Continuous operated CSTB - Degradation of EDPs mixture and biomass growth in CSTB 1. Batch ope

4.3. Results and discussion

4.3.1. Degradation of EDPs mixture and biomass growth in CSTB 1. Batch operated CSTB

4.3.1.3. Continuous operated CSTB

For continuous biodegradation of phthalate mixture by Gordonia sp. in the CSTB, the bioreactor was initially operated under batch mode for 96 h with 750 mg/L concentration of phthalate mixture which was found to be non-inhibitory to the microorganism. Figure 4.5 presents the combined profiles of biomass growth, inlet - outlet concentration and percentage removal efficiency of phthalates mixture at different HRT, viz. 48, 36 and 24 h. From Figure

4.5 a-c, it could be seen that lag phase in biomass growth and biodegradation of phthalates is absent due to their low initial concentration as well as enrichment of the culture to grow on phthalates mixture as the sole carbon source. Complete degradation of phthalate mixture was achieved during the initial batch run and within 96 h, which resulted in a high concentration of active and acclimatized biomass. Hence, the same time duration was considered favourable for switching over from batch operation to continuous operation mode of the bioreactor.

Under continuous operation mode, 100% degradation efficiency of phthalate mixture was achieved at 750 mg/L concentrations of phthalate mixture at all the three HRTs. Moreover, at 48 h HRT, complete degradation efficiency was achieved at all three inlet concentrations (750, 1250 and 1500 mg/L) of phthalate mixture. However, the cumulative biodegradation of phthalates was substantially decreased when the bioreactor was operated at 36 and 24 h HRT at the higher inlet concentrations. In case of 36 h HRT, more than 94 and 86% cumulative degradation efficiency were achieved at 1250 and 1500 mg/L inlet concentration of phthalate mixture, respectively. Whereas, only 85 and 75% cumulative degradation efficiency were achieved at 1250 and 1500 mg/L inlet concentration of phthalate mixture, respectively, at 24 h HRT. These results are consistent with the literature reported results on biodegradation of DBP, DEHP, bisphenol-A (BPA) and diclofenac (DCF) by acclimatized sludge under continuous operation; their biodegradation efficiency values substantially declined from 93-75% to 93- 50%, respectively (Boonnorat et al., 2016).

From the results of biodegradation of the individual phthalates, it is observed that LMW phthalates as compared to HMW phthalates are easily and quickly degraded by the bacterium.

Many studies have reported that PAEs with short ester hydrocarbon side chain are more easily and rapidly degraded than other EDPs with long ester side chains (Abdel daiem et al., 2012;

Fan et al., 2018a; Gavala et al., 2003; Zhu et al., 2022).

(a)

Time (h)

0 36 72 108 144 180 216 252 288 324 360 396 432

Phthalate concentration (mg/L)

0 50 100 150 200 250 300

Degradation efficiency (%)

0 20 40 60 80 100 120

Biomass concentration (mg/L)

0 100 200 300 400 500 600 700 800

DMP outlet DEP outlet DBP outlet BBP outlet DEHP outlet DnOP outlet DMP inlet DEP inlet DBP inlet BBP inlet DEHP inlet DnOP inlet DMP degradation DEP degradation DBP degradation BBP degradation DEHP degradation DnOP degradation Biomass

(b)

Time (h)

0 36 72 108 144 180 216 252 288 324 360 396 432

Phthalate concentration (mg/L)

0 50 100 150 200 250 300

Degradation efficiency (%)

0 20 40 60 80 100 120

Biomass concentration (mg/L)

0 100 200 300 400 500 600 700 800 900 1000

(c)

Time (h)

0 36 72 108 144 180 216 252 288 324 360 396 432

Phthalate concentration (mg/L)

0 50 100 150 200 250 300

Degradation efficiency (%)

0 20 40 60 80 100 120

Biomass concentration (mg/L)

0 100 200 300 400 500 600 700 800 900 1000

Figure 4.5: Time profile of biomass growth, inlet – outlet concentration of phthalates and % phthalates degradation by Gordonia sp. in the continuously operated CSTB for different HRTs:

(a) 48 h (b) 36 h and (c) 24 h.

Batch

Continuous

Batch

Continuous

Batch

Continuous

Effect of inlet loading rate

Figure 4.6 reveals the performance of CSTB under continuous operation mode, in terms of degradation rate of phthalates with respect to their inlet loading rate (ILR). In these figures, the straight line through the origin indicates a stable performance of the bioreactor under continuous operation for different ILRs. The degradation rate values that are offset from the line indicate that ILR values beyond this point are inhibitory and detrimental to the bioreactor’s performance for phthalates biodegradation. Hence, it could be surmised that maximum degradation efficiency is favored at a low ILR, suggesting that microbial activity is unaffected below 6.25 mg/Lh of DMP, DEP and DBP (LMW) (Figure 4.6 a-c). With an increase in the phthalates ILR, the phthalates degradation rate and efficiency decreased, indicating an inhibitory effect at or above 8.33 mg/Lh of ILR. In the case of HMW phthalates, in particular BBP and DnOP, an ILR value of 4.17 mg/Lh or higher reduced their biodegradation (Figure 4.6d and f). Among the three HMW phthalates, DEHP was identified as the most inhibitory substrate for the microorganism and, therefore, it is suggested to keep its concentration lower than the other phthalates to achieve a better performance with the CSTB (Figure 4.6e). From Figure 4.6g, which shows the cumulative phthalate degradation rate vs their ILR, maximum degradation rate is achieved at 31.25 mg/Lh cumulative ILR value, and above which degradation rate was not consistent with the increase in ILR value. Hence, in order to achieve efficient biodegradation under continuous operation using the CSTB, the bioreactor should be operated at less than 31.25 mg/Lh cumulative ILR of the phthalates.

(a)