Simultaneous denitrification, phosphorus recovery and low sulfate production in a recirculated pyrite-packed biofilter (RPPB)

Insights into carbon-neutral treatment of rural wastewater by constructed wetlands: A review of current development and future direction

In recent years, with the global rise in awareness regarding carbon neutrality, the treatment of wastewater in rural areas is increasingly oriented towards energy conservation, emission reduction, low-carbon output, and resource utilization. This paper provides an analysis of the advantages and disadvantages of the current low-carbon treatment process of low-carbon treatment for rural wastewater. Constructed wetlands (CWs) are increasingly being considered as a viable option for treating wastewater in rural regions. In pursuit of carbon neutrality, advanced carbon-neutral bioprocesses are regarded as the prospective trajectory for achieving carbon-neutral treatment of rural wastewater. The incorporation of CWs with emerging biotechnologies such as sulfur-based autotrophic denitrification (SAD), pyrite-based autotrophic denitrification (PAD), and anaerobic ammonia oxidation (anammox) enables efficient removal of nitrogen and phosphorus from rural wastewater. The advancement of CWs towards improved removal of organic and inorganic pollutants, sustainability, minimal energy consumption, and low carbon emissions is widely recognized as a viable low-carbon approach for achieving carbon-neutral treatment of rural wastewater. This study offers novel perspectives on the sustainable development of wastewater treatment in rural areas within the framework of achieving carbon neutrality in the future.

Optimization of "sulfur-iron-nitrogen" cycle in constructed wetlands by adjusting siderite/sulfur (Fe/S) ratio

Sulfur-siderite autotrophic denitrification (SSAD) has been proved to solve the key problem of low nitrogen removal efficiency caused by the shortage of carbon source in constructed wetlands (CWs). In this study, five vertical flow constructed wetlands (VFCWs) were constructed with different Fe/S ratios (0/0, 0/1, 1/1, 2/1 and 1/2) to optimizing SSAD process, labeled S.0, S.1, S.2, S.3 and S.4. The results showed that the best NO3--N and TN removal rates were achieved with a Fe/S ratio of 2:1 (S.3), which were 96.26 ± 1.40% and 93.63 ± 3.12%, respectively. The abundance of denitrification genes (nirS, nirK and nosZ) in S.3 was significantly increased. Illumina high-throughput sequencing analysis indicated that the abundance and diversity of microorganisms involved in the "Sulfur-Iron-Nitrogen" cycle were enriched in S.3. The current study provided that the "Sulfur-Iron-Nitrogen" cycle in CWs was optimized by adjusting Fe/S ratio, and more types of denitrifying bacteria could be enriched, thereby enhancing nitrogen removal.

Iron-containing nanominerals for sustainable phosphate management: A comprehensive review and future perspectives

Adsorption, which is a quick and effective method for phosphate management, can effectively address the crisis of phosphorus mineral resources and control eutrophication. Phosphate management systems typically use iron-containing nanominerals (ICNs) with large surface areas and high activity, as well as modified ICNs (mICNs). This paper comprehensively reviews phosphate management by ICNs and mICNs in different water environments. mICNs have a higher affinity for phosphates than ICNs. Phosphate adsorption on ICNs and mICNs occurs through mechanisms such as surface complexation, surface precipitation, electrostatic ligand exchange, and electrostatic attraction. Ionic strength influences phosphate adsorption by changing the surface potential and isoelectric point of ICNs and mICNs. Anions exhibit inhibitory effects on ICNs and mICNs in phosphate adsorption, while cations display a promoting effect. More importantly, high concentrations and molecular weights of natural organic matter can inhibit phosphate adsorption by ICNs and mICNs. Sodium hydroxide has high regeneration capability for ICNs and mICNs. Compared to ICNs with high crystallinity, those with low crystallinity are less likely to desorb. ICNs and mICNs can effectively manage municipal wastewater, eutrophic seawater, and eutrophic lakes. Adsorption of ICNs and mICNs saturated with phosphate can be used as fertilizers in agricultural production. Notably, mICNs and ICNs have positive and negative effects on microorganisms and aquatic organisms in soil. Finally, this study introduces the following: trends and prospects of machine learning-guided mICN design, novel methods for modified ICNs, mICN regeneration, development of mICNs with high adsorption capacity and selectivity for phosphate, investigation of competing ions in different water environments by mICNs, and trends and prospects of in-depth research on the adsorption mechanism of phosphate by weakly crystalline ferrihydrite. This comprehensive review can provide novel insights into the research on high-performance mICNs for phosphate management in the future.

