What is special about an anaerobic digester that sets it apart from large scale anaerobic composting as simple and wasteful as illustrated above. A digester contains the process, and in a digester the temperature, water content and other parameters are controlled for optimal processing and the container traps the escaping gases.
The question remains as to what separates the various kinds of industrial anaerobic digester and what levels of sophistication can be reached in their design. I wish to delve into this question of the difference between various kinds of anaerobic digesters… with the understanding of an amateur of course, because it has become quite complex and it is interesting too, and empowering. If you know where your waste is going when you flush the loo or bring out your wheelie bin, you are so much better equipped to take the right public action on waste treatment.
BATCH AND CONTINUOUS PROCESS
The first two major types of processing are the batch versus continuous process mode. Batch processing is simpler, and basically is the processing of one batch of organic waste in a digester where it remains till digestion is completed. To accomplish the movement of material required for the continuous process mode, various types of flow are used such as stirring, or upflow anaerobic sludge blankets through which a constant stream of highly liquid waste water allows solubles like sugars to pass through the system quickly as they process quickly, but traps particles of sewage with gravity, for up to 90 days so that they can cure fully. These digesters produce a high methane harvest.
DRY OR WET DIGESTER MATERIAL
The second major difference between digestors according to Wikipedia, is in the broad physical characteristic of the material as solid or liquid. A multistage process involving stirring movement or pumping of any kind works works with more liquid media, whereas high solids media are stackable and suit batching, because stirring semi solid material requires a high energy investment. Dryer material, composted in tunnels, produces next to no waste water.
Another difference between digesters is their different levels of complexity. This the authors divide into one and two stage processing. One stage are batch systems, which are just one sealed container, or anaerobic lagoons which are a single open dam. Two stage processes may allow the first three stages of anaerobic digestion, hydrolysis, acidogenesis and acetogenesis to occur in one container, for about 14 days, which is then heated and pumped to a second container for methanogenesis for a further 14 to 20 days.
For processing in which there is no stirring, the process can be batch or continuous. The continuous vertical plug flow digestor feeds in material from the top, which drops slowly down a cylinder, by gravity, the finer the material the faster the digestion and the sooner the digested material will exit. Batch tunnel digesters deposit material in a tunnel with airtight doors, but this broad type of digestor also can use aerobic decomposition, in which case coarse material is good as it allows aeration.
The wetter the substrate is, the more bacteria can come into contact with it, and the higher the gas production. The ideal C:N ration is 20-30:1. Excess N can produce too much ammonia and cause inhibition of methanogenesis.
Time in the digestor is another factor that is important in this industry. Note that within the digestor, the full anaerobic decomposition takes 4 – 5 weeks. Compare this to frequently stirred aerobic hot compost which takes 18 – 24 days. It is slower but in a digester but nothing like the year required for anaerobic composting at home. An upflow anaerobic sludge blanket system requires 90 days for full digestion of the slowest form of putrefaction.
Some by products of the digestive process, such as ammonia, and some nutrients normally found in plants (light metal ions like Mg, Ca ) and contaminants like aromatics and heavy metals, and some larger molecules such as long chain fatty acids may slow digestion down.
The chemical complexity of the digestor food dictates the it time to digest. The higher the digestibility of the material, the higher the gas yield from a system. The high yielding materials seem to be what we call ‘green’ material in the compost mix (food left overs, grass clippings, sewage, animal waste) except for paper which is also high yield. I’ve also found paper products to be very nutritious in the vermiculture situation as worm food.
The low yielding materials are the woody waste part of plants, high in Lignin. Lignin and cellulose natural plant polymer combinations can take longer to break down. Lignin processing bacteria (Xylophageous anaerobes), or high temperature pretreatment in or outside an industrial digestor can break down this lignin.