Operational optimization of a three-stage nitrification moving bed biofilm reactor (NMBBR) by obtaining enriched nitrifying bacteria: Nitrifying performance, microbial community, and kinetic parameters

Efficient nitrogen removal by microalgal-bacterial granular sludge-marimo coupling process

Current biological wastewater treatment processes usually have a drawback of insufficient nitrogen (N) removal, contributing to the ubiquitous eutrophication of aquatic ecosystems globally. To address such a challenging situation, this study explored an innovative microalgal-bacterial granular sludge-marimo (MBGS-MA) coupling process. The process removed 83.4 % of N with the effluent N concentration of 4.0 mg/L. With the growth of MBGS, there was a shift towards genes associated with nitrification and denitrification, and away from ammonia assimilation genes, revealing internal mechanism of the shift of N removal pathway. Contrarily, MA could use gaseous N2 with the N fixing genes in MA enriched, and the genes abundance related to assimilatory nitrate reduction were also raised under the mutualistic interactions between Proteobacteria and Cyanobacteria, which was beneficial to achieve efficient N removal. These findings may open a new horizon for developing innovative hybrid microalgal-bacterial processes aimed at high-efficiency N removal from wastewater.

Partial nitrification-denitrification and enrichment of paracoccus induced by iron-chitosan beads addition in an intermittently-aerated activated sludge system

Insufficient carbon source has become the main limiting factor for efficient nitrogen removal in wastewater treatment. In this study, an intermittently-aerated activated sludge system with iron-chitosan (Fe-CS) beads addition was proposed for nitrogen removal from low C/N wastewater. By adding Fe-CS beads, partial nitrification-denitrification (PND) process and significant enrichment of Paracoccus (with ability of iron reduction/ammonium oxidation/aerobic denitrification) were observed in the reactor. The accumulation rate of NO2--N reached 81.9 %, and the total nitrogen removal efficiency was improved to 93.9 % by shortening the aeration time. The higher activity of ammonium oxidizing bacteria and inhibited activity of nitrite-oxidizing bacteria in Fe-CS assisted system mediated the occurrence of PND. In contrast, the traditional nitrification and denitrification process occurred in the control group. The high-throughput sequencing analysis and metagenomic results confirmed that the addition of Fe-CS induced 77.8 % and 54.9 % enrichment of Paracoccus in sludge and Fe-CS beads, respectively, while almost no enrichment was observed in control group. Furthermore, with the addition of Fe-CS beads, the expression of genes related to outer membrane porin, cytochrome c, and TCA was strengthened, thereby enhancing the electron transport of Fe(Ⅱ) (electron donor) and Fe(Ⅲ) (electron acceptor) with pollutants in the periplasm. This study provides new insights into the direct enrichment of iron-reducing bacteria and its PND performance induced by the Fe-CS bead addition. It therefore offers an appealing strategy for low C/N wastewater treatment.

Achieving simultaneous nitrification and endogenous denitrifying phosphorus removal in anaerobic/intermittently-aerated moving bed biofilm reactor for low carbon-to-nitrogen ratio wastewater treatment

In this study, an anaerobic/intermittently-aerated moving bed biofilm reactor (AnIA-MBBR) was proposed to realize simultaneous nitrification and endogenous denitrifying phosphorus removal (SNEDPR) in treating low carbon-to-nitrogen (C/N) ratio wastewater. The effect of different intermittent aeration modes (short and long aeration) on nutrients' removal was investigated. With the C/N ratio around 3, the removal efficiencies of total nitrogen and phosphorus were 90% and 74%, 88% and 59%, respectively, for short aeration and long aeration. The different aeration time also altered the nutrients' degradation pathway, biofilm characteristics, microbial community, and functional metabolic pathways. The results confirmed the occurrence of aerobic denitrifiers, anoxic denitrifiers, phosphorus accumulating organisms, glycogen accumulating organisms in AnIA-MBBR systems and their synergistic performance induced the SNEDPR. These results indicated that the application of AnIA in MBBR systems was an effective strategy to achieve SNEDPR, providing better simultaneous removal performance of nitrogen and phosphorus from low C/N ratio wastewater.

