Chromium species behaviour in the activated sludge process

Unraveling the nexus of Cr (Ⅵ), Aniline, and Microbial Ecology on aniline-degrading biosystem: Removal efficiency, sludge type, microbial ecology

In order to explore the stress principle of Cr (Ⅵ) on aniline biodegradation system, a control group and experimental groups with the concentration of Cr (Ⅵ) at 2, 5, 8 mg/L were set up. The results demonstrated that Cr (Ⅵ) had minimal effects on the degradation efficiency of aniline but significantly inhibited nitrogen removal function. When Cr (Ⅵ) concentration was below 5 mg/L, the nitrification performance recovered spontaneously, while denitrification performance was severely impaired. Furthermore, the secretion of extracellular polymeric substances (EPS) and its fluorescence substance concentration were strongly inhibited with increasing Cr (Ⅵ) concentration. High-throughput sequencing revealed that the experimental groups were enriched with Leucobacter and Cr (Ⅵ)-reducing bacteria, but the abundance of nitrifiers and denitrifiers was significantly decreased compared to the control group. Overall, the effects of Cr (Ⅵ) stress at different concentrations on nitrogen removal performance were more significant than those on aniline degradation.

Fixed-bed flow experiments with supported green nZVI for the remediation of contaminated waters: Effect of pH and background solution composition

Aim of this study is to evaluate the performance of a nanocomposite material (R-nFe), consisting of nano zero valent iron (nZVI) and a cation exchange resin, for Cr(VI) reduction and heavy metals removal from contaminated water streams. The effect of pH and the background solution composition was investigated by conducting three columns tests. In Column I, the experiments were carried out using NaCl as background electrolyte while in Column II and III the background solution consisted of 50% tertiary effluents of a waste water treatment plant and 50% deionized water. The kinetics of Cr(VI) removal was found to be much faster at acidic pHs. Based on the experimental results it was calculated that the required contact time of the permeating solution with the RnFe beads for Cr(VI) removal at pH 7 is 6.5 times longer than at pH 4. The results also revealed the role of competing ions on the performance of RnFe. During the operation of the column with an inert electrolyte, such as NaCl, the RnFe bed removed an amount of Cr(VI) equivalent to 4200 mg/kg. When the column was fed with a water stream containing relatively high levels of nitrates, the RnFe removed efficiently only 190 mg/kg Cr(VI), due to the competitive consumption of nanoiron for the reduction of nitrates. The nanocomposite was found to be very efficient for the simultaneous removal of tested heavy metals, i.e. Cu, Zn, Ni, Cd and Pb, which were selectively retained in the cation exchange sites of the supporting resin matrix.

In situ bioremediation of hexavalent chromium in presence of iron by dried sludge bacteria exposed to high chromium concentration

Stability of chromium in the ferrochrome slag dumps and leachate are affected by pH, redox potential and the presence of other metallic species in the slag. It is desirable to keep chromium in slag dumps in the trivalent [Cr(III)] state because trivalent chromium is 1000 times less toxic to living organisms than the hexavalent form [Cr(VI)]. Due to the low toxicity and low mobility of Cr(III), it is recommended to convert Cr(VI) to Cr(III) wherever possible to protect the health of living organisms. In this study, the role of Cr(VI) reducing organisms for stabilising chromium in slag dumps was evaluated in the presence of iron [oxidation states Fe(II) and Fe(III)]. The study showed that stabilisation of chromium species in the trivalent state was most favourable under aerated conditions. Up to 100 mg/L Cr(VI) was reduced in less than 24 h by cultures grown under aerobic conditions in the presence of Fe(III). A much shorter time (6 h) was required to reduce the same amount of Cr(VI) in the presence of Fe(II). When oxygen was completely excluded, it was only possible to reduce 20 mg/L in about 48 h which was much slower than the removal of 100 mg/L in less than 24 h under aerated conditions. Fe(II) contributed directly to catalytic reduction of Cr(VI) reduction whereas Fe(III) was beneficial to Cr(VI) reduction up to an initial Cr(VI) concentration of 75 mg/L. Evaluation of Cr(VI) reduction kinetics showed that Cr(VI) reduction under aerobic conditions followed the non-competitively inhibited mixed-order reaction. Cr(VI) reduction in sealed reactor vessels, under anaerobic conditions, followed a modified non-competitive inhibition reaction model. The results indicate that chromium stabilisation in ferrochrome slag dumps would require maintenance of a fully aerated dump supplemented by a culture of Cr(VI) reducing organisms.

