The presence of heavy metals in municipal solid waste (MSW) is considered as prevalent global pollutants that cause serious risks to the environment and living organisms. Due to industrial and anthropogenic activities, the accumulation of heavy metals in the environmental matrices is increasing alarmingly. MSW causes several adverse environmental impacts, including greenhouse gas (GHG) emissions, river plastic accumulation, and other environmental pollution. Indigenous microorganisms (Pseudomonas, Flavobacterium, Bacillus, Nitrosomonas, etc.) with the help of new pathways and metabolic channels can offer the potential approaches for the treatment of pollutants. Microorganisms, that exhibit the ability of bioaccumulation and sequestration of metal ions in their intracellular spaces, can be utilized further for the cellular processes like enzyme signaling, catalysis, stabilizing charges on biomolecules, etc. Microbiological techniques for the treatment and remediation of heavy metals provide a new prospects for MSW management. This review provides the key insights on profiling of heavy metals in MSW, tolerance of microorganisms, and application of indigenous microorganisms in bioremediation. The literatures revealed that indigenous microbes can be exploited as potential agents for bioremediation.

Recent studies have demonstrated the ability of mealworm (Tenebrio molitor) for plastic degradation. This study is focused on changes in microbiome structure depending on diets. Microbial community obtained from oat and cellulose diet formed similar group, two kinds of polyethylene formed another group, while polystyrene diet showed the highest dissimilarity. The highest relative abundance of bacteria colonizing gut was in PE-oxodegradable feeding, nevertheless all applied diets were higher in comparison to oat. Dominant phyla consisted of Proteobacteria, Bacteroides, Firmicutes and Actinobacteria, however after PS feeding frequency in Planctomycetes and Nitrospirae increased. The unique bacteria characteristic for cellulose diet belonged to Selenomonas, while Pantoea were characteristic for both polyethylene diets, Lactococcus and Elizabethkingia were unique for each plastic diet, and potential diazotropic bacteria were characteristic for polystyrene diet (Agrobacterium, Nitrosomonas, Nitrospira).Enzymatic similarity between oatmeal and cellulose diets, was shown. All three plastics diet resulted in different activity in both, digestive tract and bacteria. The enzymes with the highest activity were included phosphatases, esterases, leucine arylamidase, β-galactosidase, β-glucuronidase, α-glucosidase, β-glucosidase, chitinase, α-mannosidase and α-fucosidase. The activity of digestive tract was stronger than cultured gut bacteria. In addition to known polyethylene degradation methods, larvae may degrade polyethylene with esterase, cellulose and oatmeal waste activity is related with the activity of sugar-degrading enzymes, degradation of polystyrene with anaerobic processes and diazotrophs.

Carbon nanotubes (CNTs) inevitably enter domestic sewage and industrial wastewater with the continuous increase of their production and application field. The potential effect of CNTs on biological wastewater treatment processes has raised wide concerns due to their biotoxicity. In the present study, the performance, microbial community and enzymatic activity of sequencing batch reactors (SBRs) were evaluated under 148-day exposure of amino-functionalized multi-walled CNTs (MWCNTs-NH₂) at 10 and 30 mg/L. The COD removal efficiency at 10 and 30 mg/L MWCNTs-NH₂ gradually reduced from 91.03% and 90.43% on day to 89.11% and 86.70% on day 148, respectively. The NH₄⁺-N removal efficiency at 10 and 30 mg/L MWCNTs-NH₂ gradually reduced from 98.98% and 98.46% on day 1 to 96.65% and 63.39% on day 148, respectively. Compared to 0 mg/L MWCNTs-NH₂, the oxygen-utilizing rate, ammonia-oxidizing rate, nitrite-oxidizing rate, nitrite-reducing rate and nitrate-reducing rate at 30 mg/L MWCNTs-NH₂ were decreased by 52.35%, 60.58%, 55.12%, 56.56% and 57.42% on day 148, respectively. The microbial reactive oxygen species and lactate dehydrogenase release on day 148 was increased by 59.71% and 55.28% at 30 mg/L MWCNTs-NH₂, respectively. The key microbial enzymatic activity related to nitrogen removal decreased with the increase of operation time under MWCNTs-NH₂ stress. The relative abundances of Nitrosomonas, Nitrosospira, Nitrospira and some denitrifying bacteria at 10 mg/L MWCNTs-NH₂ gradually reduced with an increment in operation time. The changes of nitrogen removal rate, microbial community and enzymatic activity of SBR were related to the time-cumulative nonlinear inhibition effect under long-term exposure.

