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Biodigester Design and Construction

RURAL COSTA RICA

Small-Scale Biogas Use with Biogidesters in Rural Costa Rica


Biodigester Design and Construction

Biodigester Design & Construction

Understanding the Basics of a Biodigester

sketch of the biodigester tank's profile
Above is a sketch of the profile of a biodigester to better visualize the concept. In the picture, A represents the biodigester tank where the water and manure mixture is digested by the bacteria. When working with cow manure in a biodigester of this size (1.9 meters deep X 1.5 meters wide X 3 meters long), every day you need to add 10 gallons of water and 5 gallons of manure. When working with pig manure you work with a 1:1 ratio, or rather, 5 gallons of water for the same 5 gallons of manure. In Costa Rica they use twice as much water for the cow manure because the cattle grazes on grasses, making the manure more fibrous than that of pigs. So, keep in mind that grain-fed cattle may produce less-fibrous and easily-digested manure. In the picture B and C represent the entrance and exit tubes respectively. The entrance tube should enter the tank near the bottom and the exit tube should enter the tank just beneath the first row of cement block. D and E represent the mixing tub and the collection tub respectively. The mixing tub will ideally be more than 15 gallons in volume in order to mix the water and manure thoroughly. The mixture should have a uniform consistency to facilitate optimal digestion throughout the tank. Also, in the picture, the green circles represent the bottom support pins that catch the frame of the plastic in case the water level decreases drastically. The purple circles represent the top hangers against which the frame of the plastic rests as it tries to float up to the water surface. The curved tubes that enter the tank on each end are to hold the mixing rope. The mixing rope is to have 3-5 gallon jugs half-full with sand attached. When two people tug back-and-forth on this mixing rope for a few minutes daily, the partially submerged gallon jugs break up any thick film that may gather at the surface, suffocating the bacteria in the tank below. The dotted yellow line represents the liquid level. Notice that the level comes right up to the rim of the exit tube. This parity is important, as every day that you put in 15 gallons of mixture, the exit tube, in theory, will discard the same volume into the collection tub to be used for fertilizer. The black dome that hovers over the tank is the plastic frame that fits just under the top hangers (purple circles) and holds the plastic that balloons up when the biogas, represented by the upward arrows, bubbles up from the surface of the water/manure mixture. The biogas then escapes through the PVC tubing represented by the blue line that extends above the middle of the plastic. Through this tubing the biogas is transported to the kitchen to be burned for cooking.

Biodigester Materials

Completed biodigester The biodigester design that the Santa Fe Women's Group used is a fairly simple and inexpensive one. A few of the tasks, like digging the hole, may be quite labor-intensive, and some of the tasks require certain skills, like creating cement walls and laying cement block, but the cost of the materials is relatively modest at around $300 US in Costa Rica, and the construction time is fairly short at about one week. Below is a list of the most essential materials that the Santa Fe Women's Group used for its biodigesters. In order to facilitate a good understanding of the instructions later on, all of the materials in the list include a brief description of their roles in a functioning biodigester. Some of the materials not included in the list are not as essential and can be replaced by other parts that are more convenient for your particular situation. These options will be explained as we go along.

Quantity Description
2 Cubic meters of sand to mix with the cement to make the walls of the biodigester and to fill the cement block
1 Cubic meter of rock to mix with the cement and the aforementioned sand
5.5 Meters of a strong, yet flexible, plastic sheet with at least 2.8 meters in width. This plastic will be used to contain the biogas that is produced in the biodigester tank
4 Meters of 3" PVC tubing to use for the entrance and exit tubes for the biodigester tank
9 50-kilogram sacks of cement to make the walls and the floor of the tank, as well as to fill the three rows of cement block. You may also use the cement to mount the mixing tube on the entrance tube.
60 Blocks of cement measuring 12 cm X 20 cm X 40 cm to make the three rows into which the bottom pins and the top hangers are placed.
1 1/2" PVC tubing sufficient enough to make a rectangular frame with a circumference of 16.6 meters and to carry the biogas from the biodigester to the kitchen where it will be burned
* Enough rebar to lay down the three rows of cement block along the edge of the tank
2 Tubes with a rounded elbow to put at each end of the biodigester like shown above in the picture. The mixing rope is to be threaded through these tubes.
5 Meters of thin rope to thread through through aforementioned curved tubes to mix the water and manure inside the tank
3-5 Gallon jugs half-filled with sand to be attached to the mixing rope, partially submerged in the water/manure mixture to break up any thick film accumulated at the top of the tank
20 Curved tubes for the top hangers. See the pictures below of the hangers in place to get a good idea of what these are like.
12 Straight tubes for the bottom pins. Again, see the pictures and you should be able to find something similar at a hardware store.
There are other materials that you will possibly need, like wood, nails, and sheets of tin for making the roof for the biodigester, but I won't include these materials, simply because some people will use other materials or different quanitities of the same materials. Also, the connection of the the tubing to your gas range in the kitchen will require materials as your situation demands, not simply the materials put on some list that I make. I also understand that I may not be the best at explaining all of the materials and construction steps, especially considering the limited explanatory scope of this web page, so if you have any questions or suggestions, please contact me.

