Enhancement Of Denitrification Research Articles

Denitrification enhanced by composite carbon sources in AAO- biofilter: Efficiency and metagenomics research

Nitrogen removal from domestic sewage is usually limited by insufficient carbon source and electron donor. An economical solid carbon source was developed by composition of polyvinyl alcohol, sodium alginate, and corncob, which was utilized as external carbon source in the anaerobic anoxic oxic (AAO)-biofilter for the treatment of low carbon-to-nitrogen ratio domestic sewage, and the nitrogen removal was remarkably improved from 63.2% to 96.5%. Furthermore, the effluent chemical oxygen demand maintained at 35 mg/L or even lower, and the total nitrogen was reduced to less than 2 mg/L. Metagenomic analysis demonstrated that the microbial communities responsible for potential denitrification and organic matter degradation in both AAO and the biofilter reactors were mainly composed of Proteobacteria and Bacteroides, respectively. The solid carbon source addition resulted in relatively high abundance of functional enzymes responsible for NO3−-N to NO2−-N conversion in both AAO and the biofilter reactors, thus enabled stable reaction. The carbon source addition during glycolysis primarily led to the increase of genes associated with the metabolic conversion of fructose 1.6P2 to glycerol-3P The reactor maintained high abundance of genes related to the tricarboxylic acid cycle, and then guaranteed efficient carbon metabolism. The results indicate that the composite carbon source is feasible for denitrification enhancement of AAO-biofilter, which contribute to the theoretical foundation for practical nitrogen removal application.

Enhancement of denitrification by sulfur-based carrier in sequencing batch reactor (SBR) for advanced wastewater treatment

This study was to enhance the nitrogen removal efficiency in the sequencing batch reactor (SBR) process by adding sulfur-based carriers. The nitrogen removal efficiency of the control group was compared with that of the experimental group through a two-series operation of SBR1 without carrier and SBR2 with the carrier under the condition of no external carbon source. A total nitrogen (T-N) removal efficiency of 6.6%, 72.6%, and 79.9% was observed in SBR1, SBR2 (5%), and (10%), respectively. The T-N removal efficiency was improved in the system with carriers, which showed an increase in the removal efficiency of approximately 91.7%. The results suggest that the inclusion of the carrier led to an elevation in the sulfur ratio, implying an augmented surface area for sulfur-based denitrifying microorganisms. Additionally, CaCO3 contributed essential alkalinity for sulfur denitrification, thereby preventing a decline in pH. Regardless of the carrier, the efficiency of organic matter removal surpassed 89%, indicating that the sulfur-based carrier did not adversely affect the biological reaction associated with organic matter. Therefore, autotrophic denitrification was successfully performed using a sulfur carrier in the SBR process without an external carbon source, improving the nitrogen removal efficiency.

Simultaneous denitrification enhancement and sludge reduction based on novel suspended carrier modified using activated carbon and magnetite at low carbon/nitrogen ratio

A novel suspended carrier was prepared by sticking activated carbon (AC) and magnetite (Fe3O4) onto polypropylene slices. Although this carrier could not reverse the decreased denitrification capacity trends under anoxic conditions at an influent carbon/nitrogen (C/N) ratio of 2, it enhanced denitrification by stimulating sludge reduction and accelerating electron transfer to certain extent. The carrier stuck by mixed AC/Fe3O4 exhibited better performance in terms of sludge reduction, extracellular polymeric substances (EPS) secretion, and denitrification than that merely stuck by AC and Fe3O4 at an influent C/N ratio of 2. The carrier stuck by mixed AC/Fe3O4 increased the total nitrogen removal efficiency by 24.6 % ± 12.5 % in a 72-h denitrification batch experiment compared to the common polypropylene carrier. Moreover, the carrier improved EPS secretion and nitrogen metabolism and promoted the growth of Trichococcus and some denitrifying genera. This study provides a reference for the treatment of low C/N ratio sewage.