Comparison on biological nutrient removal and microbial community between full-scale anaerobic/anoxic/aerobic process and its upgrading processes

A comparative study was conducted for the anaerobic/anoxic/aerobic (AAO) process and its two upgrading processes, five-stage Bardenpho and AAO coupling moving bed bioreactors (AAO + MBBR), using long-term operation data of six full-scale wastewater treatment plants. The three processes all had good COD and phosphorus removal performance. The reinforcing effects of carriers on nitrification were moderate at full-scale applications, while the Bardenpho was advantageous in nitrogen removal. The AAO + MBBR and Bardenpho processes both had higher microbial richness and diversity than the AAO. The AAO + MBBR favored bacteria to degrade complex organics (Ottowia and Mycobacterium) and to form biofilms (Novosphingobium), and preferentially enriched denitrifying phosphorus-accumulating bacteria (DPB) (norank_o__Run-SP154) with the highest anoxic to aerobic phosphorus uptake rates of 65.3 % - 83.9 %. The Bardenpho enriched bacteria tolerant to varied environments (Norank_f__Blastocatellaceae, norank_o__Saccharimonadales, and norank_o__SBR103), and was more suitable for the upgrading of the AAO because of its excellent pollutant removal performance and flexible operation mode.

Design strategy and mechanism of nitrite oxidation suppression of elevated loading rate partial nitritation system

There is a current need for a low operational intensity, effective and small footprint system to achieve stable partial nitritation for subsequent anammox treatment at mainstream municipal wastewaters. This research identifies a unique design strategy using an elevated total ammonia nitrogen (TAN) surface area loading rate (SALR) of 5 g TAN/m2.d to achieve cost-effective, stable, and elevated rates of partial nitritation in a moving bed biofilm reactor (MBBR) system under mainstream conditions. The elevated loaded partial nitritation MBBR system achieves a TAN surface area removal rate (SARR) of 2.01 ± 0.07 g TAN/m2.d and NO2−-N: NH4+-N stoichiometric ratio of 1.15:1, which is appropriate for downstream anammox treatment. The elevated TAN SALR design strategy promotes nitrite-oxidizing bacteria (NOB) activity suppression rather than a reduction in NOB population as the reason for the suppression of nitrite oxidation in the mainstream elevated loaded partial nitritation MBBR system. NOB activity is limited at an elevated TAN SALR likely due to thick biofilm embedding the NOB population and competition for dissolved oxygen (DO) with ammonia-oxidizing bacteria for TAN oxidation to nitrite within the biofilm structure, which ultimately limits the uptake of DO by NOB in the system. Therefore, this design strategy offers a cost-effective and efficient alternative for mainstream partial nitritation MBBR systems at water resource recovery facilities.

Simultaneous nitrification and denitrification in a novel rotating self-aerated biofilm reactor for decentralized wastewater treatment

Decentralized wastewater pollution in rural areas has become a serious problem for the rural environment. In this study, a novel rotating self-aerated biofilm reactor was developed for decentralized wastewater treatment without any aeration equipment. After the long-term operation of 110 days, the removal efficiency reached to 96.06 % (COD), 98.06 % (NH₄⁺-N), and 62.58 % (TN) in the last phase. Under high dissolved oxygen level, the simultaneous nitrification-denitrification (SND) maintained at a stable ratio of 62.53 % and the denitrification rates reached over 28.37 mg/L/h. With the organic loading rate increased, key nitrogen functional bacterial communities such as anoxic denitrifiers (Thiothrix, Flavobacterium, Pseudoxanthomonas, Aquimonas and Azoarcus) and aerobic denitrifiers (Hydrogenophaga, Zoogloea and Terrimonas) increased obviously. Overall, microbial analysis and nitrogen metabolism pathway indicated that an integration of SND process was achieved in this single reactor by the combined action of nitrification, denitrification and comammox without any aeration equipment.

Efficient partial nitrification with hybrid nitrifying granular sludge based on a simultaneous fill/draw SBR mode

In this study, a simultaneous fill/draw SBR was applied to investigate the feasibility of partial nitrification process with inoculation of matured aerobic granular sludge. The system operated stably over 120 days with the relatively high ammonium removal efficiency (≥ 98.83%) and nitrite accumulation rate (≥ 89.60%). Moreover, a hybrid flocs/granules system was formed stably after long-term operation. The nitrite-oxidizing bacteria (NOB) was suppressed effectively because of the combined effect of simultaneous fill/draw mode and intermittent aeration conditions. Furthermore, batch tests were separately tested with isolated granules (> 200 μm) and flocs (< 200 μm), showing that the specific ammonia oxidation rate of granules and flocs were 15.94 ± 2.85 and 66.77 ± 0.83 mg N/(g MLSS·h), respectively. Correspondingly, the abundance of Nitrosomonas as a typical AOB in granules (6.24%) and flocs (11.94%) was obtained via the microbial diversity analysis, while NOB was almost hardly detected in granules and flocs.