Development of an integrated treatment strategy for a leather tannery landfill leachate

This study aimed at developing an efficient multistage treatment strategy for a complex industrial landfill leachate: a leather tannery landfill leachate. Based on the leachate physicochemical characteristics, the following treatment train was delineated and tested: (i) initial biological process for removal of biodegradable organics, ammonium and alkalinity, (ii) coagulation/flocculation process for total removal of chromium and partial removal of recalcitrant organics and suspended solids, (iii) advanced oxidation process (AOP) or electrochemical AOP (EAOP) for degradation of recalcitrant organics and biodegradability enhancement, and (iv) final biological polishing step. Two initial biological treatment configurations were applied: one comprising nitrification and the other nitrification-denitrification. Coagulation/flocculation was optimized in terms of pH, coagulant dosage (iron(III) chloride) and flocculant nature and dosage. The following AOPs/EAOPs were tested: Fenton, photo-Fenton with UVA or UVC radiation (PF-UVA or PF-UVC), anodic oxidation (AO), electro-Fenton (EF) and photoelectro-Fenton with UVA radiation (PEF-UVA). The biological nitrification-denitrification was beneficial not only because it avoided the need for alkalinity addition during nitrification and decreased the amount of substrate added during denitrification, as expected. Over and above that, it reduced the acid consumption in the coagulation/flocculation, avoided the application of an additional stage comprising nitrites oxidation to nitrates prior to the AOP/EAOP, and improved the efficiency of Fenton's reaction based processes. Following nitrification-denitrification, the coagulation/flocculation was maximized at pH 3.0 and 400 mg Fe L^-1 with no flocculant addition. The PEF-UVA process was the best AOP/EAOP. The final leachate fulfilled the discharge limits into waterbodies.

Effect of chromium (VI) on the multiple nitrogen removal pathways and microbial community of aerobic granular sludge

The frequent appearance of Cr(VI) significantly impacts the microbial metabolism in wastewater. In this study, long-term effects of Cr(VI) on microbial community, nitrogen removal pathways and mechanism of aerobic granular sludge (AGS) were investigated. AGS had strong resistance ability to 1.0 mg/L Cr(VI). 3.0 mg/L Cr(VI) increased the heterotrophic-specific ammonia uptake rate (HSAUR) and heterotrophic-specific nitrate uptake rate (HSNUR) transiently, whereas 5.0 mg/L Cr(VI) sharply decreased the specific ammonia uptake rate (SAUR), specific nitrate uptake rate (SNUR) and simultaneous nitrification denitrification rate (SNDR). It was found that Cr (VI) has a greater inhibitory effect on autotrophic nitrification (ASAUR), and the maximal inhibition rate (IR) was 139.19%. Besides, the inhibition of Cr (VI) on nitrogen removal process belongs to non-competitive inhibition. Cr(VI) had a weaker negative impact on heterotrophic bacteria compared with that on autotrophic bacteria. Denaturing gradient gel electrophoresis analyses suggest that Acidovorax sp., flavobacterium sp., uncultured soil bacterium, uncultured nitrosospira sp., uncultured prokaryote, uncultured β-proteobacterium and uncultured pseudomonas sp. were the dominant species. The inhibition of Cr(VI) on nitrite-oxidizing bacteria was the strongest, followed by ammonia-oxidizing bacteria and denitrifying bacteria. Linear correlations between bacterial count and biomass-specific uptake rate were observed when the Cr(VI) concentration exceeded 3 mg/L. This study revealed the effect of Cr(VI) on nitrification is more serious than that on denitrification. Autotrophic and heterotrophic nitrification, heterotrophic denitrification and simultaneous nitrification denitrification played a significant role on nitrogen removal under Cr(VI) stress.

Speciation evolution of zinc and copper during pyrolysis and hydrothermal carbonization treatments of sewage sludges

Thermal and hydrothermal treatments are promising techniques for sewage sludge management that can potentially facilitate safe waste disposal, energy recovery, and nutrient recovery/recycling. Content and speciation of heavy metals in the treatment products affect the potential environmental risks upon sludge disposal and/or application of the treatment products. Therefore, it is important to study the speciation transformation of heavy metals and the effects of treatment conditions. By combining synchrotron X-ray spectroscopy/microscopy analysis and sequential chemical extraction, this study systematically characterized the speciation of Zn and Cu in municipal sewage sludges and their chars derived from pyrolysis (a representative thermal treatment technique) and hydrothermal carbonization (HTC; a representative hydrothermal treatment technique). Spectroscopy analysis revealed enhanced sulfidation of Zn and Cu by anaerobic digestion and HTC treatments, as compared to desulfidation by pyrolysis. Overall, changes in the chemical speciation and matrix properties led to reduced mobility of Zn and Cu in the treatment products. These results provide insights into the reaction mechanisms during pyrolysis and HTC treatments of sludges and can help evaluate the environmental/health risks associated with the metals in the treatment products.

Chromium Biosorption from Cr(VI) Aqueous Solutions by Cupressus lusitanica Bark: Kinetics, Equilibrium and Thermodynamic Studies

The present study investigated the kinetics, equilibrium and thermodynamics of chromium (Cr) ion biosorption from Cr(VI) aqueous solutions by Cupressus lusitanica bark (CLB). CLB total Cr biosorption capacity strongly depended on operating variables such as initial Cr(VI) concentration and contact time: as these variables rose, total Cr biosorption capacity increased significantly. Total Cr biosorption rate also increased with rising solution temperature. The pseudo-second-order model described the total Cr biosorption kinetic data best. Langmuir´s model fitted the experimental equilibrium biosorption data of total Cr best and predicted a maximum total Cr biosorption capacity of 305.4 mg g(-1). Total Cr biosorption by CLB is an endothermic and non-spontaneous process as indicated by the thermodynamic parameters. Results from the present kinetic, equilibrium and thermodynamic studies suggest that CLB biosorbs Cr ions from Cr(VI) aqueous solutions predominantly by a chemical sorption phenomenon. Low cost, availability, renewable nature, and effective total Cr biosorption make CLB a highly attractive and efficient method to remediate Cr(VI)-contaminated water and wastewater.