Intensified denitrification in a fluidized-bed reactor with suspended sulfur autotrophic microbial fillers

Sulfur-based autotrophic denitrification (SAD) is a promising low-carbon approach to tackle nitrate pollution. However, practical SAD reactor implementation faces challenges of slow denitrification rates and prolonged start-up periods. In this work, a fluidized-bed denitrification reactor with suspended composite fillers immobilized with elemental sulfur and SAD bacteria was constructed. The reactor reaches a steady state within the first day of operation. A denitrification rate of 0.61 g N L-1 d-1 was realized, which is 2.4-fold higher than that in the packed-bed reactor. Mixotrophic denitrification prevailed during the start-up period, while the SAD process became the predominant pathway (>70%) after several days of operation. The prevailing bacteria in the fillers, notably Thiobacillus, are enriched during denitrification operations. Overall, this study highlights the impressive denitrification capabilities of the fluidized SAD reactor with microbial fillers, providing valuable insights for enhancing denitrification techniques.

Effect mechanism of micron-scale zero-valent iron enhanced pyrite-driven denitrification biofilter for nitrogen and phosphorus removal

This study aims to explore the effect mechanism of micron-scale zero-valent iron (mZVI) to improve nitrogen and phosphorus removal in a pyrite (FeS2)-driven denitrification biofilter (DNBF) for the secondary effluent treatment. Two similar DNBFs (DNBF-A with FeS2 as fillers and DNBF-B with the mixture mZVI and FeS2 as carrier) were developed. The results showed that NO3--N, total nitrogen (TN) and PO43--P removal efficiencies were up to 91.64%, 67.44% and 80.26% in DNBF-B, which were obviously higher than those of DNBF-A (with NO3--N, TN and PO43--P removal efficiencies of 38.39%, 44.89% and 53.02%, respectively). Kinetic analysis of both PO43--P and NO3--N showed an increase in the rate constant (K) for DNBF-B compared to DNBF-A. The addition of mZVI not only improved the electron transport system activity (ETSA), but also achieved system Fe(II)/Fe(III) redox cycle in DNBF-B. In addition, the high-throughput sequencing analysis indicated that the addition of mZVI could obviously stimulate the enrichment of functional bacteria, such as Thiobacillus (11.99%), Mesotoga (7.50%), JGI-0000079D21 (6.37%), norank_f__Bacteroidetes_vadinHA17 (6.19%), Aquimonas (5.93%) and Arenimonas (3.97%). These genus played the important role in nitrogen and phosphorus removal in DNBF-B. Addition mZVI in the FeS2-driven denitrification biofilter is highly promising for TN and TP removal during secondary effluent treatment.

Advance in the sulfur-based electron donor autotrophic denitrification for nitrate nitrogen removal from wastewater

