Mixed sulfur-iron particles packed reactor for simultaneous advanced removal of nitrogen and phosphorus from secondary effluent

Amorphous Fe substrate enhances nitrogen and phosphorus removal in sulfur autotrophic process

The autotrophic denitrification of coupled sulfur and natural iron ore can remove nitrogen and phosphorus from wastewater with low C/N ratios. However, the low solubility of crystalline Fe limits its bioavailability and P absorption capacity. This study investigated the effects of amorphous Fe in drinking water treatment residue (DWTR) and crystalline Fe in red mud (RM) on nitrogen and phosphorus removal during sulfur autotrophic processes. Two types of S-Fe cross-linked filler particles with three-dimensional mesh structures were obtained by combining sulfur with the DWTR/RM using the hydrogel encapsulation method. Two fixed-bed reactors, sulfur-DWTR autotrophic denitrification (SDAD) and sulfur-RM autotrophic denitrification (SRAD), were constructed and stably operated for 236 d Under a 5-8-h hydraulic retention time, the average NO3--N, TN, and phosphate removal rates of SDAD and SRAD were 99.04 %, 96.29 %, 94.03 % (SDAD) and 97.33 %, 69.97 %, 82.26 % (SRAD), respectively. It is important to note that fermentative iron-reducing bacteria, specifically Clostridium_sensu_stricto_1, were present in SDAD at an abundance of 58.17 %, but were absent from SRAD. The presence of these bacteria facilitated the reduction of Fe (III) to Fe (II), which led to the complete denitrification of the S-Fe (II) co-electron donor to produce Fe (III), completing the iron cycle in the system. This study proposes an enhancement method for sulfur autotrophic denitrification using an amorphous Fe substrate, providing a new option for the efficient treatment of low-C/N wastewater.

Sulfur autotrophic denitrification as an efficient nitrogen removals method for wastewater treatment towards lower organic requirement: A review

The sulfur autotrophic denitrification (SADN) process is an organic-free denitrification process that utilizes reduced inorganic sulfur compounds (RISCs) as the electron donor for nitrate reduction. It has been proven to be a cost-effective and environment-friendly approach to achieving carbon neutrality in wastewater treatment plants. However, there is no consensus on whether SADN can become a dominant denitrification process to treat domestic wastewater or industrial wastewater if organic carbon is desired to be saved. Through a comprehensive summary of the SADN process and extensive discussion of state-of-the-art SADN-based technologies, this review provides a systematic overview of the potential of the SADN process as a sustainable alternative for the heterotrophic denitrification (HD) process (organic carbons as electron donor). First, we introduce the mechanism of the SADN process that is different from the HD process, including its transformation pathways based on different RISCs as well as functional bacteria and key enzymes. The SADN process has unique theoretical advantages (e.g., economy and carbon-free, less greenhouse gas emissions, and a great potential for coupling with novel autotrophic processes), even if there are still some potential issues (e.g., S intermediates undesired production, and relatively slow growth rate of sulfur-oxidizing bacteria [SOB]) for wastewater treatment. Then we present the current representative SADN-based technologies, and propose the outlooks for future research in regards to SADN process, including implement of coupling of SADN with other nitrogen removal processes (e.g., HD, and sulfate-dependent anaerobic ammonium oxidation), and formation of SOB-enriched biofilm. This review will provide guidance for the future applications of the SADN process to ensure a robust-performance and chemical-saving denitrification for wastewater treatment.

Effects of magnetite on microbially driven nitrate reduction processes in groundwater

