Retrofitting a full-scale multistage landfill leachate treatment plant by introducing coagulation/flocculation/sedimentation and ultrafiltration process steps

Considering that landfilling still remains among the most commonly used methods for the confrontation of solid wastes, effective methods should be applied to treat the leachate generated, due to its recalcitrant nature. In this work, a full-scale system consisting of two SBRs operating in parallel (350 m³ each) and two activated carbon (AC) columns operating in series (3 m³ each) was retrofitted by introducing a coagulation/flocculation/sedimentation (C/F/S) unit of 7.8 m³ and an ultrafiltration (UF) membrane of 100 m² to effectively treat landfill leachate. The raw leachate was characterized by high COD and NH₄⁺-N concentration, i.e., 3095 ± 706 mg/L and 1054 ± 141 mg/L respectively, a BOD/COD ratio of 0.22, and high concentrations of certain heavy metals. Leachate processing in this retrofitted multistage treatment system resulted in total COD removal efficiency of 89.84%, with biological treatment, C/F, UF, and AC contributing 46.31%, 4.68%, 15.98%, and 22.87% to the overall organic content removal. The retrofitted scheme achieved an overall NH₄⁺-N and TKN removal of 92.03% and 91.75% respectively, attributed mostly to the activity of an effective nitrifying community. Color number (CN) was reduced by 26.96%, 10.29%, 15.94%, and 5.39% after the activated sludge, the C/F, the UF, and the AC adsorption process respectively, corresponding to a 58.91% overall decrease. Regarding heavy metal removal, all elements examined, apart from Ni, i.e., effluent As, Cd, Co, Cr, Cu, Hg, Mg, Mn, and Pb, were below the legislative limits set by the national authorities for restricted or unrestricted irrigation. Lastly, total operating expenses (OPEX) were estimated as equal to 72,687 €/year or 6.64 €/m³.

Fertilizing-induced changes in the nitrifying microbiota associated with soil nitrification and crop yield

Ammonia oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and comammox Nitrospira (CMX) play pivotal roles in global nitrogen-cycling network. Despite its importance, the driving forces for niche specialization of these nitrifiers, as well as their relative contributions to nitrification and crop yield have not been fully understood. Here, we investigated the niche specialization and environmental prevalence of nitrifying communities, and their importance for the nitrification rate and crop yield across a gradient of nitrogen inputs in a two-decade old field experiment. The results of ¹⁵N-tracer and quantitative PCR revealed that AOB and NOB jointly determined the gross nitrification rates across mineral fertilizer treatments, whereas AOA and AOB contributed more than other nitrifiers to nitrification under with organic fertilizer amendments. Linear regression model revealed that crop yield could be linked with AOB and NOB under inorganic farming but closely associated with CMX under organic management. Amplicon sequencing of these functional genes further demonstrated that mineral and organic fertilizers have distinct influences on the β-diversity and niche breadth of these nitrifying communities, indicating that fertilization triggered niche specialization of nitrifying guilds in agricultural soils. Notably, organic fertilization enhanced the network complexity of these nitrifiers by harboring keystone taxa. Random forest analysis provide robustly evidence for the hypothesis that abundance of functional genes contributed more than a- and β-diversity of these nitrifiers for driving nitrification rates and crop yields. Collectively, these findings provide the empirical evidence for the environmental adaptation and niche specialization of nitrifying communities, and their contributions in nitrification and crop yield when confronted with long-term nitrogen inputs.

Environmental variables better explain changes in potential nitrification and denitrification activities than microbial properties in fertilized forest soils

