Microorganisms are responsible for degrading the raw leachate generated in sanitary landfills, extracting the soluble fraction of the landfill waste and biotransforming organic matter and toxic residues. To increase our understanding of these highly contaminated ecosystems, we analyzed the microbial communities in the leachate produced by three landfill cells of different ages. Using high-throughput 454 pyrosequencing of the 16S rRNA gene, we describe the structure of the leachate communities and present their compositional characteristics. All three communities exhibited a high level of abundance but were undersampled, as indicated by the results of the rarefaction analysis. The distribution of the taxonomic operational units (OTUs) was highly skewed, suggesting a community structure with a few dominant members that are key for the degradation process and numerous rare microorganisms, which could act as a resilient microorganism seeder pool. Members of the phylum Firmicutes were dominant in all of the samples, accounting for up to 62% of the bacterial sequences, and their proportion increased with increasing landfill age. Other abundant phyla included Bacteroidetes, Proteobacteria, and Spirochaetes, which together with Firmicutes comprised 90% of the sequences. The data illustrate a microbial community that degrades organic matter in raw leachate in the early stages, before the methanogenic phase takes place. The genera found fit well into the classical pathways of anaerobic digestion processes.

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Color removal was evaluated in anaerobic assays using synthetic (SE) and real (RE) effluents containing the dye Direct Black 22 (DB22). The focus was on the individual influence of two electron donors (EDs), sucrose and ethanol; and their combination with two redox mediators (RMs), lawsone and riboflavin. In addition to SE assays, the EDs were combined with sulfate which was present in the RE. Distinct behavior was observed with both assays. In the case of SE, using the EDs sucrose and ethanol resulted in DB22 removal efficiency of 93 and 83%, respectively; but only 27% in the control. The respective removal rates were 27.8‐ and 4.4‐fold higher than that of the control. Using the EDs with the RM lawsone resulted in color removal higher than that with riboflavin. Interestingly, in the ethanol + sulfate assay, late release of the last was observed, possibly due to the desulfonation reaction of the aromatic structures during the DB22 molecule breakdown. In the case of RE, no improvement was observed with either EDs; and some combinations with the RMs even resulted in decreased color removal. Therefore, the used EDs and RMs showed positive effect on color removal in SEs; but unexpectedly not in REs.

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The nitroheterocyclic 3-nitro-1,2,4-triazol-5-one (NTO) is an ingredient of insensitive explosives increasingly used by the military, becoming an emergent environmental pollutant. Cometabolic biotransformation of NTO occurs in mixed microbial cultures in soils and sludges with excess electron-donating substrates. Herein, we present the unusual energy-yielding metabolic process of NTO respiration, in which the NTO reduction to 3-amino-1,2,4-triazol-5-one (ATO) is linked to the anoxic acetate oxidation to CO2 by a culture enriched from municipal anaerobic digester sludge. Cell growth was observed simultaneously with NTO reduction, whereas the culture was unable to grow in the presence of acetate only. Extremely low concentrations (0.06 mg L–1) of the uncoupler carbonyl cyanide m-chlorophenyl hydrazone inhibited NTO reduction, indicating that the process was linked to respiration. The ultimate evidence of NTO respiration was adenosine triphosphate production due to simultaneous exposure to NTO and acetate. Metagenome sequencing revealed that the main microorganisms (and relative abundances) were Geobacter anodireducens (89.3%) and Thauera sp. (5.5%). This study is the first description of a nitroheterocyclic compound being reduced by anaerobic respiration, shedding light on creative microbial processes that enable bacteria to make a living reducing NTO.

In the present study, we evaluate the behavior of real textile wastewater treatment using a system composed of two sequential pilot-scale reactors (anaerobic followed by aerobic) during 622 days. The work focused on the competition between color and sulfate removal processes, when the hydraulic retention time (HRT) was increased in the anaerobic/aerobic reactors from 16/12 hours in phase I (PI) to 4/3 days in phase II (PII). The organic matter was successfully removed in both phases through the system, and the highest efficiency (75%) was achieved in PII. The increase in the HRT did not improve azo dye degradation under anaerobic conditions. Instead, it favored sulfate reduction, which removal efficiency increased from 26% in PI to 75% in PII. Aromatic amines were detected in the anaerobic reactor effluent and removed in the aerobic reactor.

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