Molasses as an efficient low-cost carbon source for biological Cr(VI) removal

In the present study, indigenous microorganisms from industrial sludge were used to reduce the activity of Cr(VI). Molasses, a by-product of sugar processing, was selected as the carbon source (instead of sugar used in a previous work) as it is a low-cost energy source for bioprocesses. Initially, experiments were carried out in suspended growth batch reactors for Cr(VI) concentrations of 1.5-110 mg/L. The time required for complete Cr(VI) reduction increased with initial Cr(VI) concentration. Initial molasses concentration was also found to influence the Cr(VI) reduction rate. The optimal concentration for all initial Cr(VI) concentrations tested was 0.8 gC/L. Experiments were also carried out in packed-bed reactors. Three different operating modes were used to investigate the optimal performance and efficiency of the filter, i.e. batch, continuous and SBR with recirculation. The latter mode with a recirculation rate of 0.5L/min lead to significantly high Cr(VI) reduction rates (up to 135 g/m(2)d). The results of this work were compared with those of a similar work using sugar as the carbon source and indicate that molasses could prove a feasible technological solution to a serious environmental problem.

Accumulation and fraction distribution of Ni(II) in activated sludge treating Ni-laden wastewater

The accumulation and fraction distribution of Ni(II) in sludge was determined, and their effect on the performance of sequencing batch reactor (SBR) systems was evaluated at laboratory scale. The results showed that the removal efficiencies of substrates decreased significantly with increasing feeding concentration of Ni(II) into SBRs. The concentration of Ni(II) fed into the SBRs was significantly positively correlated with the Ni(II) contents accumulated in the sludge, while it was negatively correlated with the biomass in the SBRs. The accumulated Ni(II) in the sludge was distributed mainly in the available fraction, accounting for 75.8-90.0 % of the total Ni(II) content. The accumulated content of Ni(II) in each sludge fraction could be predicted satisfactorily by the feeding Ni(II) concentrations in the solutions. As compared with the total contents and other chemical fractions, Ni(II) in the oxidizable fraction in sludge exhibited more important inhibition effects on sludge microorganisms in the SBRs.

Inhibition of the nitrification process in activated sludge by trivalent and hexavalent chromium, and partitioning of hexavalent chromium between sludge compartments

The input of wastewater treatment plants (WWTPs) may contain high concentrations of Cr(III) and Cr(VI), which can affect nitrogen removal. In the present study the influence of different Cr(III) and Cr(VI) concentrations towards activated sludge nitrification was studied. To better understand the mechanisms of Cr(VI) toxicity, its reduction, adsorption and uptake in activated sludge was investigated in a batch growth system. Quantification of Cr(VI) was performed by speciated isotope dilution inductively coupled plasma mass spectrometry. It was found that Cr(VI) concentrations above 1.0 mg L(-1) and Cr(III) concentrations higher than 50 mg L(-1) negatively affected nitrification. Speciation studies indicated almost complete reduction of Cr(VI) after 24h of incubation when Cr(VI) concentrations were lower than 2.5 mg L(-1), whereas for Cr(VI) added to 5 mg L(-1) around 40% remained unreduced. The study of the partitioning of Cr in the activated sludge was performed by the addition of Cr(VI) in concentrations of 2.5 and 5.0 mg L(-1). Results revealed that Cr was allocated mainly within the intercellular compartments, whereas intracellular and adsorbed Cr represented less than 0.1% of the Cr sludge concentrations. Cr(VI) was reduced in all compartments, the most efficiently (about 94%) within the intracellular and intercellular fractions. The extent of reduction of adsorbed Cr was 92% and 80% for 2.5 and 5.0mg of Cr(VI) L(-1), respectively. The results of present investigation provide a new insight into the toxicity of Cr species towards activated sludge nitrification, which is of significant importance for the management of WWTPs in order to prevent them from inflows containing harmful Cr(VI) concentrations.

Simultaneous bioreduction of nitrate and chromate using sulfur-based mixotrophic denitrification process

This study aims at evaluating simultaneous chromate and nitrate reduction using sulfur-based mixotrophic denitrification process in a column reactor packed with elemental sulfur and activated carbon. The reactor was supplemented with methanol at C/N ratio of 1.33 or 2. Almost complete denitrification was achieved at influent NO3(-)-N and Cr(VI) concentrations of 75 mg/L and 10mg/L, respectively, and 3.7h HRT. Maximum denitrification rate was 0.5 g NO3(-)-N/(L.d) when the bioreactor was fed with 75 mg/L NO3(-)-N, 150 mg/L methanol and 10mg/L Cr(VI). The share of autotrophic denitrification was between 12% and 50% depending on HRT, C/N ratio and Cr(VI) concentration. Effluent total chromium was below 50 μg/L provided that influent Cr(VI) concentration was equal or below 5mg/L. DGGE results showed stable microbial community throughout the operation and the presence of sulfur oxidizing denitrifying bacteria (Thiobacillus denitrificans) and Cr(VI) reducing bacteria (Exiguobacterium spp.) in the column bed.

