Starch hydrolysis in autogenerative high pressure digestion: Gelatinisation and saccharification as rate limiting steps

Autogenerative high pressure digestion (AHPD) provides an integrated biogas upgrading technology, capable of producing biogas with a CH4 content exceeding 95% at pressures up to 90 bar. Hydrolysis is generally regarded as the rate-limiting step in the anaerobic digestion of complex organic matter, governing the volatile fatty acid (VFA) production rate for subsequent conversion to methane. Starch hydrolysis rates in AHPD systems were studied and the potential risk for VFA accumulation was assessed. Under the anticipated practical moderate pressure conditions at 30 °C, experimental CH4-content of the biogas improved from 49 to 73 ± 2% at atmospheric and elevated pressure, respectively. Furthermore, no significant effect of pressure on the hydrolysis was found. Like under atmospheric pressure, gelatinisation was the rate-limiting step for particulate starch (0.05 d−1) and saccharification for gelatinised starch (0.1 d−1). Because no effect was observed on starch, an effect on the hydrolysis rate of more complex organic matter like (ligno-)cellulose is also not anticipated.