In the field of wastewater treatment, nitrate nitrogen (NO3--N) is one of the significant contaminants of concern. Sulfur autotrophic denitrification technology, which uses a variety of sulfur-based electron donors to reduce NO3--N to nitrogen (N2) through sulfur autotrophic denitrification bacteria, has emerged as a novel nitrogen removal technology to replace heterotrophic denitrification in the field of wastewater treatment due to its low cost, environmental friendliness, and high nitrogen removal efficiency. This paper reviews the advance of reduced sulfur compounds (such as elemental sulfur, sulfide, and thiosulfate) and iron sulfides (such as ferrous sulfide, pyrrhotite, and pyrite) electron donors for treating NO3--N in wastewater by sulfur autotrophic denitrification technology, including the dominant bacteria types and the sulfur autotrophic denitrification process based on various electron donors are introduced in detail, and their operating costs, nitrogen removal performance and impacts on the ecological environment are analyzed and compared. Moreover, the engineering applications of sulfur-based electron donor autotrophic denitrification technology were comprehensively summarized. According to the literature review, the focus of future industry research were discussed from several aspects as well, which would provide ideas for the application and optimization of the sulfur autotrophic denitrification process for deep and efficient removal of NO3--N in wastewater.

Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge - oxic settling anaerobic system

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRTSHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < -91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (TSHT) from ambient (19.8-25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Yobs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Yobs decreased by 32% and 46-65% at HRTSHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Yobs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRTSHT of 6 h and when TSHT was set at 35 °C. OSA implementation did not affect COD and NH4+ oxidation of the IFAS system (90-96% and 99%, respectively) and improved total nitrogen (TN) reduction (31-53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.

The enrichment of more functional microbes induced by the increasing hydraulic retention time accounts for the increment of autotrophic denitrification performance

With pyrite (FeS2) and polycaprolactone (PCL) as electron donors, three denitrification systems, namely FeS2-based autotrophic denitrification (PAD) system, PCL-supported heterotrophic denitrification (PHD) system and split-mixotrophic denitrification (PPMD) system, were constructed and operated under varying hydraulic retention times (HRT, 1-48 h). Compared with PAD or PHD, the PPMD system could achieve higher removals of NO3--N and PO43--P, and the effluent SO42- concentration was greatly reduced to 7.28 mg/L. Similarly, the abundance of the dominant genera involved in the PAD (Thiobacillus, Sulfurimonas, and Ferritrophicum, etc.) or PHD (Syntrophomonas, Desulfomicrobium, and Desulfovibrio, etc.) process all increased in the PPMD system. Gene prediction completed by PICRUSt2 showed that the abundance of the functional genes involved in denitrification and sulfur oxidation all increased with the increase of HRT. This also accounted for the increased contribution of autotrophic denitrification to total nitrogen removal in the PPMD system. In addition, the analysis of metabolic pathways disclosed the specific conversion mechanisms of nitrogen and sulfur inside the reactor.

Application of high-intensity static magnetic field as a strategy to enhance the fertilizing potential of sewage sludge digestate

Anaerobic digestion (AD) is a sustainable and well-established option to handle sewage sludge (SS), as it generates a methane-rich biogas and a digestate with potential fertilizing properties. In the past, different strategies have been proposed to enhance the valorization of SS. Among these, the application of a static magnetic field (SMF) has been poorly evaluated. This study aims to determine the effects of a high-intensity SMF (1.5 and 2 T) on the chemical composition of SS anaerobic digestate. Several strategies (i.e., number of magnetization cycles, addition of different sources and quantities of magnesium, and digestate aeration) have been applied to evaluate the possible formation of compounds with valuable fertilizing properties in the digestate. Experimental results showed that by combining different strategies promoting digestate exposure to the magnetic field it is possible to favour the reduction in the liquid phase of NH4+, NO3-, PO43-, SO42- and Mg2+ concentrations up to 28%, 38%, 34%, 39% and 31%, respectively. The XRD analyses conducted on the solid phase of the same magnetized digestate samples showed an increase in crystalline and amorphous phases of nitrogen and phosphorus compounds with fertilizing value, such as struvite. These results highlight that SMF application can increase the fertilizing potential of sewage sludge digestate and promote its valorization in a sustainable and circular perspective.

