Some European companies (Telschow, 2006; Chollet,

2011) advertise the application of special enzyme combi­nations in biogas digesters. A 30% faster digestion or a 10% higher biogas yield is reported.

Water cleanup secondary sludge is a source of enzymes. The secondary sludge consists mainly of bac­teria and the intracellular liquid of these bacteria con­tains lyses enzymes.

PRETREATMENTS

Biological Pretreatment with Enzymes

Shredded straws, bagasse and husks are seasonal products and need to be stored before being used as sub­strate in a digester. Storage with silage can be used to improve the biodigestability of the substrate. Methane yield for maize silage increased from 290l/kgVS to 330l/kgVS using the enzyme mixture Microfern (Bossuwe, 2011).

Methane yield increased from 145l/kgVS (fresh reed) to 200l/kgVS (reed silage prepared with the enzyme mixture Methaplus; Helbig, 2009). Komatsu et al. (2007) report an increase in methane yield from 280l/kgVS to 310l/kgVS for rice straw soaked in a solution of an unspecified enzyme codigested with sewage sludge.

Chemical Pretreatment

Lime (calcium hydroxide) is a relatively cheap chem­ical and calcium improves the fertility of the soil. In its production about 0.8kWh/kg high-temperature ther­mal energy is used. Gunnerson et al.; (1987) advise to compost straw with lime, water and dung. In this method a fraction of VS is lost. Raju et al., 2010 demon­strated an increase of 60% in biogas production using a pretreatment at 0.015 kg Ca(OH)2 per kilogram VS. The pretreatment with 1.5% CaOH is equivalent to an increase in retention time from 32 to 100 days (Moeller et al., 2006). Klopfenstein (1978) found for hemicellulose and cellulose an increase of 80% and 20%, respectively, for sodium hydroxide using corn­cobs as substrate. The yield increase was only 25% us­ing calcium hydroxide both for hemicellulose and cellulose.

Pretreatment with a minimum amount of dilute acids at 50—100 °C dissolves the hemicellulose and leaves a solid residue that is highly porous (Tsao,

1987) . German biogas tanks have an acid pretreatment (Sauter, 2012). Lebuhn et al. (2010) report technical dif­ficulties with the acid pretreatment and no increase in methane yield.

Schober et al. (2006) and Busch et al. (2006) describe an aerated percolation reactor followed by a methano — genese reactor. They report shorter retention times for kitchen and garden waste and maize silage compared to wet systems.

Hot Water Treatment

Raju et al. (2010) obtained a 40% increase in methane yield using a 15 min pretreatment of wheat and rape- seed straw at 75 °C.

Mechanical Pretreatment

Jerger et al., 1983 found an increase in the methane yield from 270l/kgVS for particles of hybrid poplar <8 mm to 310 l/kg VS for particles <0.8 mm. The dura­tion of the tests was 90 days. Slotyuk (Oechsner, 2012) found an increase from 230 l/kg VS for 10 mm wheat straw particles to 300 l/kg VS for 1 mm particles. The duration of the tests was 35 days (Table 13.4).

LONGER RETENTION TIMES

Doubling of the retention times increases the gas yield with 30—50% (Table 13.5). It is unfortunate that the tests were not done at optimum nutrient concentra­tions. Calculated yields for shorter retention times using Eqns (1) and (2) are compared with measured yields in Table 13.6. The standard deviation between measure­ment and calculation is 35 l/kg VS (similar to the corre­lation for longer retention times) (Table 13.7).

ENERGY CROPS

About 7% of the land used for agriculture in Germany is planted with maize destined for methane production. There are a number of other energy crops with higher production costs (Boese, 2010). Some of these crops have a higher methane yield per hectare (Table 13.8). The humus content of the soils will decrease when only maize is planted as crop (Willms et al., 2009).