Denitrification characterization of dissolved oxygen microprofiles in lake surface sediment through analyzing abundance, expression, community composition and enzymatic activities of denitrifier functional genes

Community Assembly Mechanisms of nirK- and nirS-type Denitrifying Bacteria in Sediments of Eutrophic Lake Taihu, China

Denitrifying bacteria, particularly nirK- and nirS-type, are functionally equivalent and could occupy different niches, but their community assembly mechanisms and responses to environmental heterogeneity are poorly understood in eutrophic lakes. In this study, we investigated the community assembly mechanisms of nirK- and nirS-type denitrifying bacteria and clarified their responses to sediments environmental factors in Lake Taihu, China. The quantitative real-time PCR and Illumina HiSeq-based sequencing revealed that the abundance and composition of two types of denitrifying bacterial communities varied among different sites in the sediments of Lake Taihu. The functions of these two types of denitrifying bacteria were assigned to mainly nitrogen cycling along with carbon, oxygen, and sulfur cycling, indicating their diverse ecosystems functions. Neutral community model showed that majority of nirK- and nirS-type denitrifying bacteria were neutrally distributed, while dispersal and selection were the dominant drivers in shaping community assembly of nirK-type bacteria. The community assembly of nirS-type was mainly driven by homogeneous selection. We found complex network interactions between nirK- and nirS-type denitrifying bacteria with other bacterial communities, indicating the importance of other bacterial coexistence for ecosystem functions by denitrifying bacteria in lake sediments. Keystone taxa of other bacteria showed the highest interactions with denitrifying bacteria; further, a strong significant correlation between keystone taxa with environmental factors and sediment enzyme content revealed by Mantel tests. Specially, total phosphorous was the key environmental factor determining the composition and diversity of nirK and nirS-type denitrifying bacteria in lake sediments, whereas NAR, AmoA, and NIR were the key reductase enzymes directly or indirectly affected to them. Our results provide significant insights into understanding the effects of changing nirK- and nirS-type denitrifying bacterial diversities and underlying community assembly mechanisms under changing environmental conditions in eutrophic lake ecosystems.

Optimizing nitrogen removal in advanced wastewater treatment using biological aerated filters

Reducing total nitrogen (TN) presents a significant challenge for numerous wastewater treatment facilities. In order to address this issue, the current study employed a biological aerated filter for the treatment of wastewater containing low nitrogen concentrations. Both lab-scale and pilot-scale biofilters were constructed to investigate the denitrification performance and maximum denitrification load. The findings indicated that the anaerobic denitrification process of established biofilm adhered to pseudo-first-order kinetics. The results of batch testing and continuous-flow experiments confirmed that the minimum hydraulic retention time (HRT) required for mature biofilm was determined to be 0.5 h. The optimal operating parameters were found to be as follows: influent NO3 --N concentration of 25 mg/L, HRT of 0.5 h, resulting in effluent TN levels below 1 mg/L. Under these conditions, the denitrifying load for the lab-scale I-BAF system was calculated to be 1.26 kg (TN)/(m3·d). Furthermore, it was observed that the maximum denitrifying load could reach 2.2 kg (TN)/(m3·d) when the influent NO3 --N concentration was increased to 50 mg/L while maintaining an HRT of 0.5 h. For the mature biofilter, the appropriate HRT ranged from 2 h to 0.5 h. Microbial diversity analysis revealed that the genus Enterobacter was dominant in all denitrification systems, followed by Comamonas and Rhodococcus. The operational parameters described in the paper could be recommended for a full-scale wastewater treatment facility.

