2.3. Digester setup and operation
Four 5-L digesters each with a 4 L working volume were used in
this study. These were maintained at 36 ± 1 C and mixed by an offset
bar stirrer operating continuously at a speed of 40 rpm. Each
digester was fed daily and digestate was withdrawn weekly, a sample
of which was used for analysis. Biogas production was recorded
using tipping bucket gas counters connected to a continuous datalogging
system: these were periodically calibrated by collecting
the biogas discharged in Tedlar bags (Tedlar, SKC Ltd., UK) and
measuring the volume in a water displacement gasometer, with
reported values corrected to standard temperature and pressure
(STP) of 0 C and 101.325 kPa (Walker et al., 2009). The biogas
and methane production are expressed as a 7-day average values.
Over the first 44 days of operation, the organic loading rate
(OLR) was increased from 1 to 3 g VS L1 day1 with a step increase
of 0.5 g VS L1 day1 every 11 days, and with the CM representing
20% of the mixture on a VS basis. The OLR was then maintained at
3 g VS L1 day1 giving an HRT of approximately 100 days depending
on the ratio of the two feedstocks. The proportion of CM used in
the mixed feedstock was adjusted to increase or decrease TAN concentrations
in order to maintain digester stability or induce the
required experimental conditions. Details of the operating conditions
applied are summarised in Table 1.