Integrating sulfur, iron(II), and fixed organic carbon for mixotrophic denitrification in a composite filter bed reactor for decentralized wastewater treatment: Performance and microbial community

Elucidating distinct roles of chemical reduction and autotrophic denitrification driven by three iron-based materials in nitrate removal from low carbon-to-nitrogen ratio wastewater

Effective nitrate removal is a key challenge when treating low carbon-to-nitrogen ratio wastewater. How to select an effective inorganic electron donor to improve the autotrophic denitrification of nitrate nitrogen has become an area of intense research. In this study, the nitrate removal mechanism of three iron-based materials in the presence and absence of microorganisms was investigated with Fe2+/Fe0 as an electron donor and nitrate as an electron acceptor, and the relationship between the iron materials and denitrifying microorganisms was explored. The results indicated that the nitrogen removal efficiency of each iron-based material coupled sludge systems was higher than that of iron-based material. Furthermore, compared with the sponge iron coupled sludge system (60.6%-70.4%) and magnetite coupled sludge (56.1%-65.3%), the pyrite coupled sludge system had the highest removal efficiency of TN, and the removal efficiency increased from 62.5% to 82.1% with time. The test results of scanning electron microscope, X-ray photoelectron spectroscopy and X-ray diffraction indicated that iron-based materials promoted the attachment of microorganisms and the chemical reduction of nitrate in three iron-based material coupled sludge systems. Furthermore, the pyrite coupled sludge system had the highest nitrite reductase activity and can induce microorganisms to secrete more extracellular polymer substances. Combined with high-throughput sequencing and PICRUSt2 functional predictive analysis software, the total relative abundance of the dominant bacterial in pyrite coupled sludge system was the highest (72.06%) compared with the other iron-based material systems, and the abundance of Blastocatellaceae was relatively high. Overall, these results suggest that the pyrite coupled sludge system was more conducive to long-term stable nitrate removal.

Nutrient removal and microbial community succession in moving bed biofilm reactor: Effects of influent carbon to nitrogen ratio fluctuation

This study investigated the nutrient removal and microbial community succession in moving bed biofilm reactor under stable and three levels of influent carbon/nitrogen (C/N) ratio fluctuation (± 10%, ± 20%, and ± 30%). Under the conditions of influent C/N ratio fluctuation, the removal efficiency of COD and PO43--P decreased 4.7-6.4% and 3.7-12.9%, respectively, while the nitrogen removal was almost unaffected. A sharp decrease in the content of culturable functional bacteria related to nitrogen and phosphorus removal including nitrite-oxidizing bacteria (NOB), aerobic denitrifying bacteria (DNB), and polyphosphate-accumulating organisms (PAOs) from the carrier biofilm was observed. Sequencing analysis revealed that the abundance of Candidatus Competibacter increased 10.3-25.9% and became the dominant genus responsible for denitrification, potentially indicating that nitrate was removed via endogenous denitrification under the influent C/N ratio fluctuation. The above results will provide basic data for the nutrient removal in decentralized wastewater treatment under highly variable influent conditions.

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.

Pyrrhotite-sulfur-limestone composite for high rate nitrogen and phosphorus removal from wastewater: Column study

Pyrrhotite-sulfur-limestone composite (PSLC) was prepared and PSLC autotrophic denitrification biofilter (PSLCAD) was constructed with PSLC particle (2-4.75 mm) in this study. During treating synthetic, municipal and industrial secondary effluent, PSLCAD showed good NO3--N and PO43--P removal, and the highest TON (Total oxidized nitrogen) removal rate of PSLCAD was up to 1749.91 mg/L/d. At HRT 0.5 h, and influent NO3--N 21.09 mg/L, TON removal rate was up to 1005.12 mg/L with effluent NO3--N 0.10 mg/L. PSLCAD achieved effluent PO43--P below 0.2 mg/L when influent PO43--P was around 0.5 mg/L. HRT down to 0.5 h had no negative impacts on N removal. Effluent pH below 7 was harmful to denitrification performance of PSLCAD. TON removal rate increased with influent NO3--N increasing, but influent NO3--N over 103.55 mg/L decreased NO3--N removal rate. In PSLCAD biofilter, the most dominant bacteria were Thiobacillus and Sulfurimonas, and they played the most important role in denitrification, but the abundance of heterotrophic denitrifiers was also quite high. PO43- was mainly removed through precipitate of Fe-P in PSLCAD. The synergistic effects between pyrrhotite and sulfur autotrophic denitrification were much enhanced, and that caused PSLCAD to achieve high rate N and P removal.