Nitrite-resistance mechanisms on wastewater treatment in denitrifying phosphorus removal process revealed by machine learning, co-occurrence, and metagenomics analysis

Nitrite is a key intermediate in nitrogen metabolism that determines microbial transformations of N and P, greenhouse gas (N2O) emissions, and system nutrient removal efficiency. However, nitrite also exerts toxic effects on microorganisms. A lack of understanding of high nitrite-resistance mechanisms at community- and genome-scale resolutions hinders the optimization for robustness of wastewater treatment systems. Here, we established nitrite-dependent denitrifying and phosphorus removal (DPR) systems under a gradient concentration of nitrite (0, 5, 10, 15, 20, and 25 mg N/L), relying on 16S rRNA gene amplicon and metagenomics to explore high nitrite-resistance mechanism. The results demonstrated that specific taxa were adopted to change the metabolic relationship of the community through phenotypic evolution to resist toxic nitrite contributing to the enhancement of denitrification and inhibition of nitrification and phosphorus removal. The key specific species, Thauera enhanced denitrification, whereas Candidatus Nitrotoga decreased in abundance to maintain partial nitrification. The extinction of Candidatus Nitrotoga induced a simpler restructuring-community, forcing high nitrite-stimulating microbiome to establish a more focused denitrification rather than nitrification or P metabolism in response to nitrite toxicity. Our work provides insights for understanding microbiome adaptation to toxic nitrite and giving theoretical support for operation strategy of nitrite-based wastewater treatment technology.

Seawater intrusion alters nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions: Based on Bayesian isotope mixing model and microbial functional network

Seawater intrusion is a global coastal environmental issue of great concern and significantly impacts the regional biogeochemical environment and material cycles, including nitrogen cycling. To reveal the mechanism of seawater intrusion altering nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions, the Bayesian mixing model (δ15N-NO3– and δ18O-NO3–) and 16S rDNA gene amplicon sequencing are used to establish nitrogen cycling pathways and microbial functional network. The results show that the nitrate in the coastal groundwater is from manure and septic waste (M&S, over 44 %), soil organic nitrogen (SON, over 20 %), and nitrogen fertilizer (FN, over 16 %). The hydrological interaction has promoted the coupling between material cycling and microbial community in the coastal groundwater systems. Among them, precipitation infiltration has caused the gradual decrease of specific microbes along the flow direction, such as Lactobacillus, Acinetobacter, Bifidobacterium, etc. And seawater intrusion has caused the mutations of specific microbes (Planktomarina, Clade_Ia, Wenyingzhuangia, Glaciecola, etc.) and convergence of microbial community at the salt-freshwater interface in the aquifer. In the coastal intruded aquifer systems, the nitrogen cycling pattern can be divided into oxidation and reduction processes. The oxidation process involves the enhancement of nitrification while the weakening of denitrification and anammox with the increase of aquifer depth. The reduction process consists of the enhancement of denitrification and anammox while the erosion of nitrification and ammonification with increased seawater intrusion. In addition, seawater intrusion can mitigate nitrate contamination by promoting denitrification and anammox in coastal areas.

A novel strategy to improving Rhodobacter azotoformans denitrification efficiency: Insight into the role of a two-component system NtrX/Y in denitrification regulation

Transcription factors (TFs) can manage the coordinated expression of genes clusters or multiple genes. TF was used to improve bacterial denitrification ability in this study. During denitrification, the ntrY of R. azotoformans, which encodes the sensor of NtrX/Y system, was significantly upregulated in transcription. Denitrification of the mutant △ntrY was significantly inhibited, and it was recovered after replenishing this gene to the mutant, which indicates the NtrX/Y system plays an important role in regulating bacterial denitrification. According to additional research, the NtrX/Y system regulates bacterial denitrification by directly promoting the expression of the nitrite reductase. ntrY overexpression appears to accelerate bacterial denitrification, and the introduction of a strong promoter tac in conjunction with iron supply optimization increases the rate by 72% further. This study realizes bacterial denitrification enhancement from the perspective of global transcription regulation, which provides a novel strategy for improving microbial ability to degrade pollutants.