Aerobic-anaerobic DHS reactor for enhancing denitrification in municipal wastewater treatment

Providing anaerobic environments can enhance the denitrification capacity of wastewater treatment systems. This study developed an aerobic-anaerobic downflow hanging sponge (DHS) reactor to increase denitrification. A siphon tube was integrated with a conventional DHS to create anaerobic conditions by controlling a water column inside the reactor. The siphon heights were set in the middle of the reactor, representing 2/5 of the total volume. Anaerobic conditions were maintained at the bottom (1/5), whereas aerobic conditions were maintained at the top (2/5). The middle part experienced alternating aerobic-anaerobic conditions within a siphon cycle. The hydraulic retention time (HRT) was 3 h, and the temperature was maintained at 30 °C. The study was divided into the low-loading and standard phases based on the influent soluble chemical oxygen demand (sCOD) concentration. During the low-loading phase, the aerobic-anaerobic DHS exhibited a higher sCOD removal rate (53.6 ± 26.6 %) than the control aerobic DHS (35.6 ± 31.9). However, the removal efficiencies for NH₄+-N and total nitrogen (TN) were lower, averaging 37.1 ± 24.7 % and 31.5 ± 21 %, respectively. In the standard phase, the aerobic-anaerobic DHS achieved an sCOD removal rate of 69.3 ± 14.8 % and an NH₄+-N removal rate of 17.2 ± 11.4 %. The control aerobic DHS showed higher sCOD and NH₄+-N removal at 84.7 ± 10.8 % and 54.1 ± 12.8 %, respectively, possibly due to the reduced aerobic zone volume in the aerobic-anaerobic DHS. The TN removal rates were also lower in the aerobic-anaerobic DHS (17.1 ± 11.4 %) than in the control DHS (25 ± 12.3 %). However, based on the ratio of nitrification to TN removal, the aerobic-anaerobic DHS showed a high denitrification rate of 85 %-100 %. The highest number of denitrifiers was in the middle section of the aerobic-anaerobic DHS, with Rhodocyclaceae the most prominent, followed by Comamonadaceae. Maintaining an anaerobic zone improved the denitrification capacity, indicating the potential of the aerobic-anaerobic DHS for efficient municipal wastewater treatment.

Antibiotic resistant genes profile in the surface water of subtropical drinking water river-reservoir system

To comprehensively understand antibiotic resistant genes (ARGs) profile in the subtropical drinking water river-reservoir system, this study selected Dongzhen river-reservoir system in Mulan Creek as object to investigate the spatial-temporal characteristics of ARGs diversity, bacterial host and resistance mechanism, and to analyze the key environmental factors driving ARGs profile variation. The results indicated that a total of 440 ARGs were detected in the target system, and the ARGs distribution pattern in the reservoir was attributed to autologous evolution or the comprehensive influence of feeding river system. The predominant bacterial host at different sites showed similar variations to dominated ARGs, and Proteobacteria, Actinobacteria and Bacteroidetes harbored most ARGs at phylum level, which showed the highest proportions of 74%, 37% and 35%, respectively. Antibiotic efflux was the primary resistance mechanism in all samples from wet season (45%-60%), yet the samples from dry season exhibited multiple resistance mechanisms, including inactivation (37%-52%), efflux (44%), and target alteration (43%). The total relative abundances of ARGs in the target system ranged from 0.89 × 10-2 to 1.71 × 10-2, and seasonal variation had a more significant influence on ARGs abundance than spatial variation (R = 0.68, P < 0.01). Environmental factors analysis indicated that the concentrations of nitrite nitrogen and total organic carbon were significant factors explaining ARGs number and various resistance mechanism proportions (P < 0.01), accounting for 48.7% and 61.1% of the variation, respectively; ammonia nitrogen concentration, total organic carbon concentration, temperature and pH were the significant influence factors on the relative abundance of ARGs (P < 0.05), with standardized regression weights of 0.700, 1.414, 1.447, and 1.727, respectively. In summary, in the surface water of the target system, ARGs diversity was primarily driven by ARGs horizontal transfer and antibiotics biosynthesis. Nutrients mainly promoted ARGs abundance by providing abundant energy, rather than increasing bacterial reproductive capacity.

