Robustness and microbial consortia succession of simultaneous partial nitrification, ANAMMOX and denitrification (SNAD) process for mature landfill leachate treatment under low temperature

Nitrogen removal from mature landfill leachate through enhanced Partial Nitrification-Anammox process in an innovative multi-stage fixed biofilm reactor

In the current integrated PN/A method/process for mature landfill leachate treatment, microbial inhibition and low nitrogen removal capacity are the big barriers due to high ammonia concentration and low C/N. This study aimed to evaluate the performance of a high-rate nitrogen removal lab-scale reactor, which combines pre-denitrification and Partial Nitrification-Anammox (PN/A) in a multi-stage fixed biofilm reactor (MFBR), for mature landfill leachate treatment. A nitrogen removal efficiency (NRE) of 90.43 % and an average nitrogen removal rate (NRR) of 0.94 kg/m³·d were observed at an influent NH+ 4-N concentration of 2274.39 mg/L during the last operational phase. The nitrogen mass balance showed that the nitrogen concentration gradually decreases along the course, and nitrogen was mainly removed in the aerobic chambers, in which Anammox contributed to 86.4 % of the removed nitrogen, while the front anoxic chamber is mainly used to remove NO- 3-N from the recirculation. Redundancy analysis showed that the variation in NH+ 4-N concentration along the course was the main factor affecting microbial community succession, which shows that the reactor configuration enables efficient cooperation and distribution of different microorganisms. Moreover, economic analysis of MFBR process showed that the energy consumption and carbon addition were reduced by 58.9 % and 100 %, respectively. Therefore, the MFBR established in this study, with its new configuration, achieves efficient treatment of landfill leachate in a single reactor and is environmentally friendly, and could be considered as a reference for full-scale landfill leachate treatment.

Exploiting refractory organic matter for advanced nitrogen removal from mature landfill leachate via anammox in an expanded granular sludge bed reactor

Anammox is an efficient low-carbon nitrogen removal technology for mature landfill leachate (MLL). However, it produces 11 % nitrate theoretically, which needs further removal. In this study, the mechanisms of exploiting refractory organic matter (ROM) from an MLL as an inner carbon source for advanced nitrogen removal via anammox were systematically analyzed, and the effects of hydraulic retention time on nitrogen and ROM removal/utilization were investigated. Without any external carbon source, a total nitrogen and organic carbon removal efficiency of 94.50 % and 27.12 %, respectively, were achieved, with a nitrogen loading rate of 2.4 kg N/(m³·d). The abundances of norank_f_norank_o_SBR1031, OLB13, and norank_f_A4b, which had the capacity to degrade ROM, increased from 21.63 % to 49.21 %. This study reveals that the ROM in an MLL can be exploited for synchronous advanced nitrogen and organic matter removal.

Effect of aeration modes on nitrogen removal and N2O emission in the partial nitrification and denitrification process for landfill leachate treatment

The anoxic/multi-aerobic process is widely applied for treating landfill leachate with low carbon to nitrogen ratio. In this study, the effect of two aeration modes in the aerobic phase, i.e. decreasing dissolved oxygen (DO) and increasing DO, on nitrogen removal and N₂O emission in the process were systematically compared. The results demonstrate that the aerobic phase with increasing DO mode has a positive effect on improved total nitrogen removal (78 %) under the COD/N ratio as low as 3.45 and minimized N₂O emission. DO concentration higher than 1.5 mg/L in the aerobic phase reduced nitrogen removal and led to a significant high N₂O emission in the process. Complete nitrite denitrification in the anoxic phase correlated with minimized N₂O emission. Under efficient nitrogen removal stage, N₂O emission factor was 2.4 ± 1.0 % of the total incoming nitrogen. Microbial analysis revealed that increasing DO mode increased the abundance of ammonia oxidizing bacteria and denitrifiers.

