We can save some money on insulation and use less in the wall that separates the rooms. The heat that does escape from the digesters will help heat the greenhouse.

Continuous flow digesters are constantly fed and constantly putting out fertilizer. Notice the two chambers on each end of the digester, they allow waste in and fertilizer out without allowing oxygen into the main chamber. The waste steam is dropped into the left chamber and builds up until gravity pushes the waste through the opening in the bottom into the main chamber. When gravity pushes the waste into the chamber, the waste on the other end is pushed out through a similar opening on the bottom into the right chamber. It can only build up so far before it drops out the pipe on the end.

Continuous flow digesters are commonly used because you don’t have to save up your waste until you have enough for a batch. I toured on of these methane digesters just 5 miles from my home. It is part of a large dairy farm and the chamber is 1,000,000 gallons. The methane powers a large generator and the power is sold to the power company and the heat generated is pumped into the chamber for warmth. Odorless fertilizer is pumped to the fields with irrigation equipment and the undigestable fibers are used for peat pots and bedding for the cows. I can’t remember the particulars in the funding for this project but it is a vast improvement over the stench of raw manure being pumped through irrigation sprinklers.

The only information on small family sized digesters I was able to find when I started this about 4 years ago was one excellent set of plans and data from China. They built a brick and mortar digester that was buried underground. There was also data about the elimination of the pathogens in the waste, which was almost 100%. For them it eliminated the problem of raw sewage and provided a source of gas for cooking and lights as well as treated fertilizer for the crops.

I have yet to find that site again, but now there are quite a few kits available from China. This is obviously a growing market. Besides several different styles of kits there is also equipment available for taking out the sulfur dioxide, cooking stoves and lights. Many of the newer kits feature a bladder that expands as it fills. They also expand as the methane is produced which may eliminate a separate bladder needed to store the methane in.

In light of the new kits that are available I am going to reevaluate the plans I have for my methane digesters. My plans as they stand now are a combination of a batch and continuous flow digesters. I am designing this system to reduce the amount of time needed to manage this system and have some flexibility as to when you need to empty the tank.

The waste stream is still loaded as a continuous flow so you have the convenience and functionality of loading small amounts at a time and from different systems. This model produces gas as it’s being filled and then once it is filled is left to finish the digesting while another one is starting to fill. When it is done producing gas, the fertilizer is ready to be pumped to the greenhouse, gardens, trees and lawn.

Here are some photos of my system, it is not finished and it won’t be for a while. By the time this book get to the book stores I hope to have sold my experimental Urban Croft and moved to a dry climate to continue on with Urban Crofting. Much of the world is fairly dry and with the effects of global warming will become even dryer. I expect to be able to produce all of my food with the grey water and the liquid fertilizer. The greenhouse will have to be redesigned a bit to provide protection from heat as well as the cold. Anyways, back to the digesters, which will be finished later.

Example of a Methane Digester

In these pictures 5-8, you can see what I’ve built so far. Picture 5 shows a large pipe with the cutout on the bottom. This is the filling chamber. The waste stream drops into the pipe and flows out into the main chamber through the cutout. Picture 6 shows the stirring assembly. In the chamber of a batch digester it is possible to have a mat of floating material block the flow of gas. The stirrer will help keep that from happening. Getting the stirrer into the chamber was like building a ship in a bottle. Most of it was pre-assembled but the middle upright and the top cross piece were assembled through two openings in the top.

Picture 7 shows the pipes that feed the digester. This is my design for human waste and the two long parallel pipes each connect to a toilet. The pipes will be outfitted with cam-lock fittings for ease and minimizing the exposure to that horrible poop. When the tank is full the jumper hoses that attach to those pipes will be hooked up to the other digester and the filled one capped. The short pipe that comes out of the big pipe attaches to the black pipe on the wall. Since the filling chamber is not air tight it produces sewer gas and needs to be vented outside in the same manner as your current toilet is vented. The black pipe on the wall is connected to the vent system of the building. There is also a rubber boot that attaches to the top of the large pipe around the stirrer. This seals the large pipe forcing the sewer gas out through the vent system. The white pipes carry the methane outside the room to a manifold and then would go to a bladder for storing the gas.

Picture 8 shows the digester room. It has to be kept at 95 degrees so it is heavily insulated. You can tell it has thick walls by looking at the window. The walls are double studded 12 inch thick with 12 inches of R-38 insulation. And the ceiling has 2 feet of insulation.

Well those are the big pieces now to solve some of the other issues. The digester shown above is for human waste and I said earlier that I have two systems the other for yard, barn and food scrap waste. For now let’s keep on with the human waste. Once again, it is not finished or operational as there are permits and other processes to be completed first. This is an experimental system.

One of the biggest problems with human waste is that toilets currently use 5 to 10 times too much water. This is also a big problem for a world that is starting to run low on fresh water supplies. This is not a big problem to solve, though, as there are plenty of toilets designed for RVs, boats, trains and airliners that use only a fraction of the water. They range in price from $100-$200 for an RV toilet with a foot peddle pump for flushing, $500 and up for an electric flushing marine toilet and $1200 or more for one of those toilets like an airliner that goes whoosh! These toilets do not flush a tank full every time you push the lever, but rather only flush as long as the lever is pulled. You control how much water is used.