The Effects of Shaking Duration on the Abundance and the Community of Aerobic Denitrifying Bacteria in Shrimp Pond Water and Sediment Samples

With the rapid development of intensive aquaculture, the considerable release of nitrogenous organic compounds has become a serious threat to aquatic organisms. Currently, isolating autochthonous aerobic denitrifying bacteria (ADB) from aquaculture environments is essential for the biological elimination of nitrogenous pollutants. In this study, the enrichment of ADB from shrimp pond water and sediment samples was conducted under different shaking durations. The absolute abundance of total bacteria, nosZ-type, and the napA-type ADB was measured using qPCR. High-throughput sequencing of 16S rRNA, nosZ, and napA genes was performed to reveal the community structure of bacteria and ADB, respectively. Our data revealed that absolute abundance and the community structure of the total bacteria, nosZ-type and napA-type ADB, were significantly altered under different shaking durations. Specifically, the order Pseudomonadales, possessing both nosZ and napA genes, was significantly enriched in water and sediment samples under both 12/12 and 24/0 shaking/static cycles. However, in water samples, compared to the 24/0 shaking/static cycles, the 12/12 shaking/static cycles could lead to a higher enrichment rate of aerobic denitrification bacteria indicated by the higher absolute abundance of bacteria and the higher accounting percentage of orders Oceanospirillales and Vibrionales. Moreover, although the order Pseudomonadales notably increased under the 12/12 of shake/static cycle compared to the 24/0 shaking/static cycle, considering the relative higher abundance of ADB in 24/0 shaking/static cycle, the enrichment of ADB in sediment may be efficient with the 24/0 shaking/static cycle.

Spatial and temporal analysis and quantification of pollution sources of the surface water quality in a coastal province in Vietnam

The surface water quality in coastal areas may be highly vulnerable to degradation due to various pollution sources such as seawater intrusion and anthropogenic activities. The current study sought to spatially and temporally analyze and quantify pollution sources of the surface water system in the coastal province of Tra Vinh, Vietnam. A total of 600 surface water samples were taken from 30 monitoring sites distributed over 4 spatial zones. Water samples were collected in four campaigns each year during the dry and rainy seasons from 2016 to 2020 and analyzed for 10 physiochemical and biological parameters. The analyzed data were calculated for the water quality index (WQI). Two-way analysis of variance, principal component analysis/factor analysis (PCA/FA), and multivariable regression analysis (MRA) were conducted over the whole dataset. The results showed that the WQI decreased from the inland zone to the coastal area, was greater in the river zone than in the canal zone, and was higher in the dry season than in the rainy season. The PCA/FA revealed that surface water quality was affected by at least 4 main pollution sources, including agricultural production, seawater intrusion, residential activities, and mixed sources. MRA revealed that these pollution sources explained 68.3%, 12.8%, 7.0%, and 2.7% of the total variance of the WQI, respectively. In summary, the surface water quality in the study area significantly changed spatially and temporally, depending on four pollution sources, which need to be managed properly for a better environment and sustainable development.

The nitrogen cycle and mitigation strategies for nitrogen loss during organic waste composting: A review

