Mitigating nitrite accumulation during S0-based autotrophic denitrification: Balancing nitrate-nitrite reduction rate with thiosulfate as external electron donor

Thermodynamic Inhibition of Microbial Sulfur Disproportionation in a Multisubunit Designed Sulfur-Siderite Packed Bioreactor

Sulfur disproportionation (S0DP) poses a challenge to the robust application of sulfur autotrophic denitrification due to unpredictable sulfide production, which risks the safety of downstream ecosystems. This study explored the S0DP occurrence boundaries with nitrate loading and temperature effects. The boundary values increased with the increase in temperature, exhibiting below 0.15 and 0.53 kg-N/m3/d of nitrate loading at 20 and 30 °C, respectively. A pilot-scale sulfur-siderite packed bioreactor (150 m3/d treatment capacity) was optimally designed with multiple subunits to dynamically distribute the loading of sulfur-heterologous electron acceptors. Operating two active and one standby subunit achieved an effective denitrification rate of 0.31 kg-N/m3/d at 20 °C. For the standby subunit, involving oxygen by aeration effectively transformed the facultative S0DP functional community from S0DP metabolism to aerobic respiration, but with enormous sulfur consumption resulting in ongoing sulfate production of over 3000 mg/L. Meanwhile, acidification by the sulfur oxidation process could reduce the pH to as low as 2.5, which evaluated the Gibbs free energy (ΔG) of the S0DP reaction to +2.56 kJ, thermodynamically suppressing the S0DP occurrence. Therefore, a multisubunit design along with S0DP inhibition strategies of short-term aeration and long-term acidification is suggested for managing S0DP in various practical sulfur-packed bioreactors.

Optimized start-up strategies for elemental sulfur packing bioreactor achieving effective autotrophic denitrification

The start-up efficiency of the elemental sulfur packing bioreactor (S0PB) is constrained by the slow growth kinetics of autotrophic microorganisms, which is essentially optimized. This study aims to optimize start-up procedures and offer scientific guidance for the practical applications of S0PB. Through comparing the start-up efficiencies under various conditions related to inoculation, backwashing, and EBCT, it was found that these conditions did not significantly influence start-up time, but they did impact denitrification performance in detail. Using activated sludge as the inoculum was not recommended as the 2.5 ± 0.2 mg-N/L higher nitrite accumulation and 26.0 ± 5.1 % lower TN removal rate, compared to self-enrichment. Starting with a long-to-short EBCT (1 → 0.33 h) achieved higher nitrate removal of 11.5 ± 0.6 mg-N/L and eliminated nitrite accumulation compared to constantly short EBCT (0.33 h) conditions. Daily and postponed backwashing were suggested for long-to-short EBCT and constantly short EBCT start-up, respectively. Enrichment of Sulfurimonas was beneficial for the effective nitrite reduction process.

Sustainable Recovery of Platinum Group Metals from Spent Automotive Three-Way Catalysts through a Biogenic Thiosulfate-Copper-Ammonia System

This study explores an eco-friendly method for recovering platinum group metals from a synthetic automotive three-way catalyst (TWC). Bioleaching of palladium (Pd) using the thiosulfate-copper-ammonia leaching processes, with biogenic thiosulfate sourced from a bioreactor used for biogas biodesulfurization, is proposed as a sustainable alternative to conventional methods. Biogenic thiosulfate production was optimized in a gas-lift bioreactor by studying the pH (8-10) and operation modes (batch and continuous) under anoxic and microaerobic conditions for 35 d. The maximum concentration of 4.9 g S2O32- L-1 of biogenic thiosulfate was reached under optimal conditions (batch mode, pH = 10, and airflow rate 0.033 vvm). To optimize Pd bioleaching from a ground TWC, screening through a Plackett-Burman design determined that oxygen and temperature significantly affected the leaching yield negatively and positively, respectively. Based on these results, an optimization through an experimental design was performed, indicating the optimal conditions to be Na2S2O3 1.2 M, CuSO4 0.03 M, (NH4)2SO4 1.5 M, Na2SO3 0.2 M, pH 8, and 60 °C. A remarkable 96.2 and 93.2% of the total Pd was successfully extracted from the solid at 5% pulp density using both commercially available and biogenic thiosulfate, highlighting the method's versatility for Pd bioleaching from both thiosulfate sources.

