|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
When the digester is built and ready to start. The safest way to start is to completely fill the digester and the gas storage tank with water. The floating gas tank should be an inch shorter than the water tank it will be floating in. It will be totally under water before biogas production begins. In this way there will be as little oxygen as possible in the system. Oxygen kills the biogas bacteria and under certain conditions, the mixture of oxygen and methane can be explosive. Come back 24 hours later, and if the water level has not dropped anywhere in the system, there are no leaks that need repairing.
Make sure that all gas pipes going to gas storage tanks, engines, stoves, etc., are connected but that only the gate valve going into the gas tank is open and that the water level in the digester inlet and in the digester are the same. Open the outlet valve until the water level has dropped about 5.0 cm (2.0 inches) below the level of the bottom of the digester roof. Permanently mark this level on the side of the inlet pipe. In normal use, the level of the slurry should never be too far from this mark. If it is often above the mark, the digester has been overfed or the sludge under removed and the possibility of clogging the gas pipe with scum becomes real. On the other extreme, the slurry level can drop too low. If the openings from the inlet or the overflow pipe are exposed by the slurry level dropping below their tops, biogas will escape and oxygen will get in.
According to L. John Fry, 40 days is Just about right for the amount of time any one day's load of slurry should stay in a horizontal digester. That is why the daily slurry volumes listed in the Facts and Figures section of the appendix are all 1/40 of the volume of the digesters they are going into. Another important factor in preparing the daily slurry is how much water to add to the animal and/or plant waste.
The percentage of solids in the slurry must be kept at approximately ten percent. More detailed information on this subject is in the Facts and Figures section of the Appendix. One other thing: add the slurry to the digester at the same time every day, or better yet, divide the daily slurry volume into two or three equal parts and add each part at the same time every day. This more gradual step-method of adding slurry will result in a more stable digester, which will result in more biogas.
Digesters are very sensitive. If there are lumps in the manure, if the plant waste is not in small pieces, the biogas-producing bacteria will have a hard time breaking down the waste so that methane can be produced. The more dissolved the solids are in the slurry, the higher the biogas production rates will be.
One method that can be used to solve the scum problems of using plant waste is to first crush, grind, or shred the plants. Then break down (saccharify) the plants with lime. The plants can then be used after a few days composting to make a digester slurry. Instead of going directly into the digesters, plants could be used as animal feed, especially for cud-chewing ruminant animals such as cattle and water buffalo. (Livestock can also be fed the water from sludge aging ponds, which are full of nitrogen-rich algae.)
Thirty-five degrees centigrade (96° F) is the digester temperature at which the highest rate of biogas production occurs. It is also important that there be no wild swings in the temperature of the slurry inside the digester. In addition to heating the digester, many heating systems also heat the slurry (or the water that is to be mixed with the organic matter) before the slurry goes into the digester.
A very efficient source of heat for the slurry, inside or outside the digester, is the excess engine heat from a stationary engine that is fueled by the biogas. Some systems use hot water from inexpensive solar heat collector panels. There is more information on heating digesters in Chapter Seven and on solar heating in the New Ideas section of the Appendix.
If at all possible, for the first few days, the slurry for a new digester should be sludge from a working biogas digester. This sludge will be full of biogas producing bacteria which will help get the new digester producing usable quantities of quality biogas within three to four weeks. If this is not possible, start with fresh waste. It will work ok; it will just take a few more weeks to get a gas production going that has a sufficiently high percentage of methane in it to burn on its own. When a biogas digester first starts producing gas, most of the gas is carbon dioxide, not methane.
For the first couple of months, use the sludge that is taken out of the digester to mix with the fresh waste going in. Because the digester started completely filled with water, the sludge will have a very small percentage of solids in it. It will have some biogas bacteria in it, which will help get the digester working faster and in any case, the sludge will be too weak to make a good fertilizer.
After the digester has been in operation for one or two months, the sludge can be used for fertilizer or it can continue to be used to dilute the fresh waste.
If the decision is made to use the sludge to dilute the fresh waste, the solid portion of the sludge must first be separated out.
At this point, keeping the solids no greater than ten percent of the slurry becomes very important and will always remain important. There are many ways to separate the solid from the liquid sludge, including letting the sludge run through gravel or a screen or by a series of ponds where the solid portion is raked off. Once separated, the solid portion can be dried and used as fertilizer.
