3.3 Process performance under stable and disturbed states
targeting HRT
During the HRT shortening stage (Fig. 3), the volumetric biogas
production registered was stable at around 4.4 L L-1
d-1 under 5 d HRT (Fig. 3a). The specific methane
yield was 354±27 mL CH4 g-1
VS-1, and it represented the 65% v/v of the biogas
(Table 2). This result fell within the range reported, i.e. 182-368 mL
CH4 g-1 VS-1, in
previous studies treating straw and food waste in batch reactors (Liu et
al., 2018); however, it was higher than that reported for the mono
digestion of food waste at longer HRT under thermophilic conditions (Kim
et al., 2006), implying a synergistic effect of the co-digestion
strategy. The methane yields declined by 29% (251±63 mL
CH4 g VSin-1)
alongside with the decrease of the HRT to 3 and by 78% (78±12 mL
CH4 g VSin-1) with HRT
shortened to 1.5 d, respectively.
With the decrease in the HRT, the propionate concentration only slightly
changed while the acetate concentration accumulated from 333±197 mg
L-1 to 951±577 and 2533±496 mg L-1
at HRT of 5, 3 and 1.5 d, respectively (Fig. 3c and Table 1). The
concentration of propionate at HRT of 1.5d was stable at about 224±75mg
L-1 and so it was 2-fold lower than that reported in
HRT of 5d. It was therefore obvious that the
consumption rate of acetate was the
primary rate-limiting factor at HRTs of 3 and 1.5 d. Although the
concentration of VFAs sharply increased during the shortest HRTs, it was
much lower than VFAs concentration at OLR of 17.1 g-1
VS L-1 d-1 in the first stage of
current study. Therefore, the reason behind the reduction in gas
production rate at this stage was most probably due to the excessive
microbial washout caused by the shortest HRTs, and partially because of
the impact of VFA accumulation on methanogens activity. The phenomena of
the washout effects and the inhibition of VFA have been previously
reported under HRT shortened to 5d (Algapani et al., 2016).
The VS removal efficiency with the shortening of the HRT was consistent
with the methane production results and this further verified the VFA
accumulation. Alongside with the decrease of the HRT, the removal
efficiency of VS decreased down to 5.2% at the end of experiment, which
was 11-fold less than the removal efficiency attained at an 8d HRT. The
variation of pH between different HRTs was negligible and varied between
7.2 and 7.3. To conclude, the low methane production during the shortest
HRT can be explained by i) the finite digesting time; ii) the microbiome
washout effect; and iii) methanogenesis inhibition by VFA accumulation.