Coupled pyrite and sulfur autotrophic denitrification for simultaneous removal of nitrogen and phosphorus from secondary effluent: feasibility, performance and mechanisms

The discharge standards of nitrogen (N) and phosphorus (P) in wastewater treatment plants (WWTPs) have become increasingly strict to reduce water eutrophication. Further reducing N and P in effluent from municipal WWTPs need to be achieved effectively and eco-friendly. In this study, a carbon independent pyrite and sulfur autotrophic denitrification (PSAD) system using pyrite and sulfur as electron donor was developed and compared with pyrite autotrophic denitrification (PAD) and sulfur autotrophic denitrification (SAD) systems through batch and continuous flow biofilter experiments. Compare to PAD and SAD, PSAD was more effective in simultaneous removal in N and P. At hydraulic retention time (HRT) 3 h, average effluent concentrations of total nitrogen (TN) and total phosphate (TP) of 1.40 ± 0.03 and 0.19 ± 0.02 mg/L were achieved when treating real secondary effluent with 20.65 ± 0.24 mg/L TN and 1.00 ± 0.24 mg/L TP. The improvement in simultaneous removal of N and P was attributed to the coupling of PAD and SAD in enhancing the transformation of sulfur and iron and enlarging the reaction zone in the pyrite and sulfur autotrophic denitrification biofilter (PSADB) system. Therefore, more biomass was accumulated and the microbial denitrification functional stability, including electrons transfer and consumption was enhanced on the surface of pyrite and sulfur particles in the PSADB system. Moreover, autotrophic denitrifiers (Thiobacillus and Ferritrophicum), sulfate-reducing bacteria (Desulfocapsa) and iron reducing bacteria (Geothrix), acting as contributors to microbial nitrogen, sulfur and iron cycle, were specially enriched. In addition, the leaching of iron ions was promoted, which facilitated the removal of phosphate in the form of Fe3(PO4)2·8H2O and Fe3PO4. PSADB has proven to be an efficient technology for simultaneous removal of N and P, which could meet increasingly stringent discharge standards effectively and eco-friendly.

Pyrite-based denitrification combined with electrochemical disinfection to remove nitrate and microbial contamination from groundwater

Nitrate and microbial contamination of groundwater can occur in countries that face intense urbanization and inadequate sanitation. When groundwater is the main drinking water source, as is often the case in such countries, the need to remove these contaminants becomes acute. The combination of two technologies is proposed here, a biological step to denitrify and an electrochemical step to disinfect the groundwater, thereby aiming to reduce the chemical input and the footprint of groundwater treatment. As such, a pyrite-based fluidized bed reactor (P-FBR) was constructed to autotrophically denitrify polluted groundwater. The P-FBR effluent was disinfected in an electrochemical cell with electrogenerated Cl2. Nitrate was removed with 79% efficiency from an initial 178 mg NO3- L-1 at an average denitrification rate of 171 mg NO3- L-1 d-1, with 18 h hydraulic retention time (HRT). The electrochemical unit achieved a 3.8-log reduction in total coliforms with a 41.7 A h m-3 charge density.

Pyrite and sulfur-coupled autotrophic denitrification system for efficient nitrate and phosphate removal

The inefficiency of nitrogen removal in pyrite autotrophic denitrification (PAD) and the low efficiency of PO₄^3--P removal in sulfur autotrophic denitrification (SAD) limit their potential for engineering applications. This study examined the use of pyrite and sulfur coupled autotrophic denitrification (PSAD) in batch and column experiments to remove NO₃^--N and PO₄^3--P from sewage. The effluent concentration of NO₃^--N was 0.32 ± 0.11 mg/L, with an average Total nitrogen (TN) removal efficiency of 99.14%. The highest PO₄^3--P removal efficiency was 100% on day 18. There was a significant correlation between pH and the efficiency of PO₄^3--P removal. Thiobacillus, Thiomonas and Thermomonas were found to be dominant at the bacterial genus level in PSAD. Additionally, the abundance of Thermomonas in the PSAD was greater than that observed in the SAD reactor. This result indirectly indicates that the PSAD system has more advantages in reducing N₂O.

