analysis of the waste at the completion of the stage, which revealed
an increase ofpHfrom 6.4 (end ofweek6, Table 5) to 8.0 (end of week
10, Table 5) and a significant reduction of VFA concentration.
3.4. Operation of the digester at an adapted organic load
(weeks 11–18)
Based on the good recovery from inhibition, feeding of the digester
was resumed at week 11. The amount of raw waste introduced
into the digester was progressively increased from 2 kg to
6 kg (Fig. 2). Fig. 5 shows that the biogas production increased with
the load and stabilized between 513 and 653 Nl/kg VSadded per
week during weeks 13–18 where a constant load of 6 kg of raw
waste per week was applied (dry solids average residence time
of 5 weeks).
Fig. 4 shows that the pH was quite stable during this stage,
ranging between 8.0 and 8.4, which is close to the optimal working
range (Comino et al., 2009; Macias-Corral et al., 2008). VFA content
decreased from 38.8 to 1.5 g/kg DS, indicating that the acids produced
were efficiently consumed by acetogenic and methanogenic
communities and converted into biogas (Fig. 4).
Biogas composition was also relatively stable over the 8 weeks
of this phase (Table 6). The CH4/CO2 ratio ranged between 59/41
and 63/37, thereby confirming that the process of anaerobic digestion
was conducted under good operating conditions. It should be
noted, however, that biogas composition was determined at the
end of each week (before loading/unloading operations) and therefore
did not reflect the average composition of the total biogas produced
between two feedings. Indeed, the biogas produced in the
first days after feeding, which probably contained a higher proportion
of CO2 generated by hydrolysis and acidogenesis, was continuously
vented out of the digester and therefore not analyzed. Only
the biogas produced at the end of each week, containing more
methane, was actually analyzed. This overestimation could partly
explain why the CH4/CO2 ratio in this phase of the pilot-scale study
was higher than in the BMP assays (Table 4).