Impact of Cr(VI) on P removal performance in enhanced biological phosphorus removal (EBPR) system based on the anaerobic and aerobic metabolism

Influence of Cr(VI) on P removal in enhanced biological phosphorus removal (EBPR) system was investigated with respect to the composition of poly-phosphate-accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs), the transformation of poly-β-hydroxyalkanoates (PHA) and glycogen, enzymes' activities, and the intracellular Cr. Whether EBPR system could revive after Cr(VI) shock was also explored. Results showed P removal performance was completely inhibited by Cr(VI) with the concentration more than 5 mg L(-1). PAOs were more sensitive to Cr(VI) than GAOs and the other bacteria were. PHA consumption, glycogen synthesis and adenylate kinase's activity had been inhibited by 5 mg L(-1) Cr(VI). Both adenylate kinase's activity and P removal efficiency were negatively correlated with the intracellular Cr. Recovery experiments revealed that P removal performance with 5 mg L(-1) Cr(VI) shock could revive after a 2-day recovery treatment, while systems with high level Cr(VI) (20 and 60 mg L(-1)) shock could not.

Hexavalent chromium reduction in a sulfur reducing packed-bed bioreactor

The most commonly used approach for the detoxification of hazardous industrial effluents and wastewaters containing Cr(VI) is its reduction to the much less toxic and immobile form of Cr(III). This study investigates the cleanup of Cr(VI) containing wastewaters using elemental sulfur as electron acceptor, for the production of hydrogen sulfide that induces Cr(VI) reduction. An elemental sulfur reducing packed-bed bioreactor was operated at 28-30°C for more than 250 days under varying influent Cr(VI) concentrations (5.0-50.0 mg/L) and hydraulic retention times (HRTs, 0.36-1.0 day). Ethanol or acetate (1000 mg/L COD) was used as carbon source and electron donor. The degree of COD oxidation varied between 30% and 85%, depending on the operating conditions and the type of organic carbon source. The oxidation of organic matter was coupled with the production of hydrogen sulfide, which reached a maximum concentration of 750 mg/L. The biologically produced hydrogen sulfide reduced Cr(VI) chemically to Cr(III) that precipitated in the reactor. Reduction of Cr(VI) and removal efficiency of total chromium always exceeded 97% and 85%, respectively, implying that the reduced chromium was retained in the bioreactor. This study showed that sulfur can be used as an electron acceptor to produce hydrogen sulfide that induces efficient reduction and immobilization of Cr(VI), thus enabling decontamination of Cr(VI) polluted wastewaters.

Effects of chromium on activated sludge and on the performance of wastewater treatment plants: A review

Chromium is a heavy metal of commercial importance, thus significant amounts are released in wastewaters. Chromium in wastewaters and in the aquatic environment is primarily encountered in oxidation stages +3 (Cr((III))) and +6 (Cr((VI))). Recent publications suggest that Cr((VI)) compounds are more toxic than Cr((III)) ones, while Cr((III)) has been identified as trace element, at least for complex organisms. With respect to chromium species mobility, Cr((VI)) can cross cellular membranes, which then may be oxidized to Cr((III)) and react with intracellular biomolecules. Clear conclusions cannot be derived about the critical chromium concentrations that affect activated sludge growth, as the latter is a function of a number of factors. Broadly, may be supported that activated sludge growth is stimulated at Cr((III)) concentrations up to 15 mg L(-1), above which is inhibited, with lethal doses lying above 160 mg Cr((III)) L(-1). On the other hand, literature data on Cr((VI)) effects on activated sludge are even more controversial. A number of reports support that Cr((VI)) is toxic to activated sludge at concentrations above 5 mg L(-1), while others report growth stimulation at concentrations up to 25 mg L(-1). However, all reports agree that Cr((VI)) is definitely an activated sludge growth inhibitor at higher concentrations, while 80 mg Cr((VI)) L(-1) have been identified as lethal dose. A number of factors have been identified to influence chromium toxicity on activated sludge, such as, pH, biomass concentration, presence of organic substances or other heavy metals, acclimation process, exposure time, etc. Naturally, the presence of chromium species in wastewaters may affect the performance of wastewater treatment plants often causing malfunctions, particularly for industrial wastewaters containing relatively high chromium concentrations. The present work reviews in a critical way the published literature on chromium effects on activated sludge, and on the operation of wastewater treatment plants.

