Two-stage anaerobic digestion of the organic fraction of municipal solid waste – Effects of process conditions during batch tests

Two-stage anaerobic digestion (AD) batch tests were performed using the organic fraction of municipal solid waste as substrate. Effects of different combination of initial pH (5.5, 7, and 9) and food to microorganism (F/M) ratio (from 0.5 to 6 gVS/gVS) were investigated for hydrogen and methane productions during the first and the second stage of AD, respectively. Results showed that both initial pH and F/M ratio had an impact on hydrogen yield, hydrogen production rate and duration of lag phase. The highest hydrogen yield of 29.8 mLH2/gVS was obtained at initial pH of 5.5 and F/M ratio of 6. However, the highest hydrogen production rate (65 mLH2/gVS/d) was recorded at pH of 9 and F/M ratio of 6. Increasing the initial pH from 5.5 to 9, led to shorter lag phases for all F/M ratios. Methane production from second phase was not significantly influenced by the F/M ratios tested in the first digestion phase. When compared to single-phase AD, two-stage AD tests resulted in enhanced methane production rates from 37.3 to 68.5 mLCH4/gVS/d, reducing by half both the lag phase and the time required to reach maximum methane production.