Solid-phase denitrification for water remediation: processes, limitations, and new aspects

Metagenomic insights into microbial metabolic mechanisms of a combined solid-phase denitrification and anammox process for nitrogen removal in mainstream wastewater treatment

To overcome the significant challenges associated with nitrite supply and nitrate residues in mainstream anaerobic ammonium oxidation (anammox)-based processes, this study developed a combined solid-phase denitrification (SPD) and anammox process for low-strength nitrogen removal without the addition of nitrite. The SPD step was performed in a packed-bed reactor containing poly-3-hydroxybutyrate-co-3-hyroxyvelate (PHBV) prior to employing the anammox granular sludge reactor in the continuous-flow mode. The removal efficiency of total inorganic nitrogen reached 95.7 ± 1.2% under a nitrogen loading rate of 0.18 ± 0.01 kg N·m3·d-1, and it required 1.02 mol of nitrate to remove 1 mol of ammonium nitrogen. The PHBV particles not only served as biofilm carriers for the symbiosis of hydrolytic bacteria (HB) and denitrifying bacteria (DB), but also carbon sources that facilitated the coupling of partial denitrification and anammox in the granules. Metagenomic sequencing analysis indicated that Burkholderiales was the most abundant HB genus in SPD. The metabolic correlations between DB (Betaproteobacteria, Rhodocyclaceae, and Anaerolineae) and anammox bacteria (Candidatus Brocadiac and Kuenenia) in the granules were confirmed through microbial co-occurrence networks analysis and functional gene annotations. Additionally, the genes encoding nitrate reductase (Nap) and nitrite reductase (Nir) in DB primarily facilitated nitrate reduction, thereby supplying nitric oxide to anammox bacteria for subsequent nitrogen removal with hydrazine synthase (Hzs) and hydrazine dehydrogenase (Hdh). The findings provide insights into microbial metabolism within combined SPD and anammox processes, thus advancing the development of mainstream anammox-based processes in engineering applications.

Unveiling novel pathways and key contributors in the nitrogen cycle: Validation of enrichment and taxonomic characterization of oxygenic denitrifying microorganisms in environmental samples

Microorganisms play a crucial role in both the nitrogen cycle and greenhouse gas emissions. A recent discovery has unveiled a new denitrification pathway called oxygenic denitrification, entailing the enzymatic reduction of nitrite to nitric oxide (NO) by a putative nitric oxide dismutase (nod) enzyme. In this study, the presence of the nod gene was detected and subsequently enriched in anaerobic-activated sludge, farmland soil, and paddy soil samples. After 150 days, the enriched samples exhibited significant denitrification, and concomitant oxygen production. The removal efficiency of nitrite ranged from 64.6 % to 79.0 %, while the oxygen production rate was between 15.4 μL/min and 18.6 μL/min when exposed to a sole nitrogen source of 80 mg/L sodium nitrite. Additionally, batch experiments and kinetic analyses revealed the intricate pathways and underlying mechanisms governing the oxygenic denitrification reaction by using CARBOXY-PTIO, 18O-labelled water, and acetylene to unravel the intricacies of the reaction. The quantitative polymerase chain reaction (qPCR) results indicated a significant surge in the abundance of nod genes, escalating from 7.59 to 10.12-fold. Moreover, analysis of 16S ribosomal DNA (rDNA) amplicons revealed Proteobacteria as the dominant phylum and Thauera as the main genus, with the presumed affiliation. In this study, a new nitrogen conversion pathway, oxygenic denitrification, was discovered in environmental samples. This process provides the possibility for the control of nitrous oxide in the treatment of nitrogenous wastewater.