Biosorption and biotransformation of chromium by Serratia sp. isolated from tannery effluent

A bacterium isolated from soil and sediment ofa leather tanning mill's effluent was identified as Serratia sp. by the analysis of 16S rDNA. Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX) and transmission electron microscopy (TEM) were used to assess morphological changes and confirm chromium biosorption in Serratia sp. both in a shake-flask culture containing chromium and in a tannery wastewater. The SEMEDX and the elemental analysis of the chromate-containing samples confirmed the binding of chromium with the bacterial biomass. The TEM exhibited chromium accumulation throughout the bacterial cell, with some granular deposits in the cell periphery and in the cytoplasm. X-ray diffraction analysis (XRD) was used to quantify the chromium and to determine the chemical nature of the metal-microbe interaction. The XRD data showed the crystalline character of the precipitates, which consisted of mainly calcium chromium oxide, chromium fluoride phosphate and related organo-Cr(III) complex crystals. The XRD data also revealed a strong involvement of cellular carboxyl and phosphate groups in chromium binding by the bacterial biomass. The results of the study indicated that a combined mechanism of ion-exchange, complexation, croprecipitation and immobilization was involved in the biosorption of chromium by bacterial cells in contaminated environments.

Effects of combining biological treatment and activated carbon on hexavalent chromium reduction

The objectives of the present work were: (a) to analyze the Cr(VI) removal by combining activated sludge (AS) with powdered activated carbon (PAC), (b) to analyze the effect of PAC and Cr(VI) on the growth kinetics of activated sludge, and (c) to determine if the combined method (AS-PAC) for Cr(VI) removal can be considered additive or synergistic with respect to the individual processes. Chromate removal was improved by increasing PAC concentrations in both PAC and AS-PAC systems. Cr(VI) removal using the AS-PAC system was higher than using AS or PAC. The increase of Cr(VI) caused longer lag phase and lower observed specific growth rate (μ(obs)), biomass yield (Y(X/S)), and specific growth substrate consumption rate (q(S)) of activated sludge; additionally, PAC did not enhance the growth kinetic parameters (μ(obs), Y(X/S), q(S)). Cr(VI) reduction in AS-PAC system was the result of the additive effect of each individual Cr(VI) removal process.

Effects of Cr(VI) on the performance and kinetics of the activated sludge process

The substrates removal performance, removal kinetics and the electron transport system (ETS) of sludge were investigated by sequencing batch reactors (SBR) and batch assays, respectively. Compared to the control system, significant decreases were observed in substrate removal efficiency with the Cr(VI)-feeding concentration up to 5 mg L(-1) in SBR system. And the recovery for NH4+-N removal were more difficult than that of COD after the termination of Cr(VI)-feeding. Significant inhibitory effects of Cr(VI) on the ETS activity and substrate removal kinetics were observed in the batch assays. The inhibitory effects of Cr(VI) would be overestimated on COD removal and underestimated on NH4+-N removal by the short-term batch assay as compared to the long-term operations. Additionally, significant correlations between the ETS activity and the inhibitory rates of Cr(VI) on substrate removal indicated the ETS activity can provide effective predictions on the potential performance of substrate removal in activated sludge.

Effect of chromium and cobalt ions on the expression of antioxidant enzymes in human U937 macrophage-like cells

The main concern associated with metal-on-metal (MM) hip prosthesis is the presence of metal ions, mainly chromium (Cr) and cobalt (Co), which are found both systemically and locally in the organism of patients. Previous studies revealed that Cr(III) and Co(II) ions could induce damages to proteins in macrophage-like cells in vitro, probably through the formation of reactive oxygen species (ROS). We then hypothesized that these ions can modify the expression of antioxidant enzymes in these cells. Results showed that Cr(VI) induced the protein expression of Mn-superoxide dismutase, Cu/Zn-superoxide dismutase, catalase, glutathione peroxidase, and heme oxygenase-1 (HO-1) but had no effect of the expression of their mRNA. Cr(III) have no effect on the expression of all these antioxidant enzymes. Co(II) induced the expression of both protein and mRNA of HO-1 only. In conclusion, results showed that Cr(VI), Cr(III), and Co(II) had differential effects on the expression of antioxidant enzymes in macrophage-like cells in vitro.

Speciation of Cr(VI) in environmental samples in the vicinity of the ferrochrome smelter

The impact of ferrochrome smelter on the contamination of its environment with toxic hexavalent chromium, Cr(VI), was assessed by analyzing smelter dusts, soil, grass and tree barks. For the separation of Cr(VI) from Cr(III), solid samples were treated with 0.1M Na(2)CO(3) and filtered through hydrophilic PDVF 0.45 microm filter prior to the determination of Cr(VI) by electrothermal atomic absorption spectrometry (ET-AAS). Ferrochrome smelter dust was found to contain significant levels of Cr(VI), viz. 43.5 microg g(-1) (cyclone dust), 2710 microg g(-1) (fine dust), and 7800 microg g(-1) (slimes dust) which exceeded the maximum acceptable risk concentration (20 microg g(-1)). The concentration of Cr(VI) in environmental samples of grass (3.4+/-0.2), soil (7.7+/-0.2), and tree bark (11.8+/-1.2) collected in the vicinity of the chrome smelter were higher as compared with the same kind of samples collected from uncontaminated area. The results of the investigation show that ferrochrome smelter is a source of environmental pollution with contamination factors of Cr(VI) ranging between 10 and 50.