Mechanism of suspended sludge impact on anammox enrichment in anoxic biofilm through long term operation and microbial analysis

The basic premise of anammox-technical application reliability in municipal wastewater treatment is substantially enriched anammox bacteria. To enrich the anammox, the special interaction mechanism between the suspended sludge (SS) and anoxic biofilm was investigated over three months in a partial denitrification/anammox biosystem subjected to dynamic changes in SS (absence→ presence→ absence). Results show that the introduction of SS significantly decreased the anammox nitrogen removal efficiency (83.8 ± 6.5%→ 48.7 ± 17.0%). With the presence or absence of SS, the spatial distribution of anammox bacteria within the anoxic biofilm gradually changed between the inner and outer layers, as detected by CLSM-FISH. qPCR and metagenomic sequencing show that changes in the presence and absence status of SS significantly reduced the abundance of the NO reducing functional gene, while the NO supply capacity (NO₃^-→NO) was improved, further favoring the anammox process. Batch tests and typical cycles further demonstrated that the anammox bacteria can stably acquire NO₂^-, and anammox bacteria in the anoxic biofilm competed far more NO₂^- than denitrifying bacteria according to the typical pH curve. Accordingly, the abundance of Candidatus Brocadia, as detected by high throughput sequencing, decreased in the anoxic biofilms with the introduction of SS, but greatly increased (0.82%→2.22%) after SS discharge. This study sheds new light on the high in-situ enrichment of anammox in mainstream.

An enriched ammonia-oxidizing microbiota enables high removal efficiency of ammonia in antibiotic production wastewater

High ammonia concentration hinders the efficient treatment of antibiotic production wastewater (APW). Developing effective ammonia oxidation wastewater treatment strategies is an ideal approach for facilitating APW treatment. Compared with traditional nitrification strategies, the partial nitrification process is more eco-friendly, less energy-intensive, and less excess sludge. The primary limiting factor of the partial nitrification process is increasing ammonia-oxidizing bacteria (AOB) while decreasing nitrite-oxidizing bacteria (NOB). In this study, an efficient AOB microbiota (named AF2) was obtained via enrichment of an aerobic activated sludge (AS0) collected from a pharmaceutical wastewater treatment plant. After a 52-day enrichment of AS0 in 250 mL flasks, the microbiota AE1 with 69.18% Nitrosomonas microorganisms was obtained. Subsequent scaled-up cultivation in a 10 L fermenter led to the AF2 microbiota with 59.22% Nitrosomonas. Low concentration of free ammonia (FA, < 42.01 mg L^-1) had a negligible effect on the activity of AF2, and the nitrite-nitrogen accumulation rate (NAR) of AF2 was 98% when FA concentration was 42.01 mg L^-1. The specific ammonia oxidation rates (SAORs) at 30 °C and 15 °C were 3.64 kg NH₄⁺-N·kg MLVSS^-1·d^-1 and 1.43 kg NH₄⁺-N·kg MLVSS^-1·d^-1 (MLVSS: mixed liquor volatile suspended solids). The SAOR was 0.52 kg NH₄⁺-N·kg MLVSS^-1·d^-1 when the NaCl concentration was increased from 0 to 20 g L^-1, showing that AF2 functioning was stable in a high-level salt environment. The ammonia oxidation performance of AF2 was verified by treating abamectin and lincomycin production wastewater. The NARs of AF2 used for abamectin and lincomycin production wastewater treatment were >90% and the SAORs were 2.39 kg NH₄⁺-N·kg MLVSS^-1·d^-1 and 0.54 kg NH₄⁺-N·kg MLVSS^-1·d^-1, respectively, which was higher than the traditional biological denitrification process. In summary, AF2 was effective for APW treatment via enhanced ammonia removal efficiency, demonstrating great potential for future industrial wastewater treatment.

Nitrogen removal characteristics of biofilms in each area of a full-scale AAO oxidation ditch process

Plastic carriers were installed in different areas of a full-scale anaerobic/anoxic/aerobic (AAO) oxidation ditch process, and the dynamics of nitrogen removal, biofilm morphologies, and microorganism species were investigated. The results showed that the biofilm at the front of the aerobic area (dissolved oxygen [DO] = 0.93 mg L^-1) provided the best denitrification, with specific nitrate and nitrite reduction rates of 10.16 and 3.78 mg·(g·h)^-1, respectively. The biofilm in the middle of the aerobic area (DO = 1.27 mg L^-1) exhibited the best nitrification performance, with a maximum specific ammonia oxidation rate of 3.21 mg·(g·h)^-1. The biofilm at the end of the aerobic area (DO = 0.01 mg L^-1) exhibited the highest anammox potential with a maximum specific anammox rate of 0.67 mg·(g·h)^-1. No correlation was observed between the specific nitrogen removal rates and abundance of nitrogen-removing microorganisms at the genus level. Biofilm denitrification during the process was primarily completed by heterotrophic denitrifying bacteria (Thauera, Acinetobacter, Hyphomicrobium, and Thermomonas). Aerobic denitrifying bacteria (0.19% Thauera and 0.34% Hyphomicrobium) were identified as the main denitrifying bacteria in the middle of the aerobic area. The dominant nitrifying microorganisms in the middle of the aerobic area were Nitrosomonas (0.50%) and Nitrospira (1.04%). A biofilm in the end of the aerobic area exhibited specific anammox potential, which may have been related to the dominance of 0.024% Candidatus Brocadia. Kinetic analysis revealed that adding plastic carriers to the front and middle of the aerobic area contributed to stable nitrogen removal efficiency.

