|The Biogas/Biofertilizer Business Handbook (Peace Corps, 1985)|
Many people who have written about their experiences with biogas say that cleaning the digester once a year is a good idea. But a digester does not have to be cleaned if the gas production rate is not dropping.
There are two problems that slow down gas procution that cleaning the digester can solve.
· One is a build-up of dirt and sand on the floor of the digester which cannot be digested by the biogas bacteria. This layer of dirt will cut down on the usable capacity of the digester.
· The other reason is a layer of scum floating on top of the slurry. It is floating because, even though it is organic matter, it is lighter than water. Scum forms a growing blanket on top of the slurry that takes up valuable space and does not allow the gas produced below it to rise out of the slurry and into the gas pipe.
What follows comes in large part from L John Fry's book Practical Building of Methane Power Plants for Rural Energy Independence. It describes his experience in cleaning continuous-type biogas digesters.
After about one year's operation, one of the two digesters began to produce less and less biogas. In addition to the low gas yield, the pH level was low and the sludge continued to produce gas after it was removed from the digester. Digestion was taking place outside of the digester. The digester was, in effect, being overloaded due to a reduction in available space caused by the buildup of scum. The digester had to be cleaned.
An important note: This is one time when extreme care must be taken not to have lights, cigarettes, flames, or sparks near the digester. A mixture of biogas and air, particularly in a closed or semi-closed space, plus a spark or flame, can spell EXPLOSION.
Scum is a mixture of animal hairs, skin particles, straw, and wood shavings from animal bedding, feathers, unrotted plants, and generally anything that will float. When removed and dried, it is so light that a piece 6.0 feet by 6.0 feet by 1.0 feet can be lifted with one finger. Yet it is so bound together by a layer that it can only be broken from the slurry's surface with a hoe. Scum is bound together in matted form by fine particles of sticky material brought up in the volcanic action of the bubbling digestion. It spreads evenly over the surface area of the slurry.
All digesters must have a device to separate and remove dirt and sand from the slurry before it is put in the digester.
1) For small digesters (less than 3.0 cubic meters capacity), a plastic bucket with an outlet 5.0 cm (2.0 inches) up from the bottom is suggested.
A) After the slurry has gone into the digester from the bucket's outlet, the dirt and sand that remain on the bottom of the bucket can be added to a compost pile
B) It would also be a good idea to scoop any floating matter off the top of the slurry. This floating matter could also be added to a compost pile.
2) For large digesters a corrugated dirt trap (see Diagram 7) would be more efficient.
A) As slurry flows over the corrugations, dirt will settle in the valleys.
B) After loading, one side of the dirt trap is removed, and the dirt is swept into the gully for use in a compost pile, and the side is replaced.
If animal manure is used that has been collected off of the ground, instead of concrete floors, there will be a lot of dirt in the slurry. A system that removes the dirt will be an absolute must.
A digester's cleaning door can either be on the top, on the outlet end, or on both the top and the outlet end. If it is an underground digester, the cleaning door can only be on the top (see Diagrams 5, 6, and 17).
An easy to open top-of-digester lid is pictured in Diagram 6. It is built at the inlet end of the digester and includes the gas pipe attachment. To make cleaning as easy as possible, this removable lid is made as wide as the digester.
Instead of making a lid that needs a semi-permanent, air-tight, water-tight cement seal, this removable lid uses a water seal. The gas pipe outlet is attached to an upside down concrete or metal cup that sits in a water seal. (A concrete cup will not rust but might be harder to make.) The water seal must always be kept deep enough so that the biogas does not escape unless the pressure goes too high (more than 20 cm/8.0 inches). This is done by keeping eight inches of water in the lid's water seal.
In addition to being an easy way to enter large digesters, this type of lid also serves another purpose. The gas pipe is higher above the digester than is usually the case, making it much less likely that a scum layer could block the pipe. This type of opening on top of the digester is useful, but to clean out scum and dirt a large door should also be built at the outlet end of the digester.
If just one opening is built on an above ground digester, it should be at the outlet end. Using a rubber gasket, a rustproofed metal plate is bolted onto the end of the digester (see Diagram 8). This will make the opening watertight, but not 100 percent airtight; so in normal operations the level of the slurry should never drop below the top of the scum door. Like a lid on top of a digester, a scum door should be built as wide as the digester.
Many people have abandoned biogas digesters simply and only because of scum problems. In vertical digesters, with their small surface area in proportion to volume, the buildup of scum can come to several feet in a few months. That is why vertical digesters are often built with expensive stirring mechanisms. Horizontal digesters with their large surface areas in proportion to volume, also have the problem of scum buildup, but the scum layer grows slowly in comparison to the rate of growth of scum layers in vertical digesters.
The scum layer, being as strong as a woven sleeping mat, can stop biogas from escaping the digester. This does not mean that the gas will stop being produced. If the gas cannot get out of the digester, it will make room for itself by forcing slurry out of the overflow pipe. Slurry will also be forced out of the overflow pipe if, somewhere along the gas line, the biogas is blocked by a closed valve, bent pipe, or water in a pipe. But if a blocked or otherwise closed pipe is not the cause, there is a good chance that there is a scum layer trapping gas in the digester.
The temporary solution is to break a hole in the scum so the gas can escape the digester. This can be done by pouring slurry from the overflow pipe and the outlet back into the digester's inlet. That is not a long-term solution because a scum layer big and strong enough to trap the gas is also reducing gas production by taking up lots of digester space. The long-term solution is to drain and clean the digester.
The whole digester should be cleaned once a year and if scum is a major problem the scum layer should be raked out halfway through the year. If scum has become that big a problem, time and effort should be put into reducing the amount of undigestible plant fibers that are getting into the digester.
The basic procedure for cleaning a digester is to:
· Close the gas line to the digester, remove the top-of-digester cover, and drain the digester from the outlet valve.
· After the slurry level has dropped to below the level of the scum door, the door can be safely opened and slurry removal can continue from both the outlet and the door.
· Once the liquid slurry has been drained, any remaining solid sludge and dirt must be raked, swept, or washed out.
· To make it quicker and easier for a digester to start producing gas after it has been cleaned, use the liquid portion of the slurry that was removed to restart the digester.
· The solid portion of the slurry can either be used in a compost pile or, if it is possible to grind it up, it can be reused as slurry if it is free of dirt and sand.
There are advantages to building two medium-sized digesters instead of one large digester. When it comes time to clean or repair a digester, the other digester could take the extra waste for a few days. If there is just one digester, the waste that would have been fed into the digester should be composted.