Publicação
Optimisation of sequencing batch biofilm reactors: response to filamentous microorganisms and antibacterial compounds
| Resumo: | Filamentous bulking and the presence of emergent antibacterial micropollutants in wastewaters are problems that a wastewater treatment plant (WWTP) may deal with. The present work aimed to give additional contributions to solve the problem of filamentous bulking in sequencing batch reactors (SBRs) through the addition of biofilm carriers as well as, to provide useful data for the application of biofilms in SBRs operated in the presence of antibiotics. The influence of the organic loading rate (OLR) on the overgrowth of filamentous organisms was assessed by operating three SBRs fed with different acetate loading rates: 4.3 ± 0.3 g COD L-1 day-1 (control unit), 1.0 ± 0.1 g COD L-1 day-1 and 0.5 ± 0.1 g COD L-1 day-1. The microscopic observations showed that the excessive growth of filaments only occurred in the SBR operated with the highest OLR. Acetate concentration profiles indicated that acetate removal occurred during the entire SBR cycle in the reactor facing the overgrowth of filaments, while in the others two SBRs a period of high substrate concentration followed by a period without substrate (“feast-famine” conditions) was happening. The nonexistence of “feast-famine” conditions in the SBR operated with the highest OLR probably favoured the excessive growth of filaments. The filaments were identified by culture-dependent techniques together with molecular methods as closely related to Geotrichum candidum (fungus). The study of the influence of carrier concentration in the overgrowth of the G. candidum-like filaments and in the bacterial community was conducted in SBRs without (control unit similar to the one of the previous study) and with 5 %, 10 % and 20 % carriers. The occurrence of the fungal filaments was only negligible in the SBRs with 10 % and 20 % carriers, apparently because the higher amount of sludge obtained in these reactors (suspended and biofilm) decreased the substrate availability and the carrier-to-carrier collisions established increased the shear stress. Bacterial community fingerprinting using molecular techniques showed that the bacterial groups usually present in activated sludge systems prevailed. The influence of the wastewater composition in the overabundance of the fungal filaments was also evaluated. Two SBRs were operated: the control SBR was fed with acetate (soluble organic matter) and the other SBR received wastewater containing both soluble and particulate organic matter. The results indicated that the wastewater composition influenced the overgrowth of the G. candidum-like filaments since they were detected in the SBR fed with acetate, while in the other SBR, filamentous bulking caused by Meganema perideroedesrelated filaments was identified (using culture-independent techniques). Both bulking were successfully overcome through the addition of carriers into the SBRs, suggesting that this might be an effective strategy to solve filamentous bulking. The addition of 10 % carriers solved the G. candidum-like bulking, apparently because the combined effect of decreasing the substrate availability and increasing the shear stress induced the growth of the fungus as arthroconidia instead of filaments. The suppression of the M. perideroedes-like bulking was probably related to the increase of the sludge concentration caused by the addition of 20 % carriers. The final study of this dissertation compared the microbial community composition and activity as well as the antibiotic resistance genes of SBRs operated with biofilms and receiving wastewater without and with 50 μg L-1 of tetracycline. The removal of organic carbon and nitrogen were similar in both reactors, but the bacterial composition of the biofilms (identified using culture-independent methods) differed. The presence of tetracycline decreased the fraction of Flavobacterium, Caulobacter and Zooglea-like bacteria and of members of the Sphingobacteriaceae family and increased the fraction of the Sandarakinorhabdus genus. The analysis of the tetracycline resistance (tet) genes indicated a relationship between the occurrence of tet(S) and tetracycline contamination. The occurrence of the others tested tet genes [tet(A), tet(C), tet(G), tet(M) and tet(X)] was not changed by the presence of the antibiotic. The reactor with tetracycline showed poor tetracycline removal (about 28 %): 15.8 ± 0.3 μg of the antibiotic was found sorbed to the biofilm matrix and, based on a mass balance, the remaining antibiotic removal probably occurred by metabolic processes. |
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| Autores principais: | Matos, M. I. |
