Outside of Cairo, Egypt, at the Sekem Farm and Vocational Training Center in Bilbaes, Solar CITIES was invited this past month (October 2009) to build and test our new biogas digester designs in anticipation of our upcoming Blackstone Ranch/National Geographic Emerging Explorer's Innovation Grant, to be conducted in partnership with Dr. Katey Walter in Alaska, Germany, Egypt, Israel, Tanzania, Botswana, and Rwanda (among other places!).
During this preparatory pre-grant phase we built 4 different digester designs in order to experiment with different locally available materials.
Digester 1: (A success)
We started with a traditional ARTI India biogas digester as a control, but made some modifications to bring the cost down and improve winter performance. The typical ARTI household kitchen waste digester uses two cylindrical black plastic water tanks, a 1000 liter tank to hold the bacterial-food-slurry (LE 330 = 60 dollars) in Sabtiya Market in Cairo) and a 750 liter tank (LE 270 = 50 dollars) to hold the gas as it is created. This is a "telescoping" digester, so named because the smaller top tank, inverted and placed into the larger tank, rises and falls ("telescopes") as gas is produced and used.
The second modification we made was the reduction of the inlet feedpipe size from 4 inches to 2 inches. Since 4" and 3" PVC fittings are quite expensive, and most urban dwellers have access to a food blender, we felt that the larger sizes were unnecessary. In Germany, thinking like a sacred cow, we decided to try to make the "throat" size the same as that of a small ungulate and found 2" to work quite well if the food was properly ground up with water. And whereas the cost of a 4" tank connector is LE 85 (about 17 dollars) and a 3" connector LE 35 (about 7 dollars) because demand is relatively low relative to supply, a 2" connector can be found for as little as LE 6.50 ($1.30). So the cost savings are considerable. Food merely has to be fluid enough with small enough particle sizes that it won't clog up the "throat". We also used 2" connectors for the drain on the bottom (2" valves are much less expensive than 3 or 4" valves!) and a 1" plastic Zahran connector for the fertilizer-overflow outlet (since this is digested solids in a fairly homogeneous liquid matrix). For the gas outlet we use a 1/2" plastic Zahran connector. These simple modificatons reduced the cost of construction by nearly 300 LE (half the average monthly salary in our communities) -- from about LE 1000 to LE 700, putting the digester in the range of many more families.
One note: One has to be VERY careful with the cheap blenders on the Egyptian market -- do not overload with food and avoid fibrous material -- we burned ours out within two days of use and lost our LE 125 investment. Better, perhaps to buy a more expensive blender (next category starts at LE 250) but then the loss would be higher too if it burns out. The best thing is to get Egypt to start importing and then manufacturing heavy duty kitchen waste disposal units like the Insinkerator.
The third modification we made to the traditional ARTI design was to increase the surface area available to the bacteria for the reaction. We always put broken bricks -- the kinds with holes in them (much more surface area) on the bottom of the tank, but this time we asked the students to make strips of waste cloth and sew styrofoam floats to the top and then tie small rocks to the bottom and insert 10 of these into the tank. These act as hanging vertical strips that bacteria can colonize.
The final modification we made was to wrap the digester tank with cheap black polyethylene irrigation pipe to act as a heat exchanger, connected to the hot water tank from our solar hot water system. The hot water flows down the coils under gravity from the elevated Hot Water tank and is pulled by thermosiphon into the bottom of the solar collector. The digester can then be covered with insulation.
Photo left: Irrigation tube surrounds the digester to be heated by solar hot water for elevated winter performance. Photo right: CREATING YOUR OWN SACRED COW: According to ARTI India's Dr. Anand Karve, when constructing a biogas digester, it helps to "think like a Sacred Cow" -- cows don't eat manure, and the bacteria in their stomachs don't eat manure either -- they make manure. When creating a biogas digester all you are really doing is building an artificial cow as a home for the bacteria. The bottom barrel is the cow's stomach, the top inverted barrel is the cow's intestines. A funnel is the cow's mouth into which you put "chewed up food" (food waste in a blender) and water, and the 2 " pvc tube is her throat. The liquid fertilizer outlet is her urethra; when you give her water she expunges the equivalent amount of fertile tea. The gas outlet on top of the intestines is the cow's anus. When the intestines fill with gas you open the valve and she farts clean climate friendly biogas!
Digester 1 is the simplest and least expensive or complicated biogas digester you can build. Besides the addition of the heating coils and a covering of insulation, winter performance can be improved by building a cage to hold the gas collection tank and covering the whole thing with a sheet of black plastic and then a sheet of clear plastic. Nonetheless these digester designs are open to the environment and experience heat loss and, while suitable for subtropical zones such as Egypt, would not be suitable for north temperate zones in Europe, the U.S., Canada or Alaska.