Effect of asparagine, corncob biochar and Fe(II) on anaerobic biological treatment under low temperature: Enhanced performance and microbial community dynamic

To ensure the efficiency of anaerobic biological treatment technology at lower temperature will expand the application of anaerobic reactor in practical industrial wastewater treatment. Through a batch experiment, asparagine, corncob biochar and Fe²⁺ were selected as strengthening measures to analyze the effects on the anaerobic sludge characteristics, microbial community and functional genes in the low temperature (15 °C). Results showed that after 21 days, asparagine began to promote chemical oxygen demand (COD) removal by the anaerobic treatment, with highest COD removal rate (81.65%) observed when the asparagine concentration was 1 mmol/L. When adding 3 g biochar, 25 mg/L Fe²⁺, and the combination of biochar and Fe²⁺, the COD removal rates reached to 82%, 92% and 97%, respectively. In the presence of asparagine, both biochar and Fe²⁺ alone or in combination increased the activity of protease (16.35%-120.71%) and coenzyme F₄₂₀ (5.63%-130.2%). The relative abundance of Proteobacteria and Methanobacterium increased in the presence of biochar and Fe²⁺. In addition, the KEGG results showed that the combined addition of biochar and Fe²⁺ enhanced bacterial replication and repair and promoted amino acid metabolism of archaea.

Control strategy and performance of simultaneous removal of nitrogen and organic matter in treating swine manure digestate using one reactor with airlift and micro-granule

A simultaneous partial nitritation, anammox, denitrification, and COD oxidation (SNADCO) process was used to evaluate the nitrogen and biodegradable organic matter removal of swine manure digestate based on a nitrite limitation and ammonium surplus strategy. As influent ammonium concentration increased from 500 mg/L to 2100 mg/L, the 5 day biochemical oxygen demand (BOD₅) maintained at a high removal efficiency of 95.4%. However, nitrogen removal efficiency (NRE) decreased from 90.9% to 68.2% due to the inhibition of AnAOB caused by an ammonium concentration of 2100 mg/L. The contribution of AnAOB to nitrogen removal was 75.6-86.5%, while that of denitrifying bacteria was 4.6-7.0%. In the case of COD removal, the contributions were from ordinary heterotrophic organisms and denitrifying bacteria, at 27.1-64.9% and 11.2-22.1%, respectively. The results of specific bacteria activity tests and microbial analysis showed that a highly efficient synergism between functional microorganisms is essential for the stability of the SNADCO process.

Efficient nitrogen removal from mature landfill leachate in a step feed continuous plug-flow system based on one-stage anammox process

A continuous plug-flow multistage anoxic/oxic (A/O) system based on one-stage partial nitrification coupled anammox (PNA) process with integrated fixed-film activated sludge (IFAS) was established and operated over 400 days. A step feed strategy effectively controlled free ammonia concentration and alleviated impacts on ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). During day 301-405, 98.1% of total inorganic nitrogen was removed from mature landfill leachate, whereas chemical oxygen demand (COD) removal efficiency was 52.9%. With the enrichment of AnAOB in oxic biofilm, nitrogen removal via the anammox pathway reached 94.3%-95.0%. During system operation, the dominant anammox genus shifted from Candidatus_Brocadia to Candidatus_Kuenenia. Fluorescent in situ hybridization (FISH) indicated AnAOB encapsulated by AOB colonies were mainly distributed inside of the biofilm, which promoted nitrite utilization by the anammox process. This innovative system and the results are of great value to practical applications.

Deciphering effects of humic acid in landfill leachate on the simultaneous nitrification, anammox and denitrification (SNAD) system from performance, electron transfer and microbial community

Simultaneous nitrification, anammox and denitrification (SNAD) system is effective for landfill leachate treatment. However, humic acid (HA) as both an organic pollutant and electron shuttle in landfill leachate, its effects on the SNAD system remain unknown. This study demonstrated that HA initially inhibited NH₄⁺-N removal efficiency due to HA inhibition on anammox bacteria (the lowest fell to 90.89% from 100%), but the HA inhibition was released after adaption in the SNAD system. Hence, the mechanism of releasing HA inhibition in the SNAD system was established from performance, electron transfer and microbial community. Firstly, HA could be effectively removed by an adsorption-biodegradation process in the SNAD system, which avoided deteriorated performance caused by HA accumulation. Electrochemical analysis demonstrated that HA stimulated riboflavin and flavin mononucleotide (FMN) secretion to promote electron transfer efficiency. With the improved electron transfer efficiency, ETSA and ATP values significantly increased, indicating that HA enhanced the microbial metabolism activity of the SNAD system. Further analysis by enzymatic activity assay showed that the HAO (39.68 to 69.53 U/L), AMO (242.94 to 308.36 U/L), HZO (133.73 to 169.65 U/mL), NXR (24.63 to 54.52 U/L), NAR (94.40 to 114.36 U/L) and NIR (104.40 to 123.74 U/L) activities were improved with the HA increased from 0 to 200 mg/L, manifesting that HA enhanced nitrogen metabolism in the SNAD system. Besides, more reasonable metabolic division of labor in functional bacterial and enrichment of heterotrophic bacteria achieved efficient simultaneous removal of HA and nitrogen. Overall, efficient HA biodegradation, faster electron transfer efficiency and better metabolic division of microbial communities released HA inhibition, making the SNAD system more resistant to HA stress. This study shed light on the effects of HA on the SNAD system and provided a new insight for the SNAD system to landfill leachate treatment.