Composting is a promising technology to decompose organic waste into humus-like high-quality compost, which can be used as organic fertilizer. However, greenhouse gases (N₂O, CO₂, CH₄) and odorous emissions (H₂S, NH₃) are major concerns as secondary pollutants, which may pose adverse environmental and health effects. During the composting process, nitrogen cycle plays an important role to the compost quality. This review aimed to (1) summarizes the nitrogen cycle of the composting, (2) examine the operational parameters, microbial activities, functions of enzymes and genes affecting the nitrogen cycle, and (3) discuss mitigation strategies for nitrogen loss. Operational parameters such as moisture, oxygen content, temperature, C/N ratio and pH play an essential role in the nitrogen cycle, and adjusting them is the most straightforward method to reduce nitrogen loss. Also, nitrification and denitrification are the most crucial processes of the nitrogen cycle, which strongly affect microbial community dynamics. The ammonia-oxidizing bacteria or archaea (AOB/AOA) and the nitrite-oxidizing bacteria (NOB), and heterotrophic and autotrophic denitrifiers play a vital role in nitrification and denitrification with the involvement of ammonia monooxygenase (amoA) gene, nitrate reductase genes (narG), and nitrous oxide reductase (nosZ). Furthermore, adding additives such as struvite salts (MgNH₄PO₄·6H₂O), biochar, and zeolites (clinoptilolite), and microbial inoculation, namely Bacillus cereus (ammonium strain), Pseudomonas donghuensis (nitrite strain), and Bacillus licheniformis (nitrogen fixer) can help control nitrogen loss. This review summarized critical issues of the nitrogen cycle and nitrogen loss in order to help future composting research with regard to compost quality and air pollution/odor control.

Applying the Nernst Equation to Control ORP in Denitrification Process for Uranium-Containing Nuclear Effluent with High Loads of Nitrogen and COD

Several reviews of denitrification have shown it to be an efficient process for treating high nitrate-loaded effluents from nuclear industries. However, stressful conditions adversely affect biological kinetic parameters and performance. Additionally, actual nuclear effluents contain multiple pollutants and radioactive emissions that could render implementation difficult. The objective of this study was to treat and recycle water from nuclear industries by using a mixture of blended real nuclear wastewater (BRNW). The process was carried out under physicochemical parameters control in a biological model to established a technical setup and to model the denitrification process in a real nuclear wastewater effluent. Denitrification processes were carried out in the wastewater sample under controlled ORP conditions by the Hill model to establish the kinetic model. The results show a complete elimination of nitrate by the bacteria. Indicators of biochemical reactions were used to obtain a model based on Monod and controlled ORP. The good fit of the proposed model was verified under empirical and simulated conditions. To establish optimal performance, it was necessary to add 3% v/v of methanol, as a carbon source, to remove the nitrate in BRNW. Isolation techniques confirmed that Pseudomonas spp. was the dominant bacteria. Gene expression demonstrated the lack of inhibition of the NosZ gene responsible for the reduction in nitric oxide, a “greenhouse gas”. Finally, COD and uranium were removed from the liquid by precipitation. At the end of the process, the treated effluent could potentially be reused in industrial processes, recycling most of the wastewater effluents.

Diverse key nitrogen cycling genes nifH, nirS and nosZ associated with Pichavaram mangrove rhizospheres as revealed by culture-dependent and culture-independent analyses

Mangroves are highly productive unique ecosystems harboring diverse unexplored microbial communities that play crucial roles in nutrient cycling as well as in maintaining ecosystem services. The mangrove-associated microbial communities transform the dead vegetation into nutrient sources of nitrogen, phosphorus, potash, etc. To understand the genetic and functional diversity of the bacterial communities involved in nitrogen cycling of this ecosystem, this study explored the diversity and distribution of both the nitrogen fixers and denitrifiers associated with the rhizospheres of Avicennia marina, Rhizophora mucronata, Suaeda maritima, and Salicornia brachiata of the Pichavaram mangroves. A combination of both culturable and unculturable (PCR-DGGE) approaches was adopted to explore the bacterial communities involved in nitrogen fixation by targeting the nifH genes, and the denitrifiers were explored by targeting the nirS and nosZ genes. Across the rhizospheres, Gammaproteobacteria was found to be predominant representing both nitrogen fixers and denitrifiers as revealed by culturable and unculturable analyses. Sequence analysis of soil nifH, nirS and nosZ genes clustered to unculturable, with few groups clustering with culturable groups, viz., Pseudomonas sp. and Halomonas sp. A total of 16 different culturable genera were isolated and characterized in this study. Other phyla like Firmicutes and Actinobacteria were also observed. The PCR-DGGE analysis also revealed the presence of 29 novel nifH sequences that were not reported earlier. Thus, the mangrove ecosystems serve as potential source for identifying unexplored novel microbial communities that contribute to nutrient cycling.