A collaborative effect of solid-phase denitrification and algae on secondary effluent purification

This study explored the collaborative effect on nutrients removal performance and microbial community in solid-phase denitrification based bacteria-algae symbiosis system. Three biodegradable carriers (apple wood, poplar wood and corncob) and two algae species (Chlorella vulgaris and Chlorella pyrenoidosa) were selected in these bacteria-algae symbiosis systems. Results demonstrated that corncob as the carrier exhibited the highest average removal efficiencies of total nitrogen (83.7%-85.1%) and phosphorus removal (38.1%-49.1%) in comparison with apple wood (65.8%-71.5%, 25.5%-32.7%) and poplar wood (42.5%-49.1%, 14.2%-20.7%), which was mainly attributed to the highest organics availability of corncob. The addition of Chlorella acquired approximately 3%-5% of promotion rates for nitrated removal among three biodegradable carriers, but only corncob reactor acquired significant promotions by 3%-11% for phosphorous removal. Metagenomics sequencing analysis further indicated that Proteobacteria was the largest phylum in all wood reactors (77.1%-93.3%) and corncob reactor without Chlorella (85.8%), while Chlorobi became the most dominant phylum instead of Proteobacteria (20.5%-41.3%) in the corncob with addition of Chlorella vulgaris (54.5%) and Chlorella pyrenoidosa (76.3%). Thus, the higher organics availability stimulated the growth of algae, and promoted the performance of bacteria-algae symbiosis system based biodegradable carriers.

Effects of released organic components of solid carbon sources on denitrification performance and the related mechanism

Here, a hybrid scaffold of polyvinyl alcohol/sodium alginate (PVA/SA) was used to prepare solid carbon sources (SCSs) for treating low carbon/nitrogen wastewater. The four SCSs were divided into two groups, biodegradable polymers group (including polyvinyl alcohol-sodium alginate (PS) and PS-PHBV (PP), and blended SCSs (PS-PHBV-wood chips (PPW) and PS-PHBV-wheat straw (PPS)). After the leaching experiments, no changes occurred in elemental composition and functional groups of the SCSs, and the released dissolved organic matter showed a lower degree of humification and higher content of labile molecules in the blended SCSs groups using EEM and FT-ICR-MS. The denitrification performance of the blended SCSs was higher, with nitrate removal efficiency over 84%. High-throughput sequencing confirmed PPW had the highest alpha-diversity, and the microbial community structure significantly varied among SCSs. Results of functional enzymes and genes show the released carbon components directly affect the NADH level and electron transfer efficiency, ultimately influencing denitrification performance.

Start-up of solid-phase denitrification process for treatment of nitrate-rich water in recirculating mariculture system: Carbon source selection and nitrate removal mechanism

Efficient nitrate removal from recirculating mariculture system (RMS) water is of significance since high concentration of nitrate would cause chronic health effects on aquatic organisms and eutrophication. Solid-phase denitrification (SPD) is a safer and more sustainable approach than conventional heterotrophic denitrification by dosing liquid carbon sources. Thus, its application for treating nitrate-rich RMS water was investigated in this study. Poly 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) was identified with the best nitrate removal among four kinds of carbon sources. PHBV-filled reactors started with mariculture, municipal and mixing sludges (at the ratio of 1:1) and fed with 200 mg L-1 nitrate-rich RMS water all achieved over 81% nitrate removals with a HRT of 4 days. The dissolved organic carbon concentrations of the reactors were in the range of 3-9 mg L-1. Arcobacter, Halomonas, and Psedomonas were dominant genera responsible for nitrate removal in different reactors. Metagenomic analyses indicate that both denitrification and assimilatory nitrate reduction (ANR) are the main contributors to nitrate removals. Metagenomic results illustrated nirB/D cooperated with nasA may perform ANR pathway, which transformed nitrate to ammonia for biosynthesis. These results indicate that SPD could be a safer alternative for treating nitrate-rich RMS water, and provide new insights into nitrogen metabolism pathways in SPD process.

