3.8 Anaerobic Digestion Technologies and Operation

hi this week we'll talk about anaerobic digestion technologies and their practical operation remember you can download the book for free from our webpage if you want more detailed information on anaerobic digestion this diagram looks really scary but don't worry it isn't it's a classification of different types of AD technologies we can differentiate between dry and wet systems between batch and continuous systems between thermophilic and mesophilic operating systems and between one stage and multistage technologies what I would like to go through together with you in this module is we'd like to look at wet continuous mesophilic

and one stage reactors the examples that will be discussing our floating drum fixed dome and balloon type digesters continuous actually means that we're feeding at a regular interval at the same time an equivalent volume that enters an equivalent volume will also leave the digester wet means that we'll be operating with the total solid content of sixteen percent or less so low TS so now let's look at these digesters let me start by showing you the wet continuous mesophilic fixed dome reactor this is a brick built and gas-tight plastered structure which is often put underground it

has an inlet pipe where the waste material is fed into the digester it has a digester with a volume of slurry and then it has a gas holder storage space what it further has is an outlet or what we also call compensation chamber and the overflow you might also see here that the gas pipe is at the highest point with a valve to open or close the gas flow there are different designs of fixed dome digesters and sizes can vary this is the most common reactor type in developing countries when starting the reactor for the

first time it is important to inoculate the reactor with methadone genic bacteria the easiest way to do this is by adding cow manure and water with a ratio of one to one or even easier use digestate from another reactor a rule of thumb is that it needs about ten percent of the reactor volume as cow manure to start the process but more is also fine or even better in this animation we can see how this works the waste material is mixed with water and enters the reactor and mixes with the material already in the reactor

undergoing degradation biogas is generated in the slurry through anaerobic digestion the gas bubbles then move to the top part of the reactor where gas accumulates and starts to build up pressure if the valve is closed when the valve is closed the grass pressure will increase and push down the slurry in the reactor and up into the overflow chamber when then the gas is used the gas pressure drops and the slurry level balances out to a new equilibrium here you see a sequence of pictures of the construction of a fixed stone reactor in Lesotho during the

building of the structure and the dome here the dome is just about finished and here it's being plastered this is another example from Indonesia what you see here is that the bottom part is built with concrete and the dome part is just starting to be built with bricks here is a summary of some advantages and some disadvantages for the advantages let me highlight three long lifespan absence of moving parts and the underground construction which save is space there are also disadvantages I'll also highlight a few one is that it needs specific technical skills it also

needs a special sealant to make sure that it's gas tight and another disadvantage is the fluctuating gas pressure depending on the volume of the stored gas and its use a second type is the floating drum digester you see this in this picture it has similar features the inlet the digester for the slurry the outlet and the overflow but here now the gas holder is not a fixed unit but is a moving unit here it's floating on a water jacket in some cases it could also be floating directly on the slurry with increasing gas pressure this

floating gas holder moves upwards and when gas is used the weight of the drum pushes it back down the gas drum is typically made of metal painted to protect it from corrosion it can also be made out of fiberglass reinforced plastic or galvanized sheet metal in this design there's also a guiding pole to make sure to stabilize it while it's going up and down in there's also a divider wall which helps to avoid short-circuiting of the slurry from the inlet to the outlet you see it here the digester is often placed below the ground while

the gas holder drum is above ground here is an example of such floating drum digesters from India with the metal floating gas holders or hear another also from India here are examples of small household units made out of fiberglass reinforced plastic the brown part on the top is the floating drum or here another example from Tanzania made by using the typical water tanks out of plastic and some plastic piping here are some advantages and disadvantages of the floating drum reactors advantages are that it's simple and easy to operate one big advantage is that the volume

of gas is directly visible and that means we know if the system is working or not we also have constant gas pressure or we can increase this pressure by adding some weight onto the gas holder tank some disadvantages are the high material costs for the steel drum and that these steel parts are susceptible to corrosion there are also regular maintenance costs that arise from keeping this corrosion at bay the third digester type I would like to show you is the tubular digester like this one in Costa Rica these are quite common in the Latin American

region these are long horizontal lying tubes or balloons made out of plastic and rubber the inlet and outlet are attached directly to the skin of the balloon you see this here in a schematic the balloon the inlet and the outlet and the gas pipe on the top these balloons usually need some protection from the top as well as from the bottom what we usually have is a compacted back film some advantages are low construction costs very simple to install easy to construct but some disadvantages are a relative short lifespan and susceptible to mechanical damage so

those were the three types of technologies that I wanted to share with you they are a lot of variations in design for instance this combination of a fixed structure for the digestate and then a balloon for the gas storage this is an example from Germany why don't you share with us what kind of systems you might have encountered in your region or during your travels submit them into our forum so let me summarize what we covered in this module we looked at three different type of digesters we looked at their design features and some examples

for each we listed some advantages and disadvantages in the next module will focus on maintaining the digester in operation and at the product chain the gas and the digestate thanks very much