[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.