Analysis of the Partial Nitrification Process Affected by Polyvinylchloride Microplastics in Treating High-Ammonia Anaerobic Digestates

The roles of typical emerging pollutants on N2O emissions during biological nitrogen removal from wastewater

N2O as a potent greenhouse gas often generates in the biological nitrogen removal (BNR) processes during wastewater treatment, which makes BNR become an important greenhouse gas emission source. The emerging pollutants (EPs) are ubiquitous in wastewater and they have shown to influence the BNR processes. However, the deep discussion on potential impacts of EPs on N2O emissions during BNR is rare. Moreover, the experimental parameters for EPs investigation in most of literatures are generally not in line with real-world BNR processes, which calls for deep elucidating the roles of EPs on N2O production and emission. In this work, a critical review summarizes the existing literature about influences of typical EPs on N2O emissions and associated mechanisms during BNR, and it discusses the impacts of some easily overlooked factors, such as real EPs environmental concentrations, EPs bioaccumulation, and multiple EPs coexistence on N2O emissions. This review will provide an insight into exploring and mitigating threats posed by typical EPs on N2O emissions.

Environmental sustainability assessment of a new food waste anaerobic digestion and pyrolysis hybridization system

This research conducted an environmental life cycle assessment (LCA) to evaluate an anaerobic digestion-co-pyrolysis (ADCo-Py) system in which pyrolysis was added to traditional food waste (FW) anaerobic digestion (AD) systems to treat the solid fraction and impurities separated from FW. The solid fraction, including impurities such as wooden chopsticks, plastics, eggshells, and bones, is usually incinerated, while pyrolysis can be a viable alternative to optimize FW treatment. The environmental impact of ADCo-Py was compared with stand-alone AD, pyrolysis, and ADCo-INC (AD with incineration of separated solids). The results indicated that both ADCo-Py (-1.726 kg CO2-Eq/kgFW) and ADCo-INC (-1.535 kg CO2-Eq/kgFW) outperform stand-alone AD (-0.855 kg CO2-Eq/kgFW) and pyrolysis (-0.181 kg CO2-Eq/kgFW) in mitigating global warming potential (GWP). Additionally, pretreatments were found to have the most significant influence on GWP, ecotoxicity potential (ETP), and acidification potential (AP). The two-step pretreatment in ADCo-Py, including the separation of solids and drying, significantly improved the environmental sustainability of the system when compared with standalone pyrolysis.

Polyvinyl Chloride Microplastics Facilitate Nitrous Oxide Production in Partial Nitritation Systems

Partial nitritation (PN) is an important partner with anammox in the sidestream line treating high-strength wastewater and primarily contributes to nitrous oxide (N2O) emissions in such a hybrid system, which also suffers from ubiquitous microplastics because of the growing usage and disposal levels of plastics. In this study, the influences of polyvinyl chloride microplastics (PVC-MPs) on N2O-contributing pathways were experimentally revealed to fill the knowledge gap on N2O emission from the PN system under microplastics stress. The long-term results showed that the overall PN performance was hardly affected by the low-dose PVC-MPs (0.5 mg/L) while obviously deteriorated by the high dose (5 mg/L). According to the batch tests, PVC-MPs reduced biomass-specific ammonia oxidation rates (AORs) by 5.78-21.94% and stimulated aerobic N2O production by 9.22-88.36%. Further, upon increasing dissolved oxygen concentrations from 0.3 to 0.9 mg O2/L, the degree of AOR inhibition increased but that of N2O stimulation was lightened. Site preference analysis in combination with metabolic inhibitors demonstrated that the contributions of hydroxylamine oxidation and heterotrophic denitrification to N2O production at 0.3 mg O2/L were enhanced by 18.84 and 10.34%, respectively, accompanied by a corresponding decreased contribution of nitrifier denitrification. Finally, the underlying mechanisms proposed for negative influences of PVC-MPs were bisphenol A leaching and reactive oxygen species production, which led to more cell death, altered sludge properties, and reshaped microbial communities, further resulting in enhanced N2O emission. Overall, this work implied that the ubiquitous microplastics are a hidden danger that cannot be ignored in the PN system.

