Anaerobic – aerobic sequencing batch reactor treating azo dye containing wastewater: effect of high nitrate ions and salt

In this work, the treatment of wastewater containing azo dye using anaerobic – aerobic sequencing batch reactor (SBR) based on mixed culture for its ef ﬁ cacy in decolorization and reduction in chemical oxygen demand (COD) under different operational conditions has been analyzed. Effects of hydraulic retention time (HRT), salts content and nitrate ion concentration on the rate and extent of color and COD removal through 180 days containing steady-state and acclimation periods were investigated. Solid retention time was kept constant at 20 days in all experiments. Almost complete decolorization could be achieved at dye concentrations between 5 and 500 mg/L, but the removal of COD decreased gradually from 90 to 65% with increasing dye concentration. The results indicated that color was mainly removed under anaerobic conditions and it was almost ﬁ lled out within 2 – 3 h of the anaerobic residence time with up to 98% decolorization ef ﬁ ciency. Besides, cutting the cycle time from 24 to 8 h does not have an effect on color removal. Increases in HRT provide enough time for partial mineralization of COD and intermediates in SBR system. The rates of color and COD removals decreased with increasing salt content and nitrate ion concentration in the feed wastewater.


INTRODUCTION
Textile wastewater is a complex and highly variable mixture of various pollutants such as degradable organics, dyes, nutrients, salts, sulfur, toxicants and refractory organics.
Azo dyes account for more than 50% of the dyes used in textile processing industries and are the most common synthetic dyes discharged into the environment (Khouni et al. ). Azo dyes are characterized by the presence of one or more azo bonds (-N ¼ N-) connecting aromatic rings. Different substitutions on aromatic nucleus give a structurally diverse and most versatile group of compounds which makes them recalcitrant and xenobiotic (Ali ).
Wastewaters from the textile industry can produce severe environmental problems due to their toxicity, mutagenicity and carcinogenicity effects to aquatic life and hence need to be treated before being released into the environment or any reuse program (Tony et  Azo dye removal has been studied using both pure and mixed mediums (Lourenco et al. ). Although considerable results have been achieved using pure mediums (Ghodake et al. ; Parshetti et al. ; Silveira et al. ), these seem to be not usable at full scale facilities for real textile wastewater treatment. Many works have recommended that mixed medium may be more appropriate for decolorization of azo dyes (Coughlin et al. ; Guo et al. ; Koupaie et al. b). Thus, the mixed mediums can perform tasks better than or equivalent to that of an individual pure culture without any precautions to prevent contamination (Popli & Patel ).  Color removal, especially from textile wastewaters, has been a huge challenge over the last decades, and up to now there is no single and economically attractive treatment that can effectively remove colors. Also, the reuse of water from effluents in the production process or treatment plant leads to a reduction in costs for the textile industry. In a study for possibility of wastewater reuse the authors have shown that the use of advanced oxidation processes (AOPs) can reuse the effluent water by up to 10 times in the dyeing process The primary purpose of this study was to determine the performance of an anaerobic-aerobic SBR technology for the removal of azo dye. Likewise, the effects of some operational parameters such as cycle times, high total dissolved solids (TDS) and nitrate concentration were determined.

Experimental set-up
A laboratory scale SBR system consisted of a 10 L cylindrical reactor made of plexiglass with an inner diameter of 17 cm (working volume of 8 L). The scheme of the SBR system is shown in Figure 1. The reactor was operated with cycle times of 8, 12 and 24 h and the volume exchange in each cycle was 4 L (volumetric exchange ratio 0.5). The system was first operated under anaerobic conditions with a slight mixing to obtain homogenous conditions. After that, the anaerobic step was completed. The peristaltic pump, mixer, blower and solenoid valve were controlled by a PLC time controller (Omron, Japan). Phase duration and operating condition of the system's working cycles are presented in Table 1.

