Sauter (2012) circumvents this problem by spraying digestate on top of the scum layer using water canons. The patent of Rossow (2011) claims that the scum layer can be 5 m in thickness. It is to be expected that digested particles with a high lignin content will be washed down, sink to the bottom and can be collected there using a pump. The input material is loaded into the digester through an input shaft with its exit below the lower level of the scum layer in the digester. It will be acidified for a few days in the first step of the digestion process. The acids will be transformed into methane and carbon dioxide in the other parts of the digester. Schoenberg and Linke (2012) tested a 451 scum layer digester with whole plants (Silphium perfolia — tum) as substrate. A loading rate of 8kg/(m3d) was possible with a methane yield of 215 l/kg VS.

Solid Biomass Digester

VandeVivere et al. (2003) gave an overview of anaer­obic digesters for solid biomass. These "dry" (20—40% solids) systems are used for the digestion of kitchen and garden waste and the mechanically sorted fraction of municipal waste, but can be used for most types of solid farm and food processing by-products.

The Kompogas (VandeVivere et al., 2003) system consists of a cylindrical plug flow reactor in which the fermenting wastes are mixed and moved by pad­dles. Temperature is 55 °C and retention time 15 days. Capacity is limited to 10,000 tons/a due to the maximum dimensions of the axis with paddles. Capac­ity of these systems in Europe and Qatar is more than 1 million tons/a. Methane yields are 200—300 l/kg VS (VandeVivere et al., 2003).

There are some batch systems with leachate recircula­tion. Methane yield is up to 40% less due to channeling of the leachate in the substrate (Vandevivere). Mussoline et al., 2012 obtained 175 l/kg VS for rice straw and swine manure in a digestion period of 1 year. Tempera­tures ranged from 15 to 35 °C. The cell with 6500 m3 capacity was filled with cylindrical bales. Packing density was 100 kg/m3.

Storage of straw for 6 months is expensive (25 V/ton). In Western Europe storage cum digester tanks can be used. The idea is to fill these tanks in the period July— October with shredded straw and a fraction of old digestate together with macro — and micronutrients. Mussoline (2012) demonstrated this concept during one year with rice straw bales and swine manure. Daily power generation was directly correlated to the digester temperature.

German tanks with concrete tops are 120 V/m3 and with foil tops 70 V/m3. At a packing density of 10% this amounts to an investment of 700—1200 V/ton straw stored and digested.