High-rate anaerobic decolorization of methyl orange from synthetic azo dye wastewater in a methane-based hollow fiber membrane bioreactor

Highlights

• MO decolorization achieves a high level of decolorization efficiency (∼ 100 %).

• The maximum decolorization rate is 883 mg/L/day.

• Microbial community is changed significantly after MO decolorization.

• Methanomethylovorans are dominant archaea and Moranbacteria are dominant bacteria.

• Archaea and bacteria play a synergistic role in MO decolorization in the HfMBR.

Abstract

Anaerobic biological techniques are widely used in the reductive decolorization of textile wastewater. However, the decolorization efficiency of textile wastewater by conventional anaerobic biological techniques is generally limited due to the low biomass retention capacity and short hydraulic retention time (HRT). In this study, a methane-based hollow fiber membrane bioreactor (HfMBR) was initially inoculated with an enriched anaerobic methane oxidation (AOM) culture to rapidly form an anaerobic biofilm. Then, synthetic azo dye wastewater containing methyl orange (MO) was fed into the HfMBR. MO decolorization efficiency of ∼ 100 % (HRT = 2 to 1.5 days) and maximum decolorization rate of 883 mg/L/day (HRT = 0.5 day) were obtained by the stepwise increase of the MO loading rate into the methane-based HfMBR. Scanning electron microscopy (SEM) and fluorescence in situ hybridization (FISH) analysis visually revealed that archaea clusters formed synergistic consortia with adjacent bacteria. Quantitative PCR (qPCR), phylogenetic and high-throughput sequencing analysis results further confirmed the biological consortia formation of methane-related archaea and partner bacteria, which played a synergistic role in MO decolorization. The high removal efficiency and stable microbial structure in HfMBR suggest it is a potentially effective technique for high-toxic azo dyes removal from textile wastewater.

Introduction

Azo dyes containing aromatic groups and one or more NN group, represent the largest class of dyes applied in the textile industry (dos Santos et al., 2007; Goncalves et al., 2009). However, approximately 8–20 % of the azo dyes are eventually discharged to the textile effluent due to their incomplete utilization (Chhabra et al., 2015; Wang et al., 2018), which causes high toxicity and mutagenicity in aquatic life and humans (Bras et al., 2005; Mahmood et al., 2016). Although a series of physiochemical methods for treating textile wastewater are technically feasible, they are costly (Fontenot et al., 2003). In contrast, due to its low cost and environmental friendliness, the biological treatment approach, especially anaerobic biological techniques, is a promising strategy for decolorization of textile wastewater (Georgiou and Aivasidis, 2006; Kim et al., 2008). Conventional anaerobic biological techniques, such as upflow anaerobic sludge blanket (UASB) (Bras et al., 2005), fluidized-bed loop reactor (FBLR) (Georgiou and Aivasidis, 2006) and sequencing batch reactor (SBR) (Yu et al., 2015; Yemashova et al., 2004), have been successfully used for the reductive decolorization of azo dyes. However, all these biological approaches consume large amounts of external organic carbon, such as glucose and acetate. The achievement of complete dyes decolorization and simultaneous minimization of energy and organic carbon consumption is the key breakthrough for biological treatment of textile wastewater.

Methane is a significant greenhouse gas, as well as a renewable energy and available carbon source if appropriately-managed (Modin et al., 2007; Zhu et al., 2016). Anaerobic oxidation of methane (AOM) is an important methane sink (Knittel and Boetius, 2009; Knittel et al., 2005). AOM coupled to the microbial reduction of various electron acceptors, such as sulfate (Boetius et al., 2000), nitrate (Haroon et al., 2013), nitrite (Ettwig et al., 2010), Fe(III) and Mn(IV) (Ettwig et al., 2016; Beal et al., 2009), plays a crucial role in mitigating methane emissions to the atmosphere. A recent study has shown that methane can also serve as electron donors for microbial decolorization of methyl orange (MO), a typical azo dye, but the decolorization was seriously inhibited when the concentration of MO was above 100 mg/L due to the high toxicity of azo dyes and its products to microorganisms in SBR (Fu et al., 2019). Therefore, it is necessary to use the appropriate biological technique, in which bacterial aggregates or biofilms are formed to improve the tolerance and adaptation of anaerobic bacteria to xenobiotic compounds, such as aromatic compounds (Donlon et al., 1995; Kargi and Eker, 2005). In addition, methane is poorly soluble (3.5 mg per 100 mL water) (He et al., 2013), thus improving methane transfer for microbe utilization is also very important in biological techniques involving methane-utilizing microbes.

A hollow fiber membrane bioreactor (HfMBR) is a novel and efficient technology to deliver a gaseous substrate to microorganisms (Lai et al., 2016a). Among its advantages, the membrane surface is used as a carrier for microorganism attachment, and can efficiently prevent the microorganisms from being washed out from the system (Syron and Casey, 2008; Cai et al., 2015). In addition, the continuous-flow mode used in the HfMBR can accelerate liquid discharge from the system, which avoids the accumulation of toxic products. These two features are particularly important for the removal of toxic pollutants by slow-growing anaerobes (Shi et al., 2013). To date, the methane-based HfMBR has been successfully used for efficient removal of highly toxic inorganics, such as perchlorate (Luo et al., 2015) and selenate (Lai et al., 2016b; Luo et al., 2018), and heavy metals, such as chromium (Lai et al., 2016a; Lu et al., 2018) and vanadate (Lai et al., 2018a). Meanwhile, methanotrophic archaea usually forms syntrophic consortia with partner bacteria in HfMBR, which ultimately enhance the stability and removal rates of this system to practical application level (Cai et al., 2015; Xie et al., 2017).

Therefore, the aims of this study were to (1) investigate the feasibility of applying methane-based HfMBR as a technology for MO decolorization from synthetic wastewater; (2) evaluate the MO decolorization rate and efficiency by stepwise changing the influent MO concentration and HRT in HfMBR; (3) explore the microbial morphology and community performing MO decolorization in HfMBR. The performance of the HfMBR was determined by measuring the concentration of MO. Microbial morphology in the HfMBR was analyzed by fluorescence in situ hybridization (FISH) and scanning electron microscopy (SEM) analysis. In addition, the key microorganisms performing MO decolorization were identified using a combination of quantitative real-time PCR (qPCR), high-throughput sequencing of the 16S rRNA gene and cloning library construction.