Digester 2: (A failure)
Digester 2 was an attempt to use the same materials as digester 1 but create a sealed digester with a water jacket. The 1000 liter tank, instead of being the cow's stomach, filled with slurry, was simply turned into a heatable water bath. The 750 liter tank was placed right side up this time and configured as the cow's stomach. The gas collection was to be achieved in a separate vessel, a common square 1000 liter plastic water tank, which was to be pressurized by a similar tank above it. The liquid in the "water jacket" was connected to the solar heater, creating a "bath" for the inner tank. Since the liquid is not in contact with the slurry in the smaller tank it could conceivably be mixed with anti-freeze (ethylene glycol) for cold climate application, or the fluid could be a thin vegetable oil based substance.
The concept was rather good but unfortunately we were unable to seal the screw-on lid of the 750 liter tank so that it would remain water or gas tight. We tried using large rubber gaskets, grease and even an enormous amount of silicone. Eventually all sprung leaks. This is due to the poor thread design of these water tanks. They are not meant to be completely water tight and the lids do not screw on tightly.
This was a great frustration and disappointment to us, but inspired another idea:
Digester 3: An Imminent Success
(Photos not yet available)
Instead of using the cylindrical tanks to produce the gas and the square tank as a gas collector, we've decided to repurpose the materials in the other direction -- the square tank is now the sealed digester. We cut and turn the 750 liter tank upside down, placing it into the 1000 liter tank, exactly like the ARTI digester design, only this time the 1000 liter cylindrical tank contains NO slurry, only water (with anti-freeze). The gas from the square tank is piped into the upper tank, which is the typical gas storage container that rises and falls in typical telescoping fashion. The big difference is that only gas and anti-freeze containing water are found in this open telescoping part of the system so it doesn't matter how cold they get. The bacteria stay nice and warm in the well insulated square production tank which is heated by adding solar heated water along with the food slurry.
Digester 3 would be useful in cold climate countries because it is simple to build (almost identical to the ARTI design in simplicity of gas storage and retrieval and because the gas production vessel is sealed it could be kept indoors (in the basement, under a stairwell or in a shed) while the gas collection vessel could be located outside since it would not be disturbed by cold weather. Retrieving the gas could be done using bricks or some other weight source in the typical way (Hanna and Hussain use planters to beautify the systems).
Digester 4: The Tower of Power Water Displacement System -- A Qualified Success
Digester 4's design, which is more complicated, comes from the unpleasant fact that the typical cylindrical water tanks, ubiquitous in developing countries, are very difficult to find in developed lands, and are very very expensive when they can be found. We have built telescoping digesters in Germany using 500 liter and 300 liter cylindrical rain water barrels, but finding larger sizes is very difficult. It thus became necessary to create a system that uses the easy to find and relatively cheap square cage-mesh 1000 liter plastic water tanks which come on a pallete and can be found almost everywhere. In Germany we can get them for as little as 60 Euro.
In Cairo these sorts of tanks are also extant everywhere, costing about the same price (300 LE), so we decided to purchase a black one (photo below, right) and make it the actual digestor, and pipe the gas to a separate gas collection tank made of the same material.
There are two problems with a rigid tank as gas collector. The first is the question "what will the gas be displacing?". In the telescoping system the gas collector is immersed in a water slurry and so is filled with that liquid. The gas lifts the collection vessel out of the water, and as the gas is used it descends back into the water. The water thus helps push the gas out of the tank. With a separate rigid tank you have to decide what should be in the tank, and how you will get it out. If you start with air, since methane is lighter than air, you could theoretically push the air out of the bottom. But the gas and the air will tend to mix, and this would creates either an explosive mixture when lots of gas is present (biogas is generally safe because it is already a mix of 60 % methane and 40% carbon dioxide; it becomes flammable when it combines with air) or a mix of gases too dilute to to much work (when little gas is present). The second is the question "what will replace the gas in the tank as you use the gas?" Air creates the same problems already stated.
Water is the logical choice for a medium to be displaced by the gas. Unfortunately for the gas collection phase water is heavy and gas is light. This means that if the water tank is above the gas production vessel , the water will tend to push the gas back into the production tank. Fortunately for the gas use phase water is heavy and gas is light. This means that a water tank above the gas collection vessel will push the gas out of the tank for pressurized use.
The devil is in the details, designing a system with the tanks and their inputs and outputs at the right relative heights. With no previous systems to go by, we had to design our water displacement systems from scratch. And because it took 3 weeks from the loading of the bacterial starter material (cow manure in this case) until first flammable methane production (which is when you start feeding the system exclusively kitchen waste) we had to build a system that would allow us to test various hypotheses about heights and placements that would be ready when the gas first came.