Nitrate is a common pollutant in the aquatic environment. Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are the main reduction processes of nitrate. In the relatively closed sediment environment, the competitive interaction of these two nitrate reduction determines whether the ecosystem removes or retains nitrogen. In the process of NO₃^--N bioreduction, Magnetite, which is a common mineral present in soil and other sediments can play a crucial role. However, it is still not clear whether magnetite promotes or inhibits NO₃^--N bioreduction. In this paper, the effect of magnetite on NO₃^--N bioreduction was studied by batch experiments. The results show that magnetite can increase the NO₃^--N reduction rate by 1.48 %, and can inhibit the DNRA process at the beginning of the reaction and then promote the DNRA process. Magnetite changed the microbial community structure in our experiment systems. The relative abundance of Sphingomonas, which mainly exists in a high carbon and low nitrogen environment, increased under sufficient carbon source conditions. The relative abundance of Fe-oxidizing and NO₃^--N reducing bacteria, such as Flavobacterium, increased in the absence of carbon sources but in the presence of magnetite. In addition, magnetite can significantly increase activity of the microbial electron transport system (ETS). the added microbial electronic activity of magnetite increased nearly two-fold under the same experiment conditions. The acid produced by the metabolisms of Pseudomonas and Acinetobacter further promotes the dissolution of magnetite, thus increasing the concentration of Fe (II) in the system, which is beneficial to autotrophic denitrifying bacteria and promote the reduction of NO₃^--N. These findings can enhance our understanding of the interaction mechanism between iron minerals and nitrate reducing bacteria during nitrate reduction under natural conditions.

Remediation of nitrogen polluted water using Fe-C microelectrolysis and biofiltration under mixotrophic conditions

A hybrid biofilter was established on Fe-C supported carriers aimed to enhance nitrogen removal from polluted water of low Carbon/Nitrogen (C/N) ratio. Effects of organic loadings, hydraulic retention time (HRT), additional electron donor (Fe²⁺) supplementation and operation mode on the performance of the biofilter were investigated. Results showed that up-flow operation mode was better than down-flow mode in terms of nitrate and total nitrogen (TN) removal at low COD/N. The average removal of NO₃^--N, NH₄⁺ -N and TN attained 83.1%, 84.7% and 81.2%, respectively, under the conditions of influent COD/NO₃^--N = 1.5-3.6, HRT = 10 h and up-flow operation. When the biofilter was operated under autotrophic conditions without organic compounds in influent as electron donors, the biofilter achieved a NO₃^--N removal of 46% and TN removal of 56% depending on the innate electron donors provided by the Fe-C carriers. Supplementation of Fe²⁺ in influent further promoted autotrophic denitrifying process, and the removal of NO₃^--N and TN increased to 96.3% and 84.7%, respectively, at the mol ratio of Fe²⁺/NO₃^- = 10 and HRT = 10 h. The microbial community was analyzed for the biofilm samples enriched under heterotrophic and autotrophic conditions. The Fe-C biofilter boosted the growth of a large population of mixotrophic denitrifying bacteria including Gallionella, heterotrophic denitrifying bacteria Denitratisoma, and autotrophic denitrifying bacteria Thiobacillus and Thioalkalispira. On the whole, the biofilter coupled with Fe-C micro-electrolysis provides a novel strategy to treat polluted water of low C/N under both heterotrophic and autotrophic conditions.

Performance and mechanism of synchronous nitrate and phosphorus removal in constructed pyrite-based mixotrophic denitrification system from secondary effluent

The performance and process of the constructed pyrite-based mixotrophic denitrification (POMD) system using pyrite and residual organic matters as the co-electron donors were investigated for simultaneous removal of N and P from secondary effluent. After the batch experiments, 61.80 ± 3.26% of phosphate and 99.99 ± 0.01% of nitrate were removed, and the obtained nitrate removal rate constant can reach 2.09 days^-1 in POMD system, which was significantly superior to that reported (0.95 day^-1) in pyrite-based autotrophic denitrification (PAD) system. PO₄^3--P removal was mainly achieved via chemical precipitation as FePO₄ with iron, and it was irrelevant with the initial nitrate and ammonium concentrations. High-throughput 16S rRNA gene sequencing analysis showed the coexistence of heterotrophic and autotrophic denitrifiers in the mixotrophic environment. The denitrification process could be divided into two stages according to the carbon balance and calculation of sulfate accumulation: (a) nitrate was mainly reduced heterotrophically during 12-36 h and (b) nitrate was reduced autotrophically after 36 h. The calculated proportion of heterotrophic denitrification was 58.17 ± 3.78%, which was promoted by a higher ammonium concentration. These findings are likely to be useful in understanding the mixotrophic denitrification process and developing a cost-effective technology to simultaneously remove N and P from secondary effluent. Graphical abstract.

