Sequential Anaerobic/Aerobic Treatment of Dye-Containing Wastewaters: Colour and COD Removals, and Ecotoxicity Tests

Biological treatment of real textile wastewater containing sulphate, salinity, and surfactant through an anaerobic-aerobic system

Real textile wastewater containing high salinity (up to 12.6 g·kg^-1) and surfactant (up to 5.9 mg·L^-1 of linear alkylbenzene sulfonate - LAS) was submitted to biological treatment for colour (up to 652 mg Pt-Co·L^-1) and sulphate (up to 1,568.6 mg SO₄^-2·L^-1) removal. The influence of ethanol and molasses supplementation was firstly evaluated in anaerobic batch reactors for the removal of dyes and sulphate. Subsequently, aiming to remove aromatic amines (dye degradation by-products), an anaerobic-aerobic continuous system supplemented with molasses was applied. Supplementation had no influence on colour removal (maximum efficiencies around 70%), while it improved sulphate reduction (23% without supplementation against 87% with supplementation), and conferred robustness to the reactors, which recovered quickly after higher salinity impact. The aerobic reactor removed aromatic amines when the level of surfactants was lower than 1.0 mg LAS·L^-1, but the performance of the system was hindered when the concentration was increased to 5.9 mg LAS·L^-1. Findings suggest that the supplementation of an easily biodegradable organic matter might be a strategy to overcome wastewater fluctuation in composition.

Enhanced decolorization of dyeing wastewater in a sponges-submerged anaerobic reactor

This study was conducted to assess the potential of a sponges-submerged anaerobic baffled reactor (SS-ABR) for enhancing the processing performance of azo dye-contaminated wastewater. A lab-scale four-compartment SS-ABR, with a total volume of 10 L, was operated at 30 °C for 180 days. A total of 14 polyurethane sponges were added in each compartment to treat synthetic wastewater including a commercial azo dye Hellozol HSR Reactive Black. During the entire operation, in synthetic wastewater, starch was used as a sole carbon source, and the true color level was maintained at 1050 ± 98 Pt/Co. Meanwhile, the hydraulic retention time (HRT) and total COD (T-COD) in the influent were changed to evaluate the SS-ABR treatment performance. After the start-up phase, true color and T-COD removal efficiencies were recorded as 65 ± 3% and 83 ± 2%, 68 ± 5% and 81 ± 4%, and 70 ± 5% and 84 ± 2% for HRT and influent T-COD concentration of 18.6 h and 260 mg L^-1, 14.6 h and 260 mg L^-1, and 14.6 h and 460 mg L^-1, respectively. The microbial community analysis showed that bacterial groups involved in dye degradation, such as Clostridium sp., and sulfate-reducing bacteria Desulfomonile sp. and Desulfovibrio sp. were detected prominently in the SS-ABR. Interestingly, the SS-ABR exhibited the dominance of both Geobacter sp. and Methanosarcina sp., and their occurrences in all columns were proportional to each other, revealing the formation of syntrophic relationships.

Evaluation of toxicity and mutagenicity of a synthetic effluent containing azo dye after Advanced Oxidation Process treatment

Approximately 20% industrial water pollution comes from textile dyeing process, with Azo dyes being a major problem in this scenario and requiring new forms of efficient treatment. Effluent treatments using the Advanced Oxidation Processes (AOP) are justified by the potential of application in the dyed effluent treatments once they can change the Azo dye chemical structure. Thus, this study aimed to evaluate the toxicity and mutagenic capacity of a synthetic effluent containing Amido Black 10B (AB10B) azo dye before treatment with AOP, named Gross Synthetic Effluent (GSE), and after the AOP, named Treated Synthetic Effluent (TSE). Daphnia magna and Allium cepa tests were used to evaluate acute toxicity effects and chromosomal mutagenesis, respectively. The Salmonella/microsome assay was performed to evaluate gene mutations. In silico assays were also performed aiming to identify the mutagenic and carcinogenic potential of the degradation byproducts of AB10B. There was 100% immobility to D. magna after 24 h and 48 h of treatments with TSE, showing EC₅₀ values around 5%, whereas GSE did not show acute toxicity. However, GSE induced chromosomal mutations in A. cepa test. Both GSE and TSE were not able to induce gene mutations in S. typhimurium strains. These effects can be associated with two byproducts generated with the cleavage of the azo bonds of AB10B, 4-nitroaniline and -2-7-triamino-8-hydroxy-3-6-naphthalinedisulfate (TAHNDS). In conclusion, AOP is an efficient method to reduce the mutagenicity of synthetic effluent containing AB10B and additional methods should be applied aiming to reduce the toxicity.

