In many publications, primary fermentation is described as a limiting step in the anaerobic digestion of fibre-rich biomass [Eastman JA, Ferguson JF. Solubilization of particulacte carbon during the anaerobic digeston. J WPCF. 1981;53:352-366; Noike T, Endo G, Chang J, Yaguchi J, Matsumoto J. Characteristics of carbohydrate degradation and the rate-limiting step in anaerobic digestion. Biotechnol Bioeng. 1985;27:1482-1489; Arntz HJ, Stoppok E, Buchholz K. Anaerobic hydroysis of beet pulp-discontiniuous experiments. Biotechnol Lett. 1985;7:113-118]. The microorganisms of the primary fermentation process differ widely from the methanogenic microorganisms [Pohland FG, Ghosh S. Developments in anaerobic stabilization of organic wastes-the two-phase concept. Environ Lett. 1971;1:255-266]. To optimize the biogas process, a separation in two phases is suggested by many authors [Fox P, Pohland GK. Anaerobic treatment applications and fundamentals: substrate specificity during phase separation. Water Environ Res. 1994;66:716-724; Cohen A, Zoetemeyer RJ, van Deursen A, van Andel JG. Anaerobic digestion of glucose with separated acid production and methane formation. Water Res. 1979;13:571-580]. To carry out the examination, a two-phase laboratory-scale biogas plant was established, with a physical phase separation. In previous studies, the regulation of the pH-value during the acid formation was usually carried out by the addition of sodium hydroxide [Cohen A, Zoetemeyer RJ, van Deursen A, van Andel JG. Anaerobic digestion of glucose with separated acid production and methane formation. Water Res. 1979;13:571-580; Ueno Y, Tatara M, Fukui H, Makiuchi T, Goto M, Sode K. Production of hydrogen and methane from organic solid wastes by phase separation of anaerobic process. Bioresour Technol. 2007;98:1861-1865; Zoetemeyer RJ, van den Heuvel JC, Cohen A. pH influence on acidogenic dissimilation of glucose in an anaerobic digestor. Water Res. 1982;16:303-311]. A new technology without the use of additives was developed in which the pH-regulation is executed by the pH-dependent recycling of effluent from the anaerobic filter into the acidification reactor. During this investigation, the influence of the different target pH-values (5.5, 6.0, 7.0 and 7.5) on the degradation rate, the gas composition and the methane yield of the substrate maize silage was determined. With an increase in the target pH-value from 5.5 to 7.5, the acetic acid equivalent decreased by 88.1% and the chemical oxygen demand-concentration by 18.3% in the hydrolysate. In response, there was a 58% increase in the specific methane yield of the overall system. Contrary to earlier studies, a marked increase in biogas production and in substrate degradation was determined with increasing pH-values. However, these led to a successive approximation of a single-phase process. Based on these results, pH-values above 7.0 seem to be favourable for the digestion of fibre-rich substrates.
