Cr(Vi) reduction capacity of activated sludge as affected by nitrogen and carbon sources, microbial acclimation and cell multiplication

Abstract

The objectives of the present work were: (i) to analyze the capacity of activated sludge to reduce hexavalent chromium using different carbon sources as electron donors in batch reactors, (ii) to determine the relationship between biomass growth and the amount of Cr(VI) reduced considering the effect of the nitrogen to carbon source ratio, and (iii) to determine the effect of the Cr(VI) acclimation stage on the performance of the biological chromium reduction assessing the stability of the Cr(VI) reduction capacity of the activated sludge.

The highest specific Cr(VI) removal rate (q_Cr) was attained with cheese whey or lactose as electron donors decreasing in the following order: cheese whey ≈ lactose > glucose > citrate > acetate. Batch assays with different nitrogen to carbon source ratio demonstrated that biological Cr(VI) reduction is associated to the cell multiplication phase; as a result, maximum Cr(VI) removal rates occur when there is no substrate limitation. The biomass can be acclimated to the presence of Cr(VI) and generate new cells that maintain the ability to reduce chromate. Therefore, the activated sludge process could be applied to a continuous Cr(VI) removal process.

Introduction

Chromium is a transition metal often used in several industrial processes such as petroleum refining, metal finishing industries, leather tanning, iron and steel industries, inorganic chemical production, textile manufacturing and pulp production [1]. The hexavalent form of this metal, Cr(VI), has high water solubility, it is the most toxic among chromium species, and it is a known carcinogen. Besides, trivalent chromium, Cr(III), is less soluble in water, and it is an essential dietary element [2]. Therefore, reducing Cr(VI) to Cr(III) is beneficial in eliminating the toxicity of Cr(VI) of wastewaters.

For many years, conventional Cr(VI) removal was mainly achieved by chemical reduction [3], [4], ion exchange or adsorption [5], [6]. Recently, researchers have focused attention on biodetoxification of hexavalent chromium. In contrast to the conventional methods, biodetoxification is cost-effective [7]. A great number of bacterial genera were described as capable of reducing Cr(VI) to Cr(III) including Escherichia [8], Pseudomonas [9], Bacillus [10], Shewanella [11], Serratia [12], Rhodobacter [13] and Arthrobacter [14]. Besides, the bio-reduction of Cr(VI) to Cr(III) by using mixed cultures was also reported [15], [16], [17], [18], [19], [20], [21].

The activated sludge technology has been widely applied to treat municipal and some industrial wastewaters since its operation is simple and convenient [18], [19]. In contrast to the pure cultures, the activated sludge biomass is easy to acclimate to different environments, it does not need to be manipulated under rigorous conditions, and the wastewater does not need to be sterilized before treatment [18], [19]. Another important characteristic of the activate sludge microorganisms is the ability to flocculate. The flocculated aggregates exhibit technologically acceptable sedimentations rates. Gravity sedimentation is the only economically feasible way of separating biomass from treated wastewater in full-scale treatment plants [22]. Concerning Cr(VI)-containing wastewaters, within the pH range that activated sludge systems are usually operated, the Cr(III) produced during the biological reduction of Cr(VI) could generate insoluble chromium hydroxides [2]. These hydroxides could precipitate in the secondary clarifier along with the biomass; therefore, chromium could be easily removed from wastewaters when solids are purged from the reactor. For these reasons, the removal of Cr(VI) using an activated sludge system can be a suitable technological alternative.

In order to optimize the design and operation of the biological Cr(VI) reduction process in activated sludge reactors, a thorough understanding of the characteristics of microbial transformation of Cr(VI) is needed. Many factors, such as the presence of aerobic or anaerobic conditions [23], availability of energy sources such as sulfur [24], initial hexavalent chromium concentration [9] among others, affect the microbial Cr(VI) reduction. Taking into account the nutritional requirements of the microorganisms, in a previous work it was demonstrated that the presence of a suitable carbon source is necessary to enhance the Cr(VI) reduction capacity of activated sludge [25]. In addition, other researchers have reported that the Cr(VI) reducing activity of the microbial cells may vary in the presence of different carbon sources [26], [27], [28]. Therefore, choosing an appropriate electron donor is an important factor to be considered when a biological Cr(VI) removal process is used. With regard to Cr(VI)-containing wastewaters with low organic matter content, such as electroplating, pigmentation, and wood preservation [2], the carbon source has to be supplied externally; in those cases, the addition of cheese whey (a residue from dairy industries) could be a suitable alternative due to its low cost. Besides, presence of high organic matter along with Cr(VI) can also occur when wastewaters from more than one industry are mixed [21], [29], [30]; in this case, the availability of electron donors depends on the origin of the wastewater. Although the importance of the carbon source in biological Cr(VI) removal process has been assessed using pure cultures, the knowledge about the requirements of other nutrients related to cell multiplication, such as the availability of a nitrogen source, is scarce.

Most of the laboratory scale biological systems used for the treatment of Cr(VI) containing wastewater are operated in batch mode, due to the eventual loss of active biomass as a result of metal toxicity [9]. Several authors [8], [23], [27] reported that the rate and extent of Cr(VI) reduction in batch cultures depend on the initial biomass concentration regardless of the subsequent growth. These authors postulated that the new cells generated in the presence of hexavalent chromium lose the chromate reduction capacity due to the mutagenic effects of chromium. The new mutant cells could generate a decrease in the Cr(VI) transport inside the cells as a resistance mechanism [8], [23], [27]. As a result, the Cr(VI) reduction capacity of the mutant cells would decrease. In this context, the capability of the biomass to reduce Cr(VI) is not stable. This implies that it would not be possible to continuously remove Cr(VI) on a long-term basis without intermittently reseeding the biological system. However, there are recent reports concerning stable biological Cr(VI) reduction in continuous systems which could contradict this theory [21], [31], [32].

The main objectives of the present work were: (i) to analyze the capacity of activated sludge to reduce hexavalent chromium using different carbon sources as electron donors in batch reactors, (ii) to determine the relationship between biomass growth and the amount of Cr(VI) reduced considering the effect of the nitrogen to carbon source ratio, and (iii) to determine the effect of the Cr(VI) acclimation stage on the performance of the biological chromium reduction assessing the stability of the Cr(VI) reduction capacity of the activated sludge.