Response of simultaneous sulfide and nitrate removal process on acute toxicity of substrate concentration and salinity: Single toxicity and combined toxicity

Inhibition of zinc ions in sulfur-driven autotrophic denitrification process: What is the behavior of extracellular polymeric substances?

Sulfur-driven autotrophic denitrification (SAD) process is a cost-effective and sustainable method for nitrogen removal from wastewater. However, a higher concentration of zinc ions (Zn(II)) flowing into wastewater treatment plants poses a potential threat to the SAD process. This study examined that a half maximal inhibitory concentration (IC50) of Zn(II) was 7 mg·L-1 in the SAD process. Additionally, the addition of 20 mg·L-1 Zn(II) resulted in a severe accumulation of nitrite to 150.20 ± 6.00 mg·L-1 when the initial concentration of nitrate was 500 mg·L-1. Moreover, the activities of nitrate reductase, nitrite reductase, dehydrogenase and electron transport system were significantly inhibited under Zn(II) stress. The addition of Zn(II) inhibited EPS secretion and worsened electrochemical properties. The result was attributed to the spontaneous binding between EPS and Zn(II), with a ΔG of -17.50 KJ·mol-1 and a binding constant of 1.77 × 104 M-1, respectively. Meanwhile, the protein, fulvic acid, and humic-like substances occurred static quenching after Zn(II) addition, with -OH and -C=O groups providing binding sites. The binding sequence was fulvic acid→protein→humic acid and -OH → -C=O. Zn(II) also reduced the content of α-helix, which was unfavorable for electron transfer. Additionally, the Zn(II) loosened protein structure, resulting in a 50 % decrease in α-helix/(β-sheet+random coil). This study reveals the effect of Zn(II) on the SAD process and enhances our understanding of EPS behavior under metal ions stress.

Performance and microbial mechanism in sulfide-driven autotrophic denitrification by different inoculation sources in face of various sulfide and sulfate stress

To develop a reliable sulfide (S2-) autotrophic denitrification (SAD) process under S2- and SO42- salinity stresses, the biofilm performance and microbial mechanisms were comparatively studied using different inocula of activated sludge (AS) and intertidal sediment (IS). Biofilm IS enriched more denitrification genes (0.34 %) and S2- oxidation genes (0.29 %) than those with AS. Higher denitrification performance was obtained under S2- (100 mg/L) and SO42- (5-15 g/L Na2SO4) stresses, but no significantly differences were observed under levels of 0-200 mg/L S2- and 30 g/L Na2SO4. Notably, biofilm samples in SAD systems with IS still had more S2- oxidation genes at high S2- levels of 100-200 mg/L and Na2SO4 level of 30 g/L. The key functional genus Thiobacillus accumulated well at 30 g/L Na2SO4, but was strongly inhibited at 200 mg/L S2-. The findings were advantage to SAD application under sulfide and salinity stresses.

Salt stimulates sulfide-driven autotrophic denitrification: Microbial network and metagenomics analyses

Sulfur autotrophic denitrification (SADN) is a promising biological wastewater treatment technology for nitrogen removal, and its performance highly relies on the collective activities of the microbial community. However, the effect of salt (a prevailing characteristic of some nitrogen-containing industrial wastewaters) on the microbial community of SADN is still unclear. In this study, the response of the sulfide-SADN process to different salinities (i.e., 1.5 % salinity, 0.5 % salinity, and without salinity) as well as the involved microbial mechanisms were investigated by molecular ecological network and metagenomics analyses. Results showed that the satisfactory nitrogen removal efficiency (>97 %) was achieved in the sulfide-SADN process (S/N molar ratio of 0.88) with 1.5 % salinity. In salinity scenarios, the genus Thiobacillus significantly proliferated and was detected as the dominant sulfur-oxidizing bacteria in the sulfide-SADN system, occupying a relative abundance of 29.4 %. Network analysis further elucidated that 1.5 % salinity had enabled the microbial community to form a more densely clustered network, which intensified the interactions between microorganisms and effectively improved the nitrogen removal performance of the sulfide-SADN. Metagenomics sequencing revealed that the abundance of functional genes encoding for key enzymes involved in SADN, dissimilatory nitrate reduction to ammonium, and nitrification was up-regulated in the 1.5 % salinity scenario compared to that without salinity, stimulating the occurrence of multiple nitrogen transformation pathways. These multi-paths contributed to a robust SADN process (i.e., nitrogen removal efficiency >97 %, effluent nitrogen <2.5 mg N/L). This study deepens our understanding of the effect of salt on the SADN system at the community and functional level, and favors to advance the application of this sustainable bioprocess in saline wastewater treatment.