Plant Carbon Sources for Denitrification Enhancement and Its Mechanism in Constructed Wetlands: A Review

Nitrogen pollution in water bodies is a serious environmental problem worldwide. Plant carbonsource (PCS) enhanced denitrification in constructed wetlands (CWs) for wastewater with low chemical oxygen demand to total nitrogen (COD/N) has been one of the most exciting research topics. This paper summarized the related studies with VOSviewer software and found that the major interests were denitrification performance and mechanism in CWs. This article mainly focused on the PCSs’ characteristics, denitrification rate, the influences of key environmental and operational parameters, surface morphology variation, microbial community structure, and denitrification genes. Engineering prospects and existing problems were also introduced. PCSs’ degradation consumes DO and creates favorable conditions for denitrification. The COD/N of wastewater should be maintained at 4–5 by adding PCSs, thus improving denitrification performance and reducing nitrous oxide emission. Aerobic degradation, anaerobic fermentation, dissimilatory nitrate reduction to ammonium, and sulfate reduction processes may consume the carbon released by PCSs depending on the influent quality and environmental conditions. More attention should be paid to the reduction of greenhouse gases and emerging pollutants in CWs with PCSs.

Weak electric field effect of MFC biocathode on denitrification

Microbial fuel cells (MFCs) have been reported to enhance denitrification. However, it is difficult to investigate the denitrification enhancement mechanism under weak electric field (WEF) due to the multi-parameter variation in MFCs. In this study, a series of potentiostatic bioelectrochemical systems (BESs) for wastewater with a low COD/N ratio were constructed by simulating the MFC biocathode. The denitrification effect differences of the BESs under different constant potentials (−400, −200, +200 and +400 mV vs. Ag/AgCl) were investigated. The nitrogen removal pathway was identified by exploring the source and the flow direction of electrons in the biocathode. The electron transfer method in the biocathode and the influencing mechanism of the WEF on microorganisms was elucidated by electrochemical analysis, high throughput sequencing and PICRUSt2. The results showed that the WEF in MFCs could enhance the denitrification effect by increasing the biomass, the abundance of denitrifying functional bacteria (Thauera, SM1A02, Plasticicumulans, Ignavibacterium, Methylopolylis), the expression of functional genes (nasA, narGH, narI, narV, nirk, hcp, nosZ), and Nar and Nir enzyme activities, thereby providing electrons for the electroautotrophic denitrifying bacteria (Thauera). The electron transfer method between the biofilm of denitrifying bacteria and the solid cathode electrode mainly involved direct electron transfer, and there were at least two electroactive components, which might be related to Nar and Nir activities. These results have important theoretical significance and positive support for the application of microbial electrochemical systems and the realization of the advanced treatment of wastewater with a low COD/N ratio.

Pipe Dreams: The Effects of Stream Bank Soil Pipes on Hyporheic Denitrification Caused by a Peak Flow Event

AbstractPeak flow events in gaining stream/river channels cause lung model hyporheic exchange with the banks (bank storage), which fosters beneficial reactions as polluted channel water cycles through riparian groundwater. Soil pipes are common along stream/riverbanks, and enhance exchange, yet their effect on reactions such as denitrification is unknown. We used MODFLOW with the Conduit Flow Package to simulate lung model exchange during a peak flow event in a section of stream bank/riparian soil with soil pipes, and MT3D‐USGS to estimate nitrate transport and denitrification. We varied soil matrix hydraulic conductivity (K) and first‐order reaction constant (k), as well as soil pipe density, length, and height above the initial channel water surface elevation (H). The addition of soil pipes enhanced stream bank (riparian) denitrification relative to banks without pipes, e.g., a 76% increase due to adding a single 1.5 m pipe. Denitrification increased linearly with pipe density but exhibited nonlinear trends with other parameters. Sensitivity analysis revealed length and density to be most influential. Soil pipe enhancement of denitrification was governed by hyporheic volume in most cases in our study. Exceptions included (a) coarse soil (K = 10−3 m/s) and (b) low k and H > 0. Scaling our results to the stream corridor scale estimated that five soil pipes per m cumulatively induced 3% nitrate removal along a 1 km reach. Overall, soil pipes enhanced advection of nitrate into the banks, and also increased residence times of that nitrate under certain conditions, which together enhanced denitrification. This enhancement has implications for excess nitrate management in watersheds.