| Ano: | 2014 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso restrito |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Filamentous bulking and the presence of emergent antibacterial micropollutants in wastewaters are problems that a wastewater treatment plant (WWTP) may deal with. The present work aimed to give additional contributions to solve the problem of filamentous bulking in sequencing batch reactors (SBRs) through the addition of biofilm carriers as well as, to provide useful data for the application of biofilms in SBRs operated in the presence of antibiotics. The influence of the organic loading rate (OLR) on the overgrowth of filamentous organisms was assessed by operating three SBRs fed with different acetate loading rates: 4.3 ± 0.3 g COD L-1 day-1 (control unit), 1.0 ± 0.1 g COD L-1 day-1 and 0.5 ± 0.1 g COD L-1 day-1. The microscopic observations showed that the excessive growth of filaments only occurred in the SBR operated with the highest OLR. Acetate concentration profiles indicated that acetate removal occurred during the entire SBR cycle in the reactor facing the overgrowth of filaments, while in the others two SBRs a period of high substrate concentration followed by a period without substrate (“feast-famine” conditions) was happening. The nonexistence of “feast-famine” conditions in the SBR operated with the highest OLR probably favoured the excessive growth of filaments. The filaments were identified by culture-dependent techniques together with molecular methods as closely related to Geotrichum candidum (fungus). The study of the influence of carrier concentration in the overgrowth of the G. candidum-like filaments and in the bacterial community was conducted in SBRs without (control unit similar to the one of the previous study) and with 5 %, 10 % and 20 % carriers. The occurrence of the fungal filaments was only negligible in the SBRs with 10 % and 20 % carriers, apparently because the higher amount of sludge obtained in these reactors (suspended and biofilm) decreased the substrate availability and the carrier-to-carrier collisions established increased the shear stress. Bacterial community fingerprinting using molecular techniques showed that the bacterial groups usually present in activated sludge systems prevailed. The influence of the wastewater composition in the overabundance of the fungal filaments was also evaluated. Two SBRs were operated: the control SBR was fed with acetate (soluble organic matter) and the other SBR received wastewater containing both soluble and particulate organic matter. The results indicated that the wastewater composition influenced the overgrowth of the G. candidum-like filaments since they were detected in the SBR fed with acetate, while in the other SBR, filamentous bulking caused by Meganema perideroedesrelated filaments was identified (using culture-independent techniques). Both bulking were successfully overcome through the addition of carriers into the SBRs, suggesting that this might be an effective strategy to solve filamentous bulking. The addition of 10 % carriers solved the G. candidum-like bulking, apparently because the combined effect of decreasing the substrate availability and increasing the shear stress induced the growth of the fungus as arthroconidia instead of filaments. The suppression of the M. perideroedes-like bulking was probably related to the increase of the sludge concentration caused by the addition of 20 % carriers. The final study of this dissertation compared the microbial community composition and activity as well as the antibiotic resistance genes of SBRs operated with biofilms and receiving wastewater without and with 50 μg L-1 of tetracycline. The removal of organic carbon and nitrogen were similar in both reactors, but the bacterial composition of the biofilms (identified using culture-independent methods) differed. The presence of tetracycline decreased the fraction of Flavobacterium, Caulobacter and Zooglea-like bacteria and of members of the Sphingobacteriaceae family and increased the fraction of the Sandarakinorhabdus genus. The analysis of the tetracycline resistance (tet) genes indicated a relationship between the occurrence of tet(S) and tetracycline contamination. The occurrence of the others tested tet genes [tet(A), tet(C), tet(G), tet(M) and tet(X)] was not changed by the presence of the antibiotic. The reactor with tetracycline showed poor tetracycline removal (about 28 %): 15.8 ± 0.3 μg of the antibiotic was found sorbed to the biofilm matrix and, based on a mass balance, the remaining antibiotic removal probably occurred by metabolic processes. |
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