Decolorization of textile waste water, with an emphasis on microbial treatment processes

Promovendus/a: Ken Meerbergen

Promotor(en): Prof. dr. ir. Bart Lievens, Prof. dr. ir. Jan Van Impe, Prof. dr. ir. Lise Appels

Textile wastewater is typically intensely colored, containing high concentrations of dyes, dyeing additives and diverse chemicals, some of which are non-biodegradable and/or toxic, mutagenic or carcinogenic. Therefore, it is essential to treat textile wastewater in order to remove these substances before being discharged into the environment. Over the past few decades extensive research has been performed concerning dye removal from different wastewaters using chemical and biological treatment technologies or a combination of both. Nevertheless, only little is known about the microbial ecology and microbial communities in biological wastewater treatment plants (WWTPs) treating textile wastewaters, and about the efficiency of these systems to remove recalcitrant dyes. In this PhD thesis, using reactive azo dyes as model components, several aspects were studied contributing to a better understanding of dye degradation and its removal from textile wastewater. Reactive azo dyes are an important group of toxic, recalcitrant textile dyes and represent the majority of all dyes used in the textile industry, and are therefore highly suited for this study.

First, a number of available molecular tools were implemented and evaluated to assess the microbial community composition and some important gene functions in activated sludge from (textile) WWTPs (Chapter 2). More particularly, a molecular-ecological toolbox was developed, consisting of quantitative real-time PCR (qPCR) protocols for monitoring abundance of bacterial and archaeal 16S ribosomal RNA (rRNA) genes as well as a number of functional genes involved in nitrogen removal through nitrification/denitrification. Additionally, a protocol based on 454 pyrosequencing of 16S rRNA gene amplicons was developed to assess the archaeal and bacterial community composition in activated sludge systems.

Microbial communities of activated sludge in WWTPs have been profoundly studied over the past decade. However, despite these efforts still little is known about the microbial community composition and their functioning in activated sludge from textile wastewater treatment systems. Therefore, the aim of Chapter 3 was to study the microbial community in activated sludge from well-operating textile WWTPs in comparison with municipal WWTPs over two seasons (winter and summer), and to explain observed differences by environmental variables. In total, 454-pyrosequencing of 16S rRNA gene amplicons generated 160 archaeal and 1645 bacterial Operational Taxonomic Units (OTUs, which are a surrogate for species). Results suggested that activated sludge from textile WWTPs harbors a microbial community which is different from those from municipal WWTPs. Both archaeal and bacterial richness were significantly higher for samples from municipal WWTPs compared to those from textile WWTPs. The bacterial phyla Planctomycetes, Chloroflexi, Chlorobi and Acidobacteria were more abundant in activated sludge samples from textile WWTPs, together with archaeal members of Thaumarchaeota. Additionally, sulfate-reducing bacteria were almost only detected in textile WWTPs, while nitrifying and denitrifying bacteria as well as phosphate-accumulation bacteria were more abundant in municipal WWTPs. It became also clear that microbial communities from textile WWTPs were more dissimilar than those of municipal WWTPs, possibly due to a wider diversity in environmental stresses to which microbial communities in textile WWTPs are subjected. High salinity, high organic loads and a higher water temperature were found as important variables driving the microbial community composition in textile WWTPs. In an attempt to assess how microbial communities in textile WWTPs are established, the response of activated sludge microbial communities when exposed to textile dyes was studied. To this end, we assessed the microbial community composition in activated sludge from municipal WWTPs before and after exposure to azo dyes (Reactive Violet 5 (RV5)) (Chapter 4). Molecular analysis revealed that microbial communities that become exposed to recalcitrant azo dyes shift from diverse communities towards less diverse communities harboring highly adapted taxa with azo dye-degrading activity.

Many approaches have been proposed to remove dyes from textile wastewaters, including (physico)chemical and biological methods. A combination of a chemical method to obtain partial dye degradation followed by a biological treatment is believed to be a promising method for cost-effective decolorization of colored wastewater. Therefore, the aim of Chapter 4 was to develop and evaluate a combined method of partial Fenton oxidation and biological treatment using activated sludge for decolorization of azo dyes. Using RV5 as a model dye, color removal was significantly higher when the combined Fenton treatment/activated sludge method was used, as opposed to separate application of these treatment technologies. More specifically, pretreatment with Fenton’s reagent removed 52.9, 83.9 and 91.3 % of color from a 500 mg l-1 RV5 aqueous solution within 60 min when H2O2 concentrations of 1.0, 1.5, and 2.0 mM were used, respectively. Subsequent biological treatment significantly enhanced the chemical treatment, with microbial decolorization removing 70.2 % of the remaining RV5 concentration, on average. No apparent lag phase was detected when the chemical and biological method were combined, suggesting that the dye compounds have been partially degraded to compounds readily usable by the sludge microorganisms.

Instead of combining a biological with chemical treatment technology to enhance purification of textile wastewater, another alternative is the application of microorganisms with dye-degrading capabilities. Therefore, in Chapter 5 bacterial strains capable of decolorizing and/or degrading azo dyes commonly applied in textile production (monoazo dye Reactive Orange 16 and diazo dye Reactive Green 19) were isolated and characterized from activated sludge systems used in the treatment of (textile) wastewater. Following a prescreening of 125 isolates for their decolorization potential, five strains were retained for further evaluation of decolorization rate and effects of physicochemical parameters using a microtiter plate method. Of those five strains, one strain belonging to the genus Acinetobacter (ST16.16/164) and another belonging to Klebsiella (ST16.16/034) outperformed the other tested strains. Interestingly, it was suggested that this Acinetobacter strain represents a novel species, which is closely related to Acinetobacter johnsonii. Both strains exhibited strong decolorization ability (> 80 %) within a wide temperature range (20 °C to 40 °C) and retained good decolorization activity at temperatures as low as 10 °C (especially strain ST16.16/034), offering promising perspectives on a practical level, which requires a stable enzymatic performance of the isolates during the different phases of the purification cycle (thermotolerant). Among the different pH values tested (4, 7 and 10), highest dye removal for both strains occurred at pH 7, with decolorization efficiency remaining relatively high under alkaline conditions (pH 10), and neither isolates decolorization efficiency was negatively impacted by high salt or high dye concentration. Furthermore, both strains displayed the highest rate of decolorization and were able to completely (ST16.16/034) or partly (ST16.16/164) degrade the azo dyes.

Altogether, this PhD thesis clearly increased our knowledge on the microbial ecology and microbial communities in textile WWTPs as well as the treatment of textile wastewaters. Eventually, this study should contribute to a more effective, feasible and sustainable treatment of dye contaminated wastewater.