Enhancement and mechanisms of micron-pyrite driven autotrophic denitrification with different pretreatments for treating organic-limited waters

Pyrite-driven autotrophic denitrification (PAD) represents a cheap and promising way for nitrogen removal from organic-limited wastewater, which has obtained increasing attention in recent years. However, the limited denitrification rate and unclear mechanism underlying the process have hindered the engineered application of PAD. This study aims to shed light on the impacts of different pretreatments (i.e., ultrasonication, acid-washing and calcination) on micron-pyrite surface characteristics, denitrification performance and biofilm formation during PAD in batch reactors. A series of solid-phase analyses revealed that all pretreatments could significantly promote biofilm attachment on pyrite granules, but impacted the proportion, distribution and chemical oxidation state of sulfur (S) and iron (Fe) at varying degrees. Batch tests showed that ultrasonication and acid-washing could enhance the total nitrogen reduction rate by 14% and 99%, and decrease the sulfate production rate by 51% and 42%, respectively, when compared with untreated pyrite. Microbial community analysis indicated that Thiobacillus and Rhodanobacter dominated in PAD systems. Two types of indirect mechanisms (i.e., contact and non-contact) for pyrite leaching may co-occur in PAD system, resulting in ferrous iron (Fe²⁺), thiosulfate (S₂O₃^2-) and sulfide (S^2-) as the main electron donors for denitrification. A PAD mechanism model was proposed to describe the PAD electron transfer pathway with the aim to optimize the engineered application of PAD for nitrogen removal.

Simultaneous removal of heterocyclic drugs and total nitrogen from biochemical tailwater by peracetic acid/cobalt-loaded ceramsite-based denitrification biofilter

It is difficult to achieve simultaneous and efficient removal of heterocyclic drugs (HCDs) and total nitrogen (TN) in conventional denitrification biofilter (DNBF). Inspired by the effective degradation of refractory organic matter by cobalt-based advanced oxidation process and the need for in-situ upgrading of DNBF, peracetic acid (PAA)/cobalt-loaded ceramsite-based DNBF system was constructed for the first time to treat biochemical tailwater containing HCDs. Results showed that PAA/Co-DNBF had relatively high removal rates for the four HCDs with the order of CBZ > TMP > SDZ > SMX, and the optimal DNBF was H2 with 150 μg L^-1of PAA. Overall, TN and HCDs removal increased by 178%-455% and 2.50%-40.99% respectively. When the influent concentration of NO₃^--N, COD and each HCDs of 20 mg/L, 60 mg/L and 20 μg/L, below 15 mg/L of effluent TN and the highest average removal rate of SMX (67.77%) could be achieved, under HRT of 4 h in H2. More even distribution of microbial species and low acute toxicity of effluent were also achieved. More even distribution of microbial species and low acute toxicity of effluent were also achieved. In addition, high extracellular polymeric substance (EPS) content and Gordonia after the addition of PAA contributed to the degradation of HCDs. This study supplied a potentially effective strategy for the treatment of biochemical tailwater containing HCDs and provided new insight into the advance of denitrification technology.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

New insights on simultaneous nitrate and phosphorus removal in pyrite-involved mixotrophic denitrification biofilter for a long-term operation: Performance change and its underlying mechanism

Simultaneous nitrate and phosphorus removal can be completed by pyrite- and influent organics-involved mixotrophic denitrification and chemical phosphorus removal via iron precipitation. However, so far, how their removal performances change with iron precipitation accumulation remains unclear. In this study, the differences in nitrate and phosphorus removal from municipal tailwater between volcanic and pyrite supported biofilters (V-BF, P-BF) for a long-term operation were investigated, as well as the underlying mechanism for these differences. The nitrate removal efficiencies (NREs) in P-BF were greater than those in V-BF due to the synergistic effect of influent organic and pyrite, as evidenced by comparable TOC consumption and Fe²⁺/SO₄^2- production. The NREs in P-BF were gradually lower than in V-BF as a result of bacterial cell-iron encrustation observed in TEM images, which would deteriorate microbial activity. However, the phosphorus removal efficiencies (PREs) in P-BF remained consistently higher than in V-BF, resulting from chemical phosphorus removal which was confirmed that P, Fe and O elements dominated on the pyrite surface after use by SEM-EDS. The dominant denitrifying bacteria differed significantly, autotrophic and heterotrophic denitrifying microorganisms coexisted in P-BF. The relative abundances of the narG coding gene in P-BF were higher than that in V-BF, which was consistent with the total relative abundances of identified denitrifying bacteria. Besides, the mechanism of simultaneous nitrogen and phosphorus removal in the pyrite-involved mixotrophic denitrification process has been deduced. This work has significant implications for the practical application of a pyrite-involved mixotrophic denitrification process for low C/N wastewater treatment.