Nitrogen Removal from Mature Landfill Leachate via Anammox Based Processes: A Review

Mature landfill leachate is a complex and highly polluted effluent with a large amount of ammonia nitrogen, toxic components and low biodegradability. Its COD/N and BOD5/COD ratios are low, which is not suitable for traditional nitrification and denitrification processes. Anaerobic ammonia oxidation (anammox) is an innovative biological denitrification process, relying on anammox bacteria to form stable biofilms or granules. It has been extensively used in nitrogen removal of mature landfill leachate due to its high efficiency, low cost and sludge yield. This paper reviewed recent advances of anammox based processes for mature landfill leachate treatment. The state of the art anammox process for mature landfill leachate is systematically described, mainly including partial nitrification–anammox, partial nitrification–anammox coupled denitrification. At the same time, the microbiological analysis of the process operation was given. Anaerobic ammonium oxidation (anammox) has the merit of saving the carbon source and aeration energy, while its practical application is mainly limited by an unstable influent condition, operational control and seasonal temperature variation. To improve process efficiency, it is suggested to develop some novel denitrification processes coupled with anammox to reduce the inhibition of anammox bacteria by mature landfill leachate, and to find cheap new carbon sources (methane, waste fruits) to improve the biological denitrification efficiency of the anammox system.

Simultaneous partial nitritation, anammox, and denitrification process for the treatment of simulated municipal sewage in a single-stage biofilter reactor

This study provides a feasible scheme for the treatment of municipal sewage through simultaneous partial nitritation, anammox, and denitrification (SNAD) process, which was realized in a single-stage biofilter reactor (BFR). First, the BFR was started up to enrich the anaerobic ammonium-oxidizing bacteria (AnAOB) in the upper part of the reactor through the operation mode of the top influent and bottom effluent. Then, the BFR was inoculated with activated sludge and aerated continuously at the bottom to realize the coupling of SNAD, which was accompanied by a two-point influent from the bottom and top effluent. Results indicated that the high removal efficiency of NH₄⁺-N (93.40%), total nitrogen (TN, 89.95%), and soluble chemical oxygen demand (SCOD, 92.68%) were achieved with an air-water ratio of 4.29 and hydraulic retention time (HRT) of 6 h. During the SNAD steady phase for the treatment of simulated municipal sewage with a soluble chemical organic demand to nitrogen (C/N) ratio of 2.31, low concentrations of NH₄⁺-N (4.13 mg/L), TN (6.44 mg/L), and SCOD (11.29 mg/L) were attained in the effluent. High-throughput sequencing analysis indicated that the relative abundance of Nitrosomonas, Candidatus Brocadia, and Denitratisoma were 0.77%, 0.43%, and 4.07% in the biofilm at the 0-12.5 cm zone, respectively, suggesting successful implementation of the SNAD process.

