Agricultural waste materials like straw and the solid fraction of manure are lignocellulosic materials. These materials are strong, flexible and protected against decay. They consist of cellulose, hemicellulose and lignin. Lignin cannot be converted into biogas, and

Solubilization of lignocellulosic materials is inhibited by free fatty acids produced during hydrolyzation and subsequent acidogenesis. Increasing the number of methanogenic bacteria reduces the concentration of free fatty acids. Macronutrients nitrogen and phosphate

and the micronutrients S, Ca, Mg, Fe, Ni, Co, Mo, Zn, Mn, Se and Cu (Demirel et al.., 2011) are required for the multiplication of methanogenic bacteria. Scherer et al. (1983) determined the chemical composition of a number of methanogenic bacteria (Table 13.2). Scherer

(2011) advises also on the minimum concentration of ions in a digester. The basal medium of Guengoer — Demirci et al. (2004) and the recommendations of Speece (1987) are similar. Lebuhn et al. (2010) formulated a spe­cial cocktail. They commented that for maize silage Co should be added at 0.1 mg/kg VS and sodium at 30 mg/kg VS. Speece (1987) has a recommendation for Se. Se was not limiting in the tests by Lebuhn et al.

(2010) .

Straws, husks, bagasse and woody biomass are gener­ally deficient in macro — and micronutrients. The same holds for cattle manure in Asia as cattle feed mostly on rice straw. Manures in Europe and North America have an excess of nitrogen.

Jerger et al., 1982 found a 60% increase in methane yield in half the time in batch-fed anaerobic potential assays with extra-micronutrients (Table 13.3). They also added NH4CL and KH2PO4 to reduce the C/N ratio to 15 and the C/P ratio to 75. Similar results have been obtained by Komatsu et al. (2007). They obtained a methane yield of 280 l/kg VS with sewage sludge and rice straw at a hydraulic retention time of 20 days in a continuously operating digester at 36 °C. Somayaji et al. (1994) had 240 l/kg VS in 40 days for rice straw.

The addition of micronutrients has an effect of 10—70% on the methane production.

Sewage water cleanup sludges are a source of macro and micronutrients. Average primary sewage sludge has the right concentration for Ni and Mo. Co is an order of magnitude too low. In some sludges the concentrations of Fe, Co and Ni are too low (Speece, 1988). Concentra­tions for Ca, Fe, Zn, Mn and Cu are an order of magni­tude too high for its use in agriculture (Wolf et al., 2005).

Optimum nutrient conditions are cost-effective. Industrial fertilizers should be used, lacking organic sources of nitrogen and phosphate. For each kilogram of dry lignocellulosic biomass a maximum of 40 g of urea and 20 g of phosphate are required. Human urine is a good source of nitrogen and phosphate. Human feces are also good but require storage for more than 100 days in order to prevent the spread of illnesses. Eco — san toilets (Terefe and Edstram, 1999) separate urine and feces, so that urine can be directly used and feces stored for the required period.

ADDITION OF MICROBES

Op den Camp et al. (1991) describe an acidogenic reactor with rumen-derived bacteria. A hydraulic retention time of 12 h and a solids retention time of 72 h resulted in a methane yield of 440 l/kg VS for cel­lulose and 120 l/kg VS for barley and rye straw. The

methane was produced in a second reactor separated from the first by a filter with 0.03 mm pore size. The liquid without the free fatty acids was recycled to the first reactor. Soluble lignin products (humic acids) inhibited further degradation of the straws. The German company Ares Technology is performing tests

at pilot plant scale. Typical conversion yields are around 50% (Strecker, 2012).

Weiss et al. (2009) isolated and multiplied hemicellu — lytic bacteria. These were immobilized on trace metal activated zeolite. Digestion of second-stage sludge from a biogas plant gave a methane yield of 215 l/kg VS after 34 days (35 °C) and 150 l/kg VS for the control.