Characterization of synthetic wastewater
Synthetic wastewater was prepared with ordinary tap water and dye, and glucose as sources of carbon and energy. Synthetic wastewater used throughout this survey is presented in Table 2.
The commercial basic dye used as a pollutant in the present study was C.I. Basic Red 46 (BR46), which was purchased from Alvan Sabet Co. (Hamedan, Iran). This dye is soluble in water and belongs to the cationic basic dye group. The chemical structure and other characteristics of BR46 are shown in Table 3. Dye solutions were prepared by dissolving dye in water. Glucose (contributed to chemical oxygen demand   (COD) of 1,500 mg/L after start-up) was added into the media to offer a readily biodegradable carbon source, while at the same time it provided the electrons for the reductive cleavage of the BR46 dye. All chemicals were analytical grade (Merck, Germany) and were used without any further purification.

Experiments
The activated sludge medium obtained from the sludge return line from Zanjan municipal wastewater treatment plant was applied as the seed sludge. At first the sludge was passed through a screen to remove existing gravel. Dilution was performed several times until mixed liquor suspended solids (MLSS) were adjusted to approximately 3,000 mg/L in the reactor. In order to achieve stability conditions, the COD content of the fill wastewater was kept constant at 1,000 ± 25 mg/L glucose as sources of carbon and energy. During the start-up of the reactor, HRT was kept constant for 24 h and SRT was adjusted to 20 d by removing a certain amount of sludge daily. This procedure was performed until the system was able to reach an entirely stable condition and COD removal was over 90%. After the start-up, the inlet COD into the reactor was raised to 1,500 ± 25 mg/L and in order to acclimatize the biomass, BR46 content of between 5 and 500 mg/L was added gradually. This step was continued until the system was able to return to the primary stable condition. The variations of cycle times were studied in the range of 8-24 h in the reactor. The effects of TDS and nitrate on the performance of SBR were also investigated in the range of 1,000-8,500 mg/L and 10-120 mg/L, respectively.

Analysis
Samples were withdrawn from the sample port of the reactor at predetermined time intervals and were centrifuged before analyses. The concentration of the dye was determined by measuring the absorption intensity at the maximum absorbent wavelength of BR46 (530 nm) using UV-vis spectrophotometer (DR 4000, HACH, USA). Also, COD, nitrate, total suspended solids (TSS), MLSS and TDS concentrations were determined according to Standard Methods (APHA ).

Dye and COD removal
Start-up with the glucose as growth substrate was quite prompt and high COD removal of up to 90% was achieved at two steps COD increment from 1,000 to 1,500 mg/L. Therefore, after three weeks of stable operating conditions, a biomass concentration of about 3,000 mg/L and SRT of 20 d were obtained. In the next step of study, biomass was acclimated to BR46 and the feed BR46 content was gradually increased from 5 to 500 mg/L for 68 d. The MLSS concentrations were found to vary in the range of 3,400-4,000 mg/L for the operation period. With increasing color concentration, COD of color is added for COD induced by glucose in wastewater. Figure 2 shows the detail of acclimation.
The results demonstrated that activated sludge from municipal wastewater treatment plants have high capability in color and COD removal from synthetic wastewater. As with increasing color concentration, almost all of the color was removed in the operation period. However, the removal of COD decreased gradually from 90 to 65% with increasing color concentration. The anaerobic phase provides   They have also indicated that the minimum HRT in the anaerobic phase should be 6 h for efficient color removal.
The SBR system requires long aeration periods to decrease the effluent COD concentration to less than 100 mg/L.

Effect of TDS on reactor stability
Textile effluents contain various acids, alkalis, salts, or metal ions as impurities. Thus, microbial species capable of tolerating salt stress will be important for treating such wastewaters (Ali ).

Effect on nitrate on color and COD removal
Nitrate was a typical salt species included in dye baths for promotion of dye fixation to the textile fibers and commonly found in saline textile wastewaters (Carliell et al. ; Varsha & Seema ). Therefore, it is necessary to examine their effects on azo dye decolorization. Figure