Article pubs.acs.org/est
Removal of Cr, Mn, and Co from Textile Wastewater by Horizontal Rotating Tubular Bioreactor Michaela Zeiner,†,* Tonci Rezić,‡ Bozidar Šantek,‡ Iva Rezić,§ Stephan Hann,† and Gerhard Stingeder† †
Department of Chemistry, Division of Analytical Chemistry, University of Natural Resources and Life Sciences, Muthgasse 18, 1190 Wien, Austria ‡ Department of Biochemical Engineering, Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, 10000 Zagreb, Croatia § Department of Applied Chemistry, Faculty of Textile Technology, University of Zagreb, Prilaz baruna Filipovića 28a, 10000 Zagreb, Croatia ABSTRACT: Environmental pollution by industrial wastewaters polluted with toxic heavy metals is of great concern. Various guidelines regulate the quality of water released from industrial plants and of surface waters. In wastewater treatment, bioreactors with microbial biofilms are widely used. A horizontal rotating tubular bioreactor (HRTB) is a combination of a thin layer and a biodisc reactor with an interior divided by O-ring shaped partition walls as carriers for microbial biomass. Using a biofilm of heavy metal resistant bacteria in combination with this special design provides various advantages for wastewater treatment proven in a pilot study. In the presented study, the applicability of HRTB for removing metals commonly present in textile wastewaters (chromium, manganese, cobalt) was investigated. Artificial wastewaters with a load of 125 mg/L of each metal underwent the bioreactor treatment. Different process parameters (inflow rate, rotation speed) were applied for optimizing the removal efficiency. Samples were drawn along the bioreactor length for monitoring the metal contents on site by UV−vis spectrometry. The metal uptake of the biomass was determined by ICP-MS after acidic microwave assisted digestion. The maximum removal rates obtained for chromium, manganese, and cobalt were: 100%, 94%, and 69%, respectively. in surface and ground waters as well as in surrounding soils5,6 justifying further investigations in this field. The biosorption process is a potentially promising technique for heavy metal decontamination using a wide spectrum of effective adsorbents in wastewater treatment, including agricultural waste material, various algae, bacteria, fungi, and other biomass.7,8 The application of immobilized microorganisms in various kinds of bioreactors led to satisfying results in the treatment of metal contaminated waters. Higher bioprocess productivity and stability against environmental changes are some advantages of bioreactors with a microbial biofilm on the inner surfaces over bioreactors with suspended cells. Different designs of bioreactors with microbial biofilms are applied in wastewater treatment, such as trickling filters, fluidized or packed-beds bioreactors, and thin-layer or biodisc reactors.9−13 A horizontal rotating tubular bioreactor (HRTB) designed as a combination of a thin layer and a biodisc reactor also showed
1. INTRODUCTION During the last decades, heavy metal pollution has steadily become a serious environmental problem, as a result of its toxicity and insusceptibility in the environment. The concentration must be reduced to an acceptable level before being discharged into the environment, in order to avoid threats to public health and/or affecting the quality of potable water. The World Health Organization’s Guidelines for Drinking Water Quality (1984) declared chromium, zinc, iron, mercury as well as lead to be the metals of most immediate concern.1 In the European Union, the Water Framework Directive requires good surface water status by 2015.2 According to Croatian legislatives accepted concentrations of metals in fresh water and sea are for Cr 6−20 μg/L (dependent on water category), for Co 100−2000 μg/L, and for Mn 50−1000 μg/L. Maximum effluent concentrations were stated by the U.S. Environmental Protection Agency.3 Different chemical, physical, and biological techniques are nowadays in use to remove heavy metals from wastewaters4 because high concentration of heavy metals in industrial wastewaters has negatively affected the environment, water quality, and biota living in the water. However, significant amounts of toxic elements discharged from industry are found © XXXX American Chemical Society
Received: April 24, 2012 Revised: July 30, 2012 Accepted: August 30, 2012
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dx.doi.org/10.1021/es301596g | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
good removal rates for selected metals14 in a pilot study. Its interior is divided by O-ring shaped partition walls that serve as carriers for microbial biomass.15,16 Advantages of HRTB for wastewater treatment are that biomass immobilized in biofilm is on the one hand protected from toxic metal influence and on the other hand prevented from wash out and kept in a bioreactor, which simplifies the separation step. Furthermore, a HRTB provides energy saving for mixing end aeration. One hundred rings inside the HRTB provides higher colonization surface in comparison with a normal cylindrical bioreactor. To describe the mixing processes in HRTB, three mathematical models were tested and comparison showed that the so-called spiral flow model was the most suitable to characterize the mixing performance in HRTB. This model was used further to develop a scale-up procedure by relating the adjustable model parameters with the process parameters expressed as dimensionless numbers.17−19 Industrial textile processing generates wastewater containing heavy metal contaminants because metals are most frequently used in textile wet processing even if some of them are extremely toxic to many organisms.20 Chromium containing dyes are essential for the fast black dyeing of wool and nylon but these could be replaced in the future by newly developed dyes containing less toxic metals such as iron.21 Not only chromium but other metals, such as cobalt and occasionally copper and nickel, form part of the most commonly used dyes, especially dyes for leather materials, nylon, and wool. Metal complex dyes in general offer good overall speed properties.22 Because of the wide application of metal complex dyes, metals are frequently present in textile dyeing wastewaters as free ionic metals or complex metals, which contribute to environmental concerns.23 Furthermore, classical chromium tanning procedures cause high amounts of discharged chromium.24 According to Hafez and El-Manharawy25 the chromium content in a tannery effluents ranges between 1300 and 2500 mg [Cr(VI)]/L. Therefore, the irrigation of fields with water from rivers to which this wastewater was discharged to without treatment causes a significant increase of chromium in the soil, which then poses a threat to future crop production.26 In this work, the applicability of a horizontal rotating tubular bioreactor (HRTB) for the removal of heavy metals from artificial textile wastewaters loaded with chromium, cobalt, and manganese was studied.