Recovery and dormancy of nitrogen removal characteristics in the pilot-scale denitrification-partial nitrification-Anammox process for landfill leachate treatment

The pilot-scale partial nitrification-anaerobic ammonia oxidation (PN-Anammox) process for landfill leachate treatment has been running stably for 2 years. The degradation characteristics of nitrogen removal performance of PN-Anammox in this system were discussed during shutdown, and different recovery strategies were analyzed from the perspective of economy and easy implementation. The results showed that during the 166 d dormancy period, the decrease in Anammox bacteria activity occurred earlier than that of Anammox bacteria, and both tended to slow down after 128 d. The recovery strategy of simulated wastewater was the fastest, followed by the pretreated landfill leachate recovery strategy with inoculation of some corresponding functional sludges, while the worst strategy was the direct pretreated landfill leachate recovery strategy. The recovery start-up of the pilot-scale PN-Anammox process further showed that microbial activities were difficult to recover simultaneously during operation using raw wastewater directly due to the presence of high NH₄⁺-N levels and the coupling process, which easily led to the accumulation of NH₄⁺-N or NO₂^-N, thereby inhibiting microbial activity. The addition of some functional bacteria was more conducive to the rapid recovery of microbial activity. This study provides a new strategy for the rapid recovery of microbial activity for the engineering application of the PN-Anammox process.

Emerging onsite electron donors for advanced nitrogen removal from anammox effluent of leachate treatment: A review and future applications

Partial nitrification-anammox process is promising in leachate treatment, but the 11% residue nitrate limits the total nitrogen removal efficiency. Denitrification or partial denitrification and anammox are both practical polishing processes of anammox effluent, requiring extra electron donors. Fortunately, there are organic matter, sulfide and methane in leachate or produced by leachate treatment, which can serve as onsite electron donors. In this review, the mechanisms and processes using these three kinds of electron donors for residue nitrate reduction in anammox effluent of leachate are systematically summarized and discussed. It can be concluded that, biodegradable organic matter is an effective electron donor, sulfide is a promising electron donor, methane is a potential electron donor. Two possible applications in future based on anammox treatment of fresh and mature leachate using sulfide and methane as onsite electron donors are proposed. Through sulfide reutilization, energy-saving with about 14% of aeration reduction can be achieved.

Adaptation of anammox granules in swine wastewater treatment to low temperatures at a full-scale simultaneous partial nitrification, anammox, and denitrification plant

In the anammox process, maintaining a high anammox activity at low water temperatures for stable nitrogen removal is a challenge. In this study, to verify the adaptability of anammox to low water temperatures, we investigated effects of annual temperature fluctuations on nitrogen removal in a full-scale swine wastewater treatment plant, where anammox bacteria accumulated. Annual quarters were defined as L-1 (November-January), L-2 (February-April), H-1 (April-July), and H-2 (July-October). The total nitrogen removal rate was stable at 0.08-0.11 kg-N/m³/d, even during temperature fluctuations. Removal efficiencies of biochemical oxygen demand and total nitrogen were consistently high at 95-99% and 69-81%, respectively. The anammox activity and abundance of anammox bacteria were highest in granule L-1 and lowest in granule H-2. The optimal temperature for anammox activity shifted from 35 °C in granules H-1 and H-2 to 30 °C in granules L-1 and L-2, while the latter maintained a moderate activity compared to the former at low temperature. Candidatus Jettenia asiatica was predominant, especially in granule L-2, accounting for up to 54% of the microbial community composition at the genus level. The high specific anammox activity in granule L-2 was considered to be due to the abundance of anammox bacteria and the adaptation of Ca. Jettenia asiatica to low temperature. The anammox granules adapted well to low temperatures and demonstrated high efficiency in the simultaneous partial nitrification anammox and denitrification process without heating. Thus, constructing an energy-saving and cost-effective nitrogen removal system can be considered.

Influence of seasonal temperature change on autotrophic nitrogen removal for mature landfill leachate treatment with high-ammonia by partial nitrification-Anammox process

In this study, a denitrification (DN)-partial nitritation (PN)-anaerobic ammonia oxidation (Anammox) system for the efficient nitrogen removal of mature landfill leachate was built with a zone-partitioning self-reflux biological reactor as the core device, and the effects of changes in seasonal temperature on the nitrogen removal in non-temperature-control environment were explored. The results showed that as the seasonal temperature decreased from 34°C to 11.3°C, the total nitrogen removal rate of the DN-PN-Anammox system gradually decreased from the peak value of 1.42 kg/(m³•day) to 0.49 kg/(m³•day). At low temperatures (<20°C), when the nitrogen load (NLR) of the system is not appropriate, the fluctuation of high NH₄⁺-N concentration in the landfill leachate greatly influenced the stability of the nitrogen removal. At temperatures of 11°C-15°C, the NLR of the system is controlled below 0.5 kg/(m³•day), which can achieve stable nitrogen removal and the nitrogen removal efficiency can reach above 96%. The abundance of Candidatus Brocadia gradually increased with the decrease of temperature. Nitrosomonas, Candidatus Brocadia and Candidatus Kuenenia as the main functional microorganisms in the low temperature.