Managing microbial sulfur disproportionation for optimal sulfur autotrophic denitrification in a pilot-scale elemental sulfur packed-bed bioreactor

Elemental sulfur packed-bed (S0PB) bioreactors for autotrophic denitrification have gained more attention in wastewater treatment due to their organic carbon-free operation, low operating cost, and minimal carbon emissions. However, the rapid development of microbial S0-disproportionation (MS0D) in S0PB reactor during deep denitrification poses a significant drawback to this new technology. MS0D, the process in which sulfur is used as both an electron donor and acceptor by bacteria, plays a crucial role in the microbial-driven sulfur cycle but remains poorly understood in wastewater treatment setups. In this study, we induced MS0D in a pilot-scale S0PB reactor capable of denitrifying over 1000 m3/d nitrate-containing wastewater. Initially, the S0PB reactor stably removed 6.6 mg-NO3--N/L nitrate at an empty bed contact time (EBCT) of 20 mins, which was designated the S0-denitrification stage. To induce MS0D, we reduced the influent nitrate concentrations to allow deep nitrate removal, resulted in the production of large quantities of sulfate and sulfide (SO42-:S2- 3.2 w/w). Meanwhile, other sulfur-heterologous electron acceptors (SHEAs), e.g., nitrite and DO, were also kept at trace levels. The negative correlations between the SHEAs concentrations and the sulfide productions indicated that the absence of SHEAs was a primary inducing factor to MS0D. The microbial community drastically diverged in response to the depletion of SHEAs during the switch from S0-denitrification to S0-disproportionation. An evident enrichment of sulfur-disproportionating bacteria (SDBs) was found at the S0-disproportionation stage, accompanied by the decline of sulfur-oxidizing bacteria (SOBs). In the end, we discovered that shortening the EBCT and increasing the reflux ratio could inhibit sulfide production by reducing it from 43.9 mg/L to 3.2 mg/L or 25.5 mg/L. In conclusion, our study highlights the importance of considering MS0D when designing and optimizing S0PB reactors for sustainable autotrophic sulfur denitrification in real-life applications.

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.

Synergistic removal of nitrate by a cellulose-degrading and denitrifying strain through iron loaded corn cobs filled biofilm reactor at low C/N ratio: Capability, enhancement and microbiome analysis

Optimization of nitrate removal rate under low carbon-to-nitrogen ratio has always been one of the research hotspots. Biofilm reactor based on functional carrier and using interspecific synergic effect of strains provides an insight. In this study, iron-loaded corn cob was used as a functional carrier that can contribute to the cellulose degradation, iron cycling, and collaborative denitrification process of microorganisms. During biofilm reactor operation, the maximum nitrate removal efficiency was 99.30% and could reach 81.73% at no carbon source. Dissolved organic carbon and carrier characterization showed that strain ZY7 promoted the release of carbon source. The crystallinity of cellulose I and II in carrier of experimental group increased by 31.26% and decreased by 21.83%, respectively, in comparison to the control group. Microbial community showed the synergistic effect among different strains. The vitality and metabolic activity of the target microorganisms in bioreactor were increased through interspecific bacterial cooperation.