Microbial community structure and functional prediction in five full-scale industrial park wastewater treatment plants

The development of industrial parks has become an important global trend contributing significantly to economic and industrial growth. However, this growth comes at a cost, as the treatment of multisource industrial wastewater generated in these parks can be difficult owing to its complex composition. Microorganisms play a critical role in pollutant removal during industrial park wastewater treatment. Therefore, our study focused on the microbial communities in five full-scale industrial park wastewater treatment plants (WWTPs) with similar treatment processes and capacities. The results showed that denitrifying bacteria were dominant in almost every process section of all the plants, with heterotrophic denitrification being the main pathway. Moreover, autotrophic sulfur denitrification and methane oxidation denitrification may contribute to total nitrogen (TN) removal. In plants where the influent had low levels of COD and TN, dominant bacteria included oligotrophic microorganisms like Prosthecobacter (2.88 % ~ 10.02 %) and hgcI_clade (2.05 % ~ 9.49 %). Heavy metal metabolizing microorganisms, such as Norank_f__PHOS-HE36 (3.96 % ~ 5.36 %) and Sediminibacterium (1.86 % ~ 5.34 %), were prevalent in oxidation ditch and secondary settling tanks in certain plants. Functional Annotation of Prokaryotic Taxa (FAPROTAX) revealed that microbial communities in the regulation and hydrolysis tanks exhibited higher potential activity in the nitrogen (N) and sulfur (S) cycles than those in the oxidation ditch. Sulfate/sulfite reduction was common in most plants, whereas the potential occurrence of sulfide compounds and thiosulfate oxidation tended to be higher in plants with a relatively high sulfate concentration and low COD content in their influent. Our study provides a new understanding of the microbial community in full-scale industrial park WWTPs and highlights the critical role of microorganisms in the treatment of industrial wastewater.

Enhanced nitrogen removal in immersed rotating self-aerated biofilm reactor: nitrogen removal pathway and microbial mechanism

To achieve energy-efficient treatment of the rural wastewater with satisfying performance, a novel immersed rotating self-aerated biofilm reactor (iRSABR) was proposed in this study. The iRSABR system showed better biofilm renewal performance and higher microbial activity. The effect of different regulation strategies on the iRSABR system was investigated in this study. The 70% immersion ratio and 4 r/min rotation speed (stage III) exhibited the best performance, with a total nitrogen removal efficiency of 86% and a simultaneous nitrification-denitrification (SND) rate of 76%, along with the highest electron transport system activity. The nitrogen removal pathway revealed that the SND was achieved through autotrophic/heterotrophic nitrification and aerobic/anoxic denitrification. The regulation strategy in the iRSABR system established a synergistic microbial community with main functional bacteria of nitrification (Nitrosomonas), anoxic denitrification (Flavobacterium, Pseudoxanthomonas), and aerobic denitrification (Thauera). This study highlighted the feasibility and adaptability of the iRSABR system toward energy-efficient rural wastewater treatment.

Microbial mixotrophic denitrification using iron(II) as an assisted electron donor

Mixotrophic denitrification processes have a great potential in nitrogen removal in biological wastewater treatment processes. However, so far, few studies have focused on the mixotrophic denitrification system using Fe(II) as an exclusively assisted electron donors and the underlying mechanisms in such a process remain unclear. Furthermore, the mechanisms by which microorganisms cover carbon, nitrogen, phosphorus and iron in an iron-assisted mixotrophic system remain unrevealed. In this work, we explore the feasibility of using Fe(II) as an assisted electron donor for enhancing simultaneous nitrogen and phosphorus removal via long-term reactor operation and batch tests. The results show that Fe(II) could provide electrons for efficient nitrate reduction and that biological reactions played a predominant role in these systems. In these systems Thermomonas, a strain of nitrate-reduction Fe(II)-oxidation bacterium, was enriched and accounted for a maximum abundance of 60.2%. These findings indicate a great potential of the Fe(II)-assisted mixotrophic denitrification system for practical use as an efficient simultaneous nitrogen and phosphorus removal process.