Insight into effects of organic and inorganic phosphorus speciations on phosphorus removal efficiency in secondary effluent

Most previous studies of phosphorus (P) removal focused on investigation of the soluble, and particulate P, but ignoring the difference between organic and inorganic P. In this study, the effects of various flocculants, namely polyacrylamide (PAM) and polyaluminum chloride (PAC), on flocculation efficiency in different P speciations (organic and inorganic P) were investigated. A modified method to differentiate between organic and inorganic P content in secondary effluent samples was developed. The results showed that P speciation based on organic/inorganic P (Pearson's correlation R = 0.915, p < 0.05) was more effective than those based on soluble/particulate P (p > 0.05) in evaluating the P content in secondary effluents. The liquid ³¹P nuclear magnetic resonance measurements results indicated that PAM was more effective in removing organic P (phosphonates and orthophosphate monoesters) rather than inorganic P. However, PAC was more effective in removing inorganic P (particularly orthophosphate) rather than organic P. Based on the modeled results of a response surface methodology (RSM), doses of PAM and PAC were optimized for secondary effluent containing different amounts of organic and inorganic P from the two typical wastewater treatment plants (WWTPs) in Wuhan city, China.

DL-cysteine and L-cystine formation and their enhancement effects during sulfur autotrophic denitrification

Sulfur autotrophic denitrification has been proved feasible for nitrate removal from aquatic environments and it utilizes elemental sulfur as the electron donor. A maximum denitrification rate of 194.57 mg N/L·d was achieved with biogenic sulfur as electron donor in a mixed culture collected from sulfur packed bed reactors; this rate was considerably higher than that delivered by α-S₈ or μ-S in the same mixed culture. The elemental sulfur was also tested in the pure culture of Thiobacillus denitrificans, while a lower denitrification rate was noted than in the mixed culture, bio-S (4.86 mg N/L·d) again outperformed other two elemental sulfur's. X-ray absorption near edge structure spectra were collected to examine possible metabolic intermediates during the sulfur autotrophic denitrification process. The analysis revealed the existence of two major intermediates: DL-cysteine and L-cystine. They were found to not only provide electrons but also play a critical role in promoting the elemental sulfur-mediated sulfur autotrophic denitrification process. In general, we investigated the formation and enhancement effects of sulfur intermediates in the sulfur autotrophic denitrification process.

Simultaneous removal of nitrate and hydrogen sulfide by autotrophic denitrification in nitrate-contaminated water treatment

Nitrate contamination is a risk to human health and may cause eutrophication, whereas H₂S is an undesirable constituent in biogas. In order to better understand denitrification using gaseous H₂S as electron donor, this study investigated denitrification at different molar ratios of sulfur and nitrogen (S/N ratios) and H₂S dosages. Although nitrate continued to decrease, a lag in sulfate generation was observed, implying the generation of sulfide oxidizing intermediates, which accumulated even though nitrate was in excess at lower S/N ratios of 0.19 and 0.38. More addition of H₂S could result in a longer lag of sulfate generation. Before depletion of dissolved sulfide, denitrification could proceed with little nitrite accumulation. High throughout sequencing analysis identified two major genera, Thiobacillus and Sulfurimonas, that were responsible for autotrophic denitrification. The simultaneous removal of nitrate and H₂S using a wide range of concentrations could be achieved.

Integrated sulfur- and iron-based autotrophic denitrification process and microbial profiling in an anoxic fluidized-bed membrane bioreactor

