The low consumption is due to the absence of internal mixing, in fact these plants utilise the principle of stratification as shown in the image on the side.
The fresh biomass is added at the top and the spent biomass is collected from the bottom of the digester. By operating in this way, the fresh biomass is always located on the free surface thus facilitating biogas development.
The spent biomass that settles on the bottom will be re-added into the next digester, or heated and re-added into the same digester with internal reuse.