Friday, September 11, 2009
Animation of a simple telescoping biogas digester
For this simple animation describing just how simple it is to build your own biogas digester I downloaded Google Sketchup 7 and then used Google 3D Warehouse to download a model of a digester by roilbilad130 called "Biogas untuk limbah dapur" (which is Indonesian for "Biogas for Kitchen Garbage"). I changed the food inlet pipe and the fertilizer output pipe in the model to reflect the way they work in the ARTI India systems we have been building in Cairo and Germany and set different sequence animation key frames in Sketchup and did screen capture in Snap Z Pro. Building real digester is about as easy as building a digital one in sketchup -- it really is just two plastic barrels, one inverted inside the other, with three pipes -- one to get the food in, one to take the liquid fertilizer out and one at the top to deliver the gas to your cookstove or generator. 1000 liter tanks should get you about 2 hours of cooking gas a day if you live in a warm climate. As shown, we intend to use psychrophilic bacteria at the bottom to increase cold weather production.
The psychrophils can be obtained from lake mud in the arctic circle area. We will be obtaining our samples working with Dr. Katey Walter Anthony and Laurel McFadden at the University of Alaska, Fairbanks.
Dr. Anand Karve, inventor of the household biodigester process in Pune India reminded me to "think like a sacred cow": cows eat food, not manure, he told me, and the bacteria in the cows stomach and intestines also eat that food, mixed with saliva and water. The goal is to replicate the inside of a cow's digestive tract to help the bacteria get the most energy from the food. We only use the dung/manure to"innoculate" the system on the first day because it is the easiest non-invasive way of getting the "bacterial biogas experts" out of the animals' guts and into the tank.
In many respects making biogas is similar to making yoghurt. If you have a friend who has an active culture of methanogens from their own biogas digester (or from a septic tank, or from their baby's diaper (:)) ) just put them in. If you don't use manure, however, I'm not sure what or how much to feed them in the beginning. We will let you know as soon as we do since to culture the psychrophils from the arctic mud we will need something to encourage their growth.
The process working with mesophils from animal dung, however, is started very simply by taking about 40 or 50 kg (maybe 4 to 6 10 or 15 liter buckets) of manure (we used horse manure in Germany, cow manure in Egypt, but any manure will do) and mix it into the bottom container with water (this is per 200 liters of water but we just go ahead and fill the whole thing even up to a 1000 liters of water; it may make the wait time for first flammable gas a bit longer as it takes the bacteria time to reproduce and fill that volume, but it worked fine for us as we didn't want to haul in more manure.). Then put the top barrel on and open the top valve so all the air escapes and the top barrel sinks down into the bottom barrel all the way.
You then close the valve at the top so no air can get in and just let it sit there for anywhere from 2 weeks to a month (depending on climate). During this boring period the bacteria will multiply. At first they will just produce CO2. After a few weeks open the valve and flame test with a candle (we didn't use a flashback arrestor! Doh! :) ) . The first couple of times the escaping gas will blow out the candle. Eventually, after a few days, the methane content will exceed 50%.
Once the gas starts to burn you can start feeding your digester ground up food waste (mixed in a blender with water, about 1 to 2 Kg a day, but start slowly so as not to overwhelm the bacteria; start with 200 grams then 400 the next day etc.). Soon the CH4 content at the top of the tank will exceed 60% (since CO2 is water soluble it can get up to 70%) and can be directly used in cook stoves and engines. Hope that helps explain it. It works well and is fairly simple you'll find. Give it a try !
Remember you only have to put the manure in THE VERY FIRST DAY. After this no more manure is needed (though our neighbor at Imbrahm Recycling who has a 1 million Euro commercial operation says adding some fresh manure every couple of months keeps the bacterial colony in top form. Logically the bacteria are symbionts in animal guts and, having co-evolved there, reproduce there best. In the artificial environment of a plastic tank there is no guarantee that succeeding generations of bacteria will be selected for the right traits.). To start another digester, however, you don't need manure, just the effluent from another active digester! Started with this active culture the waiting time is much shorter.
