Granulation and enrichment performance
Initial reactor operations were aimed at complete retention of the
biomass. The cycle length varied in the beginning, starting with a long
cycle length of 24 h. Both a flat base addition profile and
CO2 and H2 concentrations in the off gas
below 1 % (m/m) were used as indicators of full conversion of glucose
into organic acids. During the reaction time the biomass could grow and
consume the glucose dosed. A short settling time (2-10 min) resulted in
non-granular biomass at the start. After increasing the settling time to
3 h a visible improvement of biomass retention was achieved. After 6 d
the settling time was manually lowered as shown in figure 1 to promote
granule formation.
The granules formed during the start-up period were used as starting
material for the first enrichment. An impression of the reactor operated
at different SRT is depicted in figure 2. The start-up period served
only as inoculum production for the first enrichments and thus day 0 was
chosen to be the first day of the SRT research.
After the start-up phase the settling time was increased to 60 min and
the cycle length to 6 h, to prolong the SRT. In this first period the
SRT was uncontrolled, this meant that TSS was not actively removed from
the system. TSS could still leave the system when a maximum sludge
volume (SV) was reached (which meant that the sludge bed reached until
the effluent point of the reactor which was half way the reactor height)
and additional biomass production left the reactor with the discharge
effluent. Using this approach an SRT of 40-50 d was achieved. After
characterisation of the system, the SRT was manually controlled at 10-20
d SRT and 1-2 d SRT. During the start-up phase H2 and
CO2 were produced and measured in the biogas. After the
start-up period no H2 was measured in the biogas, the
biogas consisted solely out of CO2 (20-40% m/m) and
N2 (60-80% m/m) and no (<0.5%)
H2 or CH4 were detected. An overview of
the culture performance for the 3 SRT is shown in figure 3.
With increasing the SRT the concentration of biomass in the reactor
increased from 6 gVSS·L-1 at 1-2 d SRT to 59
gVSS·L-1 at 40-50 d SRT. An high SRT could be achieved
due to amongst others a very compact sludge bed with a SVI of 11 ± 2
ml·gTSS-1 (average ± standard deviation; n=8) and a
biomass yield of 0.11 gCODX·gCOD-1 (X
= biomass). The sludge bed became dense as it showed a strong
compressive ability. After starting the settling phase an initial sludge
bed was formed within 10 minutes. After the initial settling the sludge
bed compressed for the remainder of the settling time (total settling
time 60 minutes) resulting in a very low SVI60. An overview of system
characteristics is given in table 1. In all experimental periods the VSS
present in the effluent was low, 0.21 ± 0.18 (n=100)
gVSS·L-1, compared to the VSS concentrations present
in the reactor. The manual removed sludge was 3.1
gVSS·d-1, 1.9 gVSS·d-1 and 0
gVSS·d-1 for the 1-2 d SRT, 10-20 d SRT and 40-50 d
SRT respectively. The biomass yield was estimated over multiple cycles
for each SRT and was in the range of 0.09-0.17
gCOD·gCOD-1. The assumption was made that all VSS
could be classified as biomass as described in the material and methods.