Effect of hydraulic retention time on performance of autotrophic, heterotrophic, and split-mixotrophic denitrification systems supported by polycaprolactone/pyrite: Difference and potential explanation

Microbial interactions elucidate the mechanisms of hydraulic retention time altering denitrification pathway in a sole pyrite-based electrochemical bioreactor (PEBR)

In the current context of low-carbon wastewater treatment, pyrite-based autotrophic denitrification (PAD) has gained attention as an energy-efficient and environmentally sustainable method for nitrogen elimination. However, the limited dissolution of pyrite and the associated slow autotrophic denitrification rate restrict its practical application. To tackle this, a pyrite-based electrochemical bioreactor (PEBR) was constructed and the microbial effect of hydraulic retention time (HRT) on denitrification efficiency and sulfide or iron oxidation in the PEBR system was investigated. It was found that upon the conclusion of phase V (HRT = 12 h), the nitrate removal efficiency (NRE) reached 92.53% ± 0.96%, and the concentration of NH4+-N in the effluent reached 2.63 ± 0.57 mg/L with a minimal accumulation of NO2--N (0.03 ± 0.05 mg/L) when the optimal treatment performance was obtained. As the HRT increased, the proportion of heterotrophic denitrification decreased substantially to 1%. Desulfobacterota, a sulfate-reducing bacteria (SRB), became dominant, with a relative abundance ranging from 0.04% to 19.44%. The PAD-related genera, such as Thiobacillus and Ferritrophicum, exhibited a positive correlation with HRT, indicating that PAD was enhanced with the extension of HRT. The functional genes related to Fe2+ intracellular oxidation (e.g., korA/B) positively correlated with HRT. The positive correlation of dsrA/B with HRT highlighted the role of dissimilatory sulfate reduction (DSR) as a primary contributor to reduced sulfate production. Furthermore, the variations in the relative abundance of aprA/B for sulfate reduction with the extension of HRT reflected that HRT affected sulfate reduction probably via the APS→SO32- process. This study might shed light on the optimization of HRT in PEBR for the treatment of nitrogenous wastewater.

Current intensity and hydraulic retention time play differential roles in functional gene expression or electron transfer pathways in a pyrite-filled three-dimensional biofilm electrode reactor (P3DBER)

This study developed a pyrite-filled three-dimensional biofilm electrode reactor (P3DBER) to treat nitrate wastewater with a low carbon/nitrogen ratio. Meanwhile, the joint effect of current intensity (CI) and hydraulic retention time (HRT) on the performance of the P3DBER was investigated. Results indicated that under the optimal conditions (CI = 30 mA, HRT = 4.9 h), the total inorganic nitrogen removal efficiency (TINRE) reached a maximum of 93.5 ± 1.4%, with a low electrical consumption of 0.075 kW h/g TIN. Increasing CI under different HRTs significantly enhanced the nitrogen removal capacity of the P3DBER. However, at high CI (30 mA), prolonging HRT did not further improve the nitrogen removal efficiency. The introduction of pyrite not only increased the types of electron donors but also could effectively maintain the stability of pH in the P3DBER. Variation partitioning analysis (VPA) showed that CI had a greater impact on the microbial community/functional genes than HRT. In addition, network analysis demonstrated a strong interconnection among microorganisms/functional genes within the P3DBER. This study offers valuable information for optimizing the operating parameters of the P3DBER.

Establishment and optimization of sulfur-based autotrophic-heterotrophic denitrification biofilters for advanced post-anaerobic treatment of effluent from kitchen wastewater and landfill leachate under low temperature

Landfill leachate treatment is a major challenge in wastewater treatment. In this study, two sulfur-based autotrophic-heterotrophic denitrification biofilters (Ra biofilter with room-temperature molded filler and Rb biofilter with melt molded filler) were used to treat kitchen-landfill leachate at low temperatures. The effects of reflux ratio, concentrations of NaHCO3, and Na2S2O3 on the total nitrogen removal efficiency were analyzed, and based on response surface methodology, the optimum parameters were determined. After optimization, the total nitrogen removal efficiency for the Ra and Rb biofilters increased by 83% and 81%, respectively. Moreover, sulfur-based autotrophic denitrification accounted for more than 70% of the nitrogen removal in both biofilters. Based on high-throughput sequencing results, the functional bacteria exhibited high abundance in the Ra biofilter, indicating that the room-temperature molded filler favored the enrichment of functional bacteria. These findings were important for optimizing the operation of sulfur autotrophic-heterotrophic denitrification biofilters at low temperatures.

Deciphering the influencing mechanism of hydraulic retention time on purification performance of a mixotrophic system from the perspective of reaction kinetics

At present, eutrophication is increasingly serious, so it is necessary to effectively reduce nitrogen and phosphorus in water bodies. In this study, a pyrite/polycaprolactone-based mixotrophic denitrification (PPMD) system using pyrite and polycaprolactone (PCL) as electron donors was developed and compared with pyrite-based autotrophic denitrification (PAD) system and PCL-based heterotrophic denitrification (PHD) system through continuous flow experiment. The removal efficiency of NO3--N (NRE) and PO43--P (PRE) and the contribution proportion of PAD in the PPMD system were significantly increased by prolonging hydraulic retention time (HRT, from 1 to 48 h). When HRT was equal to 24 h, the PPMD system conformed to the zero-order kinetic model, so NRE and PRE were mainly limited by the PAD process. When HRT was equal to 48 h, the PPMD system met the first-order kinetic model with NRE and PRE reaching 98.9 ± 1.1% and 91.8 ± 4.5%, respectively. When HRT = 48 h, the NRE and PRE by PAD system were 82.7 ± 9.1% and 88.5 ± 4.7%, respectively, but the effluent SO42- concentration was as high as 152.1 ± 13.7 mg/L (the influent SO42- concentration was 49.2 ± 3.3 mg/L); the NRE by PHD system was 98.5 ± 1.7%, but the PO43--P could not be removed ideally. The concentrations of NO3--N, total nitrogen, PO43--P, and SO42- in the PPMD system also showed distinct changes along the reactor column. In addition, the microbial diversity analysis showed that prolonging HRT (from 24 to 48 h) increased the abundance of autotrophic denitrifying microorganisms in the PPMD system, ultimately increasing the contribution proportion of PAD.

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.