The following description of the process by inventor Dr. Anand Karve of ARTI India in his paper "Taking Action to Rid the World of Indoor Air Pollution" is very useful and can be found in its entirety at proceedings of the CleanAirSIGe-conference: 16 -27 July 2007:
"The current process of biomethanation, which uses feedstocks like cattle dung, human feces, distillery effluents etc. is highly inefficient, because the nutritionally available calories and nutritive value of those substances is quite low. Common sense tells us that the energy output of a system must be matched by the input. Methane has a calorific value of 11000 kcal/kg. If one wants a high output of methane from this system, it must also receive input having a correspondingly high calorific value. Nowadays, municipal solid waste (MSW) is also being used as a source of methane. Food rests in the MSW have a relatively high calorific value. But in the process currently being used MSW is subjected first to aerobic fermentation, in order to reduce its bulk. The predigested material, having very few calories left in it, is then fed into the anaerobic digester for producing methane. This is called a biphasic fermentation system. As a rule of thumb, one can state that biogas production systems operating on human or animal faeces, distillery effluent, or two phase digestion of municipal solid waste, all produce about 100 kg methane per ton of feedstock. The traditional biogas generating systems require about 40 days to complete the process. The time can be shortened by using thermophilic bacteria and digestion under higher temeperature, but the input to output ratio remains unchanged.
'Use of cattle dung as the feedstock is the main factor limiting widespread use of methane as household fuel in rural India. The present domestic biogas plant requires daily about 40 kg cattle dung (from 6 to 8 heads of cattle). Because the dung must ferment for about 40 days, the size of the biogas plant is also large. Restrictions of space, money and absence of sufficient animals prevent many aspirants for having a biogas plant based on this technology. The servicing of this plant requires mixing the dung with water to make the feedstock, filling it into the biogas plant and the disposal of about 80 to 100 liters of effluent slurry. These chores must be done daily and they are considered to be a bother by the users.
"ARTI developed in 2003 a new biogas technology which uses high calorie feedstock, consisting of starchy or sugary material. This material is capable of producing about 250 kg of methane per ton of feedstock (on a dry weight basis) and the reaction takes only 1 day to complete. In the case of a household biogas system, application of daily just 1 kg of feedstock is enough to provide a family with sufficient biogas to cook all the meals. The material that can be used as feedstock in the new biogas system consists of waste grain, seed of any plant species, oilcake of non-edible oilseeds as well as nonmarketable or nonedible fruits (wild species of ficus, overripe mango and banana). Even the flour mill can be used as feedstock.
"Because of the smaller quantity of feedstock and also because of the short reaction time, the digester size and also its price are drastically reduced. The gas holder of the domestic model of the new compact biogas plant has a capacity of just 750 to 1000 liters which is enough to cook two meals for a family of five. The user applies 1 kg feedstock in the morning and another kg in the evening. The total effluent slurry generated daily by this system is hardly 10 liters. Thus, this system does away with the daily drudgery of handling huge quantitities of cattle dung and the daily hassle of disposing of about 100 liters of spent slurry. The new biogas plant would be available at a cost ranging between Rp. 10,000 to 12,000. This technology brings cooking fuel in the form of methane within reach of every household.
"The apparatus itself consists of two plastic water tanks, which are generally available in shops selling sanitary ware and plumbing hardware. The top of each drum is cut open so that the smaller drum can nest in the larger one. The outer drum serves as the digester and the inner drum, which is placed upside down into the outer drum, serves as the gas holder. The inlet pipe for the input is a vertical pipe fitted inside the gas holder. It runs along the entire length of the gas holder. The gas outlet is also fitted on the inner drum.
"To begin with, the system is loaded with a slurry containing about 250 kg cattle dung and water. Then one waits about 2 weeks, til the gas emanation begins. The gas is tested by burning it. Once it starts producing combustible gas, one can start applying the high calorie input, as explained above.
"According to an estimate by the World Health Organisation, about 3 million people in the world die every year as a consequence of exposure to suspended particulate matter in the air, and that 85% of the deaths are due to indoor air pollution. The indoor air pollution is caused mainly by traditional cookstoves, using traditional biomass based fuels. Considering India's share in the world population, the estimated deaths due to indoor air pollution in India come to annually about 500,000. Although acute respiratory infection is the single largest category of deaths in children under 5 years of age, indoor air pollution remains a neglected topic in India, because the number of persons killed annually by polluted water is much higher than that killed by polluted air. It must, however, be emphasized, that while polluted water can be made potable by filtration, chlorination, boiling, reverse osmosis, distillation etc. there is no simple treatment to purify polluted air. It is therefore necessary to reduce the pollutant load in the air at the source itself. Methane as cooking fuel would prevent these deaths. It is nonpolluting, renewable, cheap and CO2 neutral.
"In addition to household fuel, it can also be used as fuel in internal combustion engines."
We at Solar CITIES urge you to build your own digester - even if you never used the fuel, you will find that it is much better than composting for dealing with kitchen garbage and making fertilizer!
T.H., Sybille and Kilian Culhane next to a telescoping digester they built with Hanna Fathy on his roof in Manshiyet Nasser's Zabaleen community, Cairo, Egypt.