High solids anaerobic digestion is defined in older references to this type of anaerobic digestion as simply a discriminator to separate out low solids anaerobic digestion, such as the anaerobic digestion of foul sewage and manures/ farm slurries and other naturally dilute water-borne anaerobic digestion processes, from substrate sources (feed materials) like dung, and the untreated organic fraction of municipal solid waste. As described on our page about Wet and “Dry” anaerobic digestion the transition between the two was governed by the ability of the available pump technology to move the materials through the process reliably without blockages or high pumping costs due to rapid wear.
There are now proprietary systems which have been given the name of High Solids Anaerobic Digestion which do clearly have a right to claim that they work at “high solids” contents, but don’t fit into this model. An example is shown in the video below:
In this high solids system there is a dry digestion stage, during which hydrolysis, acidogenesis, and acetogenesis is achieved, and as the temperature is raised or naturally rises high enough (above 70 degrees centigrade for long enough) the material is also sanitized. Once that occurs the material (substrate) is then put through a second stage which would appear to be a traditional pumped liquid stage similar to many other two stage anaerobic digestion systems.
This is quite unusual for such MSW systems as the commonly accepted wisdom has been that organic fraction MSW/ yard waste hydrolyses rapidly and does not need the cost of a two stage digestion process, and yet using a two stage process, for manures and sewage sludge according to many researchers, does tend to lead to a more efficient second stage.
Two Stages or One?
The stages of the anaerobic degradation process through hydrolysis, acidogenesis, to acetogenesis and finally methanogenesis, each have slightly different optimum operating conditions. This results in a compromise in the operating conditions for single vessel digesters. Separating the stages of the process into separate vessels, such that the hydrolysis/acid-forming stages are completed out separately from the methane-forming stage, can result in more efficient processing. However, there are cost implications from having the necessary additional vessels. However, many researchers report that the higher rates of mass reduction, and lower residence times that can be achieved, may outweigh these disadvantages.
Where two stages are used, the most common form of staging is where the waste is treated in a solid state, as in the video above, during the hydrolysis stage when it is sprayed with a percolate and the liquor from this hydrolysis is then treated in a standard wastewater/ methanogenesis stage.
The benefits of reducing complexity when dealing with waste, accounts for the prevalence of the relatively simple single vessel systems on the market. Most clients seek the very lowest initial expenditure before gaining revenue from each project. This may not necessarily be the wisest purchasing policy, but until investors develop greater confidence in anaerobic digestion as a profitable investment this will no doubt continue.