|wet fermentation||plug-flow fermentation||dry fermentation (SSAD)|
|range of substrates|
|tolerance to impurities|
|tolerance to inhibitors|
|risk of process failure|
And the winner is …
Most of the biogas plants worldwide are built based on liquid-type anaerobic digestion (AD), wherein biomass (usually animal dung) and water are mixed in equal amounts to form a slurry in which the content of total solids (TS) is about 10-15%. While this model is suitable for small-scale biogas plants it becomes a challenge in large commercial plants where it necessitates the use of large quantities of water every day, often in water-scare areas.
Solid state digestion (SSAD), dry fermentation, as opposed to the liquid digestion described above, dramatically reduces the need to dilute the biomass before using it for digestion. SSAD can handle dry, stackable biomass with a high percentage of solids (40-60%). A SSAD plant consists of gas-tight chambers called fermenter boxes working in batch-mode that are periodically loaded and unloaded with solid biomass. Liquid digestate or percolate produced during digestion is recirculated and sprinkled on the dry biomass to ensure process continuity and stability.
The solid state fermentation/dry digestion distinguishes itself by the usability of a wide range of organic input substrates, including very dry or long-fibred ones that can be digested. Contaminants or impurities within the material will not affect the process of anaerobic digestion or the plant operation. Even energy rich liquid substrates can be integrated in the process without any problems. The uncomplicated close to nature technology ensures a quick and stable degradation process.
The operation of a dry/SSAD digestion plant mostly automated. Only small maintenance work, beside charging and discharging the fermenter boxes after 18-21 days, has to be done manually. Whereas liquid digestion is often known as a 24/7 job, operating a dry digestion plant is a comparable lightweight job in terms of both time and effort. Just as time-consuming in liquid systems can be the pre-processing of the biomass to get it in the right particle size for feeding into the liquid digestion tanks.
In the next part of the comparison series, we will specifically look at operating costs.