The integrated sulfur- and Fe⁰-based autotrophic denitrification process in an anoxic fluidized-bed membrane bioreactor (AnFB-MBR) was developed for the nitrate-contaminated water treatment in order to control sulfate generation and avoid alkalinity supplement. The nitrate removal rate of the AnFB-MBR reached 1.22 g NO₃^--N L^-1d^-1 with NO₃^--N ranging 40-200 mg L^-1 at hydraulic retention times of 1.0-5.0 h. The denitrification in the integrated system was simultaneously carried out by sulfur- and Fe⁰-oxidizing autotrophic denitrifiers. The effluent sulfate generation was decreased by 29.3-70.3% and 31.2-50.9% due to the functional role of Fe⁰-based denitrification in the integrated system. Alkalinity produced by Fe⁰-oxidizing autotrophic denitrification could compensate for the alkalinity consumption by sulfur-based autotrophic denitrification. The sulfur- and Fe⁰-oxidizing autotrophic denitrifying bacterial consortium was composed mainly of bacteria from Thiobacillus, Sulfurimonas, and Geothrix genera. The integrated modes leads to a harmonious co-existence of sulfur- and Fe⁰-oxidizing denitrifying microbes, which may make a difference to the functional performance of the bioreactor. Overall, the integrated sulfur- and Fe⁰-based autotrophic denitrification could overcome the shortcomings of excess sulfate generation and external alkalinity supplementation compared to the sole sulfur-based autotrophic denitrification, indicating further potential for the technology in practice.

Simultaneous removal of nitrate and phosphate from wastewater by siderite based autotrophic denitrification

The potential of simultaneous removal of nitrate and phosphate from wastewater by a single anaerobic Fe(II) oxidizing denitrifiers (the strain PXL1) was assessed using siderite biofilters under different influent TOC concentrations and hydraulic retention times (HRTs) over a 160-day trial. Higher TOC concentrations promoted NO₃^- removal, while there was no significant influence on PO₄^3- removal. Lowering down HRT from 10 h to 5 h did not significantly influence NO₃^- and PO₄^3- removal. The NO₃^- removal performance and microbial community structure in the biofilters indicated that NO₃^- was reduced to N₂ by both strain PXL1 and heterotrophic Acidovorax delafieldii. Iron content analysis of the used siderite along the biofilters showed that PO₄^3- removal was improved by the bio-oxidation of Fe(II) in siderite to Fe(III) via the strain PXL1. The coexistence of the strain PXL1 and natural siderite in nitrate-contaminated aquifers provides a practical technology for in situ remediation of nutrient contaminated waterbodies.

Shift in microbial community structure of anaerobic side-stream reactor in response to changes to anaerobic solid retention time and sludge interchange ratio

A laboratory scale nutrient removal activated sludge system coupled with an anaerobic side-stream reactor was operated for 300 days treating real urban wastewater. A significant decrease in sludge production was obtained increasing the anaerobic solid retention time (SRT_ASSR) and decreasing the sludge interchange ratio (IR). In this study, the microbial community structure was analyzed and compared with the sludge reduction performance. Quantitative polymerase chain reaction analyses encoding 16 ribosomal RNA and functional genes revealed a wide diversity of phylogenetic groups in each experimental period, resulting from long solids retention time and recirculation of sludge under aerobic, anoxic and anaerobic conditions. However, decreasing SRT_ASSR from 10 to 2.5d and increasing IR from 27 to 100%, an increasing selection of both fermenting bacteria able to release extracellular polymeric substances and hydrolyze organic matter and slow growing bacteria involved in nutrient removal were detected and linked to the sludge reduction mechanisms.

A modified BAF system configuring synergistic denitrification and chemical phosphorus precipitation: Examination on pollutants removal and clogging development

The performance of a BAF system configuring simultaneous chemical phosphorus precipitation in the pre-denitrification stage was examined using a continuously operated setup to treat real domestic wastewater. The effects of using no chemical, dosing sole Fe(2+), and dosing combined Fe(2+), PAM, and NaHCO3 in the pre-denitrification tank were assessed by monitoring COD, nitrogen, and phosphorus removal and hydraulic headloss development in the BAF column. Though dosing sole Fe(2+) significantly enhanced phosphorus removal, it would consume alkalinity through hydrolysis and form smaller-sized sludge flocs in the pre-denitrification tank, and hence resulted in affected NH4(+)-N, insoluble COD, and SS removal in the BAF. Dosing combined Fe(2+), PAM, and NaHCO3 can enhance sludge flocculation to form larger flocs and compensate alkalinity consumption. It exhibited sound performance on COD, nitrogen, and phosphorus removal, and led to less frequent BAF backwashing by slowing clogging development in the BAF filter layer.