Lessons learnt from a pilot study on residual dye removal by an aerated treatment wetland

Effect of Typha angustifolia, feeding modes and intermittent aeration on the performance of hybrid constructed wetland systems treating Reactive Black 5 diazo dye wastewater

The current research assessed the effectiveness of four hybrid constructed wetland (HCW) systems for the remediation of synthetic dye wastewater with Reactive Black 5 (RB5) azo dye. All HCW systems had identical configurations, consisting of a horizontal CW followed by a vertical CW, and operated under diverse conditions such as the presence of plants (Typha angustifolia), feeding modes (batch and continuous) and intermittent aeration (4 h day-1). Anaerobic-aerobic conditions simulated within the HCW systems were crucial in removing the pollutants from synthetic dye wastewater. The planted HCW system operating in continuous-continuous feeding mode exhibited better colour reduction (91%). The planted HCW system with continuous-continuous feeding mode and intermittent aeration achieved better COD (90%) and BOD5 (99%) removals, and a significant reduction in dye degradation intermediates generated from RB5 reduction. Typha angustifolia had a beneficial effect on pollutant removals, attributed to the enhanced microbial density and biomass activity in the plant rhizosphere. The microbial density and biomass activity in the rhizosphere region of the planted system were (116 ± 28) × 104 CFU mL-1 and 1074 mg COD g VSS-1 day-1, respectively, notably higher than those in the non-rhizosphere region of the planted system ((186 ± 97) × 102 CFU mL-1 and 794 mg COD g VSS-1 day-1) and the unplanted system ((91 ± 35) × 102 CFU mL-1 and 772 mg COD g VSS-1 day-1). The introduction of continuous feeding mode and intermittent aeration in vertical units further improved pollutant removals in the HCW system. This improvement could be attributed to the steady feeding approach inherent in continuous mode operation, which may result in less toxicity to the system and the degradation of organic compounds under more favourable aerobic conditions.

Bioremediation technologies for remediation of dyes from wastewater

The colored dyes are extensively applied in diverse industrial setups such as textiles, paper, leather, and cosmetics. The unutilized dyes are released in the waste and pose a serious menace to the environment, ecological balance, and human health. Because of their chemical nature, they are extremely resistant to common methods of treatment and often persist in the aquatic environment. A sustainable and eco-friendly approach for treating dye-contaminated wastewater is "bioremediation." This manuscript aims to discuss the exclusive role of diversified microorganisms and plants, immobilized microbial cells/enzymes, microbial consortia, nanomaterials, and combination approaches in the bioremediation of dyes. It also provides a comprehensive understanding of different bio-remedial technologies used to remove dyes from wastewater. In addition, the underlying mechanisms affecting the efficacy of bio-remedial technologies, the latest breakthroughs, challenges, and potential solutions in scaling up, and prospects in this area are also explored. We also detail the noteworthiness of genetic engineering in different bioremediation technologies to solve the issues associated with dye contamination in wastewater and its removal from the environment.

Room temperature fabrication of cobalt mullite for the snappy adsorption of cationic and anionic dyes

Cobalt mullite adsorbent for the robust adsorption performance toward Victoria Blue (VB) and Metanil Yellow (MY) is fabricated by the sol-gel method at room temperature using dipropylamine as a structure-directing agent. The synthesized adsorbent is characterized by XRD, FT-IR, and HRTEM. From these analyses, it is found that dipropylamine binds with the alumina and cobalt oxide, which makes it into tetrahedral to octahedral form. This interaction causes the formation of cobalt mullite. It is observed that trigonal alumina and orthorhombic cobalt mullite are interlinked to form a hybrid network. The special feature of adopting this adsorbent for the adsorption of VB and MY is that it has a large amount of Brønsted acid site because of the octahedral coordination of Al and Co. The large availability of acid sites in the framework and hybridization of two different network systems favors robust adsorption. The rate of adsorption (K₂ = 0.00402 g/mg.min for VB and K₂ = 0.004 g/mg.min for MY) and adsorption capacity (Q_e = 102.041 mg/g for VB and Q_e = 19.0406 mg/g for MY) are greater for VB than MY. This may be due to the more steric factor involved in MY than VB. Thermodynamic parameter indicated that the adsorption of VB and MY is spontaneous, endothermic, and increased randomness in the adsorbent-adsorbate interface. The results from the enthalpy value (ΔH° = 65.43 kJ/mol for VB and ΔH° = 44.729 kJ/mol for MY) revealed that the chemisorption is involved in the adsorption process.

