Impact of SRT on the efficiency and microbial community of sequential anaerobic and aerobic membrane bioreactors for the treatment of textile industry wastewater
Introduction
Textile industry wastewater is highly variable in nature due to the use of multiple processes especially during several washing and rinsing cycles [1]. The world market share of azo dyes are around 60–70% [2], [3]. In dyeing operation, around 20–50% of the dyes stay in the water phase, which lead to serious pollution of water resources. The textile industry wastewaters contain several other auxiliary chemicals in addition to dyes, which make the generated wastewater quite complex and variable [3], [4].
Different physico-chemical [5] as well as biological methods [6] are used for textile wastewater treatment. Physico-chemical processes are generally considered expensive and sometimes non-environmentally friendly due to the generation of high amount of sludge and secondary pollution. Biological processes are considered to be environmentally friendly approaches compared to physico-chemical methods due to the possibility of complete removal of organic pollutants with lower cost [7], [8]. Therefore, biological processes should be preferred or they should be used in combination with other physico-chemical treatment processes, such as ozonation, membrane filtration and chemical precipitation.
In recent years, several types of membranes have been increasingly used for wastewater treatment [9], [10], [11], [12], [13]. The membrane bioreactor (MBR) technology combines biological treatment process and membrane technology for more effective wastewater treatment. In the MBR, both micro- and ultra-filtration membranes (0.03–0.4 μm) can be used for complete physical retention of microorganisms [14]. Due to higher performance compared to classical activated sludge processes, MBR processes have been used increasingly for textile wastewater treatment [8].
Yurtsever et al. [8] observed that anaerobic MBR (AnMBR) performed better compared to aerobic MBR (AeMBR) in terms of color removal performance, however, literature studies concentrated on AeMBR [15] and only few studies have been performed on AnMBR treating textile wastewater [8], [16], [17].
In textile wastewater, sulfate concentration is generally quite high due to its addition to dye bath for the adjustment of ionic strength [18]. In our previous study, we have observed that AnMBR performs well for sulfate-rich textile wastewater treatment. The presence of sulfide in the bioreactor caused accumulation of metal-sulfide precipitate on membrane, which may increase the fouling. Although sequential AnMBR and AeMBR is effective in the treatment of textile industry wastewater treatment [19], both the biological and the filtration performances should be optimized.
AnMBRs have several advantages, i.e. biogas production, lower sludge production, whereas higher fouling tendencies and lower operational fluxes restrain their wide spread application [8], [20], [21]. Supplementation of mixed liquor with granular or powdered activated carbon (PAC) may alleviate the membrane fouling and increase the operational fluxes of AnMBRs [21], [22], [23]. Additionally solid retention time (SRT), one of the most important operational parameters in biological processes [20], [24], may have impact on biological and filtration performances and its effect on sequential anaerobic sulfate-reducing and aerobic sulfide-oxidizing MBRs have not been studied before.
Powdered activated carbon (PAC) is widely used as an adsorbent due to significant porous structure [23]. Addition of PAC to the MBRs decreases soluble and colloidal organic concentration and reduces fouling, especially pore blocking. Also, PAC addition can increase the shear effect of aeration on the membrane and may reduce cake formation. Although PAC addition on the fouling alleviation has been extensively studied in municipal wastewater treating aerobic MBRs treating, studies on industrial wastewaters treating AnMBRs are limited [25].
Several studies have been conducted to associate the fouling problems in MBRs with the microbial community using different molecular biological techniques [17], [26], [27], [28]. New generation sequencing, specially, Illumina Miseq system becomes prevalent with some advantages such as presenting relatively low abundance of microbial community, time efficiency and decreased cost day by day. Illumina Miseq was recently performed in psychrophilic (15 °C) AnMBR treating domestic wastewater for the evaluation of membrane biofilm development [29].
This study aims at investigating the influence of SRT on the robustness of sequential anaerobic sulfate-reducing and aerobic sulfide-oxidizing MBRs treating synthetic textile wastewater. Also, fouling control by PAC addition has been emphasized. Additionally, the bacterial community changes in both MBRs were studied.
Section snippets
Anaerobic and aerobic membrane bioreactors
In the study, a sequential process consisting AnMBR and AeMBR was operated at different SRTs for longer than 200 days. The total and the working volumes of the plexiglass AnMBR (11 × 14 × 37 cm) were 5.7 L and 4 L, respectively. The total and working volumes of AeMBR (8 × 14 × 38 cm) were 4.3 L and 2.5 L, respectively. A sequential operating mode was adopted, i.e. the wastewater was fed to the first AnMBR and its effluent was used as a feed for the AeMBR (Fig. S1).
Both MBRs were equipped with flat sheet
Results and discussions
The impact of SRT on the process performance of sequentially operated MBRs was evaluated for sulfate-containing textile wastewater treatment. The impacts of SRT on the MBRs were discussed separately considering both removal and filtration performances.
Conclusions
The impacts of SRT on the performance of sequentially operated anaerobic sulfate reducing and aerobic sulfide oxidizing MBRs were investigated. The average COD removal performance (86–65%) and sulfate removal performance (95–82%) decreased at short SRTs due to decreased biomass concentration. Complete decolorization was attained in AnMBR at all SRTs (>99%). The filterability of AnMBR sludge was 4–8 times lower compared to that of AeMBR. Metal-sulfide and Ca-P precipitates are the main inorganic
Acknowledgment
This study was funded by Scientific and Technological Research Council of Turkey (TUBITAK Project No: 113Y336) and Istanbul Medeniyet University Research Fund (Project No: F-KDT-2016-780).
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