Achieving superior nitrogen removal in an air-lifting internal circulating reactor for municipal wastewater treatment: Performance, kinetic analysis, and microbial pathways

Anticipated growth in living standards has accentuated higher requirements for effluent quality from municipal wastewater treatment. In this study, an air-lifting internal circulating reactor with a high internal circulation ratio (36:1) was established to treat municipal wastewater with a long-term operation. In the bioreactor, the average effluent chemical oxygen demand, total nitrogen, and ammonium nitrogen could be 13.1, 5.7, and lower than 1 mg/L, respectively. Further analysis of nitrogen removal showed that traditional nitrification and denitrification, simultaneous nitrification and denitrification (SND), and nitrogen assimilation accounted for 27.4 %, 68.7 %, and 3.9 % respectively. The proportion of aerobic bacteria (Saprospiraceae) and facultative bacteria (Comamonadaceae) were significantly increased, indicating a higher capacity for organic degradation in the reactor. The relative abundance of denitrifying bacteria and bacterial groups with SND (Comamonadaceae) increased. These results suggested the air-lifting internal circulating reactor could be a viable and efficient option for superior nitrogen removal in wastewater treatment.

Simultaneous nitrification and denitrification in a novel rotating self-aerated biofilm reactor for decentralized wastewater treatment

Decentralized wastewater pollution in rural areas has become a serious problem for the rural environment. In this study, a novel rotating self-aerated biofilm reactor was developed for decentralized wastewater treatment without any aeration equipment. After the long-term operation of 110 days, the removal efficiency reached to 96.06 % (COD), 98.06 % (NH₄⁺-N), and 62.58 % (TN) in the last phase. Under high dissolved oxygen level, the simultaneous nitrification-denitrification (SND) maintained at a stable ratio of 62.53 % and the denitrification rates reached over 28.37 mg/L/h. With the organic loading rate increased, key nitrogen functional bacterial communities such as anoxic denitrifiers (Thiothrix, Flavobacterium, Pseudoxanthomonas, Aquimonas and Azoarcus) and aerobic denitrifiers (Hydrogenophaga, Zoogloea and Terrimonas) increased obviously. Overall, microbial analysis and nitrogen metabolism pathway indicated that an integration of SND process was achieved in this single reactor by the combined action of nitrification, denitrification and comammox without any aeration equipment.

A comprehensive review on mixotrophic denitrification processes for biological nitrogen removal

Biological denitrification is the most widely used method for nitrogen removal in water treatment. Compared with heterotrophic and autotrophic denitrification, mixotrophic denitrification is later studied and used. Because mixotrophic denitrification can overcome some shortcomings of heterotrophic and autotrophic denitrification, such as a high carbon source demand for heterotrophic denitrification and a long start-up time for autotrophic denitrification. It has attracted extensive attention of researchers and is increasingly used in biological nitrogen removal processes. However, so far, a comprehensive review is lacking. This paper aims to review the current research status of mixotrophic denitrification and provide guidance for future research in this field. It is shown that mixotrophic denitrification processes can be divided into three main kinds based on different kinds of electron donors, mainly including sulfur-, hydrogen-, and iron-based reducing substances. Among them, sulfur-based mixotrophic denitrification is the most widely studied. The most concerned influencing factors of mixotrophic denitrification processes are hydraulic retention times (HRT) and ratio of chemical oxygen demand (COD) to total inorganic nitrogen (C/N). The dominant functional bacteria of sulfur-based mixotrophic denitrification system are Thiobacillus, Azoarcus, Pseudomonas, and Thauera. At present, mixotrophic denitrification processes are mainly applied for nitrogen removal in drinking water, groundwater, and wastewater treatment. Finally, challenges and future research directions are discussed.

Effects of the aeration mode on nitrogen removal in a compact constructed rapid infiltration system for advanced wastewater treatment