Elemental sulfur-driven autotrophic denitrification process for effective removal of nitrate in mariculture wastewater: Performance, kinetics and microbial community

To date, there is a lack of systematic investigation on the elemental sulfur-driven autotrophic denitrification (SDAD) process for removing nitrate (NO3--N) from mariculture wastewater deficient in organic carbon sources. Therefore, a packed-bed reactor was established and continuously operated for 230 days to investigate the operation performance, kinetic characteristics and microbial community of SDAD biofilm process. Results indicate that the NO3--N removal efficiencies and rates varied with the operational conditions including HRT (1-4 h), influent concentrations of NO3--N (25-100 mg L-1) and DO (0.2-7.0 mg L-1), and temperature (10oC-30 °C), in the ranges of 51.4%-98.6% and 0.054-0.546 g L-1 d-1, respectively. Limestone could partially neutralize the produced acidity. Small portions of NO3--N were converted to nitrite (<4.5%) and ammonia (<2.8%) in the reactor. Operational conditions also influenced the production of acidity, nitrite and ammonia as well as sulfate. Shortening HRT and increasing influent NO3--N concentration turned the optimal fitting model depicting the NO3--N removal along the reactor from half-order to zero-order. Furthermore, the NO3--N removal was accelerated by a higher temperature and influent NO3--N concentration and a lower HRT and influent DO concentration. Microbial richness, evenness and diversity gradually decreased during the autotrophic denitrifier enrichment cultivation and the reactor start-up and operation. Sulfurimonas constituted the predominate genus and the primary functional bacteria in the reactor. This study highlights the SDAD as a promising way to control the coastal eutrophication associated with mariculture wastewater discharge.

Denitrifying Woodchip Bioreactors: A Microbial Solution for Nitrate in Agricultural Wastewater-A Review

Nitrate (NO3-) is highly water-soluble and considered to be the main nitrogen pollutants leached from agricultural soils. Its presence in aquatic ecosystems is reported to cause various environmental and public health problems. Bioreactors containing microbes capable of transforming NO3- have been proposed as a means to remediate contaminated waters. Woodchip bioreactors (WBRs) are continuous flow, reactor systems located below or above ground. Below ground systems are comprised of a trench filled with woodchips, or other support matrices. The nitrate present in agricultural drainage wastewater passing through the bioreactor is converted to harmless dinitrogen gas (N2) via the action of several bacteria species. The WBR has been suggested as one of the most cost-effective NO3--removing strategy among several edge-of-field practices, and has been shown to successfully remove NO3- in several field studies. NO3- removal in the WBR primarily occurs via the activity of denitrifying microorganisms via enzymatic reactions sequentially reducing NO3- to N2. While previous woodchip bioreactor studies have focused extensively on its engineering and hydrological aspects, relatively fewer studies have dealt with the microorganisms playing key roles in the technology. This review discusses NO3- pollution cases originating from intensive farming practices and N-cycling microbial metabolisms which is one biological solution to remove NO3- from agricultural wastewater. Moreover, here we review the current knowledge on the physicochemical and operational factors affecting microbial metabolisms resulting in removal of NO3- in WBR, and perspectives to enhance WBR performance in the future.

Nitrate Bioreduction under Cr(VI) Stress: Crossroads of Denitrification and Dissimilatory Nitrate Reduction to Ammonium

This study explored the response of NO₃^--N bioreduction to Cr(VI) stress, including reduction efficiency and the pathways involved (denitrification and dissimilatory nitrate reduction to ammonium (DNRA)). Different response patterns of NO₃^--N conversion were proposed under Cr(VI) suppress (0, 0.5, 5, 15, 30, 50, and 80 mg/L) by evaluating Cr(VI) dose dependence, toxicity accumulation, bioelectron behavior, and microbial community structure. Cr(VI) concentrations of >30 mg/L rapidly inhibited NO₃^--N removal and immediately induced DNRA. However, denitrification completely dominated the NO₃^--N reduction pathway at Cr(VI) concentrations of <15 mg/L. Therefore, 30 and 80 mg/L Cr(VI) (R₄ and R₆) were selected to explore the selection of the different NO₃^--N removal pathways. The pathway of NO₃^--N reduction at 30 mg/L Cr(VI) exhibited continuous adaptation, wherein the coexistence of denitrification (51.7%) and DNRA (13.6%) was achieved by regulating the distribution of denitrifiers (37.6%) and DNRA bacteria (32.8%). Comparatively, DNRA gradually replaced denitrification at 80 mg/L Cr(VI). The intracellular Cr(III) accumulation in R₆ was 6.60-fold greater than in R₄, causing more severe oxidant injury and cell death. The activated NO₃^--N reduction pathway depended on the value of nitrite reductase activity/nitrate reductase activity, with 0.84-1.08 associated with DNRA activation and 1.48-1.57 with DNRA predominance. Although Cr(VI) increased microbial community richness and improved community structure stability, the inhibition or death of nitrogen-reducing microorganisms caused by Cr(VI) decreased NO₃^--N reduction efficiency.