Investigation of an intermittently-aerated moving bed biofilm reactor in rural wastewater treatment under low dissolved oxygen and C/N condition

An intermittently-aerated moving bed biofilm reactor (MBBR) was proposed for nitrogen and carbon removal from low C/N synthetic rural wastewater. In purposes of low energy consumption and costs, the intermittent aeration modes were changed and the dissolved oxygen was reduced gradually during the operation. The results showed that effluent concentrations of ammonia nitrogen and chemical oxygen demand were lower than 15 and 50 mg/L, respectively, even under microaerobic condition (0.1-1.0 mg/L). Meanwhile, the simultaneous nitrification-denitrification was achieved by intermittent aeration. The activity of functional bacteria was still high and the proportion of autotrophic biomass increased significantly under intermittent micro-aeration mode, which improved the nitrification performance. Aerobic denitrifier Hydrogenophaga, anoxic denitrifier Thiothrix, and heterotrophic nitrifier such as Rhodobacter were enriched in the intermittently micro-aerated MBBR, which will provide an applicable solution for rural wastewater treatment under low C/N and costs.

Occurrence and risk assessment of typical PPCPs and biodegradation pathway of ribavirin in wastewater treatment plants

A large number of pharmaceuticals and personal care products (PPCPs) persist in wastewater, and the consumption of PPCPs for COVID-19 control and prevention has sharply increased during the pandemic. This study investigated the occurrence, removal efficiency, and risk assessment of six typical PPCPs commonly used in China in two wastewater treatment plants (WWTPs). Ribavirin (RBV) is an effective pharmaceutical for severely ill patients with COVID-19, and the possible biodegradation pathway of RBV by activated sludge was discovered. The experimental results showed that PPCPs were detected in two WWTPs with a detection rate of 100% and concentrations ranging between 612 and 2323 ng L^-1. The detection frequency and concentrations of RBV were substantially higher, with a maximum concentration of 314 ng L^-1. Relatively high pollution loads were found for the following PPCPs from influent: ibuprofen > ranitidine hydrochloride > RBV > ampicillin sodium > clozapine > sulfamethoxazole. The removal efficiency of PPCPs was closely related to adsorption and biodegradation in activated sludge, and the moving bed biofilm reactor (MBBR) had a higher removal capacity than the anoxic-anaerobic-anoxic-oxic (AAAO) process. The removal efficiencies of sulfamethoxazole, ampicillin sodium, ibuprofen, and clozapine ranged from 92.21% to 97.86% in MBBR process and were relatively low, from 61.82% to 97.62% in AAAO process, and the removal of RBV and ranitidine hydrochloride were lower than 42.96% in both MBBR and AAAO processes. The discrepancy in removal efficiency is caused by temperature, hydrophilicity, and hydrophobicity of the compound, and acidity and alkalinity. The transformation products of RBV in activated sludge were detected and identified, and the biodegradation process of RBV could be speculated as follows: first breaks into TCONH₂ and an oxygen-containing five-membered heterocyclic ring under the nucleosidase reaction, and then TCONH₂ is finally formed into TCOOH through amide hydrolysis. Aquatic ecological risks based on risk quotient (RQ) assessment showed that PPCPs had high and medium risks in the influent, and the RQ values were all reduced after MBBR and AAAO treatment. Ranitidine hydrochloride and clozapine still showed high and medium risks in the effluent, respectively, and thus presented potential risks to the aquatic ecosystem.

Bioaugmentation for low C/N ratio wastewater treatment by combining endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) in the continuous A2/O - MBBR system