Wastewater from textile digital printing as a substrate for microalgal growth and valorization

This study aims at evaluating an innovative biotechnological process for the concomitant bioremediation and valorization of wastewater from textile digital printing technology based on a microalgae/bacteria consortium. Nutrient and colour removal were assessed in lab-scale batch and continuous experiments and the produced algae/bacteria biomass was characterized for pigment content and biomethane potential. Microbial community analysis provided insight of the complex community structure responsible for the bioremediation action. Specifically, a community dominated by Scenedesmus spp. and xenobiotic and dye degrading bacteria was naturally selected in continuous photobioreactors. Data confirm the ability of the microalgae/bacteria consortium to grow in textile wastewater while reducing the nutrient content and colour. Improvement strategies were eventually identified to foster biomass growth and process performances. The experimental findings pose the basis of the integration of a microalgal-based process into the textile sector in a circular economy perspective.

Investigation of functional performance of treatment systems for textile wastewater in selected textile industries in Tanzania

Textile industrialization is an integral part of the economic growth in Tanzania. However, the corresponding wastewater from textile treatment processes consists of dyes and auxiliaries associated with acute toxicological impacts. This necessitates an investigation of the functional performance of the industrial treatment systems used before effluent discharge. The study primarily accesses the catalog of industrial dyes and the functionality of the treatment system at Arusha, Morogoro and Dar es Salaam vis-à-vis the effluent physicochemical properties. The analytical study reveals disperse (42%), vat (34%) and reactive (26%) as the most used industrial dyes. The physicochemical properties of the quantified wastewater reveal a significant amount of and phosphorus which was consequent to the high turbidity, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) apart from the color at the different sampling points. Although the treatability of the wastewater was 90% efficient using an activated carbon system (237.33 ± 0.67 mg/L). Similarly, the use of aerated constructed wetlands shows efficiency in the remediation of the recalcitrant having a value of 12.13 ± 0.89^b mg/L (90%) and 13.22 ± 0.15^a mg/L (94%). Thereafter, needful recommendations were suggested based on the physicochemical properties of the textile wastewater and to improve the functionality of the treatment systems in the respective industries.

Degradation of Azo Dyes: Bacterial Potential for Bioremediation

The use of dyes dates to ancient times and has increased due to population and industrial growth, leading to the rise of synthetic dyes. These pollutants are of great environmental impact and azo dyes deserve special attention due their widespread use and challenging degradation. Among the biological solutions developed to mitigate this issue, bacteria are highlighted for being versatile organisms, which can be applied as single organism cultures, microbial consortia, in bioreactors, acting in the detoxification of azo dyes breakage by-products and have the potential to combine biodegradation with the production of products of economic interest. These characteristics go hand in hand with the ability of various strains to act under various chemical and physical parameters, such as a wide range of pH, salinity, and temperature, with good performance under industry, and environmental, relevant conditions. This review encompasses studies with promising results related to the use of bacteria in the bioremediation of environments contaminated with azo dyes in the most diverse techniques and parameters, both in environmental and laboratory samples, also addressing their mechanisms and the legislation involving these dyes around the world, showcasing the importance of bacterial bioremediation, specialty in a scenario in an ever-increasing pursuit for sustainable production.