Biosorption of chromium, copper and zinc by wine-processing waste sludge: single and multi-component system study

Wine-processing waste sludge (WPWS) has been shown to have powerful potential for sorption of some heavy metals (i.e., chromium, lead and nickel) in single-component aqueous solutions. But although most industrial wastewater contains two or more toxic metals, there are few sorption studies on multicomponent metals by WPWS. This study has two goals: (i) conduct competitive adsorption using Cr, Cu and Zn as sorbates and examine their interaction in binary or ternary systems; and (ii) determine the effects of temperature on the kinetic sorption reaction. The sludge tested contained a high amount of organic matter (38%) and had a high cation exchange capacity (CEC, 255 cmol(c)kg(-1)). Infrared analysis reveals that carboxyl is the main functional group in this WPWS. The (13)C NMR determination indicates alkyl-C and carboxyl-C are major organic functional groups. At steady state, there are about 40.4% (Cr), 35.0% (Cu) and 21.9% (Zn) sorbed in the initial 6.12 mM of single-component solutions. Only pseudo-second-order sorption kinetic model successfully describes the kinetics of sorption for all experimental metals. The rate constants, k(2), of Cr, Cu and Zn in single-component solutions are 0.016, 0.030 and 0.154 g mg(-1)min(-1), respectively. The sorption of metals by WPWS in this competitive system shows the trend: Cr>Cu>Zn. Ions of charge, hydrated radius and electronic configuration are main factors affecting sorption capacity. The least sorption for Zn in this competitive system can be attributed to its full orbital and largest hydrated radius. Though the effect of temperature on Zn sorption is insignificant, high temperature favors the other metallic sorptions, in particular for Cr. However, the Cr sorption is lower than Cu at 10 degrees C. The Cr sorption by WPWS can be higher than that of Cu at 30 degrees and 50 degrees C.

About the performance of Sphaerotilus natans to reduce hexavalent chromium in batch and continuous reactors

The hexavalent chromium biological reduction constitutes a safe and economical detoxification procedure of wastewaters containing Cr(VI). However, little research has been done to evaluate Cr(VI) tolerance and reduction capacity of microbial cultures under different growth conditions. The aims of this work were (a) to evaluate the capacity of Sphaerotilus natans to reduce Cr(VI) to Cr(III) in a continuous system limited in carbon and energy source or in nitrogen source, (b) to evaluate the toxic effect of Cr(VI) on this microorganism, (c) to carry out a complete analysis of Cr(VI) reduction by S. natans not only in continuous regime but also in batch system, and (d) to model the obtained results mathematically. S. natans exhibited great resistance to Cr(VI) (19-78 mg l(-1)) and optimal growth in continuous and batch systems using a mineral medium supplemented only with citric acid as organic substrate. In carbon- and energy-limited continuous systems, a maximum percentual decrease in Cr(VI) by 13% was reached for low influent Cr(VI) concentration (4.3-5.32 mg Cr(VI)l(-1)); the efficiency of the process did not notoriously increase as the length of cellular residence time was increased from 4.16 to 50h. A nitrogen-limited continuous operation with a cellular residence time of 28.5h resulted in a Cr(VI) decrease of approximately 26-32%. In batch system, a mathematical model allowed to predict the Cr(VI) concentration as a function of time and the ratio between the initial Cr(VI) concentration and that of the biomass. High concentrations of initial Cr(VI) and biomass produced the highest performance of the process of Cr(VI) reduction reached in batch system, aspects which should be considered in detoxification strategies of wastewaters.

The use of microemulsions to remove chromium from industrial sludge

In this work microemulsion systems were used to remove chromium from leather tannery sediments. The sludge was treated by a solid-liquid extraction process (acid digestion). The effects of particle size, digestion temperature and digestion time with regards to the efficiency of chromium removal were considered. The raw sludge (3 Mesh) was dried, grounded and sieved. Particles with 3, 14, 65, 100, 200, and 325 Mesh were evaluated. Sludge digestion solutions were prepared using each studied granule size at 25 degrees C, 70 degrees C, and 95 degrees C. Microemulsion extraction experiments to remove chromium III from the acid digestion solution were made according to a Scheffé Net experimental design methodology, using microemulsion systems inside the Winsor II region (System I) and inside the microemulsion region (Winsor IV--System II). A statistical treatment was used to obtain the isoresponse plots. Chromium extraction percentages were up to 73.3% for System I and up to 93.4% for System II.