Insights into solid phase denitrification in wastewater tertiary treatment: the role of solid carbon source in carbon biodegradation and heterotrophic denitrification

The practical application of solid phase denitrification (SPD) was hindered by either poor water quality from natural plant-like materials or high cost of pure synthetic biodegradable polymers. In this study, by combining polycaprolactone (PCL) with new natural materials (peanut shell, sugarcane bagasse), two novel economical solid carbon sources (SCSs) named as PCL/PS and PCL/SB were developed. Pure PCL and PCL/TPS (PCL with thermal plastic starch) were supplied as controls. During the 162-day operation, especially in the shortest HRT (2 h), higher NO₃^--N removal was achieved by PCL/PS (87.60%±0.06%) and PCL/SB (87.93%±0.05%) compared to PCL (83.28%±0.07%) and PCL/TPS (81.83%±0.05%). The predicted abundance of functional enzymes revealed the potential metabolism pathways of major components of SCSs. The natural components entered the glycolytic cycle by enzymatical generation of intermediates, while biopolymers being converted into small molecule products under specific enzyme activities (i.e., carboxylesterase, aldehyde dehydrogenase), together providing electrons and energy for denitrification.

New insights into hexavalent chromium exposure in electron donor limited denitrification: bio-electron behavior

The bio-electron behavior (electron production, transmission, and consumption) response to a typical heavy metal, hexavalent chromium, was unraveled in the electron donor limited system (EDLS) and electron donor sufficient system (EDSS). Nicotinamide adenine dinucleotide and adenosine triphosphate production were reduced by 44% and 47%, respectively, due to glucose metabolism inhibition, leading to NO₃^--N declining to 31% in EDLS. The decreased electron carrier contents and denitrifying enzymes activity inhibited electron transmission and consumption in both EDLS and EDSS. Additionally, electron transfer and antioxidant stress abilities were reduced, further hindering the survival of denitrifiers in EDLS. The lack of dominant genera (Comamonas, Thermomonas, and Microbacterium) in EDLS was the primary reason for poor biofilm formation and chromium adaptability. The decreased expression of enzymes related to glucose metabolism caused the imbalance of electron supply, transport, and consumption in EDLS, adversely impacting nitrogen metabolism and inhibiting denitrification performance.

Denitrification performance and bacterial ecological network of a reactor using biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as an electron donor for nitrate removal from aquaculture wastewater

Nitrate accumulation is a common phenomenon in aquaculture that can lead to eutrophication of surrounding water bodies. This study used poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as a carbon source and substrate and performed a microbial co-occurrence network ecological analysis to elucidate the denitrification processes in two packed-bed reactors with different salinities. The denitrification rate reached maximum values of 0.438 and 0.446 kg m^-3 d^-1 in reactor I (salinity 0 ‰) and reactor II (salinity 20 ‰), respectively. Although ammonia was formed in both systems based on dissimilation nitrate reduction to ammonia (DNRA), the concentration was very low (2.47 ± 1.99 and 2.84 ± 1.79 mg L^-1); moreover, the nitrite content was average (1.01 ± 0.87 and 0.96 ± 0.86 mg L^-1). These results suggested that denitrification dominated in both reactors. PHBV generally presented a stable release of DOC, although a sharp increase was observed in the start-up period of reactor II. 16S rRNA results showed that reactor I had richer microbial diversity than reactor II. Among the top ten taxa, Betaproteobacteria was the dominant class in reactor I while Gammaproteobacteria was the dominant class in reactor II. In the stable period, Thauera and Denitromonas was the most abundant genera in reactor I and reactor II, respectively. In addition, the bacterial co-occurrence network showed that reactor I had a more complex node and edge network and faster start-up time compared to reactor II; however, reactor II had a more stable nitrogen removal capacity. Higher expression of NorB and NosZ genes in reactor II indicated higher efficient denitrification in seawater system. The SEM and FTIR showed bacterial development and materials surface erosion. These findings verified the denitrification performance and niche differences between freshwater and seawater environments.