Current status of textile wastewater management practices and effluent characteristics in Tanzania

Textile wastewater from wet processing units is a major environmental problem. Most chemicals, including dyes, are only partly consumed, resulting in highly colored wastewater containing a variety of chemicals released into the environment. This paper gives information on the current management of textile wastewater in Tanzania. A semiquantitative analysis was done to identify the main types of chemicals used in wet processing units, wastewater characteristics and existing wastewater treatment methods in the textile industry. The performance evaluation of the existing wastewater treatment plants is also discussed. The advantages of integrating constructed wetlands with the existing treatment facilities for textile wastewater are explained. It has been observed that pretreatment and dying/printing of the fabrics are the main two processes that produce wastewater in many textile companies. Main pollutants are chemicals used from pretreatment and materials removed from de-sizing, bleaching and scouring processes. Dyes, printing pigments and dye auxiliaries are the main pollutants from the dyeing/printing process. Most of the textile companies in Tanzania are equipped with effluent treatment plants. Wastewater treatment plants have basically similar units, which are coagulation-flocculation, sedimentation through clarifiers and aerobic reactor. However, their effluents do not meet discharge limits stipulated by the Tanzania Bureau of Standards (TBS).

Hydroxyl and sulfate radical-based oxidation of RhB dye in UV/H2O2 and UV/persulfate systems: Kinetics, mechanisms, and comparison

The degradation kinetics and mechanisms of Rhodamine B (RhB) dye by ^•OH and SO₄^•- based advanced oxidation processes were investigated. The ^•OH and SO₄^•- radicals were generated by UV photolysis of hydrogen peroxide and persulfate (i.e., UV/H₂O₂ and UV/PS), respectively. The effects of initial solution pH, RhB concentration, oxidant dosage, Fe²⁺ concentration, and water matrices were examined. The results showed that the degradation of RhB followed pseudo-first-order kinetics in both processes, with the UV/H₂O₂ process exhibiting better performance than that of the UV/PS process. Acidic conditions were favorable to the degradation of RhB in both systems. Increasing the oxidant dosage or decreasing the contaminant concentration could enhance the degradation of RhB. Photo-Fenton-like processes accelerated the performance when Fe²⁺ was added into both systems. The removal efficiency of RhB was inhibited upon the addition of humic substances. The addition of Cl^- displayed no significant effect and promoted RhB degradation in UV/H₂O₂ and UV/PS systems, respectively. The presence of NO₃^- promoted RhB degradation, while H₂PO₄^- and C₂O₄^2- showed an inhibitory effect on both UV/H₂O₂ and UV/PS processes. Radical scavenging tests revealed the dominant role of SO₄^•- radicals in the UV/PS system. Furthermore, the evolution of low molecular weight organic acids and NH₄⁺ during the degradation of RhB in these two processes were compared. Both UV/H₂O₂ and UV/PS systems led to similar formation trends of NH₄⁺ and some ring-opening products (e.g., formic acid, acetic acid, and oxalic acid), suggesting some analogies in the decay pathways of RhB by ^•OH and SO₄^•--induced oxidation processes.

Recent Advances in the Application, Design, and Operations & Maintenance of Aerated Treatment Wetlands

This paper outlines recent advances in the design, application, and operations and maintenance (O&M) of aerated treatment wetland systems as well as current research trends. We provide the first-ever comprehensive estimate of the number and geographical distribution of aerated treatment wetlands worldwide and review new developments in aerated wetland design and application. This paper also presents and discusses first-hand experiences and challenges with the O&M of full-scale aerated treatment wetland systems, which is an important aspect that is currently not well reported in the literature. Knowledge gaps and suggestions for future research on aerated treatment wetlands are provided.

Effects of nZVI dosing on the improvement in the contaminant removal performance of constructed wetlands under the dye stress