What we did first was to build a three story tall gas collection/water-displacement-and-replacement system. The bottom two tanks would serve as gas collection vessels. Both were completely filled with water. The gas would be piped to either one of the two tanks -- the white tank on the bottom in the picture, or the white tank in the middle of the picture, and would enter from the top portion of the front of the tank. Our experience in Germany on our porch with the ARTI style open digester, with the telescoping collector kept still by a pile of bricks, was that since methane is lighter than air, it prefers to rise. Rather than displace water from a tank at the same height as the digester, it simply forced water up and out of the open digester itself. Fearing a similar result we put a valve on the outflow pipe of the sealed digester and made the food intake pipe wide (back to 4" PVC!) and tall. And we mounted one of the collection tanks ABOVE the gas production vessel. This way we had our bases covered. The bottoms of either of the tanks could be variably connected to a one inch polypropylene pipe coming from the bottom of the top tank, which we left empty. This one inch pipe had a valve on it at the bottom, before it entered the gas collector.
Photos: The three story "power of tower" was used for several weeks of testing. It was determined that a three story system was un-necessary and ineffective and that a two story system would work better.
In front of the 1 inch pipe, coming from a T, we placed a swinging door check valve and a half-inch rubber hose going up to the top of the top tank. We filled this hose with a column of water all the way to the top. The theory was that as gas pressure built up in the collection tank in would force the water down inside the tank. This would then push water through the check valve and cause the column of water to spill over into the top tank. As the gas filled the collection tank the displaced water, in turn, would fill the top tank. When the gas collection tank was full with gas the top tank would be filled with the water displaced. To use the gas we would simply open the valve on the 1 inch pipe connecting them and the water pouring into the bottom of the lower tank would force the gas up and out and into the kitchen. The cycle would repeat itself as we produced more gas.
The devil, as we've noted, is in the details. When the black square collection vessel started producing gas we found that it didn't have the pressure necessary to force water out of the middle tank and up into the top tank. Instead the water, even though coming from the top of the gas collection tank, forced its way back into the gas production vessel. To solve this problem we installed a check valve at the outlet of the gas production vessel but found that most of the time the water pressure was greater than the gas pressure and it kept most of the gas stuck in the gas production vessel (though some did make it up to occupy the very top of gas collection vessel, which we found out when we opened it and did a flame test -- it created a small but surprising explosion, which I call "doing a Katey Walter".)
Photo: Solar CITIES intern and MBA student Mike Rimoin experiments with the placement of a check valve.
We then moved the check valve to the top of the gas collection vessel and found this did improve the amount of gas going into the tank but it still wasn't fast enough for practical use. By contrast, when we put the hose into the top of the bottom collection tank, the gas bubbled in fairly vigorously. And because the inlet to the collection vessel was near the same height as the outlet from the gas production vessel, no water would back up into the gas production tank. It appeared that by keeping the slurry overflow valve shut and loading the feed inlet pipe with water, a positive pressure was created that pushed the gas into collection tank. It also appeared that some of that pressure was able to push displaced water up and into the water collection/pressure tank. However as we left Egypt we didn't get full results on how effective the recycling system works.
Photo shows the construction and placement of the water displacement system described above. The student is shown filling the water tube before placing it into the top tank.
Nonetheless, it appears that this system will work and could have broad application in temperate and cold climates. As shown in the picture below, the final configuration is to have the gas production tank (the ground mounted black square tank to the left, which is filled with slurry and gets the food waste) connected to the top of the sealed gas collection tank (also ground mounted and thus at the same height). This bottom of this collection vessel is connected to an identical tank mounted above it in two ways: 1) a one inch (green polypropylene) pipe with a valve connect it to the bottom of the top tank. 2) a half inch hose with a check valve connected to a T and filled with water connects it to the top of the top tank. To the left of the system is our polythylene heat absorber and plastic tank solar hot water system to provide the hot water needed to keep the reaction going in winter. The black production vessel still needs to be insulated, and the system should be fairly complete. The gas collection vessel does not need to be insulated and can be filled with water containing anti-freeze for cold-climate performance.
Photo: After much testing, we finally hit upon a recycling water displacement design that seems to work. One of its advantages is that the water displacement/gas collection tanks can be filled with a fixed quantity anti-freeze and the sealed digester can be insulated and kept warm by adding solar heated water when it is fed. The system requires three 1000 liter (1 ton) tanks which cost about 60 Euro each (about 90 dollars), so the base cost with plumbing is about 350 to 400 dollars. The solar heater adds another 300 to 400 dollars to the price. It is hoped that the use of psychrophilic bacteria will obviate the need for a solar heater.