Effect of hexavalent chromium on the activated sludge process and on the sludge protozoan community

The objectives of this study were the determination of chromium effects to the performance of an activated sludge unit and the investigation of the response of the activated sludge protozoan community to Cr(VI). Two bench scale activated sludge reactors were supplied with synthetic sewage containing Cr(VI), at concentrations from 1 up to 50 mg L(-1). Protozoan species were identified and were related to the system efficiency. Variations in the abundance and diversity of the protozoan species were observed under various chromium concentrations. High removal rates of organics and nutrients were observed after the acclimatization of the activated sludge, which were related to the initial chromium(VI) concentration. Chromium(VI) removal efficiency was high in all cases. The protistan community was affected by the influent chromium content. Dominance of sessile species was observed in the reactor receiving 5 mg L(-1) influent chromium, whereas co-dominance of sessile and carnivorous species was observed in the reactors receiving higher chromium concentrations.

Removal of total phenols from olive-mill wastewater using an agricultural by-product, olive pomace

The purpose of this study was to investigate the sorption of total phenols, which are contained in olive-mill wastewater (OMWW), on solid by-products of olive pomace processing mills. Preliminary batch experiments were conducted using three different types of olive pomace, dried olive pomace (OP-1), dried and solvent extracted olive pomace (OP-2) and dried, solvent extracted and incompletely combusted olive pomace (OP-3). According to the results, OP-3 showed high performance for total phenols sorption and stability. For sorbent concentration of 10gL(-1) and sorbate concentration of 50mgL(-1), more than 40% of initial total phenols concentration was removed. Sorption kinetics was well described by the pseudo-second order rate model (R2>0.99). Total phenols removal efficiency was improved by increasing sorbent concentration and solution's pH or decreasing particle size of the sorbent material. The Langmuir and Freundlich isotherms sufficiently described OP-3 sorption capacity for the concentration range studied (50-500mgL(-1)). Fixed bed sorption experiments showed that lower flow rates and smaller particle size of sorbent resulted in longer column exhaustion time and higher initial removal efficiency. Experiments with thermally or chemically regenerated OP-3 showed that sorption capacity deteriorated after regeneration.

Hexavalent chromium removal by viable, granular anaerobic biomass

Hexavalent chromium in industrial wastewater is a major concern due to its extreme toxicity. This study investigates the removal of Cr(VI) using viable anaerobic granular biomass as a biosorbent. The effect of Cr(VI) concentration on biogas content and COD removal using batch studies indicated that the phase II (methanogenic-rich) culture was more sensitive than the phase I (acidogenic-rich) culture. Toxicity indices for both cultures using COD removal were developed based on linear-log interpolation. The median inhibition Cr(VI) concentration (IC(50)), for phase II cultures was found to be 263mg/L, while that for phase I cultures was 309mg/L. A sorption study was conducted on viable and non-viable (dried) phase I-rich biomass: both followed the Langmuir model. In addition, the biosorption capacity for metabolically inhibited biomass was 25% less indicating some level of cellular uptake associated with Cr(VI) removal. This study demonstrated the potential for a two-phase anaerobic treatment system for a Cr(VI)-contaminated effluent.

Column study on the adsorption of Cr(III) and Cr(VI) using Pumice, Yarikkaya brown coal, Chelex-100 and Lewatit MP 62

In the present study, adsorption of Cr(III) and Cr(VI) on Pumice (Pmc), Yarikkaya (YK) brown coal, Chelex-100, and Lewatit MP 62 is examined at room temperature and at initial chromium concentration of 1.0 x 10(-3) mol/L. Column method was carried out as a function of pH, concentration of Cr(III) and Cr(VI) ions, volume of samples and flow rate. The experimental data were evaluated by Freundlich and Langmuir isotherm models. The dynamic breakthrough capacities of the adsorbents for Cr(III) and Cr(VI) were calculated. The maximum chromium sorption occurred at 5 mL/min flow rate and 25 mL volume for all adsorbents. The results showed that the two readily available adsorbents namely Pmc and YK, were suitable for removing chromium from aqueous solution.

Modelling Cr(VI) removal by a combined carbon-activated sludge system

The combined carbon-activated sludge process has been proposed as an alternative to protect the biomass against toxic substances in wastewaters; however, the information about the effect of powdered-activated carbon (PAC) addition in activated sludge reactors for the treatment of wastewaters containing Cr(VI) is limited. The objectives of the present study were: (a) to evaluate the removal of hexavalent chromium by (i) activated sludge microorganisms in aerobic batch reactors, (ii) powdered-activated carbon, and (iii) the combined action of powdered-activated carbon and biomass; (b) to propose mathematical models that interpret the experimental results. Different Cr(VI) removal systems were tested: (S1) biomass (activated sludge), (S2) PAC, and (S3) the combined activated carbon-biomass system. A Monod-based mathematical model was used to describe the kinetics of Cr(VI) removal in the system S1. A first-order kinetics with respect to Cr(VI) and PAC respectively, was proposed to model the removal of Cr(VI) in the system S2. Cr(VI) removal in the combined carbon-biomass system (S3) was faster than both Cr(VI) removal using PAC or activated sludge individually. Results showed that the removal of Cr(VI) using the activated carbon-biomass system (S3) was adequately described by combining the kinetic equations proposed for the systems S1 and S2.