Improved lakeshore sediment microenvironment and enhanced denitrification efficiency by natural solid carbon sources

At present, the high level of nitrate in tailwater of wastewater treatment plants (WWTPs) has continued to cause serious environmental problems. In current study, wood chips and bark, serving as natural solid carbon source (NSCS), were applied to enhance the denitrification capacity of lakeshore sediment to remove superabundant nitrate in tailwater. The results show that NSCS had a positive effect on denitrification process for lakeshore sediment and increased the total phosphorus removal capacity by 212%–601%. Comparatively, wood chips were better in enhancing denitrification ability of lakeshore sediment for a longer time. At the 70th day, the nitrate elimination (NEL) of the wood chip-sediment integrated system (WSIS) was 20% and 9% higher than the control system (CS) and bark-sediment integrated system (BSIS), respectively. For the WSIS, the microbial activity and the abundance of nicotinamide adenine dinucleotide (NADH) were significantly increased, and the sediment anaerobic zone was rapidly formed, which were conducive to the denitrification of the sediment. At the DNA level, both groups presented an inconsistency in the abundance of functional denitrification genes with the NEL. At the enzyme level, wood chips significantly increased the nitrate and nitrite reductase activities of the sediment, and promoted denitrification. From the perspective of microbial communities, wood chips highly promoted the abundance of norank_f_BIrii41 Proteobacteria and Ruminclostridum more than other denitrifying and potential denitrifying bacteria in the process of sediment denitrification enhancement.

Enhancement of denitrification in biofilters by immobilized biochar under low-temperature stress

Efficient removal of nitrate under low temperature is challenging because of the reduction of the microbial activity. This study successfully explored the promotion on the performance of denitrification utilizing the immobilized biochar in biofilters under low temperature (6 ± 2 °C). The results showed that the immobilized biochar increased the denitrification rate by 76.8% and decreased the nitrous oxide emissions by 82.5%. Mechanistic studies revealed that the immobilized biochar increased the activities of the denitrifying enzymes and three enzymes involved in glycolysis. Furthermore, the immobilized biochar elevated the activity of the electron transport system by 31.8%. Finally, structural equation model explained that the increase of nitrate reductase activity was a crucial factor to enhance the total nitrogen removal efficiency in biofilters with immobilized biochar. Overall, the use of immobilized biochar can be a novel strategy to enhance nitrogen removal and reduce greenhouse gas emissions in biofilters under low temperature.

Enhanced nitrogen removal via iron‑carbon micro-electrolysis in surface flow constructed wetlands: Selecting activated carbon or biochar?

The iron-assisted autotrophic denitrification was plagued by passivation when introduced in surface flow constructed wetlands (SFCWs). Iron‑carbon micro-electrolysis (Fe/C-M/E) could facilitate the transfer of electrons during the utilization of iron. In this study, iron scraps coupling with activated carbon and biochar were applied to explore the effects of carbon materials on autotrophic denitrification. The results showed that TN removal rate in the SFCW with iron scraps and activated carbon (SFCW-IAC) and the SFCW with iron scraps and biochar (SFCW-IBC) were improved by 31.61% ± 8.18% and 14.09% ± 7.15%, and N2O fluxes were reduced to 2.73 and 3.12 mg m−2 d−1, respectively. The greater iron mass loss rate (0.91%) was confirmed in SFCW-IAC. Microbial community analysis reported that autotrophic denitrification and iron related genera were increased. This study proved that activated carbon was more suitable than biochar to Fe/C-M/E for denitrification enhancement and N2O emission reduction.

