Biological Nitrogen Removal through Nitritation Coupled with Thiosulfate-Driven Denitritation

Synchronous in-situ sludge reduction and enhanced denitrification through improving electron transfer during endogenous metabolisms with Fe(Ⅱ) addition

The creation of large amounts of excess sludge and residual nitrogen are critical issues in wastewater biotreatment. This study introduced Fe(II) into an oligotrophic anaerobic reactor (OARFe) that was implemented to modify an anoxic-oxic process to motivate in-situ sludge reduction and enhance denitrification under an effective electron shuttle among organic matter, nitrogen, and Fe. The addition of 15 mg L-1 Fe(II) resulted in a sludge reduction efficiency reached 32.0% with a decreased effluent nitrate concentration of 33.3%. This was mostly attributed to the electron transfer from Fe(II) to organic matters and nitrogen species in OARFe. The participation of Fe(II) led to the upregulation of Geothrix and Terrimonas, which caused active organic matter hydrolysis and cell lysis to stimulate the release of extracellular polymeric substances (EPS) and substance transfer between each layer of EPS. The higher utilization of released bioavailable dissolved organic matter improved endogenous denitrification, which can be combined with iron autotrophic denitrification to realize multiple electron donor-based nitrogen removal pathways, resulting in an increased nitrate removal rate of 58.2% in the absence of external carbon sources. These functional bacteria associated with the transformation of nitrogen and carbon and cycling between ferrous and ferric ions were enriched in OARFe, which contributed to efficient electron transport occurred both inside and outside the cell and increased 2,3,5-triphenyltetrazolium chloride electronic transport system activity by 46.9%. This contributed to the potential operational costs of chemical addition and sludge disposal of Fe-AO being 1.9 times lower than those of conventional A2O processes. These results imply that the addition of ferrous ions to an oligotrophic anaerobic zone for wastewater treatment has the potential for low-cost pollution control.

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.

Biological nitrogen removal from low carbon wastewater

Nitrogen has traditionally been removed from wastewater by nitrification and denitrification processes, in which organic carbon has been used as an electron donor during denitrification. However, some wastewaters contain low concentrations of organic carbon, which may require external organic carbon supply, increasing treatment costs. As a result, processes such as partial nitrification/anammox (anaerobic ammonium oxidation) (PN/A), autotrophic denitrification, nitritation-denitritation and bioelectrochemical processes have been studied as possible alternatives, and are thus evaluated in this study based on process kinetics, applicability at large-scale and process configuration. Oxygen demand for nitritation-denitritation and PN/A is 25% and 60% lower than for nitrification/denitrification, respectively. In addition, PN/A process does not require organic carbon supply, while its supply for nitritation-denitritation is 40% less than for nitrification/denitrification. Both PN/A and nitritation-denitritation produce less sludge compared to nitrification/denitrification, which saves on sludge handling costs. Similarly, autotrophic denitrification generates less sludge compared to heterotrophic denitrification and could save on sludge handling costs. However, autotrophic denitrification driven by metallic ions, elemental sulfur (S) and its compounds could generate harmful chemicals. On the other hand, hydrogenotrophic denitrification can remove nitrogen completely without generation of harmful chemicals, but requires specialized equipment for generation and handling of hydrogen gas (H2), which complicates process configuration. Bioelectrochemical processes are limited by low kinetics and complicated process configuration. In sum, anammox-mediated processes represent the best alternative to nitrification/denitrification for nitrogen removal in low- and high-strength wastewaters.

Identification of nitrogen-incorporating bacteria in a sequencing batch reactor: A combining cultivation-dependent and cultivation-independent method

Nitrogen-incorporating bacteria in activated sludge play important roles in nitrogen removal in sequencing bactch reactor (SBR), but the active microorganisms and their interactions in the complex community are rarely revealed. Herein, a combining cultivation-dependent and cultivation-independent methods associated with DNA-stable-isotope probing (SIP) was applied to determine the microbes responsible for nitrogen-incorporating in SBR. Results revealed that Cytophagaceae and Sphingobacteriales were identified to be involved in nitrification, and Anaerolineae, Plasticicumulans and Elusimicrobia were responsible for denitrification. Cultivable nitrobacter and denitrifiers were isolated from the activated sludge, but they did not participate in the nitrogen-incorporating based on the SIP results. Additionally, the molecular ecological network analysis indicated that the SIP-identified nitrogen-incorporating bacteria exhibited more links with the intra-community, which might explain the failure of isolating these active bacteria. These findings add understanding of the removal of nitrogenous compounds drived by nitrogen-incorporating bacteria in actual wastewater treatment process.

Feasibility test of autotrophic denitrification of industrial wastewater in sequencing batch and static granular bed reactors

In order to evaluate the efficacy of using reduced sulfur species in lieu of conventional substrates, a sequencing batch reactor (SBR) was used to develop an autotrophic denitrifying culture which in turn was used to seed a static granular bed reactor (SGBR) for continuous flow treatment. Both bioreactors were able to quickly acclimate to the anoxic environment and achieve stable autotrophic denitrification within several weeks of being placed in operation. The seed for the SBR was obtained from operating basins at the Cedar Rapids plant. MiSeq analysis showed the presence of the autotrophic denitrifier Thiobacillus in the seed from the sulfur oxidation basin; however, Shinella and Sulfurovum became the dominant autotrophic denitrifiers in the SBR. Both the SBR and SGBR achieved excellent nitrate removal (i.e., >95%) with stoichiometric amounts of thiosulfate added to the synthetic influent. The results of this feasibility study suggest that anaerobic granules from the UASB at the plant serve as good seed biomass for autotrophic denitrification when augmented by sulfur oxidation basin and sulfide scrubber biomass, and that reduced sulfur species at the plant (or augmented with an external sulfur source) can serve as electron donors for nearly complete denitrification. PRACTITIONER POINTS: Autotrophic denitrification of industrial wastewater was investigated to evaluate reduced sulfur species as electron donor for nitrogen removal. An autotrophic denitrifying culture was cultivated in an SBR, and continuous autotrophic denitrification was accomplished in an SGBR. No increase in head loss was observed in the SGBR, and it was able to operate without the need for backwashing in more than 200 days of operation. Reduced sulfur was demonstrated to be a sufficient electron donor for nearly complete denitrification. MiSeq analysis resolved primary species responsible for autotrophic denitrification in this study.