yeast extract: 0.3; and Tripton: 0.3 which represents artificial wastewater. The medium was sterilized at 121 °C for 20 min. To determine the nature of the microbial consortium the morphological characteristics of the bacteria communities were examined and filamentous bacteria dominated in the biofilm and cocci and rods n the suspended biomass. At the moment, 5−7 isolates were separated for exact determination which is still ongoing. 2.2. Bioreactor and Experimental Set Up. The HRTB used was constructed as a 2.0 m long stainless steel tube with a diameter of 0.25 m, the interior being divided by O-ring shaped partition walls with a diameter of 0.19 m. The distance between the partition walls was 0.02 m and the liquid volume in the HRTB was 15 L, respectively. Aeration occurred through the central tube fixed in the axis of HRTB with a constant airflow rate of 152 L/h throughout the entire experiment, whereby the inlet airflow rate was controlled by a flow-meter and a manometer. One fifth of the bioreactor was filled with wastewater. Biofilm liquid and air intervals continuously changed with bioreactor rotation providing additional biofilm aeration. The systems for sampling broth and biofilm were placed along the bioreactor at 0.40 m intervals (Figure 1).15
Figure 1. Scheme of the horizontal rotating tubular bioreactor15.
The required amount of suspended bacterial biomass (7.5 L) for the inoculation of HRTB was obtained by batch cultivation in a stirred tank bioreactor. After 24 h of feeding, the process started at a rate of 1 L/h with a bioreactor rotation speed of 10 min−1. It took 15 days from the inoculation to obtain a stable biofilm in the HRTB before it was considered ready for performing studies using different combinations of process parameters: medium inflow rate (0.5, 1.0, and 2.0 L/h) and bioreactor rotation speed (5, 15, and 30 rpm). The special bioreactor construction of HRTB allows working with higher rotation speed than the usually applied maximum 5 rpm without significant effluence on the biological reaction rate; the biofilm stability is not disturbed by these higher rotation speeds. The inlet glucose concentration and the metal ion concentrations (125 mg/L for CrVI (as Cr2O72−) Co2+, and Mn2+) were constant. The reactor was operated in sterile conditions. After the experiments, the biomass was disposed of in compliance with local regulations, that is sterilization and anaerobic digestion. 2.3. Biofilm Thickness. Biofilm thickness measurements were done by a modified Venkataraman and Ramanujam method.29 The modifications included using graphite powder instead of chalk powder and a microscope with micrometric scale instead of a projector. The mass of the biofilm was determined by collecting a sample from the inner bioreactor
2. EXPERIMENTAL SECTION 2.1. Microorganism, Medium, and Growth Conditions. In this study, the culturable method was used for mixed microbial culture selection27 instead of unculturable methods based on physical properties of bacteria. The mixed microbial culture used was isolated from ground soil samples at Kaštela Bay28 industrial area (iron, vinyl, and cement factory) located near the town of Split, Croatia, on the Adriatic Coast. A synthetic medium was used for the isolation of the mixed microbial culture from soils samples containing the following ingredients (g/L): glucose, 10; FeSO 4 ·7H 2 O, 0.62; CuSO4·5H2O, 0.49; NiSO4·6H2O, 0.56; ZnSO4·7H2O, 0.55; CoCl2·6H2O, 0.50; MnSO4·H2O, 0.39; K2Cr2O7, 0.20; Pbacetate·3H2O, 0.23; yeast extract, 0.3; and Tripton, 0.3 (pH 4.40). In this research, the isolated mixed microbial culture was cultivated at room temperature (20 ± 1 °C) in a synthetic medium with the following composition (g L−1): glucose: 10; CoCl2·6 H2O: 0.505; MnSO4·H2O: 0.384; K2Cr2O7: 0.354; B
dx.doi.org/10.1021/es301596g | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 1. Validation Data for the UV−vis Determination of Co, Cr, and Mn in Artificial Wastewater Co limit of detection limit of decision limit of quantification range of determination linearitya r2 repeatability intermediate precision (within day) reproducibility (day-to-day) recovery uncertainty of measurement a
mg/L mg/L mg/L mg/L mg/L % % % % %
Cr
0.030 0.061 0.091 0.091−6.3 y = 0.0507x + 0.1101 0.9996