Two-stage anaerobic process benefits removal for azo dye orange II with starch as primary co-substrate

Two-stage anaerobic system (S1: R1 (acidogenic phase) + R2 (methanogenic phase)) and the one-stage control (S0) were established to investigate the effect of phase separation on the removal of an azo dye orange II, i.e., Acid Orange 7 (AO7), with starch as the primary co-substrate. Although final AO7 removal from two systems showed no statistical differences, the first-order rate constants for AO7 removal (k_AO7-) and sulfanilic acid (SA) formation (k_SA) were higher in S1. Kinetic analysis showed that k_AO7- and k_SA in S1 were 2.7-fold and 1.7-fold of those in S0, respectively, indicating the benefit of phase separation to the AO7 reduction. However, this benefit only appeared in the period with influent AO7 concentrations higher than 2.14 mM. Otherwise, this advantage would be hidden due to the longer HRT (5 d) and sufficient electron donor (1.0 g starch L^-1). Within S1, R1 only contributed about 10% of the entire AO7 removal, and k_AO7- in R1 (0.172 h^-1) was much lower than in R2 (0.503 h^-1). The methanogenic phase rather than acidogenic phase was the main contribution to AO7 removal, because the influent of R2 had more available electron donors and suitable pH condition (pH 6.5-7.0) for the bio-reduction process.

Evaluation of an eventual ecotoxicity induced by textile effluents using a battery of biotests

Textile industry is considered as one of the important factors of the economic growth in Tunisia. However, this prominent role has certainly some drawbacks mainly represented by the huge amounts of textile wastewaters generated that become a real menace to nature. Many previous studies showed the purifying potential of some activated sludge and bacteria (Pseudomonas putida) to decolourize textile effluents. However, in many cases, decolourization of wastewaters is not necessary associated with detoxification, generating a real risk for the ecosystem in general. We evaluated in this work the induced toxicity of a textile effluent before and after its treatment with activated sludge followed by P. putida, using a battery of biotests. This study proved the detoxifying power of the activated sludge according to most of ecotoxicity tests. The treatment with P. putida did not improve the quality of the effluent; on the contrary, it could increase its toxicity. Daphnia magna and Raphidocelis subcapitata appear to be the most sensitive organisms in assessing eventual toxicity caused by this kind of wastewaters.

Reductive decolourisation of sulphonated mono and diazo dyes in one- and two-stage anaerobic systems

This work assessed the application of one- and two-stage mesophilic anaerobic systems to colour removal of sulphonated mono and diazo dyes with ethanol as electron donor. The dyes Congo Red (CR), Reactive Black 5 (RB5) and Reactive Red 2 (RR2) were selected as model compounds and tested separately in seven different periods. The one-stage system (R(1)) consisted of a single up-flow anaerobic sludge blanket (UASB) reactor, whereas the two-stage system (R(2)) consisted of an acidogenic UASB reactor (R(A)), a settler and a methanogenic UASB reactor (R(M)). For CR and RB5, no remarkable difference was observed between the colour removal performance of both anaerobic systems R(1) and R(2). The experiments with RR2 revealed that R(2) was more efficient on colour removal than R(1), showing efficiencies almost 2-fold (period VI) and 2.5-fold (period VII) higher than those found by R(1). Additionally, R(2) showed a higher stability, giving a good prospect for application to textile wastewaters. Finally, the acidogenic reactor (R(A)) had an important role in the overall decolourisation achieved by R(2) during the experiments with CR and RB5 (>78 %), whereas for RR2, a more recalcitrant dye, R(A) was responsible for up to 38 % of the total colour removal.