Endogenous partial denitrification (EPD) and denitrifying phosphorous removal (DPR) were combined in a novel A²/O - MBBR (Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor) system for low carbon/nitrogen (C/N) ratio wastewater treatment. The DPR performance was compared and the nutrient metabolism was elucidated based on the optimization of hydraulic retention time (HRT, 4-12 h) and nitrate recycling (R, 200%-600%). In the continuous-flow, the nitrate (NO₃^-) denitrification accompanied by nitrite (NO₂^-, via EPD) accumulation with the nitrate-to-nitrite transformation ratio (NTR) of 35.87%-43.31% in the anoxic zones. At HRT of 12 h with R of 500%, batch test initially revealed the DPR mechanism using both NO₃^- and NO₂^- as electron acceptor, where denitrifying phosphorus accumulation organisms (DPAOs) and denitrifying glycogen accumulation organisms (DGAOs) were the main contributors for EPD with incomplete denitrification (NO₃^- → NO₂^-). Furthermore, stoichiometry-based functional bacteria analysis displayed that higher bioactivity of DPAOs (NO₂^-→N₂, 57.30%; NO₃^-→N₂, 35.85%) over DGAOs (NO₃^-→N₂, 6.85%) facilitated the anoxic NO₃^- reduction. Microbial community analysis suggested that Cluster I of Defluviicoccus-GAO group (∼4%) was responsible for stable NO₂^- accumulation performance via EPD, while increased Accumulibacter-PAO group (by ∼15%) contributed to the advanced nutrient removal. Based on the achievement of NO₂^- accumulation, the application feasibility of integrated EPD - DPR - Anammox for deep-level nutrient removal was discussed.

Elucidating nitrifying performance, nitrite accumulation and microbial community in a three-stage plug flow moving bed biofilm reactor (PF - MBBR)

A three-stage plug flow moving bed biofilm reactor (PF - MBBR, consisting of three identical chambers of N1, N2 and N3) was proposed for nitrifier enrichment using synthetic wastewater. During the stable operation, the average NH₄⁺-N effluent was 0.67 mg/L and NH₄⁺-N removal was as high as 97.19% with the nitrite accumulation ratio (NAR) of 54.23%, although the biofilm thickness and biomass both presented downward trends from N1 (296 μm, 2280 mg/L), N2 (248 μm, 1850 mg/L) to N3 (198 μm, 1545 mg/L). Particularly, the comparative results of three stages revealed that N2 showed the optimum NH₄⁺-N removal (77.27%) and NAR (75.21%) in the continuous-flow, while NAR of N3 unexpectedly maintained a high level of 65.83% in the batch test, suggesting that ammonia oxidizing bacteria (AOB) accounted for absolute advantage over nitrite oxidizing bacteria (NOB). High-throughput sequencing initially verified different distribution of bacterial community structure, where N2 was far away from N1 and N3 with the lowest community richness and community diversity (operational taxonomic units (OTUs): 454(N2)<527(N3)<621(N1)). Proteobacteria (77.60%-83.09%), Bacteroidetes (1.66%-3.66%), Acidobacteria (2.28%-4.67%), and Planctomycetes (1.19%-6.63%) were the major phyla. At the genus level, AOB (mainly Nitrosomonas) accounted for 5.08% (N1), 20.74% (N2) and 14.24% (N3) while NOB (mainly Nitrospira) increased from 0.14% (N1), 7.06% (N2) to 4.91% (N3) with the total percentages of 5.22%, 27.80% and 19.15%. Finally, the application feasibility of MBBR optimization linked with nitrite (NO₂^--N) accumulation for deep-level nutrient removal was discussed.

Effect of mixing intensity on biodegradation of phenol in a moving bed biofilm reactor: Process optimization and external mass transfer study

In this work, an effort has been made to design the process variables and to analyse the impact of mixing intensity on mass transfer diffusion in a moving bed biofilm reactor (MBBR). A lab-scale MBBR, filled with Bacillus cereus GS2 IIT (BHU) immobilized-polyethylene biocarriers, was employed to optimize the process variables, including mixing intensity (60-140 rpm), phenol concentration (50-200 mg/L), and hydraulic retention time (HRT) (4-24 h) using response surface methodology. The optimum phenol removal of 87.64 % was found at 100 rpm of mixing intensity, 200 mg/L of phenol concentration, and 24 h of HRT. The higher mixing intensity improved the substrate diffusion between the liquid phase and the surface of the biofilm. The external mass transfer coefficients were found in the range of 1.431 × 10^-5-1.845 × 10^-5 m/s. Moreover, the detection of catechol and 2-hydroxymuconic semialdehyde revealed that the Bacillus sp. followed the meta-cleavage pathway during the biodegradation of phenol.

Designing Multi-Stage 2 A/O-MBR Processes for a Higher Removal Rate of Pollution in Wastewater

Multi-stage A/O-MBR processes were designed to improve wastewater treatment efficiency; three different designs were carried out and compared in this study. The 2(A/O)-MBR process, i.e., with two sets of anoxic/oxic tanks in series, showed better effluent quality than A/O-MBR and 3(A/O)-MBR processes. The removal rates of COD, NH4+-N, TP and TN were 95.29%, 89.47%, 83.55% and 78.58%, respectively, complying satisfactorily with China’s urban sewage treatment plant pollutant discharge standards. In terms of membrane fouling, the 3(A/O)-MBR process demonstrated the lowest fouling propensity. The microbial community structure in each bioreaction tank was analyzed, the results from which matched with the process efficiency and fouling behavior.

