Sanitary landfilling is still considered worldwide as one of the most common methods applied for the management of the municipal solid waste. As a consequence, vast amounts of landfill leachate are generated annually, which are characterized by variability in physicochemical composition, owing to the stabilization process that occurs over the years. However, sustainable management of landfill leachate is a challenging issue, due to diverse chemical composition and high concentration in heavy metals and xenobiotics. Despite the fact that several studies have been reported on the biotreatment of landfill leachate, only in recent years has the microbial composition in such systems have been examined. In the present review, the key role of the microbial ecology involved in depurification and detoxification of landfill leachate in activated sludge and anaerobic systems is interpreted and ecological considerations influencing landfill leachate treatment are stated. Apart from the assessment of landfill toxicity on certain model organisms, this work provides an extensive overview on microbial communities performing key biological processes during landfill leachate treatment, including nitrification-denitrification, anammox and anaerobic digestion. Moreover, microbial aspects affecting nutrient removal efficiency in such biosystems are discussed.

The evaluation of effluent wastewater quality mainly relies on the assessment of conventional bacterial indicators, such as fecal coliforms and enterococci; however, little is known about opportunistic pathogens, which can resist chlorination and may be transmitted in aquatic environments. In contrast to conventional microbiological methods, high-throughput molecular techniques can provide an accurate evaluation of effluent quality, although a limited number of studies have been performed in this direction. In this work, high-throughput amplicon sequencing was employed to assess the effectiveness of chlorination as a disinfection method for secondary effluents. Common inhabitants of the intestinal tract, such as Bacteroides, Arcobacter and Clostridium, and activated sludge denitrifiers capable of forming biofilms, such as Acidovorax, Pseudomonas and Thauera, were identified in the chlorinated effluent. Chloroflexi with dechlorination capability and the bacteria involved in enhanced biological phosphorus removal, i.e., Candidatus Accumulibacter and Candidatus Competibacter, were also found to resist chlorination. No detection of Escherichia indicates the lack of fecal coliform contamination. Mycobacterium spp. were absent in the chlorinated effluent, whereas toxin-producing cyanobacteria of the genera Anabaena and Microcystis were identified in low abundances. Chlorination significantly affected the filamentous bacteria Nocardioides and Gordonia, whereas Zoogloea proliferated in the disinfected effluent. Moreover, perchlorate/chlorate- and organochlorine-reducing bacteria resisted chlorination.

In this work, a sand filtration-activated carbon adsorption system was evaluated to remove the fungicide content of a biologically treated effluent. The purification process was mainly carried out in the activated carbon column, while sand filtration slightly contributed to the improvement of the pollutant parameters. The tertiary treatment system, which operated under the batch mode for 25 bed volumes, resulted in total and soluble COD removal efficiencies of 76.5 ± 1.5% and 88.2 ± 1.3%, respectively, detecting total COD concentrations below 50 mg/L in the permeate of the activated carbon column. A significant pH increase and a respective electrical conductivity (EC) decrease also occurred after activated carbon adsorption. The total and ammonium nitrogen significantly decreased, with determined concentrations of 2.44 ± 0.02 mg/L and 0.93 ± 0.19 mg/L, respectively, in the activated carbon permeate. Despite that, the initial imazalil concentration was greater than that of the fludioxonil in the biologically treated effluent (i.e., 41.26 ± 0.04 mg/L versus 7.35 ± 0.43 mg/L, respectively). The imazalil was completely removed after activated carbon adsorption, while a residual concentration of fludioxonil was detected. Activated carbon treatment significantly detoxified the biologically treated fungicide-containing effluent, increasing the germination index by 47% in the undiluted wastewater or by 68% after 1:1 v/v dilution.

Despite that diclofenac has been embodied to the European watch list of priority substances of concern, studies on diclofenac biodegradation are limited and the diversity of diclofenac-degrading microbiota remains unknown. In this work, an immobilized cell biofilter was constructed and operated to evaluate its effectiveness to depurate high strength diclofenac wastewater and to identify the diclofenac-degrading community accommodated in activated sludge by employing high-throughput sequencing techniques. After a two-month adaptation period, biofilter removal efficiencies reached values as high as 97.63 ± 0.62%, whereas utilization of diclofenac in the immobilized cell biofilter led to a drastic pH decrease. Based on Illumina sequencing, the major bacterial taxa identified in the immobilized cell biofilter were members of the species Granulicella pectinivorans and Rhodanobacter terrae, followed by members of the species Castellaniella denitrificans, Parvibaculum lavamentivorans, Bordetella petrii, Bryocella elongata and Rhodopseudomonas palustris. The ability of such taxa to utilize a wide range of carbon sources and to effectively adapt under acidic conditions seemed to be the main parameters, which favored their prevalence in the immobilized cell biofilter. In addition, Wickerhamiella was the predominant fungal taxon in the immobilized cell biofilter, which appears to be actively involved in diclofenac degradation in activated sludge systems.