Mixotrophic denitrification using pyrite and biodegradable polymer composite as electron donors

The denitrification performance of a novel mixotrophic system using pyrite (FeS₂) and biodegradable polymer composite (PLA/PHBV/rice hulls, PPRH) as electron donors was investigated. After 12-day operation, the average nitrate removal rate (16.3-40.6 mg-N/L/d) in the mixotrophic system was 37% higher than the combined rate in the single heterotrophic and autotrophic system. The XPS analysis identified the formation of SO₄^2-, S^2- and Fe(Ш) on the pyrite surface during mixotrophic operation. The predicted microbial function analysis by PICRUSt2 revealed that the genes involved in S-oxidation, denitrification and carbon fixation were notably enriched in the mixotrophic system, indicating the increasing contribution of autotrophic S-oxidizing denitrification to total nitrate removal. Moreover, network analysis suggested the synergistic interactions among heterotrophic denitrifiers, S-oxidizing denitrifiers, sulfate reducers, Fe(II)-oxidizing denitrifiers and Fe(Ш) reducers. This study provides novel insights into the molecular mechanism of C, N, S and Fe cycle in the pyrite/PPRH based mixotrophic denitrification system.

Pyrite-assisted denitrification in recirculated biofilter tolerates pH lower than 5

This study demonstrates the feasibility of pyrite-assisted denitrification to treat mildly acidic wastewaters and proposes a convenient solution for nitrogen removal from acidic mining waters. Pyrite is a highly available and low-cost electron donor for autotrophic denitrification, whereas mining waters represent a source of nitrogen (N) contamination due to the use of N-containing explosives during mining operations and cyanide (CN)-based compounds for ore processing. In this study, the denitrification performance of a recirculated pyrite-packed biofilter (RPPB) treating simulated N-contaminated acidic mining waters is evaluated under decreasing pH conditions. The effect of feed organic carbon on the autotrophic process has been also investigated. N removal efficiencies (NRE) > 60% could be achieved at feed pH ≥ 4.5 and HRT ≥ 5 h. Decrease of influent pH to 3.0 reduced the NRE to 20%. The addition of 10 and 20 mg/L of ethanol to the influent did not significantly impact NRE of the RPPB. PRACTITIONER POINTS: Pyrite-assisted denitrification is proposed for treating acidic wastewaters. Nitrogen removal >60% was maintained at feed pH as low as 4.7. Decrease of feed pH to 3 strongly inhibited denitrification. The presence of organic carbon in the feed did not affect the autotrophic process.

Vertical-flow constructed wetland based on pyrite intensification: Mixotrophic denitrification performance and mechanism

Deep nitrogen removal from low-carbon wastewater is a pressing water treatment challenge as of yet. Eight sets of vertical-flow constructed wetland (VFCW) intensified by pyrite were designed and applied to treat with low C/N ratio wastewater in this research. The results showed that the addition of pyrite (100% added) significantly promoted TN removal with an efficiency higher than 27.05% under low C/N ratio conditions, indicating that mixotrophic denitrification was achieved in VFCW. Microbial analysis showed that the community structure and diversity of microorganisms were changed significantly, and the growth of autotrophic (Thiobacillus) and heterotrophic bacteria (Thauera) concomitantly enhanced. It is recommended that the addition amount of pyrite is 75% of the wetland volume, meantime, mixing evenly with 25% high porosity substrate (such as activated carbon, volcanic stone, etc.), which could enhance the effective adhesion of microorganisms and their contact area with pyrite, ultimately improve the denitrification capacity of the VFCW.