The effects of undulating seasonal temperature on the performance and microbial community characteristics of simultaneous anammox and denitrification (SAD) process

The effects of undulating seasonal temperature change (USTC) (10.1 °C-31.8 °C) on the N and carbon removal efficiency of simultaneous anammox and denitrification (SAD) were investigated, and the recovery performance of SAD was simulated. Results showed that 15 °C was the critical temperature of SAD for N and carbon removal under USTC from summer to winter. The removal efficiency of NH₄⁺-N was improved in the final stage after temperature rise, but still lower than that in summer after long-term low temperature inhibition. The contribution of anammox to N removal was more than denitrification. The abundance of anammox bacteria (AnAOB) in SAD reactor was 8.8%-11.7% from summer to autumn. Candidatus Kuenenia replaced Candidatus Brocadia as the main AnAOB gradually. Finally, AnAOB abundance increased from 4.2% to 6.6% after recovery, and the abundance of denitrifying bacteria (DB) became the highest, which mainly includes Thauera and Hydrogenophaga.

Rapid start-up and stable maintenance of partial nitrification-anaerobic ammonium oxidation treatment of landfill leachate at low temperatures

The current research regarding anaerobic ammonium oxidation (anammox) for the treatment of landfill leachate mainly focuses on a temperature range of 30-35 °C. However, achieving and maintaining anammox at lower temperatures would widen its application for the treatment of landfill leachate. This study, attempts to address this issue by using a combined process involving an upflow anaerobic sludge blanket (UASB), anoxic/oxic (A/O) reactor, anammox reactor (ANAOR), and anaerobic sequencing batch reactor (ASBR) to enrich anammox bacteria at relatively low temperatures. The rapid start-up of the partial nitrification-anammox process for landfill leachate treatment was achieved and maintained at 13-22 °C. The experiment was divided into phase 1 (20-22 °C) and phase 2 (13-15 °C). The results showed that 87.1% of the chemical oxygen demand (COD), 97.4-97.7% of the ammonium nitrogen (NH₄⁺-N), and 93.3-94.7% of the total nitrogen (TN), were removed. At least 29.3% and 11.4% of NH₄⁺-N was removed through anammox in phases 1 and 2, respectively, with an accumulation NO₂^--N ratio of 86.1-88.6%. Candidatus Kuenenia was the dominant anammox bacteria in the anammox process. A low temperature of 13-15 °C did not affect ammonia oxidizing bacteria (AOB), and their relative abundance in the A/O reactor ranged from 27.29% to 33.22%.

Microbial interspecific interaction and nitrogen metabolism pathway for the treatment of municipal wastewater by iron carbon based constructed wetland

In order to explore the pollutant removal performance and interspecific interaction in constructed wetland (CW) with Fe₀-C filler, constructed wetland with Fe₀-C filler (CW-Fe) and with ceramsite filler (CW-C) were set up. Besides, the nutrients removal and interspecific interaction were analyzed, and the results showed that total nitrogen (TN) removal efficiency of CW-Fe system without carbon source was lower than that in CW-C system though CW-Fe system could convert macro-molecular organic matter into micro-molecular organic matter. However, ammonia nitrogen (NH₄⁺-N) increase was observed in CW-Fe system with better total phosphorus (TP) removal performance. High-throughput sequencing showed that the microbial richness and abundance of Bacteroides, Firmicutes, Chlorofeli and Actinobacteria in the CW with Fe₀-C filler was significantly higher than with ceramsite filler. The interaction between two CWs was significantly different, and the functional enzymes abundance of nitrate nitrogen (NO₃^--N) to NH₄⁺-N transformation in CW-Fe system significantly increased.