The inexpensive RV toilet needs to be virtually right over the digester because it has no pump, so its use will be very limited. The marine toilets with the electric pump have a longer range and can probably be 20 feet or so away from the digester, it also comes with a built in macerating blade built into the pump which chops up everything. The big whoosh toilet has a powerful pump and can pump across the house but it may not have the chopper blade. There are transfer pumps that have the chopper blade and can pump quite a ways.

My greenhouse and digester room are 120 feet across my back yard from my house. I was going to use the big whoosh to get things half way across the yard to a tank and a transfer pump in the tank to get my waste all the way to those cam-lock fittings on the digesters.

Anybody with a boat or an RV has already learned how to and what it is like to manage human waste in a similar fashion to what it will be like with methane digesters. It’s not the part of going camping that we look forward to but if it was so bad the campgrounds would be empty. Don’t be afraid of the horrible poop, there’s a lot worse things like spending all your time growing you own food and still not getting the nutrients you’ve earned.

There is one more thing about human and animal waste, in that it has already had some of the energy removed from it. In the case of the commercial million gallon digester up the road, they found that manure from cows was ok. But when they started adding some food type waste from commercial food processing plants the amount of methane increased greatly. So don’t be afraid to send the leftover soup that went bad down the marine toilet.

As we finish up the discussion on human waste it is important to note that there are laws regarding the handling of such waste. There are also provisions for experimental systems and I will have to pursue that avenue later. All along my plan has been to use the human waste on non-food crops to start with. This could be feed stock for the ethanol production, possible animal feed, as well as growing crops to put into the digesters. Even if the laws prevent the use on food crops there may still be a way to eventually get those micro-nutrients into our food. We could for instance fertilize the lawn with the fertilizer from the human waste digester and use the grass clippings in the digester for animal and yard waste which would be OK. At the very least we would be able to keep those micros in our circle of life.

Now let’s get to the non-toxic side of things. This waste stream includes animal waste, yard clippings, food scraps. This is the stuff that would wind up in most peoples compost piles and does not carry the stigmatism that human waste does. It has also not been processed by machines and chewed into bits by us humans. We are going to have to process our waste into small pieces and add a liquid to achieve that slurry state needed by methane digesters.

There is an off the shelf tool for this job already and that is a chipper/shredder. You want to get the one with a mini hammer mill inside. The chipper blade chops the bigger branches and the hammer mill part is where you put leafs. A hammer mill has a bunch of small blades on several horizontal shafts, while some chippers just have a few rotating blades. Hammer mills are the industry standard for tearing the crap out of anything, even up to car motors and big pieces of steel.

With much of the yard waste, clippings and animal waste it makes sense to let it dry out until it’s time to load the digester and then shred it and add a liquid. Some of the possible liquids available in our urban crofting are the spent mash from the ethanol still and cheese whey from the goats, besides mixing to make the slurry they each have suspended digestible solids. My plan was to load this like a batch digester. If you look closely at the pictures you will see a 6 inch cap on the top of the tank. The tank could be loaded by bucket and large custom funnel or possibly the diaphragm pump use to empty out the digesters. The optimum place to have your digester room would be in a basement so gravity could be utilized in loading.

One of the last problems is how to get the fertilizer to the plants, easily. My plan is to pump from the chamber into a drum in the greenhouse with a decent sized valve on the bottom and pour into a watering bucket. I already use a watering can for liquid fertilizer and it doesn’t take too long to do. As far as fertilizing your gardens, fruit trees and lawn goes, the pump will go a fair distance, after that a cart and tank behind a riding mower or yard cart. There are two kinds of pumps available and they are trash pumps and diaphragm pumps. You will find both at rental stores so you can try them out first to see what will work best.

Some other problems may be encountered while digesting concerning the health of your organisms that eat the waste and break things down. One of the problems I’ve read about deals with the carbon-nitrogen ratio. If you put in too much stuff high in nitrogen like grass clippings you will create a toxic climate for your organisms. My plan is to vary the input stream. There is plenty of info on the net. If they can do it why can’t you?

Last, and Certainly not Least, is Safety

You are starting a project that involves creating and using an explosive gas! This is something that deserves great respect but shouldn’t cause fear. Most homes are already outfitted for this explosive gas, another name for methane is natural gas and it heats most people’s houses and water.

Gas leak monitors, fire proof room, spark less heater and lighting, outside gas storage are just a few of the concerns that need to be addressed once we start building our digesters. From what I’ve read the most dangerous time with methane digesters is during start up when we are starting to produce methane and there is still oxygen in the tank. Fortunately we will have a source of non-explosive carbon dioxide which could be used to purge the tanks at start up.

Reprinted with permission from Urban Crofting: A Whole New Circle of Life Without Oil by Cliff Williams and published by Outskirt Books, 2013. Buy this book from our store: Urban Crofting.


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