Machine learning-based model construction and identification of dominant factor for simultaneous sulfide and nitrate removal process

Accurate water quality prediction models are essential for the successful implementation of the simultaneous sulfide and nitrate removal process (SSNR). Traditional models, such as regression and analysis of variance, do not provide accurate predictions due to the complexity of microbial metabolic pathways. In contrast, Back Propagation Neural Networks (BPNN) has emerged as superior tool for simulating wastewater treatment processes. In this study, a generalized BPNN model was developed to simulate and predict sulfide removal, nitrate removal, element sulfur production, and nitrogen gas production in SSNR. Remarkable results were obtained, indicating the strong predictive performance of the model and its superiority over traditional mathematical models for accurately predicting the effluent quality. Furthermore, this study also identified the crucial influencing factors for the process optimization and control. By incorporating artificial intelligence into wastewater treatment modeling, the study highlights the potential to significantly enhance the efficiency and effectiveness of meeting water quality standards.

Effects of inorganic ions on autotrophic denitrification by Thiobacillus denitrificans and on heterotrophic denitrification by an enrichment culture

Salinity of nitrate-laden wastewaters, such as those produced by metal industries, tanneries, and wet flue gas cleaning systems may affect their treatment by denitrification. Salt inhibition of denitrification has been reported, while impacts of individual ions remain poorly understood whilst being relevant for wastewaters where often the concentration of a single ion rather than the salts varies. The aim of this study was to determine the inhibition by inorganic ions (Na+, Cl-, SO42- and K+) commonly present in saline wastewaters on denitrification and reveal its potential for the treatment of such waste streams, like those produced by NOx-SOx removal scrubbers. The inhibitory effects were investigated for both heterotrophic (enrichment culture) and autotrophic (T. denitrificans) denitrification in batch assays, by using NaCl, Na2SO4, KCl and K2SO4 salts at increasing concentrations. The half inhibition concentrations (IC50) of Na+ (as NaCl), Na+ (as Na2SO4) and Cl- (as KCl) were: 4.3 ± 0.3, 7.9 ± 0.5 and 5.2 ± 0.3 g/L for heterotrophic, and 1-2.5, 2.5-5 and 4.1 ± 0.3 g/L for autotrophic denitrification, respectively. Heterotrophic denitrification was completely inhibited at 20 g/L Na+ (as NaCl), 30 g/L Na+ (as Na2SO4) and 30 g/L Cl- (as KCl), while autotrophic at 8 g/L Na+ (as NaCl), 10 g/L Na+ (as Na2SO4) and 15 g/L Cl- (as KCl). In both cases, Cl- addition had the most important role in decreasing denitrification rate, while Na+ at 1 g/L stimulated autotrophic denitrification but rapidly inhibited the rate at higher concentrations. Nitrite reduction was less inhibited by the ions than nitrate reduction and both the osmotic pressure and the toxicity of the single ions played key roles in the overall inhibition of denitrification. Eventually, both autotrophic and heterotrophic denitrification showed potential for the treatment of a saline wastewater from a NOx-SO2 removal scrubber from a pulp mill.

Effects of salinity on nitrite and elemental sulfur accumulation in a double short-cut sulfur autotrophic denitrification process

Double short-cut sulfur autotrophic denitrification (DSSADN) coupled with Anammox is of great significance in the low-carbon treatment of nitrogen-containing wastewater. In order to achieve high salinity autotrophic nitrogen removal, the effects of different salinities on the accumulation characteristics of NO₂^--N and S⁰ and microorganisms in DSSADN process were studied. The results showed that the effect of salinity on the DSSADN process could be categorized into the stimulation, stable, and inhibition. When the salinity gradually increased to 2.5 %, the highest NO₂^--N production rate (NiPR) and S⁰ production rate (S⁰PR) of DSSADN were 0.54 kg/(m³·d) and 1.1 kg/(m³·d) respectively. NiPR and S⁰PR gradually decreased as the salinity increased to more than 3 %. However, salinity had a relatively low impact on nitrite accumulation efficiency and S⁰ accumulation efficiency, which were 80 % and 81.5 %, respectively, when the salinity reached 5 %. Salinity has a great influence on the structure and abundance of microbial communities in the system.