The short and long-term effect of polystyrene nanoplastics on nitrifying sludge at high nitrite concentrations

The effect of nanoplastics (NPs) on nitrite oxidation bacteria (NOB) community in treating high-strength wastewater remains unclear, which seriously affects the stability of nitrogen removal process. In this study, highly active nitrifying sludge was enriched and exposed to 50 nm polystyrene NPs (PS-NPs) for short-term (1, 100, 500, and 1000 mg/L, 1.5 hr) and long-term (1, 10, 100 mg/L, 40 days) at high nitrite concentration. In contrast to previous studies, our results showed that the exposures to PS-NPs had little effect on nitrifying performances. After long-term exposure, the protein/polysaccharide ratios in extracellular polymeric substances (EPS) were positively correlated with PS-NPs concentrations (0.78-0.99). The produced reactive oxygen species (ROS) were gradually removed, and PS-NPs higher than 10 mg/L caused damage to membrane integrity. Long-term exposure for 40 days increased the community diversity and caused significant differences between the control and exposed communities. The control group were dominated by Nitrobacter and Exiguobacterium, while the exposure group was dominated by Bacillus, Mycobacterium, and Nitrospira. A noticeable shift in the NOB community from Nitrobacter (26.5% to 3.4%) to Nitrospira (1.61% to 14.27%) was observed. A KEGG analysis indicated a decrease in cell growth and death, cell motility and energy metabolism. It appeared that NOB could adapt to PS-NPs stress through enhanced secretion and removal of oxidative damage. Overall, this study provided new insights into the response mechanism of NOB to PS-NPs exposure.

Responses of activated sludge under a short-term exposure to facial scrub microbeads: implications from treatment performance and higher-life microbial population dynamics

In this study, four identical laboratory-scale sequencing batch reactors (SBRs) were continuously operated with different concentrations of microbeads (MBs) (5,000-15,000 MBs/L) to investigate the stress-responses of activated sludge under the MB exposure. It was found that the overall treatment performance (organic removal) of SBRs was fairly affected by short-term exposure to low levels of MBs; however, it was adversely affected as the concentration of MBs increased. The average concentration of mixed liquor suspended solids and heterotrophic bacteria in the reactor fed with 15,000 MBs/L were 16 and 30% less than in the pristine control reactor, respectively. Batch experiments further demonstrated that fairly low concentrations of MBs favored the development of dense microbial structures. Further increasing the MB concentrations to 15,000 MBs/L, however, distinctly weakened the settling performance of sludge. Morphological observations revealed suppressed uniformity, strength, and integrity of flocs reactors with the addition of MBs. Microbial community analyses revealed that the abundance of protozoan species declined 37.5, 58, and 64%, respectively, when SBRs were exposed to 5,000; 10,000; and 15,000 MBs/L as compared with the control reactor. The present work provided new insight into the possible effects of MBs on the performances and operational parameters of activated sludge.

Microplastics alter nitrous oxide production and pathways through affecting microbiome in estuarine sediments

Increasing microplastics (MPs) pollution in estuaries profoundly impacts microbial ecosystems and biogeochemical processes. Nitrous oxide (N₂O), a powerful greenhouse gas, is an important intermediate product of microbial nitrogen cycling. However, how MPs regulate N₂O production and its pathways remain poorly understood. Here, impacts of traditional petroleum-based and emerging biodegradable MPs on microbial N₂O production and its pathways were studied through dual-isotope (¹⁵N-¹⁸O) labeling technique and molecular methods. Results indicated that both traditional petroleum-based and emerging biodegradable MPs promoted sedimentary N₂O production, whereas pathways varied. Biodegradable polylactic acid (PLA) MPs displayed greater promotion of N₂O production than petroleum-based MPs, polyvinyl chloride (PVC) and polyethylene (PE), of which PLA promoted through nitrifier nitrification (NN) and heterotrophic denitrification (HD), PE through nitrifier denitrification and HD, and PVC through NN. By combining the analysis of N₂O production rates with sediment chemical and microbiological properties, we demonstrated that the enrichment of nitrifying and denitrifying bacteria, as well as related functional genes directly and/or indirectly increased N₂O production primarily by interacting with carbon and nitrogen substrates. Different response of nitrogen cycling microbes to MPs led to the difference in N₂O increase pathways, of which nitrifying bacteria significantly enriched in all MPs treatments due to the niches provided by MPs. However, part of denitrifying bacteria significantly enriched in treatments containing PLA and PE MPs, which may serve as organic carbon substrates. This work highlights that the presence of MPs can promote sedimentary N₂O production, and the emerging biodegradable MPs represented by PLA may have a greater potential to enhance estuarine N₂O emissions and accelerate global climate change.