Elucidating the biofilm properties and biokinetics of a sulfur-oxidizing moving-bed biofilm for mainstream nitrogen removal

The sulfide-oxidizing autotrophic denitrification (SOAD) process offers a feasible alternative to mainstream heterotrophic denitrification in treating domestic sewage with insufficient organics. Previously SOAD has been successfully applied in a moving-bed biofilm reactor (MBBR). However, the biofilm properties and biokinetics are still not thoroughly understood. The present study was therefore designed to investigate these features of sulfur-oxidizing biofilms (SOBfs) cultivated in a lab-scale MBBR under stable operation for over a year. The biofilms developed were 160 μm thick, had an uneven and porous surface on which elemental sulfur (S⁰) accumulated, and the SOB biomass was highly diverse. The bioprocess kinetics were evaluated through 12 batch experiments. The results were interpreted by adopting a two-step sulfide oxidation model (sulfide→S⁰ and S⁰→ sulfate) with all specific rates having a linear regression coefficient of R² > 0.9. Moreover, the inhibitory kinetic analysis revealed that 1) the maximum treatment capacity (about 480 mg S/(m²·h) and 80 mg N/(m²·h)) was observed at low sulfide level (40 mg S/L), while higher sulfide level (60-150 mg S/L) showed increasing inhibition on the oxidation of both sulfide and sulfur and denitrification. 2) The denitritation activity decreased by up to 43% when free nitrous acid reached a maximum of 8.6 μg N/L, whereas the oxidation of sulfide and sulfur did not have any significant effect. Interestingly, two physiologically diverse SOB groups were found in this special biofilm. The mechanisms of the cooperation and competition for electron donors and acceptors between these two SOB clades are proposed. The results of this study greatly enhance our understanding of the design and optimization of SOAD-MBBR for mainstream nitrogen removal.

A comparative study on denitrifying sludge granulation with different electron donors: Sulfide, thiosulfate and organics

A comparative study on denitrifying sludge granulation with different electron donors (sulfide, thiosulfate and organics) was carried out. Longer time was spent on sulfide-denitrifying granular sludge (DGS) cultivation (88 days) than thiosulfate- and organics-DGS cultivations (57 days). All the three DGS were characterized in terms of particle size distribution, sludge settling ability (indicated by sludge volume index and settling velocity), permeability (indicated by fractal dimension) and extracellular polymeric substances (EPS, including polysaccharide and protein) secretion. Sludge productions in the three DGS-reactors were also monitored. The key functional microorganisms in three granular reactors were revealed via high through-put pyrosequencing analysis. Batch tests were performed to measure the denitrification activities of each DGS, including both denitratation (NO₃^- → NO₂^-) and denitritation (NO₂^- → N₂). We found that thiosulfate-driven denitrifying sludge granulation (TDDSG) should be the most efficient and compact technology for effective BNR in municipal wastewater treatment. The findings of this study suggests the TDDSG could further increase the nitrogen removal potential in an enhanced sulfur cycle-driven bioprocess for co-treatment of wet flue gas desulfurization wastes with fresh sewage depending on three short-cut biological reactions, including: 1) short-cut biological sulfur reduction (SO₄^2-/SO₃^2- → S₂O₃^2-); 2) thiosulfate-driven denitritation (S₂O₃^2- + NO₂^- → SO₄^2- + N₂↑); and 3) nitritation (NH₄⁺ + O₂ → NO₂^-).

Free sulfurous acid (FSA) inhibition of biological thiosulfate reduction (BTR) in the sulfur cycle-driven wastewater treatment process

A sulfur cycle-based bioprocess for co-treatment of wet flue gas desulfurization (WFGD) wastes with freshwater sewage has been developed. In this process the removal of organic carbon is mainly associated with biological sulfate or sulfite reduction. Thiosulfate is a major intermediate during biological sulfate/sulfite reduction, and its reduction to sulfide is the rate-limiting step. In this study, the impacts of saline sulfite (the ionized form: HSO₃^- + SO₃^2-) and free sulfurous acid (FSA, the unionized form: H₂SO₃) sourced from WGFD wastes on the biological thiosulfate reduction (BTR) activities were thoroughly investigated. The BTR activity and sulfate/sulfite-reducing bacteria (SRB) populations in the thiosulfate-reducing up-flow anaerobic sludge bed (UASB) reactor decreased when the FSA was added to the UASB influent. Batch experiment results confirmed that FSA, instead of saline sulfite, was the true inhibitor of BTR. And BTR activities dropped by 50% as the FSA concentrations were increased from 8.0 × 10^-8 to 2.0 × 10^-4 mg H₂SO₃-S/L. From an engineering perspective, the findings of this study provide some hints on how to ensure effective thiosulfate accumulation in biological sulfate/sulfite reduction for the subsequent denitrification/denitritation. Such manipulation would result in higher nitrogen removal rates in this co-treatment process of WFGD wastes with municipal sewage.