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.

Enrichment of Autotrophic Denitrifiers From Anaerobic Sludge Using Sulfurous Electron Donors

This study compared the rates and microbial community development in batch bioassays on autotrophic denitrification using elemental sulfur (S0), pyrite (FeS2), thiosulfate (S2O3 2-), and sulfide (S2-) as electron donor. The performance of two inocula was compared: digested sludge (DS) from a wastewater treatment plant of a dairy industry and anaerobic granular sludge (GS) from a UASB reactor treating dairy wastewater. All electron donors supported the development of a microbial community with predominance of autotrophic denitrifiers during the enrichments, except for sulfide. For the first time, pyrite revealed to be a suitable substrate for the growth of autotrophic denitrifiers developing a microbial community with predominance of the genera Thiobacillus, Thioprofundum, and Ignavibacterium. Thiosulfate gave the highest denitrification rates removing 10.94 mM NO3 - day-1 and 8.98 mM NO3 - day-1 by DS and GS, respectively. This was 1.5 and 6 times faster than elemental sulfur and pyrite, respectively. Despite the highest denitrification rates observed in thiosulfate-fed enrichments, an evaluation of the most relevant parameters for a technological application revealed elemental sulfur as the best electron donor for autotrophic denitrification with a total cost of 0.38 € per m3 of wastewater treated.

Shortcut nitrification-denitrification and biological phosphorus removal in acetate- and ethanol-fed moving bed biofilm reactors under microaerobic/aerobic conditions

This study investigated the feasibility of coupling simultaneous partial nitrification and denitrification (SPND) to biological phosphorus removal in continuous-flow intermittently-aerated moving bed biofilm reactors (MBBRs) fed with different carbon sources, i.e. ethanol and acetate. Bacterial cultivation at pH 8.2 (±0.2), 26-28 °C and SRT of 4 day and microaerobic/aerobic MBBR operation allowed to achieve average dissolved organic carbon (DOC), total inorganic nitrogen (TIN) and P-PO₄^3- removal efficiencies (REs) of 100%, 81-88% and 83-86% at HRT of 1 day, dissolved oxygen (DO) range of 0.2-3 mg L^-1 and feed C/N and C/P ratios of 3.6 and 11, respectively. Acetate supplementation favored a diversified microbial community, while overgrowth of heterotrophs was observed when increasing feed C/N ratio in ethanol-fed MBBR. Illumina sequencing displayed the presence of putative phosphorus accumulating organisms (PAOs) such as Hydrogenophaga and Pseudomonas in MBBR biofilm and suspended biomass, whereas no typical NOB was identified during the study.

Effect on simultaneous removal of ammonia, nitrate, and phosphorus via advanced stacked assembly biological filter for rural domestic sewage treatment