Recent advances in impacts of microplastics on nitrogen cycling in the environment: A review

Nitrogen cycling plays a decisive role in biogeochemistry, and largely depends on microbial driven nitrogen transformation. The environmental problems caused by microplastics are becoming more serious, and the analysis and control of its pollution in the environment have become a research hotspot in the field. The nitrogen transformation and nitrogen cycling in the environment are mainly driven by microorganisms in the environment, and the existence of microplastics can affect the microbial population, abundance and type, thus affecting the transformation of nitrogen. The effect of microplastics on microorganisms involved in nitrogen transformation is briefly described. This paper mainly reviews the research progress on the impacts of microplastics on nitrogen transformation and nitrogen cycling in water, soil, sediment and sewage sludge. Microplastic type, size and concentration can cause obvious difference in the impacts of microplastics on nitrogen transformation. Then, response and mechanism of microplastics to microorganism mediated nitrogen transformation and nitrogen cycling are introduced. Processes of nitrogen transformation are affected by interfering with microorganism diversity and structure, enzyme activities and related coding genes and oxygen flux. Additionally, additives released from microplastics can also affect the microbial activity. However, mechanisms of microplastics on environmental nitrogen transformation and nitrogen cycling are not fully understood due to the lack of relevant research. There are effective strategies to evaluate complex environmental systems, prolong action time, strengthen multi factor and multi-level research, and assist molecular biology and stable isotope technology. This review article can provide valuable insights into the impact of microplastics on microorganisms mediated nitrogen transformation processes and evaluate the impact on ecological and environmental health.

Size dependent impacts of a model microplastic on nitrification induced by interaction with nitrifying bacteria

Two sizes of polystyrene (PS) were compared to investigate their impact on nitrification. The smaller PS (50 nm) had a higher impact than the larger PS (500 nm). Lower NO₂^- and NO₃^- accumulation was observed in the 50 nm PS treatment. There was no significant difference in DIN concentration between the control and 500 nm PS treatments. PS treatment did not have a significant influence on the specific ammonia oxidation rate, but the specific nitrite utilization rate was the lowest in the 50 nm PS treatment. The changes in transcript levels of amoA gene did not correspond well with the observed changes in DIN concentrations, suggesting that the effects of 50 nm PS treatment might be unrelated to biological phenomena, for which an actual uptake of PS is needed. The fluorescent images revealed that the smaller PS can easily access bacterial cells, which corroborated the results of inhibition of nitrification by the smaller PS. Notably, most of the PS particles did not penetrate bacterial cells, suggesting that the observed effects of 50 nm PS on nitrification might be due to disruption of the membrane potential of the cells.

Improving methane production from algal sludge anaerobic fermentation by peroxydisulfate (PDS) pretreatment

This study investigated the potential of improving methane production from algal sludge anaerobic digestion by peroxydisulfate (PDS) pretreatment. The results show that with PDS dosage at 0.02 g PDS/g algal sludge TSS, PDS added system has highest accumulative methane production after 60 days fermentation. The accumulative methane production was 1.08, 1.15, 1.14, 1.13, 1.08, 0.76, and 0.15 times as compared with control, at 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 g PDS/g algal sludge TSS added, respectively. The SCOD in the system was keep increasing with the increment of PDS dosage after 120 min pretreatment. The algal sludge dewatering rate was increased with adding of PDS as pretreatment. The addition of PDS has inhibited the activities of microbes involved in digestion, while the short chain fatty acids production was improved after 3 days digestion. One-substrate model can be used to simulate the methane yield. The hydrolysis rate was decreased after dosing with PDS, while highest actual and predicted accumulative methane yield was occurred at 0.02 g PDS/g algal sludge TSS. Proteobacteria has higher percentage when the PDS was not higher than 0.1 g PDS/g algal sludge TSS, Acetothermia has higher percentage at 0.01 g PDS/g algal sludge TSS. The microcystin-LR (MC-LR) in algal sludge was largely removed after digestion, including the intracellular MC-LR. The higher PDS dosage could cause heavy metal release from algae cell to the digestate during fermentation. The addition of PDS to algal sludge can improve the accumulative methane production and mitigate microcystin concentration.

Fate and effects of microplastics in wastewater treatment processes

Microplastics (MPs) have garnered growing attention of researchers, as they are proved to be hazardous to the environment and humans. Wastewater treatment plants (WWTPs) are deemed as an important releasing source of MPs to the environment, and thus it is of significance to study the behavior of MPs in WWTPs. In this review, the fate of MPs in WWTPs and their effects on different wastewater treatment processes have been comprehensively discussed. Studies have shown that the secondary treatment is the most efficient process to remove MPs from wastewaters with a removal rate around 98%. The presence of MPs can increase reagent addition dosage, inhibit nitrogen conversion rate, and cause membrane fouling in wastewater treatment processes. Besides, the influences of MPs on activated sludge mainly exert on nitrification and denitrification processes, sludge digestion, and microbial communities. However, it is worth noting that different methods have been employed to determine the concentrations of MPs in WWTPs. As a result, the removal performance on MPs in WWTPs is difficult to be accurately assessed. Moreover, complicated interaction among MPs and other environmental pollutants may expand the impacts of MPs on wastewater treatment processes, which still remains insufficiently investigated. Therefore, this review has also proposed some knowledge gaps existing in present MP studies in WWTPs, and would provide reference to alleviate the adverse effects of MPs for future research.