Denitrification enhancement by electro-adsorption/reduction in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) with copper electrode

The green and efficient removal of nitrate (NO3−) in groundwater is a primary concern nowadays, and membrane capacitive deionization (MCDI) is an emerging technology for the removal of nitrate (NO3−) from water. In this study, a novel electrochemical system for nitrate denitrification removal was established, wherein the economic non-noble metal copper was used as the electrode material to achieve harmless removal of nitrate in a single electrochemical cell. The effects of applied voltage, initial NO3− concentration, and co-existing matters on NO3− denitrification removal during electro-adsorption/reduction system were deeply investigated. The results showed that the NO3− denitrification removal increased with raised voltage and in proportion to the initial NO3− concentration within certain limits, wherein the removal rate reached a maximum of 53.3% in the single-solute solution of 200 mg L−1 NaNO3 at 1.8 V. Nevertheless, overhigh voltage or initial NO3− concentration would have a negative effect on nitrate removal, which was caused by multiple factors, including side reactions in the solution, fouling of activated carbon fiber and anion exchange membrane, and corrosion of copper electrode. The presence of NaCl also had a negative effect on the removal of nitrate, which was mainly caused by fouling of ACF/IEM and redox reaction on account of the chloride ions. This study provides a potential economical alternative for the NO3− denitrification removal to achieve a more environmentally friendly outcome.

Temporal enhancement of denitrification in bioirrigated estuarine sediments

Temporally intensive observations of sediment biogeochemical fluxes were used to examine the effects of water column oxygen depletion and associated loss of sediment bioirrigation on denitrification rates. In the tidal Choptank River, a subestuary of the Chesapeake Bay, coupled nitrification/denitrification was identified as the main pathway for the production of N2 gas in the sediment. Although denitrification rates were stimulated by high rates of bioirrigation, the overall efficiency of the process sharply declined as temperature increased and bottom water O2 declined. Consequently, there was a transition from nitrogen remineralization resulting in N2 gas production in winter to complete recycling of remineralized NH4+ back to the water column in summer. Bioirrigation rate estimates using a bromide tracer showed the same pattern as those derived from combining diffusive pore water O2 fluxes with intact core incubations. These bioirrigation estimates were consistent with peak abundance of small spionid polychaetes in early spring with populations declining sharply into summer. Following deposition of the spring algal bloom, bioirrigation was more important than diffusive oxygen transport, increasing the depth of habitat for microbial denitrification. Low bottom water oxygen (~ 3 mg L−1) in summer was accompanied with a loss of bioirrigation and very low denitrification efficiency. Denitrification efficiency was shown to be sensitive to bottom water oxygen concentrations even in the absence of hypoxia or anoxia.

Denitrification enhancement by electro-sorption/reduction using a layered metal oxide electrode loaded with Pd-Cu nanoparticles

The green and efficient removal of nitrate (NO3−) in groundwater is a major challenge for safe water supply. In this study, palladium (Pd) and copper (Cu) nanoparticles were loaded on nickel-aluminium (NiAl)-layered metal oxide (LMO) to improve the catalytic activity of electro-sorption/reduction of NO3− by electrodes. The Pd-Cu bimetallic nanoparticles can effectively improve the transmission of electrons in electrodes. In addition, denitrification with the NiAl-LMO electrode loaded with Pd-Cu was superior to that with LMO loaded with Pd. Furthermore, electro-sorption/reduction was a more effective denitrification mode, which improved the efficiency and produced fewer by-products. The electro-sorption process not only enhanced denitrification, but also improved the selectivity of reduced to nitrogen. Thus, our study provides a green capacitive deionization (CDI) electrode material and efficient electric adsorption mode to enhance the denitrification.

