One-stage H2 and CH4 and two-stage H2+ CH4 production from grass silage and from solid and liquid fractions of NaOH pre-treated grass silage

Abstract

Abstract In the present study, mesophilic CH4 production from grass silage in a one-stage process was compared with the combined thermophilic H2 and mesophilic CH4 production in a two-stage process. In addition, solid and liquid fractions separated from NaOH pre-treated grass silage were also used as substrates. Results showed that higher CH4 yield was obtained from grass silage in a two-stage process (467 ml g−1 volatile solids (VS)original) compared with a one-stage process (431 ml g−1 VSoriginal). Similarly, CH4 yield from solid fraction increased from 252 to 413 ml g−1 VSoriginal whereas CH4 yield from liquid fraction decreased from 82 to 60 ml g−1 VSoriginal in a two-stage compared to a one-stage process. NaOH pre-treatment increased combined H2 yield by 15% (from 5.54 to 6.46 ml g−1 VSoriginal). In contrast, NaOH pre-treatment decreased the combined CH4 yield by 23%. Compared to the energy value of CH4 yield obtained, the energy value of H2 yield remained low. According to this study, highest CH4 yield (495 ml g−1 VSoriginal) could be obtained, if grass silage was first pre-treated with NaOH, and the separated solid fraction was digested in a two-stage (thermophilic H2 and mesophilic CH4) process while the liquid fraction could be treated directly in a one-stage CH4 process.