The role of sulfur cycle and enzyme activity in dissimilatory nitrate reduction processes in heterotrophic sediments

The dissimilatory nitrate (NO₃^-) reduction processes (DNRPs) play an important role in regulating the nitrogen (N) balance of aquatic ecosystem. Organic carbon (OC) and sulfur are important factors that influence the DNRPs. In this study, we investigated the effects of sulfur cycle and enzyme activity on DNRPs in the natural and human-modified heterotrophic sediments. Quarterly monitoring of anaerobic ammonium oxidation, denitrification (DNF), and dissimilatory NO₃^- reduction to ammonium (DNRA) in sediments was conducted using ¹⁵N isotope tracing method. qPCR and high-throughput sequencing were applied to characterize the DNF and DNRA microbial abundances and communities. Results showed that instead of the OC, the glucosidase activity (GLU) was the key driver of the DNRPs. Furthermore, instead of the ratio of OC to NO₃^-, the GLU and the ratio of OC to sulfide (C/S) correctly indicated the partitioning of DNRPs in this study. We deduced that the sulfur reduction processes competed with the DNRPs for the available OC. In addition, the inhibitory effect of sulfide (final product of the sulfur reduction processes) on the DNRPs bacterial community were observed, which suggested a general restrictive role of the sulfur cycle in the regulation and partitioning of the DNRPs in heterotrophic sediments.

Manganese oxidation and prokaryotic community analysis in a polycaprolactone-packed aerated biofilm reactor operated under seawater conditions

Biogenic manganese oxides (BioMnOx) have been receiving increasing attention for the removal of environmental contaminants and recovery of minor metals from water environments. However, the enrichment of heterotrophic Mn(II)-oxidizing microorganisms for BioMnOx production in the presence of fast-growing coexisting heterotrophs is challenging. In our previous work, we revealed that polycaprolactone (PCL), a biodegradable aliphatic polyester, can serve as an effective solid organic substrate to enrich Mn-oxidizing microbial communities under seawater conditions. However, marine BioMnOx-producing bioreactor systems utilizing PCL have not yet been established. Therefore, a laboratory-scale continuous-flow PCL-packed aerated biofilm (PAB) reactor was operated for 238 days to evaluate its feasibility for BioMnOx production under seawater conditions. After the start-up of the reactor, the average dissolved Mn removal rates of 0.4-2.3 mg/L/day, likely caused by Mn(II) oxidation, were confirmed under different influent dissolved Mn concentrations (2.5-14.0 mg/L on average) and theoretical hydraulic retention time (0.19-0.77 day) conditions. The 16S rRNA gene amplicon sequencing analysis suggested the presence of putative Mn(II)-oxidizing and PCL-degrading bacterial lineages in the reactor. Two highly dominant operational units (OTUs) in the packed PCL-associated biofilm were assigned to the genera Marinobacter and Pseudohoeflea, whereas the genus Lewinella and unclassified Alphaproteobacteria OTUs were highly dominant in the MnOx-containing black/dark brown precipitate-associated biofilm formed in the reactor. Excitation-emission matrix fluorescence spectroscopy analysis revealed the production of tyrosine- and tryptophane-like components, which may serve as soluble heterotrophic organic substrates in the reactor. Our findings indicate that PAB reactors are potentially applicable to BioMnOx production under seawater conditions.