The use of dynamic membranes as a low-cost alternative for conventional membrane for the treatment of landfill leachate (LFL) was investigated in this study. For this purpose a lab-scale, submerged pre-anoxic and post-aerobic bioreactor configuration was used with nylon mesh as dynamic membrane support. The study was conducted at ambient temperature and LFL was fed to the bioreactor in gradually increasing concentration mixed with tap water (from 20% to 100%). The results of this study demonstrated that lower mesh pore size of 52 μm achieved better results in terms of solid-liquid separation performance (turbidity <10 NTU) of the formed dynamic membrane layer as compared to 200 and 85 μm meshes while treating LFL. Consistently high NH₄⁺-N conversion efficiency of more than 98% was achieved under all nitrogen loading conditions, showing effectiveness of the formed dynamic membrane in retaining slow growing nitrifying species. Total nitrogen removal reached more than 90% however, the denitrification activity showed a fluctuating profile and found to be inhibited by elevated concentrations of free nitrous acid and NO₂⁻-N at low pH values inside the anoxic bioreactor. A detailed metagenomic analysis allowed a taxonomic investigation over time and revealed the potential biochemical pathways involved in NH₄⁺-N conversion. This study led to the identification of a dynamic system in which nitrite concentration is determined by the contribution of NH₄⁺ oxidizers (Nitrosomonas), and by a competition between nitrite oxidizers (Nitrospira and Nitrobacter) and reducers (Thauera).

The performance, microbial community and enzymatic activity of sequencing batch reactors (SBRs) were investigated under 75-day exposure of different Cu(II) concentrations. Cu(II) at 0–5 mg/L had no distinct impact on the chemical oxygen demand (COD) and nitrogen removal, oxygen-uptake rate (OUR), nitrification and denitrification rate, and microbial enzymatic activity. The inhibitory effects of Cu(II) at 10 and 30 mg/L on the nitrogen removal rate, OUR, and microbial enzymatic activity of SBR increased with an increment in operation time due to the Cu(II) biotoxicity and the accumulation of Cu(II) in activated sludge. The changes of microbial reactive oxygen species production, lactate dehydrogenase release, catalase activity and superoxide dismutase activity demonstrated that Cu(II) at 10 and 30 mg/L broke the equilibrium between the oxidation and antioxidation processes in microbial cells and also damaged the cytomembrance integrity, which could affect the COD and nitrogen removal performance and change normal microbial cell morphology. The Cu(II) in the influent could be removed by the microbial absorption and accumulated in the activated sludge under long-term exposure. The microbial community displayed some distinct changes from 0 to 30 mg/L Cu(II). In contrast with 0 mg/L Cu(II), Nitrosomonas, Nitrospira and some denitrifying bacteria obviously decreased in relative abundance under long-term exposure of 10 and 30 mg/L Cu(II).

Anaerobic digestion of thermally hydrolyzed sludge was an important method for sludge treatment. But a large amount of rejected water (TRW) containing high ammonia was produced, which was difficult to treat. In this study, two-stage reactors were used for TRW treatment using partial nitrification-anammox technology. The results demonstrated that nitritation initiated rapidly. The NH₄⁺–N conversion load reached 1300 mg N/(L·day) and could be further improved. The consumption of NH₄⁺–N and the formation of NO₂⁻–N were linear with time. So, the right ratio of NH₄⁺–N/NO₂⁻–N can achieve by controlling time for anammox. Dissolved oxygen and sludge concentration had important effects on nitritation. Increasing dissolved oxygen or sludge concentration can shorten the reaction time. Nitrosomonas were the dominant ammonia-oxidizing bacteria, and nitrite-oxidizing bacteria were not detected. Aerobic treatment achieved more than 50% chemical oxygen demand removal in TRW, and this water was used in an anammox reactor. Accumulated flocculent sludge could inhibit anammox activity but could be removed by increasing the flow velocity. The anammox reactor load was above 600 mg N/(L·day); even under high conductivity (18.2 ms/cm) condition, the load reached 320 mg N/(L·day), and the total nitrogen removal rate was greater than 85% under stable condition. After approximately 190 days of operation, the abundance of anammox bacteria decreased from 29% to less than 10%, but the reactor operated stably. The results demonstrated two-stage reactors were suitable for ammonia removal in TRW using partial nitrification-anammox technology.

Several regions of the world suffer from water quality degradation caused mainly by anthropogenic interference, domestic sewage discharge, industrial discharges, and contaminants from agricultural activities, which increase the amount of nutrients into water such as nitrogen and phosphorus intensifying the process of eutrophication. Considered as one of the main problems faced by supply water sources in the world, eutrophication represents a global phenomenon that also affects the socioeconomic and environmental sector. In the present study, a bacterial pool formed by the Nitrosomonas europae, Nitrobacter winogradskyi, and Paracoccus denitrificans strains was used to remove the compounds NH₃, NO₂⁻, NO₃⁻, P, and PO₄³⁻, real water samples from human supply at time 0 to 24 h. At the final time (24 h), the bacterial pool obtained total removal (100%) of all compounds analyzed. With the removal of nutrients from water, there was also reduction in the amount cyanobacteria and saxitoxins of water. Thus, the bacterial pool can be applied as a remediators of micropollutants such as compounds N and P, since their performance in removel the compounds was satisfactory. The removal of micropollutants from water bodies aims to decrease the nutrients responsible for cyanobacteria blooms, eutrophication, as well as improve water quality, optimize, and reduce water treatment costs, benefiting both the aquatic ecosystem and human health.