Biodigester Construction

Pouring the cement walls of the biodigester Now that you know a bit about how a biodigester works (you may have already known much more than I) and the basic materials needed, you'll have any easier time making sense of the following construction directions. To build the biodigester, first you need to dig the hole. The hole should be 1.5 meters wide by 3 meters long (or longer if you're capable of greater production, but the measurements of the plastic indicated here are for a biodigester of 3 meters in length) by 1.3 meters deep (the three rows of cement block around the edges should make an overall depth of 1.9 meters). Once the hole is fixed to the proper dimensions, you need to dig the two ditches—one for the entrance tube, and one for the exit tube. (See the picture below to see the entrance ditch in the forefront with the 3" PVC tube placed within) The entrance ditch should be dug at about a 45° angle, entering the tank as close to the bottom as possible, leaving no more than 30 centimeters between the point of entrance and the biodigester's floor. The exit tube should be dug at roughly a 30° angle with the ditch entering the tank no more than 40 centimeters from the top of what should at this point be a 1.3 meters-deep hole. The entrance and exit tubes can be placed in now or after creating the cement walls, making sure that they are sealed firmly within the cement walls and do not stick out too far into the tank. Also, you need to make sure that the exit tube is exposed above ground level by at least 40 centimeters. (This is roughly where you will accomodate the water/manure level when you fill the tank) The top of the entrance tube should be at least 70 centimeters above the top of the top of the tank at this point.

biodigester tank with two of the three rows of cement block Then you need to create the cement walls. The amount of materials used may vary in this step, as different people may use different measurements when mixing cement. The Women's Group used roughly a cubic meter of rock and two meters of sand to mix with nine 50-kilogram sacks of cement to pour the cement walls and to fill the rows of cement block.

Once the cement walls are ready, you can lay the cement block around the rim of the tank. In the first row place one plastic peg in every other space between the blocks about halfway up the height of the block. The plastic pegs should stick out at least a few inches, for they will support the plastic frame if the water/manure level in the tank recedes. As you are placing the first row, make sure to place the tube for the shaking rope on each of the short ends of the tank, exactly in the middle. (See picture below to see the placement of the tube that holds the rope) Then, in the second row of block, the PCV hooks are to be placed in every space between each block. At least half of the hook needs to jut out into the interior of the tank, for these hooks are important to maintain the plastic frame submerged below the water/manure level within the tank. Then, you can place the third row, whose purpose is to allow room between the top hooks and the top of the last row of cement block in order to ensure that the water/manure level remains higher than the plastic frame that creates the water seal, trapping the biogas within the biodigester. At this point, your work on the tank is just about done. All that remains is the cement floor, which will prevent the water/manure mixture from seeping through the bottom of the tank. The floor can be made with a cement mixture with sand and/or rock, requiring about one full sack of cement.

biodigester tank with bottom pins, bottom hangers, and mixing rope Now that you're done with the tank, you can build the roof that protects the biodigester from the elements. I am not going to explain how to build shelter for your biodigester, as there are a number of different ways of doing it with several combinations of a vast array of materials. However, I will tell you that it is important to cover the biodigester tank entirely, and perhaps a little bit more, because direct contact with intense sunshine can deteriorate the plastic cover. Also, a lot of rainfall seeping in between the cement block and the floating plastic frame will dilute the manure/water mixture.

Another part that you can make at this point is the loading tank. This is also a part that can be made with whatever you have available. Put in general terms, though, the loading tank should be big enough to mix the 15 gallons of manure/water added daily. The container is to be mounted on the entrance tube. The Women's Group hollowed out the bottoms of barrels to then mount them on the entrance tubes using cement. Since you need to mix the water and the manure well to get a consistent liquid mixture, it is important to be able to cover the hole of the entrance tube. In order to do this, you can use some sort of plug that fits a 3" PVC tube, and then employ a chain to be able to pull the plug without reaching into the manure mixture every time you load. Another way to plug the hole, and perhaps a better one, is to fit a valve to the entrance tube that you can easily open and close. When not loading, the the entrance tube should be closed at all times. This will prevent rainfall from entering the biodigester tank and diluting the manure/water mixture inside.

