Treatment of textile wastewater using sequential sulfate-reducing anaerobic and sulfide-oxidizing aerobic membrane bioreactors

Highlights

• High sulfate (95%) and color removals (99%) were observed in sulfate reducing MBR.

• High sulfate reduction rates were observed at COD/sulfate ratio of 2.0.

• Sulfide was completely oxidized in the aerobic MBR with little fouling.

• Metal–sulfides were the main inorganic foulants in AnMBR.

• Ca–P precipitates were observed in cake layer of the AeMBR.

Abstract

This study aims at evaluating the performance of sequential sulfate-reducing anaerobic and sulfide-oxidizing aerobic membrane bioreactors (MBRs) for the treatment of synthetic textile wastewater. The process performance was evaluated under varying concentrations of COD (1000–2000 mg/L), NaCl (500–1000 mg/L), and sulfate (500–1500 mg/L), but keeping dye (Remazol Brilliant Violet 5R) concentration constant at 200 mg/L. In sulfate reducing anaerobic MBR (AnMBR), COD removal efficiency remained between 80% and 85%. The sulfate reduction efficiency was directly related with COD/sulfate ratio. Almost complete decolorization was attained in the AnMBR, whereas slight recolorization was observed in the AeMBR due to autooxidation of aromatic amines. EDS analyses illustrated that cake layer in AnMBR contained much higher amounts of S, Fe and Cu due to formation of metal–sulfide precipitates, whereas Ca and P were the major inorganic elements in AeMBR. In the cake layers of both MBRs, high molecular weight soluble organics were observed by gel permeation chromatographic analyses together with the identification of proteins and polysaccharides by the FT-IR analyses. Capillary suction time, specific resistance to filtration and supernatant filterability tests illustrated that AnMBR sludge had less filterability and stable operation was possible with a flux of around 5 LMH.

Introduction

In textile industry, water is primarily utilized during the dyeing and finishing operations. The textile industry may be accepted as one of the most polluting industrial sectors considering both volume and composition of wastewater generated [1]. Over one million-ton of dye is produced annually and around 50% of the produced dye is used in textile industries [2]. Azo dyes are the most widely used class of dyes with word market share of 60–70% [1], [3] and around 20–50% of the applied dyes remain in the aqueous phase in dyeing operation, leading to colorization of the effluent stream. In addition to dyes, several auxiliary chemicals are used in the dying process and the produced wastewater is quite complex and variable in characteristics [4], [5]. The composition of textile wastewater depends very much on the process, coloring matters, dyestuffs and the accompanying chemicals. The release of dye containing effluents into the environment is undesirable due to serious potential environmental problems linked with the dyes and their breakdown products, i.e. aesthetic deterioration and the carcinogenic nature of aromatic amines generated as by-products of anaerobic azo-dye biodegradation [6].

Although aerobic treatment of azo dyes is quite difficult due to its recalcitrant nature and toxicity to microorganisms [7], under anaerobic conditions azo dyes are used as electron acceptors and are readily cleaved generating aromatic amines [6], [7], [8]. Contrary to the azo-dyes, aromatic amines are generally stable under anaerobic conditions whereas they are aerobically biodegradable [6], [8], [9].

Membrane bioreactor (MBR) technology combines the activated sludge process with membrane filtration process. With the use of micro- or ultra-filtration membranes (pore size: 0.05–0.4 µm), complete physical retention of microorganisms are achieved [10]. Therefore, in the past two decades, remarkable progress has been achieved on the MBR technology all over the world and become attractive option for the treatment and reuse of industrial and municipal wastewaters.

Although anaerobic bioreactors are more efficient for color removal, aerobic MBR has been generally used in several lab and pilot scale studies [11] and limited number of studies are available in the literature on the textile wastewater treatment using anaerobic MBR (AnMBR) [7], [8], [12]. In our previous studies [7], [8], we have illustrated that anaerobic MBR (AnMBR) can be successfully used for the treatment of textile wastewater. Even at high salinity conditions, AnMBR showed high efficiency and aerobic MBR (AeMBR) following the AnMBR was responsible for the effluent polishing as well as aromatic amine degradation [8]. Textile wastewaters generally have high conductivities, even up to 9 mS/cm, due to high concentrations of NaCl and some other inorganics added to increase the dye fixation by fabric [13]. Additionally, sulfate concentration in the textile wastewater may reach to very high levels as sulfate is added to the dye bath for ionic strength adjustment [14]. Sulfate concentration may also increase due to sulfuric acid use for the neutralization of alkaline textile wastewater before biological treatment. In the presence of sulfate, sulfate reducing conditions will develop, which completely changes microbial community and affects the color removal performance [14], [15]. Additionally, sulfide formed under sulfate reducing conditions should be further oxidized biologically. In the literature, there are limited number of studies on the treatment of textile wastewater using AnMBRs [7], [8] and sulfidogenic MBRs. To the best of our knowledge, there is no study on the treatment of textile wastewater using sequential sulfate-reducing anaerobic and sulfide-oxidizing aerobic MBRs. Hence, this study aims at evaluating the efficiency of sequential sulfate reducing and sulfide oxidizing MBRs for the treatment of synthetic textile dye wastewater under varying operational conditions. Also, a special emphasis have been given for the investigation of fouling in the MBRs as the presence of sulfide may completely change the fouling characteristics due to possible metal–sulfide precipitate formation in the bioreactors.