Single-stage autotrophic nitrogen removal offers advantages of low energy and carbon consumptions. Based on previous work about a novel composite membrane aerated biofilm (CMAB), two microbial entrapping patterns (mixed and stratified patterns) were evaluated for their applicability to artificially regulate the spatial distribution of distinct microbial aggregates for single-stage autotrophic nitrogen removal. Experimental results showed that the stratified pattern caused little accumulation of NO₂⁻ and NO₃⁻, which leads to a superior nitrogen removal performance compared with the mixed pattern. Candidatus Kuenenia was found to be the major anammox bacterium in the gel film of the mixed pattern and the outer film of the stratified pattern. In contrast, Nitrosomonas, as a representative genus of ammonia-oxidizing bacteria, was substantially enriched in the inner film of the stratified pattern and the gel film of the mixed pattern. Finally, modeling results further confirmed the advantages of the stratified pattern with respect to the formation of rational microbial and nutrient profiles in gel films. The ratio of partial nitrification and anammox film thicknesses should remain below 3:2 to obtain a high fraction of anammox bacteria and to avoid NO₂⁻ accumulation. Increasing O₂ surface loading does not affect microbial profiles, but can greatly promote the TN removal performance only in the stratified pattern. Overall, the stratified pattern should be employed to achieve optimal microbial profiles and nitrogen removal efficiency.

The single-stage partial nitritation–anammox (PN/A) process is severely limited by a long start-up time and unstable removal efficiency. In this study, PN/A was developed in 67 days in a novel packed bed equipped with porous bio-carriers by gradually increasing the influent nitrogen loading rate (0.15–0.73 kg-N m⁻³·d⁻¹) and controlling the dissolved oxygen (< 1.2 mg L⁻¹). An average ammonium nitrogen removal efficiency (ARE) and total nitrogen removal efficiency (TNR) of 87.01 and 72.41%, respectively, were obtained. This represents a reliable alternative method of achieving rapid PN/A start-up. The results of 16S rRNA sequencing showed that Proteobacteria and Planctomycetes, with which ammonia-oxidizing bacteria and anammox bacteria were affiliated, accounted for 38.8%, representing the dominant phylum in the system after acclimation. The abundance of Nitrosomonas and Candidatus Brocadia increased by 16 and 1.79%, respectively. The results of metagenomics and metatranscriptomics revealed that the nitrite oxidation process was blocked by the transcriptional suppression of nitrite oxidoreductase and the entire nitrogen metabolism process was dominated by the partial nitritation and anammox process. Moreover, a high abundance of heterotrophic bacteria with potential for nitrogen removal was detected. In the nitrogen cycle, a widespread nitrite-accumulated denitrification helps to form a nitrite loop, which promotes the efficiency of total nitrogen removal. This is crucial for further improving the nitrogen removal mechanism in the PN/A system.

This study aimed to elucidate the boundaries of ammonia and organic loading rates that allow for nitritation in continuous flow phosphorylated-polyvinyl alcohol entrapped cell reactors and to clarify the community structure of microorganisms involving nitrogen transformation in the gel bead matrices. At operating bulk dissolved oxygen concentration of 2 mg/L, nitritation was accomplished when the total ammonia nitrogen (TAN) loading rate was ≥ 0.3 kgN/m³/d. At TAN loading rates of ≤ 0.2 kgN/m³ /d, complete oxidation of ammonia to nitrate took place. Nitritation performance dropped with increased chemical oxygen demand (COD) loading rates indicating limitation of nitritation reactor operation at some COD loading conditions. 16S rRNA gene amplicon sequencing revealed that the uncultured Cytophagaceae bacterium, Arenimonas, Truepera, Nitrosomonas, Comamonas, unclassified Soil Crenarchaeotic Group, and uncultured Chitinophagaceae bacterium were highly abundant taxa in the reactors’ gel bead matrices. qPCR with specific primers targeting amoA genes demonstrated the coexistence of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea, and Comammox in the gel bead matrices. AOB was likely the main functioning ammonia-oxidizing microorganisms due to the amoA gene being of highest abundance in most of the studied conditions. Nitrite-oxidizing microorganisms presented in less relative abundance than ammonia-oxidizing microorganisms, with Nitrobacter rather than Nitrospira dominating in the group. Results obtained from this study are expected to further the application of nitritation entrapped cell reactors to real wastewater treatment processes.