Mix-cultured aerobic denitrifying bacterial communities reduce nitrate: Novel insights in micro-polluted water treatment at lower temperature

Three mix-cultured aerobic denitrifiers were screened from a source water reservoir and named HE1, HE3 and SU4. Approximately 72.9%, 68.6% and 66.2% of nitrate were effectively removed from basal medium, respectively, after 120 h of cultivation at 8 °C. The nitrogen balance analysis revealed about one-fifth of the initial nitrogen was converted into gaseous denitrification products. According to the results of Biolog, the three microfloras had high metabolic capacity to carbon sources. The dominant genera were Pseudomonas and Paracoccus in these bacterial communities based on nirS gene sequencing. Response surface methodology elucidated that the denitrification rates of identified bacteria reached the maximum under the following optimal parameters: C/N ratio of 7.51-8.34, pH of 8.03-8.09, temperature of 18.03-20.19 °C, and shaking speed of 67.04-120 rpm. All results suggested that screened aerobic denitrifiers could potentially be applied to improve the source water quality at low temperature.

The interactive effects of the seawater intrusion-affected zones and types of waterways on the surface water quality from the coastal Tien Giang Province, Vietnam

Although inland surface water bodies have been studied intensively, few studies have looked at the interactive effects of seawater intrusion and waterway types on the water quality. The current study aimed to (1) assess the inland water quality as affected by waterway types and seawater intrusion-affected zones, (2) examine the longitudinal dynamics of the water quality, and (3) quantify the contributive percentage of pollution sources in the coastal Tien Giang Province, Vietnam. A total of 680 surface-water samples were taken from 34 sites distributed over the Tien River and its tributary canals from 2015 to 2019. The water samples were analyzed for 16 physical, chemical, and biological parameters, which were used for water quality index (WQI) estimation and subjected to two-way ANOVA and principal component analysis/factor analysis (PCA/FA). The WQI in both waterway types tended to get better from the downstream to the upstream zone with an improving rate of WQI faster in the River (from 79 to 88) than in the canals (from 82 to 85). The PCA/FA showed that water from the two waterway types could be polluted by six main pollution sources, one of which was derived from the seawater intrusion, one from aquaculture, and the others from agricultural, residential, and industrial activities. In brief, the inland surface water quality of a coastal area was interactively influenced by spatial distance and waterway types, transferring various pollutants in and out of the inland area.

Metagenomic Profile of Microbial Communities in a Drinking Water Storage Tank Sediment after Sequential Exposure to Monochloramine, Free Chlorine, and Monochloramine

Sediment accumulation in drinking water storage facilities may lead to water quality degradation, including biological growth and disinfectant decay. The current research evaluated the microbiome present in a sediment after sequential exposure to monochloramine, free chlorine, and monochloramine. Chemical profiles within the sediment based on microelectrodes showed evidence of nitrification, and monochloramine slowly penetrated the sediment but was not measurable at lower depths. A metagenomic approach was used to characterize the microbial communities and functional potential of top (0-1 cm) and bottom (1-2 cm) layers in sediment cores. Differential abundance analysis revealed both an enrichment and depletion associated with depth of microbial populations. We assembled 30 metagenome-assembled genomes (MAGs) representing bacterial and archaeal microorganisms. Most metabolic functions were represented in both layers, suggesting the capability of the microbiomes to respond to environmental fluctuations. However, niche-specific abundance differences were identified in biotransformation processes (e.g., nitrogen). Metagenome-level analyses indicated that nitrification and denitrification can potentially occur simultaneously in the sediments, but the exact location of their occurrence within the sediment will depend on the localized physicochemical conditions. Even though monochloramine was maintained in the bulk water there was limited penetration into the sediment, and the microbial community remained functionally diverse and active.