Simultaneous nitrate and phosphate removal based on thiosulfate-driven autotrophic denitrification biofilter filled with volcanic rock and sponge iron

This study constructed two thiosulfate-driven autotrophic denitrification biofilters filled with volcanic rock (VR-BF), sponge iron and volcanic rock (SIVR-BF), respectively. The nitrate removal load (3200 g/m³/d) and efficiency (98 %) of SIVR-BF were higher than those of VR-BF. The removal of phosphate in SIVR-BF was mainly through forming FePO₄ and Fe₃(PO₄)₂(OH)₂. Sulfur and iron cycles in SIVR-BF contributed to Fe (II)/Fe (III) electron shuttle, as well as S^2-, S⁰, Sn^2- electron buffer and energy storage, which improved nitrate removal and electron utilization. The formation of multi-path collaborative denitrification dominated by sulfur autotrophic denitrification (64.2 ∼ 89.6 %) in SIVR-BF. The other denitrification pathways, such as iron autotrophic denitrification, which buffered pH and reduced sulfate production. Thiobacillus (38.6 %) and Ferritrophicum (25.3 %) were the dominant genus of VR-BF and SIVR-BF, respectively, which played crucial roles in autotrophic denitrification of iron and sulfur. SIVR-BF was a promising process to realize iron-sulfur coupling autotrophic denitrification and phosphate removal.

Thiosulfate as external electron donor accelerating denitrification at low temperature condition in S0-based autotrophic denitrification biofilter

This study was carried out to determine the inhibition of low temperature on the performance of S⁰-based autotrophic denitrification (S⁰-SAD) biofilter, and proposed to enhance the nitrate removal efficiency with thiosulfate as external electron donor. With the decline of temperature from 30 °C to 10 °C at 0.25 h of empty bed contact time (EBCT), the nitrate removal rate presented a logarithmical drop, and the effluent nitrate dramatically increased from 9.19 mg L^-1 to 15.13 mg L^-1. EBCT was prolonged until 0.33 h for 20 °C, 0.66 h for 15 °C and 1.5 h for 10 °C, respectively, to maintain the effluent nitrate below 10 mg L^-1. Such excessive variation of EBCT for different temperature is undoubtedly incredible for practical engineering. Thiosulfate, as the external electron donor, was adopted to compensate the efficiency loss during temperature decrease, which significantly prompted nitrate removal rate to 0.59, 0.53 and 0.31 kg N m^-3 d^-1 at 20 °C, 15 °C and 10 °C conditions, respectively, even at a short EBCT of 0.25 h. It not only acted as compensatory electron donor for nitrate removal, but also promoted the contribution of elemental sulfur via accelerating the DO consumption and extended larger effective volume of S⁰-layer for denitrification. Meanwhile, the significant enrichment of Sulfurimonas and Ferritrophicum provided biological evidences to the enhancement process. However, the incomplete consumption of thiosulfate was observed especially at EBCT of 0.25 h and 10 °C, and the thiosulfate runoff needs to be concerned in case of contaminating the effluent. Herein, approximately extending EBCT to 0.66 h and decreasing thiosulfate dosage were conducted simultaneously, thereby achieving 100% thiosulfate utilization efficiency and expected nitrate removal. This study provided a fundamental guidance to design and operate S⁰-SAD biofilter in response to seasonal temperature variation for practical engineering.

Effects of different reduced sulfur forms as electron donors in the start-up process of short-cut sulfur autotrophic denitrification

In this study, two short-cut sulfur autotrophic denitrification (SSADN) reactors were initiated using different reduced sulfur forms as electron donors and their effects on the start-up speed of the SSADN process, NO₂^--N accumulation characteristics, and microbial community were investigated. Results revealed that during the same period, due to the relatively slow S⁰ dissolution rate, the NO₂^--N production rate realized by microorganisms in S⁰-SSADN (NO₂^--N production rate (NPR), 174 mg/(L·d)) was significantly slower than S^2--SSADN (NPR, 679 mg/(L·d)). The NO₂^--N accumulation efficiency (NAE) was maintained > 80%, which was significantly higher than S^2--SSADN. In the SSADN system using different reduced sulfur forms, the microbial community structure and abundance considerably differed. The main sulfur-oxidizing bacteria (SOB) in S⁰-SSADN were Sulfurimonas (6.5%) and Thiobacillus (5.3%). The main SOB species in S^2--SSADN was Thiomonas (13.6%). Thermomonas played an important role in the two reactors as an important NO₃^--N denitrifying bacteria species.