If the ten percent solids in the sludge are not separated out before the sludge is used to dilute the fresh waste, the slurry in the digester will, after a while, get too thick. Less and less gas will be produced and eventually it will be necessary to clean the digester out and start over again, long before it is time for the once a year cleaning that even most well-run digesters will need. Do not let the sludge fool you; it may look very watery, but it is full of solids and plant fibers (even if only manure is used) suspended in the liquid.
In any case, when the sludge, solid and/or liquid, is used for fertilizer, it will have to be aired out for a couple of weeks in shallow ponds before it becomes safe to use as a fertilizer. In that time, the parts of the sludge that are toxic, that can kill plants and fish, will evaporate into the air, and oxygen (which fish need) will mix with the sludge. (There is more on using sludge in Chapter Eight.)
During a biogas digester's start-up period the methane content of the gas is very low. Even if the gas will burn, the flame will go out when you take the match away. Do not try to save or use the gas, but remember not to smoke cigarettes when the low quality gas is released, or the result might be a burned face. Do not let all the low quality gas escape; leave some pressure in the system.
When fresh slurry was added to the demonstration model digester, the valve from the digester to the separate gas storage tank had to be closed because otherwise the process of removing sludge and adding fresh slurry made the gas tank fall quickly and then rise quickly.
Probably the most important thing to remember is not to let the level of the slurry drop below the top of the openings for the inlet or overflow pipe.
It can be very disappointing to see big bubbles of biogas escaping from the digester. Do not light a match to the bubbles to see if it is biogas; you might burn yourself, or if the flame gets inside the digester, the digester could explode. When the gate valve from the digester to the gas storage tank was closed before taking out the old sludge and putting in the new slurry, there were no more wild swings in gas pressure or loss of gas from the digester.
The daily routine started with mixing the organic waste with water. A weighing scale makes it easy to get the right combination of waste and water. But if a scale is not available, weigh a bucket full of the usual waste on a friend's scale, in order to know the weight of a particular volume. Because a liter of water weighs one kilogram, it will be easy to figure out how much water or liquid sludge to mix with the waste (see chart in Appendix). The liquid sludge will not weigh too much more than water, but different kinds of waste will weight different amounts for the same volume.
Another method for getting the correct mix of solids and liquids in the slurry is to measure the specific gravity of the slurry. Details on this method are in the Facts and Figures section of the Appendix.
After the slurry was mixed, the valve between the digester and the gas storage tank was closed. Then after checking to see what the slurry level was in the inlet, the sludge was removed. Next fresh slurry was added and if the inlet started to overflow before it was all in, more sludge was removed until the rest of the slurry could be added with the level of the slurry in the inlet equal to the mark 5.0 cm (2.0 inches) below the level of the bottom of the digester roof. Last but not least, the gate valve to the gas tank was reopened. Our experience was that if the gate valve from the digester is left closed for even half an hour, enough gas is produced to force slurry out of the digester.
The daily loading of slurry and removal of sludge should be followed by a regular routine of checks and preventive maintenance of the whole biogas system that include such things:
· checking the gas pipes for leaks and condensed water,
· checking the condensation traps to make sure they have enough water,
· checking gas storage tanks to make sure the water tanks have enough water and the gas tanks can move freely without tilting,
· checking that the gas pressure gauge is working correctly,
· checking engines and any other equipment fueled by the biogas, e checking the sludge ponds to make sure that all is as it should be.
The reason for all the regular checking is that preventive maintenance costs less in time and money than it costs to repair something that has broken down.
A wood cover on the inlet, with a rock or concrete hollow block on top to hold it in place, will keep children from falling into big digesters and rain from flooding digesters of all sizes.
The top of the overflow pipe should be 5.0 cm/2.0 inches lower than the bottom of the digester roof for two reasons. If slurry is forced out of the digester, it will be digested sludge coming out of the overflow pipe, not undigested slurry coming out of the inlet. Also, with the top of the overflow pipe lower than the bottom of the digester roof, slurry cannot rise high enough to block the gas pipe.
A biogas system is truly a small business and, like any business, good management is needed to keep it working right. Study how the system works, experiment with ways of improving it. If a biogas system is left to run itself with only minimal involvement on the part of the owner(s), it will not be a profit-making business.