[Joerg]

What qualifies under semi-dry digestion?

Best to differentiate by the level of total solids rather than calling it ‘wet’, ‘semi-dry’ or ‘dry’.

So low-solids AD would be up to around 15-20% TS of the infeed; high-solids above 20% TS. For high-solids AD systems there are various process designs. For example, corn silage digesters can still operate at the higher-end of low-solids or lower end of high-solids TS.
Plug-flow type AD system typically requires TS >30% of the infeed to be able to form the ‘plug’. High-solids batch AD processes operate >35-40% TS of the infeed.

So, the feedstock characteristics play an important part in deciding which AD process type is most suitable to select. For example, food scraps co-collected with yard waste would both be suitable for high-solids plug-low and batch-type processes.

[Natalia Bourenane]

Joerg, thank you very much for your detailed answer. Would you say wet digesters are more popular? Do you have any references for dry or semi-dry digesters?

[Joerg]

Hi Natalia, from my experience it is less a question of which AD process type is more or less popular but rather what are the characteristics of the feedstock(s) that then govern the selection of the process type.

So if you start with a feedstock that has a high TS, let’s say 30+ percent, then high-solids AD processes such as plug-flow or batch would be preferrable.

We did a study for a client back in 2016 that included a survey of all municipal organics AD installations world-wide. So for source separated organics and organic fraction of MSW. We found that the low-solids AD installations were about equal in number to high-solids AD installations. Back then High-solids batch represented about 20 percent of the all high-solids AD installations.

For ‘semi-dry’ AD -I would say between 15 and 25% TS- there are some providers that have specialized on this range, most notably for corn silage applications or an other AG-feestocks. For these applications slow-moving mixer designs are typically used for a CSTR design (e.g., offered by Agraferm, ArgiComp, among others).

[Natalia Bourenane]

Got it Joerg, thank you very much for your reply!