The discharge of ammonia-nitrogen (NH₃-N), total nitrogen (TN), chemical oxygen demand (COD), and total phosphorus (TP) in rural sewage usually exceeds the Pollutant Discharge Standard for Urban Sewage Treatment Plants (GB18918-2002). Efficient and cost-effective removal of these pollutants cannot be simultaneously realized using conventional rural sewage treatment methods. Thus, an assembled biological filter (D50 × W50 × H113 cm), including a phosphorus removal layer filled with solid polymeric ferric sulfate and alternating aerobic-anaerobic layers, is proposed herein. The aerobic (anerobic) layers were filled with zeolite (zeolite and composite soil) at different intervals. This system was used for the treatment of synthetic sewage having COD: 122.0-227.0 mg/L; NH₃-N: 29.1-47.0 mg/L; TN: 28.0-58.0 mg/L; and TP: 2.0-3.8 mg/L. Based on optimal operation conditions (40 L/h reflow rate, without artificial aeration, and 12-h operation cycle), the system showed NH₃-N, TN, COD, and TP removal efficiencies of 87.1  ±  8.1, 83.4  ±  7.9, 91.0  ±  9.4, and 80.0  ±  6.4%, respectively. Further, in the pilot-scale test, under the same optimal parameters, the removal efficiencies of NH₃-N, TN, COD, and TP were 78.9  ±  8.1, 75.4  ±  7.9, 82  ±  9.4, and 76  ±  6.4%, respectively. Furthermore, in the different functional units of the system, a large number of functional bacteria capable of efficiently facilitating the simultaneous removal of the different pollutants from sewage were identified. Therefore, this proposed system, which complies with current environmental discharge regulations, can be a more sustainable approach for the treatment of unattended rural sewage.

Pathways regulating the enhanced nitrogen removal in a pyrite based vertical-flow constructed wetland

In this study, two vertical constructed wetland using natural pyrite (P-VFCW) and quartz sand (C-VFCW) as substrate were constructed, and the enhanced nitrate removal mechanism by pyrite was further investigated. Results showed that the nitrate removal efficiency (NRE) of P-VFCW was 4% higher than that of C-VFCW with a C/N of 0. Interestingly, the difference on NRE between CWs markedly enlarged with C/N and hydraulic retention time (HRT) simultaneously increasing. At a COD/N of 6 and HRT of 24 h, the effluent average NO₃^--N and NO₂^--N concentrations in P-VFCW and C-VFCW were 2.36 ± 2.64 mg/L/1.34 ± 1.28 mg/L, 9.20 ± 6.91 mg/L/5.57 ± 3.68 mg/L, respectively, revealing pyrite could promote heterotrophic denitrification and avoid nitrite accumulation. After the whole operation, a better growth of Canna indica occurred in P-VFCW. High-throughput sequencing implied that denitrifying bacteria (Comamonas), iron oxidation and reduction microorganism (Thiobacillus) and the rhizosphere microorganism differed in CWs.

Effect of calcinated pyrite on simultaneous ammonia, nitrate and phosphorus removal in the BAF system and the Fe2+ regulatory mechanisms: Electron transfer and biofilm properties

To efficiently remove nitrogen and phosphorus from secondary effluent with low values of COD/TN, a novel biological aerated filter (BAF) utilizing calcined pyrite with a large specific surface area (SSA) and pore diameter (PD) was designed to address this challenge. From the perspective of nutrients removal performance, and the corresponding effluent total nitrogen (TN) and PO₄^3--P in the calcined pyrite autotrophic denitrification (CPAD) process decreased from 40.21 to 1.07 mg/L to 1.22 and 0.14 mg/L, respectively. Furthermore, the nutrients removal kinetics analysis showed that the CPAD and pyrite autotrophic denitrification (PAD) processes could be fitted with Half-order and Zero-order reactions via kinetics analysis, respectively, indicating that the TN removal performance of CPAD processes was better than that of the PAD process. Moreover, CPAD combined with sulfur autotrophic denitrification (SAD) processes was fitted by First-order reaction, and the TN removal performance was further enhanced over the CPAD process. From the perspective of microregulation, Fe²⁺ production in the PAD and CPAD processes could accelerate the electron transfer rate by increasing electron transport system activity (ETSA) and reducing electrochemical impedance spectroscopy (EIS). Moreover, Fe²⁺ stimulated microbes to produce more proteins (PN) and C₁₀-HSL, which improved biofilm stability and interspecific communication processes. Notably, nitrifiers and autotrophic denitrifiers were simultaneously enriched via detection of high-throughput sequencing of 16 S rRNA genes, which verified the feasibility of simultaneous nitrification and autotrophic denitrification. Therefore, BAF with calcined pyrite and sulfur as composite fillers have a considerable advantage in nutrients removal.