Unveiling organic loading shock-resistant mechanism in a pilot-scale moving bed biofilm reactor-assisted dual-anaerobic-anoxic/oxic system for effective municipal wastewater treatment

Microbial biomass and activity are frequently subjected to organic loading shock (OLS) from decentralized municipal wastewater. A hybrid moving bed biofilm reactor-assisted dual-anaerobic-anoxic/oxic system (D-A²MBBR) was established by integrating dual-anaerobic-anoxic/oxic with moving bed biofilm reactor to resist OLS for stable nutrients removal. The D-A²MBBR achieved 91.57% of chemical oxygen demand, 93.33% of ammonia-nitrogen, 80.20% of total nitrogen and 92.68% of total phosphorus removal, respectively, under the fluctuation of organic loading rate from 417.9 to 812.0 g COD m^-3 d^-1. The 16S rRNA gene sequencing revealed that Gemmobacter (7.28%) was identified as dominating anoxic denitrifying genus in oxic chamber, confirming the coexistence of aerobic and anaerobic/anoxic micro-environments. This circumstance boosted simultaneous nitrification-denitrification and phosphorus removal and the microbial community evolution inside the multilayer biocarrier-attached biofilms. In general, the D-A²MBBR was able to provide unique, cooperative and robust bacterial consortia to form a buffer against OLS, and ensuring effluent stability.

Analysis of nitrifying bacteria growth on two new types of biomass carrier using respirometry and molecular genetic methods

This work addresses the testing of two newly produced biomass carriers (micro- and nanofibers) and one commercially available AnoxKaldnes™ K3 carrier in a laboratory post-nitrification reactor. The carriers were prepared under parameters suitable for high-quality biomass adhesion to their surface, and each was characterized by its specific structures. As part of the evaluation of the biofilms using respirometry and molecular genetic methods, the carriers were assessed in terms of their effectiveness and comparability. The rate of biofilm development was dependent on the structure and surface properties of the individual carriers. The results showed that the biofilm most strongly adhered to nanofiber carriers, where nitrating bacteria's slower but more abundant development occurred. Microfiber carriers were more stable, but a diverse internal structure may be unsuitable in a populated carrier's early stages. The AnoxKaldnes™ K3 carriers showed the slowest growth of biofilm, but the monitored nitrifying bacteria were abundant after an extended time. AOB representatives are likely to prefer an environment with a high amount of biomass and a large active area. Conversely, NOB representatives thrive better in a slowly forming biofilm. The methods used to monitor biofilm are challenging to compare directly, but they do complement each other, which aids in verifying the individual test results. Developing new types of biomass carriers with the potential for high-quality adhesion of microorganisms is a prerequisite for the expansion of highly efficient biotechnological processes, especially for wastewater treatment.

Heterotrophic nitrification and aerobic denitrification by a novel Acinetobacter sp. TAC-1 at low temperature and high ammonia nitrogen

A novel strain was isolated from swinewastewater and identified as Acinetobacter sp. TAC-1 based on its phylogenetic and phenotypic characteristics. The strain TAC-1 was found to have a high ability to metabolize ammonium-N under low temperature condition. The strain TAC-1 could remove approximately 94.6% of ammonium-N (400 mg/L), 93.3% of nitrate-N (400 mg/L) and 42.4% of nitrite-N (400 mg/L) at 5 °C. The functional genes nitrate reductase gene (narG) and nitrite reductase gene (nirK, nirS) were successfully amplified by qPCR, further evidencing the heterotrophic nitrification and aerobic denitrification capability of Acinetobacter sp. TAC-1. The transcriptome data confirmed that the membrane transport protein and unsaturated fatty acid dehydrogenase-related genes of the strain TAC-1 were significantly up-regulated at 5 °C, enabling it to survive low temperatures. The high nitrogen removal ability at 5 °C makes this strain have a good application prospect.

Comparisons of nitrite accumulation, microbial behavior and nitrification kinetic in continuous stirred tank (ST) and plug flow (PF) moving bed biofilm reactors

Two types of continuous stirred tank moving bed biofilm reactors (ST-MBBR) and plug flow MBBR (PF-MBBR) were compared for nitrification. PF-MBBR showed strong shock resistance to temperature, and ammonium oxidation ratio (AOR) was 9.63% higher than that in the ST-MBBR, although the average biomass and biofilm thickness of ST-MBBR were 7.32-18.59%, 9.44-14.06% higher than those in the PF-MBBR. Meanwhile, a lower nitrite accumulation ratio (NAR) was observed (54.88%) in the PF-MBBR than the ST-MBBR (78.92%) due to different operation modes, and the divergence was demonstrated by the microbial quantitative analysis. Nitrification kinetics revealed that the temperature coefficient (θ) in the ST-MBBR (1.068) was much higher than that in the PF-MBBR (1.006-1.015), proving the contrasting nitrification performances caused by temperature shock. According to the Monod equation, the half-saturation coefficient (K_N) in the ST-MBBR was 0.19 mg/L while it varied around 0.12-0.24 mg/L in the PF-MBBR, revealing various NH₄⁺ affinity owing to different biofilm thickness and microbial composition. Finally, MBBR optimization related to operation mode, temperature, and free ammonium (FA) inhibition for nitrite accumulation was discussed.