In this study, different dosages of nanoscale zero-valent iron (nZVI) were used to improve the nitrogen removal efficiency in CWs under different C/N ratios and dye stress conditions. The addition of nZVI enhanced the dye and nitrogen removal efficiencies in constructed wetlands (CWs) through chemical reduction and biological denitrification processes. However, total nitrogen (TN) and dye removal efficiencies firstly increased and then decreased with the increases of the nZVI dosage and influent COD/N (C/N) ratio. Under the influent C/N ratio of 5, the higher TN removal efficiencies (80.2%, 55.1%, and 69.14% under 25 mg/L, 50 mg/L, and 75 mg/L dye concentration, respectively) and higher COD removal efficiencies (48.3%, 74.95%, and 30.76% under 25 mg/L, 50 mg/L, and 75 mg/L dye concentration, respectively) were obtained in CWs by adding the optimal nZVI dosage (0.1 g/L). The dye removal efficiencies in CWs with nZVI at C/N = 1 (75%-91%) and at C/N = 5 (81%-97%) were all significantly higher than that in CWs without nZVI (60%-82%). Moreover, the functional bacteria for nitrogen removal in denitrification and the dye degradation (Zoogloea and Acinetobacter) were enriched in CWs with 0.1 g/L nZVI.

Decontamination and disinfection of wastewater by photocatalysis under UV/visible light using nano-catalysts based on Ca-doped ZnO

Nano-catalysts based on ZnO-Ca x% (with x = 0, 0.1, 0.3, 0.5, 0.7, and 1.0 mol % Ca²⁺) were synthesized with a bio-friendly adaptation of the sol-gel method using gelatin as template. These materials were characterized by Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Micro-Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), N₂ physisorption, photoacoustic absorption spectroscopy (PAS), and photoluminescence spectroscopy (PL). The Raman results indicated that the signal, attributed to an E₁(LO) mode at 580 cm^-1, was characteristic of oxygen vacancies that decreased with the increased Ca²⁺ content in doped oxides. This agreed with the PL results, which showed that the green emission centered at 510 nm and attributable to structural defects in ZnO decreased for Ca-doped ZnO. Our oxides are constituted by nanoparticles with rod-like and spherical morphologies. All the nano-catalysts exhibited the band gap characteristics of semiconductor materials around 3.0 eV. ZnO-Ca 1.0% exhibited the best photocatalytic performance for degradation of Methyl Orange (MO) model dye, degrading about 82% after 240 min of UV-Vis irradiation at pH 7.0. The reaction mechanism was influenced by hydroxyl (OH) and superoxide (O₂^-) radicals and mainly by active holes (h⁺). This doped oxide also demonstrated efficiency in wastewater disinfection against heterotrophic bacteria and total coliforms, exhibiting a potential use as an antimicrobial agent for the treatment of hospital wastewater. Furthermore, our nanoparticles did not show significant cytotoxic effects on L929 fibroblast cells.

A re-circulating horizontal flow constructed wetland for the treatment of synthetic azo dye at high concentrations

A re-circulating horizontal flow constructed wetland (RHFCW) system was developed in a greenhouse. This system was operated with Typha domingensis to study the phytoremediation capacity of this macrophyte species in different developing stages for synthetic textile wastewater with the pollutant type, the amaranth (AM) azo dye. Experiments were applied with a fixed flow rate Q = 10 L/h corresponding to a theoretical residence time of 3 h. The synthetic feeding to the RHFCW container was re-circulated back until the required water quality was achieved. The performance of this pilot-scale system was compared to an unplanted RHFCW. The effect of the initial dye concentration was studied using four dye concentrations (10, 15, 20, and 25 mg/L). The following parameters pH, color, COD, BOD₅, NO₃^-, NO2^-, and NH₄⁺ were monitored during treatment. The maximum efficiencies obtained for discoloration, COD, NO₃^-, and NH₄⁺ were 92 ± 0.14%, 56 ± 1.12%, 92 ± 0.34%, and 97 ± 0.17% respectively. Experiences demonstrate a decrease of removal efficiencies of studied parameters with the increase of dye concentrations, leading to an increase of the duration of treatment. Changes in activities of antioxidant enzymes (superoxide dismutase (SOD), guaiacol peroxidase (GPX), catalase (CAT), ascorbic peroxidase (APX), and glutathione reductase (GR)) and their relation to plant defense system against stress were studied. Enzymes were evaluated in leaves of T. domingensis during the remediation of the azo dye (amaranth). During treatment, an increase of enzymes activities was observed in accordance with the high removal efficiency.