Co-occurrence of autotrophic and heterotrophic denitrification in electrolysis assisted constructed wetland packing with coconut fiber as solid carbon source

Aiming at the problems of lack of carbon sources for nitrogen removal and low phosphorus removal efficiency of constructed wetlands (CWs) in treating wastewater treatment plant (WWTP) effluent, an electrolysis assisted constructed wetland (E-CW) with coconut fiber as substrate and solid carbon sources was constructed. The synthetic secondary effluent was used as the influent of the E-CW with a wastewater treatment capacity of 140 L d^-1. The total nitrogen (TN) and the total phosphorus (TP) removal efficiency of the E-CW with coconut fiber treating WWTP effluent were 69.4% and 93.3%, respectively, which were 54.3% and 88.2% higher than those of CW with coconut fiber and no electrolysis. The removal efficiency of TN was 39.9% higher than that of E-CW with gravel. The current intensity had significant effect on nitrogen removal efficiency and the release of carbon sources from coconut fiber. When current intensity increased from 0.25 A to 1.00 A, the TN removal efficiency and nitrate removal rate increased by 21.1% and 0.21 mg L^-1 h^-1, respectively, and the volatile fatty acids (VFAs) released from coconut fiber increased by 57.7 mg L^-1. The 16S rRNA high-throughput sequencing results indicated that the main functional nitrogen-removing microbes were Hydrogenophaga, Thauera, Rhodanobacteraceae_norank, Xanthobacteraceae_norank, etc. Multiple paths including autotrophic denitrification with hydrogen and Fe²⁺ as electron donors and heterotrophic denitrification were achieved in the system. Meanwhile, the main functional lignocellulose degradation microbes were enriched in the system, including Cytophaga_xylanolytica_group, and Caldilineaceae. Because electrolysis created a favorable environment for them to release carbon sources from coconut fiber. This study provided a new perspective for advanced nutrients removal of WWTP effluent in CWs.

The intrinsic relevance of nitrogen removal pathway to varying nitrate loading rate in a polycaprolactone-supported denitrification system

Up to date, the intrinsic association of nitrate loading rate (NLR) with treatment performance of solid-phase denitrification (SPD) systems is still ambiguous. To address this issue, three continuous up-flow bioreactors were configured. They were packed with polycaprolactone (PCL) under a filling ratio of 30%, 60% or 90% and were operated under a varying NLR of 0.34 ± 0.01-3.99 ± 0.12 gN/(L·d). Results showed that the denitrification efficiency was high (RE > 96%) and stable except the case with the highest NLR, which was mainly attributed to the lack of available carbon sources. At the phylum or genus level, most of the detected dominant bacterial taxa were either associated with organics degradation or nitrogen metabolism. The difference in bacterial community structure among the three stages was mainly caused by NLR rather than the filling ratio. Moreover, as the NLR got higher, the Bray-Curtis distance between samples from the same stage became shorter. By the results of gene or enzyme prediction performed in PICRUSt2, the main nitrogen metabolism pathways in these reactors were denitrification, dissimilatory nitrate reduction to ammonium (DNRA), assimilatory nitrate reduction to ammonium (ANRA) and nitrogen fixation. Moreover, aerobic and anaerobic nitrate dissimilation coexisted in the systems with the latter playing a dominant role. Finally, denitrification and DNRA occurred under both high and low NLR conditions while nitrogen fixation and ANRA preferred to occur under low NLR environments. These findings might help guide practical applications.