Efficient degradation of polyacrylate containing wastewater by combined anaerobic-aerobic fluidized bed bioreactors

Polyacrylate containing wastewater (PCW) is the typical sewage discharged by the textile industry. It has extremely poor biodegradability, and chemical methods were used conventionally as the only way for treating PCW. This study is demonstrating a novel biological method. In batch experiment monod kinetics was applied to the experimental data, which indicated that anaerobic treatment used for PCW is feasible. The pilot-scale experiment combined a Spiral Symmetry Stream Anaerobic Bioreactor (SSSAB) and an air-lift external circulation vortex enhancement nitrogen removal fluidized bed bioreactor (AFB). The COD and NH₄⁺-N removal reached up to 95.2% and 96.6%, respectively, which were higher than the value obtained by other chemical methods. High-throughput sequencing analysis indicated that the relative abundance of Proteobacteria, Firmicutes and Bacteroidetes increased, which contribute to the degradation of PCW. Therefore, PCW can be degraded efficiently by using a SSSAB-AFB technique and thus provides an alternative to the chemical methods.

Combined effects of volume ratio and nitrate recycling ratio on nutrient removal, sludge characteristic and microbial evolution for DPR optimization

The optimization of volume ratio (V_An/V_A/V_O) and nitrate recycling ratio (R) in a two-sludge denitrifying phosphorus removal (DPR) process of Anaerobic Anoxic Oxic-Moving Bed Biofilm Reactor (A²/O-MBBR) was investigated. The results showed that prolonged anaerobic retention time (HRT_An: 1.25→3.75 hr) exerted favorable effect on chemical oxygen demand (COD) removal (57.26%→73.54%), poly-β-hydroxyalkanoates (PHA) synthesis (105.70→138.12 mgCOD/L) and PO₄^3- release (22.3→38.9 mg/L). However, anoxic retention time (HRT_A) and R exhibited positive correlation with PHA utilization (43.87%-81.34%) and denitrifying phosphorus removal (DPR) potential (ΔNO₃^-/ΔPO₄^3-: 0.57-1.34 mg/mg), leading to dramatical TN removal variations from 68.86% to 81.28%. Under the V_An/V_A/V_O ratio of 2:6:0, sludge loss deteriorated nutrient removals but the sludge bioactivity quickly recovered when the oxic zone was recovered. The sludge characteristic and microstructure gradually transformed under the dissolved oxygen (DO) control (1.0-1.5→1.5-2.0 mg/L), in terms of sludge volume index (SVI: 194→57 mL/gVSS), median-particle-size (D₅₀: 99.6→300.5 μm), extracellular polymeric substances (EPS) (105.62→226.18 mg/g VSS) and proteins/polysaccharides (PN/PS) ratio (1.52→3.46). Fluorescence in situ hybridization (FISH) results showed that phosphorus accumulation organisms (PAOs) (mainly Cluster I of Accumulibacter, contribution ratio: 91.79%-94.10%) dominated the superior DPR performance, while glycogen accumulating organisms (GAOs) (mainly Competibacter, contribution ratio: 82.61%-86.89%) was responsible for deteriorative TN and PO₄^3- removals. The optimal HRT_A and R assembled around 5-6.5 hr and 300%-400% based on the PHA utilization and DRP performance, and the oxic zones also contributed to PO₄^3- removal although it showed low dependence on DO concentration and oxic retention time (HRT_O).

Deep-level nutrient removal and denitrifying phosphorus removal (DPR) potential assessment in a continuous two-sludge system treating low-strength wastewater: The transition from nitration to nitritation