Carbon sources derived from maize cobs enhanced nitrogen removal in saline constructed wetland microcosms treating mariculture effluents under greenhouse condition.

This study investigated an alternative carbon source derived from maize cobs (MCs) to enhance nitrogen removal in saline constructed wetlands (SCWs). The main objectives were to select the proper pretreatment method of MCs for rapid carbon release; and to investigate the effects of maize cob pieces (i.e. MCP) and three addition levels of maize cob lixiviums (i.e. L-MCL, M-MCL and H-MCL) on nitrogen purification performance and microbial characteristics of SCWs. Results showed NaOH pretreatment enhanced carbon release of MCs in seawater (from 7.5±0.4 mgCOD g-1 to 16.4±0.2 mgCOD g-1). The 80-d trial showed SCWs with M-MCL addition performed well on nitrogen removal: NO3-N, 88.8±11.6%; NO2-N, 91.1±3.5%; TAN, 96.5±1.6%; TIN, 89.8±10.4%; with 2mgL-1 effluent COD. Denitrification parameters confirmed MCL to be a high quality carbon source: denitrification potential (PDN)=0.16 gN gCOD-1; heterotrophy anoxic yield coefficient (YH)=0.54 gCOD gCOD-1. The MCP and H-MCL treatments improved substrate dehydrogenase activity, indicating a higher microbial activity in these SCWs. Sequencing analysis revealed that, regardless of addition manners, carbon sources from MCs changed the rhizosphere microbial community. At genus level, Anaerophaga (10.1%), Granulosicoccus (8.2%) and Sulfurimonas (6.6%) dominated in SCWs under MCP treatment. Increased MCL addition levels improved the relative abundance of Vibrio, Malonomonas and Caldithrix, suggesting the enhancement of denitrification. Relative high proportions of Desulfotignum and Desulfovibrio, and Sulfurimonas were observed in MCP and H-MCL SCWs, implying that sulfate reduction occurred in SCWs with excess carbon sources.

Effect of potassium on nitrate removal from groundwater in agricultural waste-based heterotrophic denitrification system

Heterotrophic denitrification based on solid carbon sources has been widely investigated for nitrogen removal in recent years. In this study, the response of the heterotrophic denitrification process under different K+ concentrations was clarified. Additionally, the denitrification enhancement mechanism was revealed and resource utilization of agricultural waste was achieved. A series of batch tests were conducted to study the effect of different K+ concentrations on the denitrification performance, dissolved organic matter (DOM) dissolution and microbial community structure. Results demonstrate that the threshold of K+ concentration for the NO3−-N and NO2−-N reduction rates were 229.78 ± 25.80 and 159.10 ± 24.60 mg-K/L, respectively. Excitation-emission matrix (EEM) analysis identified the main DOM components associated with tyrosine-like, tryptophan-like and humic-like substances, as well as illustrated the evolutionary behavior and utilization of DOM. High throughput 16S rRNA gene sequencing indicates that a K+ concentration of 229.78 ± 25.80 mg-K/L exhibited the highest diversity of functional species associated with fermentation and denitrification. The genera Pseudomonas and Thiobacillus were the unique denitrifiers at this K+ concentration. The correlation of K+ concentration, DOM dissolution of different regions and microorganism structure were analyzed using correlation matrix and PCA, and the appropriate K+ concentration of different functional microorganisms survival was optimized by this analysis method.

A biophysiological perspective on enhanced nitrate removal from decentralized domestic sewage using gravitational-flow multi-soil-layering systems

