Investigation on the fate of benzotriazoles and benzothiazoles during biological wastewater treatment

Abstract

A major problem concerning wastewater treatment nowadays is the elimination of organic micropollutants from raw municipal and industrial wastewater. Many groups of compounds, such as surfactants, personal care products, pharmaceuticals, estrogens, perfluorinated compounds, phthalate acid esters and others are thoroughly examined concerning their occurrence and removal from wastewater as well as their ecotoxicity to living organisms. During this study benzotriazoles (BTRs) and benzothiazoles (BTHs) were examined regarding their biological removal from sewage. BTRs and BTHs are used in many industrial and every day products, leading to their presence in wastewater. Their frequent detection in surface water indicates their inadequate elimination during wastewater treatment. So far, little is known about the biodegradation rates of BTRs and BTHs by suspended and attached biomass and about their removal efficiencies in different biological wastewater treatment systems. The main goals of this study was a) to investigate the fate of BTRs and BTHs during biological wastewater treatment, as well as the role of biodegradation and sorption on their removal and b) to compare BTRs and BTHs removal efficiency in different biological treatment systems (activated sludge system, AS; moving bed biofilm reactor system, MBBR; hybrid moving bed biofilm reactor system, HMBBR). More specifically, 1H-benzotriazole (BTR), 5-chlorobenzotriazole (CBTR), xylytriazole (XTR), 4-methyl-1H-benzotriazole (4TTR), 5-methy-1H-lbenzotriazole (5TTR) and 2-hydroxy-benzothiazole (OHBTH) were studied and experiments were conducted in three steps. In the first step, BTRs and BTHs sorption and biodegradation onto activated sludge (AS) was investigated in batch experiments. Experiments with sterilized AS showed no abiotic transformation of these compounds, while their sorption constants ranged between 87 (XTR) and 220 L Kg-1 (BTR). Regarding the biodegradation experiments, the influence of different conditions was examined as to the target compounds treatment with AS. The presence of easily degradable organic compounds enhanced their biodegradation, showing that these compounds are mainly removed as a result of co metabolism. The half lives calculated in batch experiments varied between 6.5 h for OHBTH to 47 h for CBTR. The different SRT of AS did not seem to influence biodegradation of target compounds. Concerning the fate of target compounds in full-scale STPs, the application of appropriate equations showed that the examined compounds are expected to be partially removed mostly through aerobic biodegradation, while sorption poorly contributes to their elimination from sewage (less than 3%). In the second experimental part, the biodegradation of BTRs and BTHs in lab-scale AS and MBBR systems was studied. Both systems were able to remove target compounds at different rates. Removal efficiencies ranged from 43% to 76% for BTR, 8% to 69% for 4TTR, 0% to 53% for 5TTR, 42% to 49% for CBTR, 9% to 43% for XTR and 80% to 97% for OHBTH. The attached biomass (MBBR) presented higher biodegradation constants (kbio, L gSS-1 d-1) compared to suspended biomass (AS). The operational parameters of each system seemed to strongly influence the microbial community that was developed, leading to fluctuation in removal in each system. The biomass developed in the MBBR system presented higher specific removal rates of the target compounds. In general, specific removal rates in the MBBR system reached 11.9 (BTR), 15.1 (4TTR), 14.4 (5TTR), 11.3 (CBTR), 9.7 (XTR) and 13.6 (OHBTH) µg of micropollutant removed per g of biomass per day. Two experimental cycles were conducted for the MBBR system, testing the influence of organic loading on the removal capacity of the system. According to the results, higher micropollutants removal rates were obtained when the MBBR system was operated under low organic loading conditions. In the last experimental part of this PhD Thesis, a HMBBR system was used and the removal efficiency of target compounds was investigated. According to the results, the total removal rates obtained were 75% (BTR), 41% (4TTR), 57% (5TTR), 61% (CBTR), 74% (XTR) and 81% (OHBTH). Biodegradation of target compounds occurred mainly in the first reactor of the HMBBR, while the second reactor contributed significantly to the removal of the most resistant compounds (4TTR). The contribution of each type of biomass that co-exists in a HMBBR systems was examined, by using biodegradation constants calculated for each type of biomass in batch experiments. For three compounds (OHBTH, BTR and XTR), the main removal mechanism was biodegradation by AS in the first bioreactor. For CBTR and 5TTR, biodegradation by AS and biofilm was almost equal in both bioreactors, while 4TTR was mainly removed by the biofilm developed in the second bioreactor. Possible by-products were investigated with batch biodegradation experiments. In total, twenty-two transformation products were tentatively identified; hydroxylation, oxidation and methylation were the main reaction mechanisms. When compared to systems examined in the second experimental part, the HMBBR performance was similar to a low loaded pure MBBR system and more efficient than AS and MBBR systems operating under the same HRT and organic loading conditions. The following chapters structure this dissertation: Chapter 1 includes a short literature review on the main wastewater treatment processes used in this study and the target micropollutants investigated, as well as the objectives and the outline of this PhD Thesis. In Chapter 2, the experimental procedures and analytical methods are described. In Chapter 3, the results of this study are presented and discussed, while Chapter 4 summarizes the most important conclusions as well as suggestions for future research. Thereupon, supplementary data is presented as well as the three publications in scientific journals that came out of this study.