Microbial explanation to performance stratification along up-flow solid-phase denitrification column packed with polycaprolactone

In this study, the fluctuating profiles of physicochemical and microbial characterizations along different filling heights of continuously up-flow solid-phase denitrification (SPD) columns packed with polycaprolactone (PCL) were investigated. It was found both the PCL filling area and non-filling area made significant contributions to treatment performance and denitrification mainly occurred near the bottom of the filling column. Nitrate displayed a high proportional removal (≥98.7%) among all the cases except the one with the lowest filling ratio (FR30) and highest NLR (3.99 ± 0.12 gN/(L·d)), while nitrite and ammonium displayed a weak accumulation in final effluents (nitrite ≤ 0.40 mg/L; ammonium ≤ 0.98 mg/L). The intensity of PCL hydrolysis in the top substrate was stronger than those in the middle or bottom. Both dissimilatory nitrate reduction to ammonium (DNRA) and microbial lysis contributed to ammonium accumulation, and nitrate was mainly removed via traditional denitrification and DNRA. JGI_0000069-P22_unclassified and Gracilibacteria_unclassified might contribute to denitrification.

Various electron donors for biological nitrate removal: A review

Nitrate (NO₃^-) pollution in water and wastewater has become a serious global issue. Biological denitrification, which reduces NO₃^- to N₂ (nitrogen gas) by denitrifying microorganisms, is an efficient and economical process for the removal of NO₃^- from water and wastewater. During the denitrification process, electron donor is required to provide electrons for reduction of NO₃^-. A variety of electron donors, including organic and inorganic compounds, can be used for denitrification. This paper reviews the state of the art of various electron donors used for biological denitrification. Depending on the types of electron donors, denitrification can be classified into heterotrophic and autotrophic denitrification. Heterotrophic denitrification utilizes organic compounds as electron donors, including low-molecular-weight organics (e.g. acetate, methanol, glucose, benzene, methane, etc.) and high-molecular-weight organics (e.g. cellulose, polylactic acid, polycaprolactone, etc.); while autotrophic denitrification utilizes inorganic compounds as electron donors, including hydrogen (H₂), reduced sulfur compounds (e.g. sulfide, element sulfur and thiosulfate), ferrous iron (Fe²⁺), iron sulfides (e.g. FeS, Fe_1-xS and FeS₂), arsenite (As(Ш)) and manganese (Mn(II)). The biological denitrification processes and the representative denitrifying microorganisms are summarized based on different electron donors, and their denitrification performance, operating costs and environmental impacts are compared and discussed. The pilot- or full-scale applications were summarized. The concluding remarks and future prospects were provided. The biodegradable polymers mediated heterotrophic denitrification, as well as H₂ and sulfur mediated autotrophic denitrification are promising denitrification processes for NO₃^- removal from various types of water and wastewater.

Hexavalent Chromium Removal and Prokaryotic Community Analysis in Glass Column Reactor Packed with Aspen Wood as Solid Organic Substrate

Microbial hexavalent chromium (Cr(VI)) reduction is a promising method for Cr(VI)-laden wastewater treatment. However, the soluble organic substrate required for heterotrophic microbial Cr(VI) reduction necessitates constant supervision, and an excessive supply of soluble organic substrate can result in deterioration of the quality of the effluent. In this study, we evaluated aspen wood, a low-cost lignocellulose biomass, as a solid organic substrate for heterotrophic Cr(VI) reduction. A laboratory-scale aspen wood-packed glass column reactor inoculated with activated sludge was operated for 148 days for evaluation. Following reactor operation, an effective average dissolved Cr(VI) removal rate of 0.75 mg L^-1 h^-1 was confirmed under an average dissolved Cr(VI) loading rate of 0.90 mg L^-1 h^-1. Subsequently, 16S ribosomal ribonucleic acid gene amplicon sequencing analysis revealed that the dominant prokaryotic operational taxonomic units detected in the reactor were associated with prokaryotic lineages with the capacity for lignocellulose biodegradation, Cr compound resistance, and Cr(VI) reduction. Proteobacteria and Chloroflexi were two major prokaryotic phyla in the reactor. Our data indicate that aspen wood is an effective solid organic substrate for the development of simplified, effective, and low-cost microbial Cr(VI)-removing reactors.