Now you can prepare the plastic cover for the tank. First you need to place the plastic flat on a clean floor. (Rocks and other debris may damage the plastic) Once the plastic is placed flat on the ground and cut to the dimensions of 5.5 meters X 2.8 meters (the plastic that the Women's Group comes in rolls 2.8 meters wide, so trimming the sides was not necessary), you can use a permanent marker to draw a inscribed rectangle 20 centimeters inside the edge of the plastic. (See picture below) Then, you can cut a pentagonal shape out of the plastic on each of the four corners of the plastic. The pentagon is to have each side 10 centimeters long. (Save the plastic pentagons to be used later) These cuts will help you pass the PVC tubing through the plastic slits that you will make. To make these slits for the PVC frame, you need to apply PVC rubber cement just outside the rectangle that you already inscribed 20 centimeters inside the plastic. Start applying the glue on one side as you then fold the edge of the plastic to stick right where you made your mark 20 centimeters in. Once done with the first side, do the other three the same way, and your end result should be a rectangular plastic with four slender pockets around the edges with the corners exposed through which you will pass the PVC tubing frame.

sketch of biodigester plastic dimensions

point at which the biogas escapes from the middle of the biodigester plastic Then, you need to make the hole in the middle of the plastic. To do this, fold the plastic like you would a blanket, doing two folds (the stack is then 4 sheets thick). Then, find the corner of the stacked plastic that is equivalent to the very center of the plastic sheet and take a very small snip at the very tip with a scissors. Unfold the plastic and you will see a small square hole of roughly one square centimeter. Then, use two of the plastic pentagons that you cut from the plastic and cut them into squares with 10 centimeters on each side. Then, cut a hole in the center of the plastic squares equivalent to the hole in the middle of the large plastic sheet. Then, use your glue to paste the square patches on each side of the plastic with the holes lined up. Then, you need to insert a valve that allows the flow of the biogas from the plastic bag to the 1/2" PVC tubing. To do this you can use a setup with both male and female valves and washers on each side of the plastic, the outside of which is shown in the picture here. Make sure that everything fits nicely and that you do not puncture any part of the plastic during this stage.

biodigester setup that brings the biogas to the kitchen Then, you can prepare the PVC tubing frame that fits the plastic within the biodigester tank. To do this you can cut your 1/2" PVC tubing into pieces barely shorter than the measure of the inside of the rows of the cement block. The rest of the length (only a centimeter or two for each side) will be made up by the elbow connectors that connect the four pieces of tubing. Once the tubing is cut you can gently guide the tubing through the pockets made around the edges of the plastic. Once in place you can join the pieces of tube with the elbow connectors. Then, you can acoomodate the plastic underneath the top hangers in the tank (the water level in the tank is preferably up near the top hangers at this point). Now you are ready to connect the rest of the PVC tubing to the valve made in the middle of the plastic. For this you will need to utilize another elbow to direct the tubing towards the kitchen. Then, slightly away from the biodigester, but still within the confines of the shelter that you built, you will need to create the safety water seal, the gas switch, and the biogas filter. In order to do this you will need a 2-liter Coke bottle, into which you insert a tube that will be submerged at least 2" under water. This water seal allows the biogas to escape if too much pressure is built up within the plastic. Then, you can fit a switch to the PVC tubing to shut off the flow of gas to the kitchen when not in use. Then, in a 1" tube that you can fit to the 1/2" PVC tubing with the correct adapters as shown in the picture, you can stuff a few pieces of steel wool into the tubing in order to filter out some of the impurities in the biogas. Putting in the steel wool, and replacing it every few months, will prevent chemically-induced marks on the bottoms of pots and pans. Then, you can place the rest of the tubing necessary to carry the biogas to the kitchen.

biogas burning on the biogas range The connection of the tubing to your gas range (now biogas range) is not necessarily difficult, but the methods will vary, depending on the gas range that you have and the other materials you are using. I trust that you will be able to make the connection in the kitchen based on your own possiblities and preferences. Once the the connection is made in the kitchen (or perhaps before if you were very anxious), you can raise the water level in the tank about 10 centimeters above the top hangers and start putting in the manure/water mixture. You can start by putting in the 15 gallons every day of a 2:1 water-to-manure ratio for grazing livestock, or the 10 gallons of mixture of a 1:1 ratio for grain-fed livestock. Bit by bit, the tank will attain the proper proportion of water and manure, and within roughly 3 weeks you will have an inflated plastic cover and you can start cooking with your new biodigester.

If you have any questions about the construction or maintenance of this style of biodigester, visit our Biogas and Biodigester FAQ's page. If you want to know more about the Santa Fe Women's Group's biogas project, visit our biogas page. If you have any specific questions about the construction of this biodigester style contact us. If you want to explore other recommended online resources consider the following:


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