The configuration and the effective operation of constructed rapid infiltration (CRI) systems are of significance for advanced wastewater treatment. In this study, a novel CRI system was developed with a compact structure consisting of two stages, i.e., oxic and anoxic stages. The CRI system was continuously operated for about 140 days under different aeration modes, i.e., tidal flow, continuous aeration, and intermittent aeration. Nitrogen removal was not desirable with tidal flow due to the insufficient oxygen supply in the oxic stage for nitrification, while continuous aeration could achieve good performance for chemical oxygen demand (COD), ammonium, total nitrogen (TN), and total phosphorus (TP) removal. By comparison, the CRI system operated with intermittent aeration was more favorable due to the effective removal ability for pollutants and relatively lower energy demand. The microbial community analysis revealed that Proteobacteria was the dominant phylum in both oxic and anoxic stages of the developed CRI system. Functional microbial groups (Plasticicumulans, Pseudomonas, and Nitrospira in the oxic stage; Thauera, Candidatus_Competibacter, and Dechloromonas in the anoxic stage) were identified for the mediation of carbon, nitrogen, and phosphorus in the system. This study evaluated the feasibility and the optimal aeration mode of the developed CRI system for advanced wastewater treatment, which could satisfy the requirement for the high standard of effluent quality.

Denitrification performance of sulfur-based autotrophic denitrification and biomass‑sulfur-based mixotrophic denitrification in solid-phase denitrifying reactors using novel composite filters

Both the elemental sulfur-based autotrophic denitrification (ESAD) and the biomass‑sulfur-based mixotrophic (simultaneous autotrophic and heterotrophic) denitrification processes (BSMD) are efficient methods for removing nitrate from wastewater. However, a comparative analysis of the denitrification capacity of the BSMD and ESAD in the packed bed reactors is still lacking. In this paper, corncob powder was selected as the biomass source to prepare biomass‑sulfur-based composite filter (BSCF) for the BSMD process. The denitrification performances of the three identical lab-scale bioreactors packed with varying elemental sulfur-based composite filters (ESCFs) were compared under varying loading conditions, and the optimal ESCF of the ESAD system was 2:1 by weight ratio of sulfur powder to shell powder. In pilot-scale experiments, the results showed that BSCF could decrease the sulfate productivity and gave better denitrification performance than the ESCF with the optimal nitrate removal rate (NRR) of 504 ± 12.3 mg NO₃^--N·L^-1·d^-1. In addition, the two-stage flushing strategy (for the removal of aged sludge) can effectively improve the denitrification capacity, while the denitrification will be inhibited when the influent dissolved oxygen concentration was higher than 4.5 mg L^-1. Moreover, the heterotrophs and autotrophs were abundant in the reactors. Over time, the abundance of autotrophs increased while that of heterotrophs decreased. Overall, BSCF could be a promising and economic technology to improve the effluent quality.

Effectiveness of agricultural waste in the enhancement of biological denitrification of aquaculture wastewater

Nitrogen pollution in aquaculture wastewater can pose a significant health and environmental risk if not removed before wastewater is discharged. Biological denitrification uses external carbon sources to remove nitrogen from wastewater; however, these carbon sources are often expensive and require significant energy. In this study, we investigated how six types of agricultural waste can be used as solid carbon sources in biological denitrification. Banana stalk (BS), loofah sponge (LS), sorghum stalk (SS), sweet potato stalk (SPS), watermelon skins (WS) and wheat husk (WH) were studied to determine their capacity to release carbon and improve denitrification efficiency. The results of batch experiments showed that all six agricultural wastes had excellent carbon release capacities, with cumulative chemical oxygen demands of 37.74-535.68 mg/g. During the 168-h reaction, the carbon release process followed the second-order kinetic equation and Ritger-Peppas equation, while carbon release occurred via diffusion. The kinetic equation fitting, scanning electron microscopy, and Fourier transform infrared spectroscopy results showed that LS had the lowest c_m and the maximum t_1/2 values and only suffered a moderate degree of hydrolysis. It also had the lowest pollutant release rate and cumulative chemical oxygen demand, as well as the most efficient removal of total phosphorous (TP) and total nitrogen (TN). Therefore, we concluded that LS has the lowest potential risk of excess carbon release and capacity for long-lasting and stable carbon release. The WS leachate had the highest TN contents, while the SPS leachate had the highest TP content. In the 181-h denitrification reaction, all six agricultural wastes completely removed nitrate and nitrite; however, SS had the highest denitrification rate, followed by LS, WH, BS, SPS, and WS (2.16, 1.35, 1.35, 1.34, 1.34, and 1.01 mg/(L·h), respectively). The denitrification process followed a zero-order and first-order kinetic equation. These results provide theoretical guidance for effectively selecting agricultural waste as a solid carbon source and improving the denitrification efficiency of aquaculture wastewater treatment.

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.