Influence mechanism of filling ratio on solid-phase denitrification with polycaprolactone as biofilm carrier

In this study, three up-flow fixed-bed bioreactors were constructed with three different filling ratios (filling volume/effective volume: 30%, 60% and 90%) of polycaprolactone (PCL). Above 98% of nitrate removal efficiency was achieved with low accumulations of nitrite and ammonium for each filling ratio. Low filling ratio of PCL had extensive folds and pores that favored the attachment and growth of microorganisms; however, excessive biomass restrained nitrate specific reduction rate (NaSRR). The most dominant genera were Comamonas (0.80-57.64%), Stenotrophomonas (2.59-54.39%), Acidovorax (7.32-23.55%), Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium (0.30-19.74%) and Thermomonas (0.12-14.58%). Nitrate reductase (EC 1.7.99.4), nitrite reductase (EC 1.7.2.1) and nitric oxide reductase (EC 1.7.2.5) predicted by PICRUSt2 were abundant in high influent nitrate load (NaL). According to the analysis of carbon balance model, the utilization rate (η) of PCL showed a highly positive correlation with influent NaL, indicating reducing filling ratio or HRT might be an effective measure to save cost for nitrate removal.

Research progress in solid carbon source-based denitrification technologies for different target water bodies

Nitrogen pollution in water bodies is a serious environmental issue which is commonly treated by various methods such as heterotrophic denitrification. In particular, solid carbon source (SCS)-based denitrification has attracted widespread research interest due to its gradual carbon release, ease of management, and long-term operation. This paper reviews the types and properties of SCSs for different target water bodies. While both natural (wheat straw, wood chips, and fruit shells) and synthetic (polybutylene succinate, polycaprolactone, polylactic acid, and polyhydroxyalkanoates) SCSs are commonly used, it is observed that the denitrification performance of the synthetic sources is generally superior. SCSs have been used in the treatment of wastewater (including aquaculture wastewater), agricultural subsurface drainage, surface water, and groundwater; however, the key research aspects related to SCSs differ markedly based on the target waterbody. These key research aspects include nitrogen pollutant removal rate and byproduct accumulation (ordinary wastewater); water quality parameters and aquatic product yield (recirculating aquaculture systems); temperature and hydraulic retention time (agricultural subsurface drainage); the influence of dissolved oxygen (surface waters); and nitrate-nitrogen load, HRT, and carbon source dosage on denitrification rate (groundwater). It is concluded that SCS-based denitrification is a promising technique for the effective elimination of nitrate-nitrogen pollution in water bodies.

Effect of filling ratio and backwash on performance of a continuous-flow SPD reactor packed with PCL as carbon source

In this study, three up-flow fixed-bed bioreactors, named as A, B, and C, packed with polycaprolactone (PCL) under different filling ratios (31%, 62%, and 93%, respectively), were investigated over a long period (96 days). During the stable period, the mean effluent NO 3 - - N concentrations in reactors A, B, and C were 1.35 ± 0.50, 1.07 ± 0.41, and 1.03 ± 0.27 mg/L, respectively, which showed the removal of NO 3 - - N was not closely related to filling ratio (p > 0.05, one-way ANOVA). However, it was found that biomass in reactor A was 2.13 and 5.55 times in B and C, respectively. Excessively thick biofilm refrained the enzymatic hydrolysis of PCL and biofilm's specific denitrification rate (SDNR). Backwash stimulated organic matter release and enabled biofilm to restore its denitrification activity. The maximum cycle of backwash was 6 days for the lowest filling ratio reactor. Additionally, the utilization rates for denitrification were 83.3%, 86.4%, and 60.5% in reactors A, B, and C, respectively, which was higher after backwash than before backwash. PRACTITIONER POINTS: Excessively thick biofilm refrained the enzymatic hydrolysis of PCL. Backwash stimulated organic matter release and enabled biofilm to restore its denitrification activity. The maximum cycle of backwash was 6 days for the lowest filling ratio reactor. A higher utilization rate of PCL for denitrification was observed after backwash.