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Inoculation with a 2.5-month stored AGS led to different biomass profiles in the two SBRs, with distinct settling characteristics. SBR1 presented a peak in the SVI5 and SVI30 values on day 48 (177 and 98 mL gTSS−1, respectively), subsequently stabilizing along period A at higher levels than those registered in SBR2 (Table 2). Although SVI was more stable in the latter during period A, the shock loads applied in periods B–C caused a sharp increase in the SBR2 SVI values, high levels being maintained throughout the subsequent period D (Table 2). As a result of this settleability shift, TSS levels decreased from 7 to 5 gTSS L−1 in SBR2 after period C, in contrast with SBR1, which maintained an average value of 5 gTSS L−1 throughout periods A–D (Table 2). Accordingly, while the sludge retention time (SRT) levels in SBR1 were mostly stable until the end of period D (9 days, on average), in SBR2, the initial average sludge age of 18 days deteriorated to 6 days after the shock loads were applied (Table 2). Subsequently, upon introduction of acetate as 50% and 100% of the carbon source load in the feed (periods E and F, respectively), biomass settleability was significantly reduced in both reactors (Table 2), reaching maximal SVI5 and SVI30 values of 592 and 282 mL gTSS−1 in SBR1, and 400 and 255 mL gTSS−1 in SBR2, respectively, on day 299. This substrate switch led to an almost complete biomass washout, resulting in biomass concentrations around 1.5 gTSS L−1 in both SBRs. Yet, after adaptation to the new carbon source, SBR2 resumed biomass accumulation along period F, with SVI5 and SVI30 levels gradually decreasing to final values of 225 and 123 mL gTSS L−1, respectively.

Table 2

Summary of average and standard deviation values registered for the TSS, sludge volume index after 5 and 30 min settling (SVI5 and SVI30, respectively) and SRT in each experimental period for the two SBRs

PeriodSBR1
SBR2
TSS (g L−1)SVI5
SVI30
SRT (days)TSS (g L−1)SVI5
SVI30
SRT (days)
(mL gTSS−1)(mL gTSS−1)
4.8 ± 1.0 119 ± 20 72 ± 10 9 ± 6 7.4 ± 1.4 85 ± 11 51 ± 7 18 ± 10 
4.8 ± 0.0 111 ± 5 72 ± 1 7 ± 1 6.3 ± 0.9 110 ± 11 64 ± 7 13 ± 3 
5.8 ± 0.2 99 ± 8 65 ± 6 7 ± 1 7.3 ± 0.2 91 ± 2 57 ± 1 6 ± 1 
3.9 ± 0.0 103 ± 13 64 ± 9 9 ± 6 4.6 ± 0.0 138 ± 15 85 ± 9 6 ± 4 
5.1 ± 0.7 357 ± 123 237 ± 81 2 ± 0 4.9 ± 0.5 180 ± 46 112 ± 29 6 ± 4 
2.0 ± 0.9 502 ± 105 310 ± 53 2 ± 1 4.4 ± 1.1 284 ± 71 179 ± 51 9 ± 6 
PeriodSBR1
SBR2
TSS (g L−1)SVI5
SVI30
SRT (days)TSS (g L−1)SVI5
SVI30
SRT (days)
(mL gTSS−1)(mL gTSS−1)
4.8 ± 1.0 119 ± 20 72 ± 10 9 ± 6 7.4 ± 1.4 85 ± 11 51 ± 7 18 ± 10 
4.8 ± 0.0 111 ± 5 72 ± 1 7 ± 1 6.3 ± 0.9 110 ± 11 64 ± 7 13 ± 3 
5.8 ± 0.2 99 ± 8 65 ± 6 7 ± 1 7.3 ± 0.2 91 ± 2 57 ± 1 6 ± 1 
3.9 ± 0.0 103 ± 13 64 ± 9 9 ± 6 4.6 ± 0.0 138 ± 15 85 ± 9 6 ± 4 
5.1 ± 0.7 357 ± 123 237 ± 81 2 ± 0 4.9 ± 0.5 180 ± 46 112 ± 29 6 ± 4 
2.0 ± 0.9 502 ± 105 310 ± 53 2 ± 1 4.4 ± 1.1 284 ± 71 179 ± 51 9 ± 6 

The indicated SRT corresponds to the average of the SRT values calculated for each week (weekly SRT) along the respective period.

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