Removal of Cr(VI) from industrial wastewaters by adsorption Part I: determination of optimum conditions

Aim of this study is the determination of the Cr(VI) removal efficiency of treated pine sawdust and also to find out the thermodynamic and kinetic parameters of Cr(VI) removal process in batch systems. Sawdust has been treated with 1,5-disodium hydrogen phosphate before the adsorption experiments. The effects of initial concentration of Cr(VI) ion, temperature, amount of adsorbent and pH of the solution on adsorption have been investigated. Optimum conditions for adsorption were determined as T=40 degrees C, sawdust dose=4 g, pH 2, by using the results of these experiments and an additional set of experiments was performed under these optimum conditions in order to see the change in the adsorption efficiency. Removal of chromium ion was found as highly dependent on pH and initial Cr(VI) concentration of the solution. In order to find out thermodynamic and kinetic parameters equilibrium adsorption models were applied. Although experimental data confirm with both Langmuir and Freundlich isotherm models, they suit most on Langmuir isotherms. Adsorption rate constant was determined from Lagergren equation. Equilibrium constants, adsorption free energy, enthalpy and entropy change values were also determined. It was found that adsorption process follows first order kinetic and adsorption of Cr(VI) on sawdust has the spontaneous nature.

Microbial reduction of hexavalent chromium by landfill leachate

The reduction of hexavalent chromium (Cr(VI)) in municipal landfill leachates (MLL) and a non-putrescible landfill leachate (NPLL) was investigated. Complete Cr(VI) reduction was achieved within 17 days in a MLL when spiked with 100 mg l(-1) Cr(VI) or less. In the same period, negligible Cr(VI) reduction was observed in NPLL. In MLL, Cr(VI) reduction was demonstrated to be a function of initial Cr(VI) concentration and bacterial biomass and organic matter concentrations. The bacteria were observed to tolerate 250 mg l(-1) Cr(VI) in MLL and had an optimal growth activity at pH 7.4 in a growth medium. The MLL also possessed an ability to sequentially reduce Cr(VI) over three consecutive spiking cycles.

XANES study of Cr sorbed by a kitchen waste compost from water

A kitchen waste compost was used to sorb Cr for various times from water containing either Cr(NO3)3 or CrO3 in different concentrations. Scanning electron microscopy (SEM) results show that the composts have been partially oxidized by Cr(VI) during the sorption experiments. X-ray absorption near edge structure (XANES) simulation suggests that about 54.1-61.0% Cr sorbed on the compost is in form of organic Cr(III) through ionic exchange process with the rest being existent as Cr(NO3)3 in the Cr(III) sorption case; no Cr(OH)3 is observed or expected because the solution pH after sorption experiments is <or= 3.31. However in the Cr(VI) sorption cases, about 18.0-24.9% of the total sorbed Cr is in form of Cr(OH)3 precipitate due to the Cr(VI) chemical reduction by the compost and a contaminant increase in solution pH to >or= 5.94. Moreover, organic Cr(III) represents about 51.7-69.0% of the total sorbed Cr, and the rest (6.1-28.5%) is Cr(VI).

Investigation of Cr(VI) reduction in continuous-flow activated sludge systems

The aim of this research was to investigate hexavalent chromium, Cr(VI), reduction by activated sludge and to evaluate the use of continuous-flow activated sludge systems for the treatment of Cr(VI)-containing wastewater. Three series of experiments were conducted using two parallel lab-scale activated sludge systems. During the first experiment, one system was used as a control, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5mg l(-1). For all concentrations added, approximately 40% of the added Cr(VI) was removed during the activated sludge process. Determination of chromium species in the dissolved and particulate phase revealed that the removed Cr(VI) was sorbed by the activated sludge flocs mainly as trivalent chromium, Cr(III), while the residual chromium in the dissolved phase was mainly detected as Cr(VI). Activated sludge ability to reduce Cr(VI) was independent of the acclimatization of biomass to Cr(VI) and it was not affected by the toxic effect of Cr(VI) on autotrophic and heterotrophic microorganisms. During the second experiment, both systems were operated under two different hydraulic residence time (theta equal to 20 and 28h) and three different initial organic substrate concentration (COD equal to 300, 150 and 0mg l(-1)). Cr(VI) reduction was favored by an increase of theta, while it was limited by influent COD concentration. Finally, at the last experiment the effect of anoxic and anaerobic reactors on Cr(VI) reduction was investigated. It was observed that the use of an anoxic zone or an anaerobic-anoxic zone ahead of the aerobic reactor favored Cr(VI) reduction, increasing mean percentage Cr(VI) reduction to almost 80%.

Microbial Reduction of Hexavalent Chromium in Landfill Leachate

Landfill leachates were found to exhibit reducing properties whereby chromium(vi) was converted into chromium(iii). The reduction is attributed to a microbial process in the presence of high concentrations of organic materials in the leachates. Nonputrescible landfill leachate (NPLL) was found to reduce CrVI to a lesser extent than the municipal landfill leachate (MLL). Microbial reduction of CrVI was also found to occur under alkaline conditions in extracts from a cementitious waste. A 55% reduction of CrVI was achieved by enriching the extract with bacteria and organic material.