Removal of Nutrients From Anaerobically Digested Swine Wastewater Using an Intermittent Cycle Extended Aeration System

Swine wastewater contains high concentrations of organic compounds, nutrients (nitrogen and phosphorus), heavy metals, and residual antibiotics, amongst others, that have negative impacts on the water environment. The main aim of this work was to remove nutrients from anaerobically digested swine wastewater using an intermittent cycle extended aeration system (ICEAS). The effects of operational parameters such as cycle time, organic loading rate, C/N ratio, and aeration/mixing ratio on the pollutant removal efficiencies of ICEAS were studied and compared with the performance of a conventional sequencing batch reactor (SBR). The following optimal conditions were obtained: cycle time, 6 h; organic loading rate, 0.86 kg COD m-3 day-1; C/N ratio, 2.49-2.82; and aeration/mixing ratio, 1.57. The pH was maintained in the range of 6.0-8.0. The total organic carbon (TOC), total nitrogen (TN), ammonium (NH4 +), total phosphorus (TP), and color removal efficiencies of ICEAS were higher than those of the conventional SBR, with removal efficiencies of 95.22, 88.29, 97.69, 85.81, and 97.84%, respectively, compared to 94.34, 81.16, 94.15, 77.94, and 96.95%, respectively, observed in the SBR. TOC, TN, NH4 +, TP, and the color removal efficiencies of ICEAS were higher by 0.88, 7.13, 3.54, 7.87, and 0.95%, respectively, than the conventional SBR. The good results from this study show that ICEAS is a promising technology for the removal of organic contaminants and nutrients from anaerobically digested swine wastewater and that the effluent water quality meets the Vietnamese discharge standard (QCVN 62-MT:2016/BTNMT) for swine wastewater effluents.

Advanced nitrogen removal from mature landfill leachate via partial nitrification-Anammox biofilm reactor (PNABR) driven by high dissolved oxygen (DO): Protection mechanism of aerobic biofilm

A novel partial nitrification-Anammox biofilm reactor (PNABR) operated under high dissolved oxygen (DO) with pre-anoxic - aerobic - anoxic operational mode was developed for efficient denitrogenation from mature landfill leachate. With DO concentration gradually increasing to 4.03 ± 0.03 mg/L, the ammonia oxidation rate (AOR) was enhanced to 25.8 mgNH4+-N/(L h), while nitrite oxidation bacteria (NOB) was inhibited effectively by alternating free ammonia (FA) and oxygen starvation. DO micro-distribution revealed that estimated 1900 μm of aerobic biofilm could protect anammox biofilm underneath from being inhibited by high DO. qPCR analysis further suggested that ammonia oxidation bacteria (AOB) abundance in whole biofilm was 6.12 × 109 gene copies/(g dry sludge), which was twice than found in the floc. Anammox bacteria accounted for 2.39% of total bacteria in whole biofilm, contributing 90.0% to nitrogen removal. Nitrogen removal rate (NRR) and nitrogen removal efficiency (NRE) finally reached 396.6 gN/(m3 d) and 96.1%, respectively.

Low temperature advanced nitrogen and sulfate removal from landfill leachate by nitrite-anammox and sulfate-anammox

Under anaerobic conditions, ammonium (NH₄⁺) can react with nitrite (NO₂^-) and sulfate (SO₄^2-), termed nitrite-anammox (NirAnammox) and sulfate-anammox (Sulfammox), respectively. However, how to remove NH₄⁺ and SO₄^2- together from leachate is unclear. In this study, NirAnammox and Sulfammox cooperatively achieved nitrogen and sulfate removal from leachate using a biological process at low temperature (14-15 °C). NH₄⁺, total nitrogen (TN), and SO₄^2- concentrations in the influent were 610-700, 670-900, 1870-1920 mg/L, respectively, and 10 ± 1, 35 ± 3, and 897.7 ± 10 mg/L, respectively, in the effluent. Sulfammox, and NirAnammox (including partial nitrification) removed 44.2% and 35.46% of the NH₄⁺, respectively. Therefore, because leachate contains high concentrations of NH₄⁺ and SO₄^2-, NirAnammox and Sulfammox can easily occur together, with nitrogen removal by Sulfammox being more than NirAnammox. The relative abundance of dominant bacteria of the Sulfammox were 10-20 times that of Candidatus Kuenenia (NirAnammox) in each reactor. Organic matter negatively affected NirAnammox, but not Sulfammox. Dissolved oxygen negatively affected both.

