Effect of acclimatization of microorganisms to heavy metals on the performance of activated sludge process

Zinc and copper supplements enhance trichloroethylene removal by Pseudomonas plecoglossicida in water

The effects of two microelements, zinc and copper, on the aerobic co-metabolic removal of trichloroethylene (10 mg/L) by the isolate Pseudomonas plecoglossicida were investigated. The strain was previously isolated from a petroleum-contaminated site using toluene (150 mg/L) as substrate. Different concentrations (1, 10 and 100 mg/L) of microelements provided with SO42- and Cl- were tested. The results showed the supplement of Zn2+ and Cu2+ at the low concentration (1 mg/L) significantly enhanced cell growth. The removal efficiencies for toluene and trichloroethylene were also enhanced at the low concentration (1 mg/L) of Zn2+ and Cu2+. Compared to the control without zinc supplement, higher concentrations of zinc (10 and 100 mg/L) enhanced the removal efficiencies for both toluene and trichloroethylene in the first three days but showed some inhibitory effect afterward. However, the higher concentrations of Cu2+ (10 and 100 mg/L) always showed inhibitory to the toluene removal while showing inhibitory to the TCE removal after three days. For both Zn2+ and Cu2+, the anions SO42- and Cl- did not show significant difference in their effects on the toluene removal. A possible mechanism for Zn2+ and Cu2+ to enhance the removal of toluene and trichloroethylene would be their involvement in toluene oxygenase-based transformation processes. In addition, high concentrations of Zn2+ and Cu2+ ions could be removed from the liquid by the cells accordingly. The results imply a potential of supplementing low concentrations of zinc and copper to enhance bioremediation of the sites co-contaminated with toluene and trichloroethylene.

Bioremediation of organic/heavy metal contaminants by mixed cultures of microorganisms: A review

Although microbial remediation has been widely used in the bioremediation of various contaminants, in practical applications of biological remediation, pure cultures of microorganisms are seriously limited by their adaptability, efficiency, and capacity to handle multiple contaminants. Mixed cultures of microorganisms involve the symbiosis of two or more microorganisms. Such cultures exhibit a collection of the characteristics of each microorganism species or strain, showing enormous potential in the bioremediation of organic or heavy metal pollutants. The present review focuses on the mixed cultures of microorganisms, demonstrating its importance and summarizing the advantages of mixed cultures of microorganisms in bioremediation. Furthermore, the internal and external relations of mixed culture microorganisms were analyzed with respect to their involvement in the removal process to elucidate the underlying mechanisms.

A sensitive, wide-ranging comprehensive toxicity indicator based on microbial fuel cell

An innovative indicator for toxicity detection based on microbial fuel cells, average current inhibition rate (ACIR) was proposed. It was applied to the toxicity evaluation of three typical specific pollutants in petrochemical wastewater including copper(II), 2,4-dichlorophenol (2,4-DCP) and pyridine. ACIR which considered the entire process of toxic effects was proved to be more sensitive and wide-ranging than the conventional indicators. The linear detection ranges were 0.3-100 mg/L of copper(II), 0.4-1000 mg/L of 2,4-DCP, and 0.1-1000 mg/L of pyridine. The median effective concentrations of the three toxicants were 34.32, 36.18 and above 1000 mg/L, respectively. By contrast, using a conventional indicator such as the voltage inhibition rate, the calculation results consistently change with the exposure time. Based on the response time, the toxicity will be difficult to distinguish under high concentrations. An analysis of the microbial community in anode chamber showed that electrogenic bacteria such as Geobacter and Arcobacter significantly decreased with 2,4-DCP and pyridine under all tested concentrations. A principal component analysis was conducted, the results of which showed that the microbial community shifted from left to right with the increase concentration of copper(II) and 2,4-DCP. An increase of ACIR was noticed to be in accordance with the reduction of electrogenic bacteria.

Mixed sulfate-reducing bacteria-enriched microbial fuel cells for the treatment of wastewater containing copper

Microbial fuel cells (MFCs) have been widely investigated for organic-based waste/substrate conversion to electricity. However, toxic compounds such as heavy metals are ubiquitous in organic waste and wastewater. In this work, a sulfate reducing bacteria (SRB)-enriched anode is used to study the impact of Cu²⁺ on MFC performance. This study demonstrates that MFC performance is slightly enhanced at concentrations of up to 20 mg/L of Cu²⁺, owing to the stimulating effect of metals on biological reactions. Cu²⁺ removal involves the precipitation of metalloids out of the solution, as metal sulfide, after they react with the sulfide produced by SRB. Simultaneous power generation of 224.1 mW/m² at lactate COD/SO₄^2- mass ratio of 2.0 and Cu²⁺ of 20 mg/L, and high Cu²⁺ removal efficiency, at >98%, are demonstrated in the anodic chamber of a dual-chamber MFC. Consistent MFC performance at 20 mg/L of Cu²⁺ for ten successive cycles shows the excellent reproducibility of this system. In addition, total organic content and sulfate removal efficiencies greater than 85% and 70%, respectively, are achieved up to 20 mg/L of Cu²⁺ in 48 h batches. However, higher metal concentration and very low pH at <4.0 inhibit the SRB MFC system. Microbial community analysis reveals that Desulfovibrio is the most abundant SRB in anode biofilm at the genus level, at 38.1%. The experimental results demonstrate that biological treatment of low-concentration metal-containing wastewater with SRB in MFCs can be an attractive technique for the bioremediation of this type of medium with simultaneous energy generation.