Multi-soil-layering (MSL) system with brick-wall pattern structure and gravitational flow can be used for decentralized rural domestic sewage treatment. The capability of soil for contaminant removal is maximized within soil mixture blocks (SMBs). However, the performance of removing nitrate was still not ideal during operation. To improve its performance in MSL system, the relationship between biophysiological characteristics of denitrifying species and operating conditions was studied. Microbial species diversity of activated sludge and soil samples were analyzed. The significant effects of independent factors and their interactions on microbial species diversity and denitrifying species abundance were revealed on the basis of factorial analysis. The results indicated activated sludge in SMBs played a key role in increasing the richness of denitrifying species in MSL system. Slow-release poly (butylene succinate) (PBS) had the most dominant positive effect on increasing denitrifying species abundance. Submersion had significantly positive effect on species richness in SMBs. These three factors, including activated sludge, PBS in SMBs, and submersion condition had different significant effects on microbial responses. They were favorable for denitrification and ensuring a better removal efficiency of nitrate and total nitrogen. The porous zeolites were served as the habitats for most of aerobic bacteria to form biofilms, which could promote the oxygen consumption in both sewage and system to improve denitrification in SMBs. The results could help on the enhancement of denitrification in MSL system from biophysiological insights. It can provide a sound strategy for using MSL system with great performance on contaminant removal.

Control Measure Effects on the Effluent Quality from Extensive Green Roofs Based on Stabilized Sludge Recycling

Substrate type is an important factor affecting the quality of water deriving from rainfall onto extensively green roofed areas. Here, stabilized sludge was used as the main nutrient component of the substrate combined with biochar and a dual-substrate structure. Five green roof pilot facilities were constructed, and the effect of control measures on effluent quality was analyzed. The results showed that the stabilized sludge dosage was 3%, and the annual average mass concentrations of total nitrogen (TN) and nitrate nitrogen (NO3--N) were 3.27 mg·L-1 and 1.61 mg·L-1. The use of stabilized sludge as a nutrient component under real rainfall and temperature conditions in Shanghai did not cause significant leaching of TN and NO3--N. In order to further improve the quality of the effluent, biochar was used as an amendment measure. As a result, the concentrations of TN and NO3--N in the effluent were decreased to 2.16 mg·L-1 and 1.38 mg·L-1, respectively. Using an adsorption layer of pumice can alleviate the leaching of total phosphorus (TP) and chemical oxygen demand (COD). For the total nitrogen budget of each pilot facility, the retained TN was about 55% of the original TN after one year of operation. Thus, stabilized sludge could be used as a nutrient substrate to meet the long-term requirements of plants. In the substrate with biochar, the retained TN in the substrate and the NO3--N concentration in the effluent was decreased, which was related to the mineralization of organics during dry periods and the enhancement of denitrification during rainfall periods. Stabilized sludge was not a polluting source of N but was a source of P. Using biochar and a dual-substrate structure can effectively reduce the TN and COD load of the tested green roof facilities.

Denitrification in a low carbon environment of a constructed wetland incorporating a microbial electrolysis cell

The main aim of this study was to explore the possibility of denitrification under low carbon conditions in a microbial electrolysis cell (MEC) integrated into a constructed wetland (CW). To our knowledge, this is the first systematic study of the incorporation of an MEC into a CW for enhancing denitrification. CWs are an efficient and low-cost technology for wastewater treatment facing problems in denitrification under low carbon conditions. Two CW-microcosms were fabricated: one included an MEC arrangement with granular graphite electrodes attached to a power source and the other with a microcosm fabricated with normal stone gravel as a control. These microcosms were fed with synthetic wastewater containing 50 mg/l of nitrate-nitrogen (NO3-N) with different ratios of organic carbon. Carbon to nitrogen ratio (C: N) were kept to 2:1, 1:1 and 0.5:1. The maximum concentration of organic carbon was 100 mg/l, subsequently decreased to 50 mg/l to 25 mg/l. The highest percentage removal achieved from CW-MEC in C: N ratios of 2:1 were 69.3% on 0.583 mA applied current, whereas the control microcosm’s removal efficiency on the same C:N was 66.2%. The outcomes of this study show that the NO3-N removal was always higher in the MEC integrated CW compared to the normal CW. The study concludes that the higher denitrification is possible by incorporation of an MEC into a constructed wetland under low carbon condition. This approach can provide a new direction for denitrification enhancement in a CW.