Electron storage and resupply modes during sulfur cycle enhanced nitrogen removal stability in electrochemically assisted constructed wetlands under low temperature

In this work, an electrochemically assisted vertical flow constructed wetland (E-VFCW) achieved efficient PO₄^3--P (92.9-96.6%), NO₃^--N (50.8-91.8%) and TN (38.8-73.1%) removal from synthetic sewage effluent within 1-12 h at 12 °C. Abiotic reduction, Fe(II)-, S- and H₂-dependent denitrification, as well as coupling of fermentation, acetogenesis and heterotrophic denitrification might facilitate NO₃^--N removal in the E-VFCW. Particularly, electron resupply for NO₃^--N reduction by the in-situ deposited FeS, FeS₂ and S⁰ in the E-VFCW would occur during electron supply-demand disequilibrium situations (e.g., lower HRT or temperature). Stoichiometric results suggested that 21.7-278.7 mmol e^- d^-1 from the in-situ deposited S contributed to NO₃^--N reduction under HRT of 1-6 h at 12 °C, which improved the resilience capabilities of the E-VFCW to temperature and nitrogen loads fluctuations. Overall, this work provides new insights into the modes of S cycle mediating NO₃^--N conversions in the E-VFCW under low temperature.

A continuous-flow combined process based on partial nitrification-Anammox and partial denitrification-Anammox (PN/A + PD/A) for enhanced nitrogen removal from mature landfill leachate

A novel continuous-flow combined process of partial nitrification, Anammox (PN/A) and partial denitrification-Anammox (PD/A) was established to achieve enhanced nitrogen removal from landfill leachate. The NH₄⁺-N transformation rate and NO₂^--N accumulation rate in the PN reactor reached 93.4% and 91.5%, respectively. The nitrite generated from the PN reactor was combined with influent (38%) and fed into the Anammox reactor. The nitrate produced in the Anammox reactor was then discharged to PD/A reactor, where nitrate was transformed to nitrite and removed via Anammox. Under a COD/NO₃^--N ratio of 4.0, the NO₃^--N-to-NO₂^--N transformation ratio (NTR) and Anammox contribution rate reached 60.4% and 57.1% in PD/A reactor. The final effluent TN concentration was 15.7 mg/L, and the efficiency of TN removal could reach 98.8%. By combining PN/A with PD/A, enhanced nitrogen removal from landfill leachate was achieved successfully with an external carbon source addition (COD/NH₄⁺-N) of 0.28.

Simultaneous partial nitrification, anammox and denitrification (SNAD) process for nitrogen and refractory organic compounds removal from mature landfill leachate: Performance and metagenome-based microbial ecology

In this study, a simultaneous partial nitrification, Anammox and denitrification (SNAD) bioreactor was constructed for mature landfill leachate treatment, which exhibited favorable NH₄⁺-N (98.9-99.9%), TN (90.7-94.9%) and bio-refractory organic compounds (46.2-67.7%) removal efficiencies. Stoichiometric analysis demonstrated that the synergy of ammonium-oxidizing bacteria and Anammox bacteria dominated TN removal (96.1-97.2%). NO₃^--N produced in Anammox could be further reduced through (partial) denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The results highlighted that humic-like and their intermediates might serve as the electron donor for these (partial) denitrifiers and DNRA bacteria to remove NO₃^--N, and could be effectively removed from mature landfill leachate in SNAD bioreactor. Metagenomic characterization further demonstrated that phyla Chloroflexi, Chlorobi and genera Nitrosomonas, Ignavibacterium and Aminiphilus might be responsible for such humic-like degradation. Overall, this work offers new insights into the metagenome-based bioinformatic roles for the previously understudied microorganisms in SNAD bioreactor for mature landfill leachate treatment.