Because of increases in atmospheric nitrogen (N) deposition worldwide, nutrient imbalances and phosphorus (P) limitations in soil are aggravated, with the result that P fertilizer applications to terrestrial ecosystems worldwide may increase. Nitrification and denitrification in soil are major sources of nitrous oxide emissions, especially in soils treated with fertilizers. However, few researchers have studied how forest soils respond to nutrient additions, so we are not sure how the potential nitrification and denitrification activities (PNA and PDA, respectively) and microbial communities involved in these processes might respond when N and P are added to temperate and subtropical forest soils. We investigated how the PNA, PDA, the abundances and community compositions of nitrifiers and denitrifiers, and environmental properties, including soil pH, soil total and dissolved organic carbon, total and available N and phosphorus P, changed when N and/or P were added to subtropical and temperate forest soils. We quantified the abundance, and analyzed the composition, of functional marker genes of nitrifiers (ammonia-oxidizing bacteria and archaea amoA) and denitrifiers (nirK and nirS) using quantitative PCR and sequencing, respectively. We found that the PNA and PDA in the subtropical soil increased when P was added and PNA in the temperate forest soil increased when either N or P was added. The PNA and PDA were positively correlated with the abundance of ammonia-oxidizing bacteria and nirK-denitrifiers, respectively, in the subtropical forest soil but were not correlated with changes in corresponding community compositions in either of the forest soils. The soil total N to total P ratio explained most of the variabilities in the PNA and PDA in the subtropical forest soils, and the soil exchangeable ammonium concentrations and pH were the main controls on the PNA and PDA, respectively, in the temperate forest soils. Our results indicate that soil environmental conditions have more influence on variations in the PNA and PDA in forest soils fertilized with N and P than the corresponding microbial properties.

Understanding the relationships between grazing intensity and the distribution of nitrifying communities in grassland soils

Nitrifying microbes are of critical importance in regulating efficient nitrogen (N) cycling, which plays a crucial role in plant productivity and maintaining soil sustainability. Long-term different intensities of grazing can strongly influence the microbial communities, while our understanding of the complex nitrifying community in the grazed grassland soil environment is still limited. To investigate whether and how long-term grazing with different intensities influence soil nitrifying communities, high-throughput sequencing and quantitative PCR analyses were performed on soil samples from permanent grassland soils under four grazing intensities: 0 (G0), 1.5 (G1), 6 (G2) and 9 (G3) sheepha^-1. Results showed that the G3 treatment significantly reduced the soil nutrient content and increased the soil bulk density, changes that are not sustainable in the long run. The G1 treatment, on the other hand, significantly increased the soil nutrient content and would improve soil fertility. Some functional microbes were specifically enriched after long term grazing, like Nitrospirae (phylum) to Nitrospira (class) in the G2 samples and Chromatiales (order) to Nitrosococcus (genus) in the G3 soils. The numerically dominant Nitrosococcus watsonii lineage of ammonia oxidizing bacteria (AOB) was observed in this grassland soil. The redundancy analysis (RDA) together with the structural equation modeling (SEM) analysis showed that grazing intensity was important in mediating the distribution of soil microorganisms and affected nitrifying communities by impacting soil physicochemical characteristics (e.g., bulk density, NH₄⁺-N). These results showed the shifts of nitrifying communities across different grazing intensities, and could aid in the determination of an optimal grazing intensity for these grazed grassland soils.

Nitrite oxidizing bacteria (NOB) dominating in nitrifying community in full-scale biological nutrient removal wastewater treatment plants

Nitrification activities and microbial populations of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were investigated in 10 full-scale biological nutrient removal wastewater treatment plants in Xi’an, China. Aerobic batch tests were used to determine the nitrifying activities while fluorescence in situ hybridization was used to quantify the fractions of AOB and NOB in the activated sludge. The results showed that nitrifying bacteria accounted for 1–10% of the total population. Nitrosomonas and Nitrospira were the dominant bacteria for AOB and NOB respectively. Moreover, the average percentage of AOB was 1.27% and that of NOB was 4.02%. The numerical ratios of NOB/AOB varied between 1.72 and 5.87. The average ammonium uptake rate and nitrite uptake rate were 3.25 ± 0.52 mg (NH₄⁺–N)/g(VSS) h and 4.49 ± 0.49 mg (NO₂⁻–N)/g(VSS) h, respectively. Correspondingly, the activity of NOB was 1.08–2.00 times higher than that of AOB. Thus, NOB was the dominating bacteria in nitrifying communities. The year-round data of Dianzicun (W6) also expressed a similar trend. Since NOB had higher activities than that of AOB, a large nitrite oxidation pool could be formed, which guaranteed that no nitrite would be accumulated. Therefore, stable nitrification could be achieved. A conceptual model was proposed to describe the population variation of AOB and NOB in a nitrifying community.