Effect of copper on the performance and bacterial communities of activated sludge using Illumina MiSeq platforms

The anaerobic-anoxic-aerobic (A2O) process is a highly efficient sewage treatment method, which uses complex bacterial communities. However, the effect of copper on this process and the bacterial communities involved remains unknown. In this study, a systematic investigation of the effect of persistent exposure of copper in the A2O wastewater treatment system was performed. An A2O device was designed to examine the effect of copper on the removal efficiency and microbial community compositions of activated sludge that was continuously treated with 10, 20, and 40 mg L(-1) copper, respectively. Surprisingly, a decrease in chemical oxygen demand (COD) and ammonia nitrogen (NH4N) removal efficiency was observed, and the toxicity of high copper concentration was significantly greater at 7d than at 1d. Proteobacteria, Bacteroidetes, Acidobacteria, Chlorobi, and Nitrospirae were the dominant bacterial taxa in the A2O system, and significant changes in microbial community were observed during the exposure period. Most of the dominant bacterial groups were easily susceptible to copper toxicity and diversely changed at different copper concentrations. However, not all the bacterial taxa were inhibited by copper treatment. At high copper concentration, many bacterial species were stimulated and their abundance increased. Cluster analysis and principal coordinate analysis (PCoA) based on operational taxonomic units (OTUs) revealed clear differences in the bacterial communities among the samples. These findings indicated that copper severely affected the performance and key microbial populations in the A2O system as well as disturbed the stability of the bacterial communities in the system, thus decreasing the removal efficiency.

Culturable microbial groups and thallium-tolerant fungi in soils with high thallium contamination

Thallium (Tl) contamination in soil exerts a significant threat to the ecosystem health due to its high toxicity. However, little is known about the effect of Tl on the microbial community in soil. The present study aimed at characterizing the culturable microbial groups in soils which experience for a long time high Tl contamination and elevated Hg and As. The contamination originates from As, Hg and Tl sulfide mineralization and the associated mining activities in the Guizhou Province, Southwest China. Our investigation showed the existence of culturable bacteria, filamentous fungi and actinomyces in long-term Tl-contaminated soils. Some fungal groups grow in the presence of high Tl level up to 1000 mg kg⁻¹. We have isolated and identified nine Tl-tolerant fungal strains based on the morphological traits and ITS analysis. The dominant genera identified were Trichoderma, Penicillium and Paecilomyces. Preliminary data obtained in this study suggested that certain microbes were able to face high Tl pollution in soil and maintain their metabolic activities and resistances. The highly Tl-tolerant fungi that we have isolated are potentially useful in the remediation of Tl-contaminated sites.

Effects of multi-metal toxicity on the performance of sewage treatment system during the festival of colors (Holi) in India

The present study investigated the effects of heavy metals (Ni, Zn, Cd, Cu, and Pb) toxicity on the performance of 18 MLD activated sludge process-based sewage treatment plant (STP) during celebration of Holi (festival of colors in India). The composite sampling (n = 32) was carried out during the entire study period. The findings show a significant decrease in chemical oxygen demand removal efficiency (20%) of activated sludge system, after receiving the heavy metals laden wastewater. A significant reduction of 40% and 60% were observed in MLVSS/MLSS ratio and specific oxygen uptake rate, which eventually led to a substantial decrease in biomass growth yield (from 0.54 to 0.17). The toxic effect of metals ions was also observed on protozoan population. Out of the 12 mixed liquor species recorded, only two ciliates species of Vorticella and Epistylis exhibited the greater tolerance against heavy metals toxicity. Furthermore, activated sludge shows the highest metal adsorption affinity for Cu, followed by Zn, Pb, Ni, and Cd (Cu > Zn > Pb > Ni > Cd). Finally, this study proves the robustness of activated sludge system against the sudden increase in heavy metal toxicity since it recovered the earlier good quality performance within 5 days.

Temporal and spatial inhibitory effects of zinc and copper on wastewater biofilms from oxygen concentration profiles determined by microelectrodes

To understand the temporal and spatial toxic effect of heavy metals on the microbial activities of biofilms, microelectrodes were used to measure the inhibitory oxygen (O(2)) concentration profiles resulted from the effects of zinc (Zn(2+)) and copper (Cu(2+)). Using the O(2) microprofiles as bases, the spatial distributions of net specific O(2) respiration were determined in biofilms with and without treatment of 5 mg/L Zn(2+) or 1 mg/L Cu(2+). Results show that microbial activities were inhibited only in the outer layer (∼400 μm) of the biofilms and bacteria present in the deeper sections of the biofilms became even more active. The inhibition caused by the heavy metals was evaluated by two methods. One was derived from the oxygen influx at the interface and the other was based on the integral of the oxygen consumption calculated from the entire O(2) profile. The two methods yielded significantly different results. We argue that the integral method results in more accurate assessment of toxicity than the surface flux determination.