Sulfur and iron cycles promoted nitrogen and phosphorus removal in electrochemically assisted vertical flow constructed wetland treating wastewater treatment plant effluent with high S/N ratio

Phosphate (PO₄^3--P) and nitrate (NO₃^--N) in the effluent of wastewater treatment plants are the predominant sources of eutrophication. In this study, a bench-scale electrochemically assisted vertical flow constructed wetland (E-VFCW) was developed, which exhibited favorable PO₄^3--P (89.7-99.4%), NO₃^--N (82.7-99.6%), and TN (51.9-93.7%) removal efficiency in tertiary wastewater treatment. In addition, little N₂O accumulation (0.32-2.19% of △NO₃^--N) was observed. The study further elucidated that PO₄^3--P was removed mainly in the anode chamber by co-precipitation (Fe⁽ⁿ⁺⁾OH-PO₄) and adsorption (FeOOH-PO₄) pathways. Multi-pathway of NO₃^--N reduction was proposed, with 13.9-30.2% of NO₃^--N predominantly eliminated in the anode chamber by ferrous-dependent NO₃^--N reduction bacteria. In the cathode chamber, electrons storage and resupply modes during S cycle exerted crucial roles in NO₃^--N reduction, which enhanced the resilience capabilities of the E-VFCW to shock loadings. Stoichiometric analysis revealed that 3.3-6.6 mmol e^-/cycle were stored in the form of S⁰, FeS, and FeS₂ in the E-VFCW under longer HRT or higher current density. However, the deposited S resupplied 19.6% and 28.3% of electrons for NO₃^--N reduction under shorter HRT (1 h) or lower current density (0.01 mA cm^-2). Moreover, ferrous-driven NO₃^--N-reducing or DNRA bacteria also promoted NO₃^--N elimination in the cathode chamber. These findings provide new insight into the coupling interactions among S, Fe and H cycles, as well as N and P transformations in electrochemically assisted NO₃^--N reduction systems.

Temperature dependence of denitrification microbial communities and functional genes in an expanded granular sludge bed reactor treating nitrate-rich wastewater

The temperature dependence of denitrification was investigated for high nitrate nitrogen denitrification in an expanded granular sludge bed (EGSB) reactor. The optimal reaction temperatures were 15-35 °C in which nearly complete denitrification was achieved with the removal of COD maintained over 80%. Nitrite accumulation was observed at 10 °C indicating the incomplete denitrification at low temperature. However, almost complete denitrification was even accomplished as high as 52 °C. High-throughput sequencing detected a total of 84 bacterial genera and 7 phyla, and temperature variation resulted in the shift of microbial community structure and diversity. Proteobacteria thrived while Firmicutes and Bacteroidetes were inhibited by temperature stress. The predominance of Halomonas and the significant decrease of Azoarcus at low temperature indicated a more important role of these two genera in denitrification in an EGSB reactor. The results of qPCR indicated that temperature exerted effects on the abundance of denitrification function genes, nirK, nirS, narG, and nosZ, due to the shift of the bacterial community. This study provided a comprehensive understanding of temperature effects on the denitrification process in an EGSB reactor treating high concentration nitrate wastewater.

Potential Use of Dimocarpus longan Seeds as a Flocculant in Landfill Leachate Treatment

Landfill leachate is a highly polluted and generated from water infiltration through solid waste produced domestically and industrially. In this study, a coagulation–flocculation process using a combination of Polyaluminium chloride (PACl) as a coagulant and Dimocarpus longan seed powder (LSP) as coagulant aid was used in treating landfill leachate. LSP has been tested as the main coagulant and as coagulant aid with PACl. As the main coagulant, the optimum dosage and pH for PACl were 5 g/L and 6, respectively, with removal efficiencies of 67.44%, 99.47%, and 98% for COD, SS, and color, respectively. For LSP as the main coagulant, results show that LSP is not effective where the removal efficiencies obtained for COD, SS, and color were 39.40%, 22.20%, and 28.30%, respectively, with the optimum dosage of 2 g/L and pH 4. The maximum removal efficiencies of COD, SS, and color were 69.19%, 99.50%, and 98.80%, respectively, when LSP was used as coagulant aid with PACl. Results show that using LSP as coagulant aid was found to be more effective in the removal of COD, SS, and color with less PACl dosage. The PACl dosage was decreased from 5 to 2.75 g/L when LSP was used as a coagulant aid. Cost estimation for using PACl alone and using LSP as the coagulant aid showed a reduction in the cost of approximately 40% of the cost of using PACl alone. Overall, this study confirmed the efficiency of LSP to be used as a natural coagulant aid in leachate treatment.