In a continuous two-sludge denitrifying phosphorus removal (DPR) process of anaerobic anoxic oxic - moving bed biofilm reactor (AAO - MBBR), nitritation was practicable through the combined regulation of high temperature (T: 30-32 °C), short hydraulic retention time (HRT: 8 h) and low dissolved oxygen (DO: 1.0-1.5 mg/L). The system lasted for 90 days with stable nitrite accumulation ratio (NAR > 60%), and the total inorganic nitrogen (TIN) removal was 7% higher than complete nitrification. Ammonia oxidizing bacteria ((AOB) 6.18-9.41%) responsible for nitritation showed a clear relationship with NAR, but Nitrospira (2.11% → 2.35%) gradually outcompeted Nitrobacter (1.19% → 0.31%) under higher temperature. During the transition from nitration to nitritation, the DPR potential (characterized by ΔPO₄^3-/ΔNO_x^-) increased by 11.90% while the energy requirement of poly-β-hydroxyalkanoates (PHA) and glycogen (Gly) decreased by 12.58% and 14.50%, respectively, contributing to higher TIN (84.83%) and TP (97.45%) removals. DPR batch tests using different electron acceptors (NO₃^- .vs. NO₃^- + NO₂^-) revealed that removing 1 mg PO₄^3- only consumed 7.12 ± 0.25 mg PHA via NO₃^- + NO₂^- (.vs. 8.50 ± 0.12 mg PHA via NO₃^-) and 16% carbon source was saved although the DPR capability was suppressed as NO₂^- concentration exceeded 15 mg/L. Based on the achievement of nitritation, the feasibility of integrated DPR - Anammox in the AAO - MBBR system for deep-level nutrient removal was discussed.

Nutrient metabolism, mass balance, and microbial structure community in a novel denitrifying phosphorus removal system based on the utilizing rules of acetate and propionate

The effect of acetate (HAc) and propionate (HPr) on denitrifying phosphorus removal (DPR) was evaluated in a novel two-sludge A²/O - MBBR (anaerobic/anoxic/oxic - moving bed biofilm reactor) system. Results showed that it was the carbon source transformation and utilization especially the composition of poly-β-hydroxyalkanoates (PHA) (mainly poly-β-hydroxybutyrate (PHB) and poly-bhydroxyvalerate (PHV)) decided DPR performance, where the co-exist of HAc and HPr promoted the optimal nitrogen (85.77%) and phosphorus (91.37%) removals. It facilitated the balance of PHB and PHV and removing 1 mg NO₃^- (PO₄^3-) consumed 3.04-4.25 (6.84-9.82) mgPHA, where approximately 40-45% carbon source was saved. Mass balance revealed the main metabolic pathways of carbon (M_An,C (consumed amount in anaerobic stage) and M_A-O,C (consumed amount in anoxic and oxic stages): 66.38-76.19%), nitrogen (M_DPR,N (consumed amount in DPR): 57.01-65.75%), and phosphorus (M_WS,P (discharged amount in waste sludge): 81.05-85.82%). Furthermore, the relative abundance and microbial distribution were assessed to elucidate DPR mechanism (e.g. Accumulibacter, Acinetobacter, Dechloromonas, Competibacter, and Defluviicoccus) in the A²/O reactor and nitrification performance (e.g. Nitrosomonas, Nitrosomonadaceae and Nitrospira) in the MBBR. Carbon source was demonstrated as the key point to stimulate the biodiversity and bioactivity related to DPR potential, and the operational strategy of carbon source addition was proposed based on the utilizing rules of HAc and HPr.

Roles of nitrate recycling ratio in the A2/O - MBBR denitrifying phosphorus removal system for high-efficient wastewater treatment: Performance comparison, nutrient mechanism and potential evaluation

The long-term effect of nitrate recycling ratios (R = 100%-500%) on the denitrifying phosphorus removal (DPR) characteristics was studied in a novel two-sludge system, which coupled Anaerobic Anoxic Oxic (A²/O) with Moving Bed Biofilm Reactor (MBBR) for simultaneous nitrogen (N) and phosphorus (P) removals. During the 220 days' operation, effluent COD (30.87-45.15 mg/L) can meet the discharge standard completely, but N and P removals were significantly affected by the R-value, including COD_intra removal efficiency (COD_intra-Re: 56.09-85.98%), TN removal (TN-Re: 52.06-80.50%), anaerobic PO₄^3- release (PO₄^3--An: 10.66-29.02 mg/L) and oxic PO₄^3- absorption (PO₄^3--O: 2.22-6.26 mg/L). Meanwhile, N and P displayed close correlation with the ΔPO₄^3-/ΔNO₃^- ratio of 4.20-4.41 at R = 300%-400%, resulting in the high-efficient anoxic poly-β-hydroxyalkanoates (PHA) utilization (ΔPHA_A: 64.88 mgCOD/gVSS). Based on the stoichiometry methodology, at R of 300%-400%, the percentages of phosphorus accumulation organisms (PAOs) and glycogen accumulating organisms (GAOs) contributed to ΔPHA_An (ΔGly_An) were 71.7%, 28.3% (61.3%, 38.7%) in the anaerobic stage, respectively, while N denitrification rate (NDR_A: 3.91-3.93 mg N/(gVSS·h)) and P uptake rate (PUR_A: 3.76-3.90 mg P/(gVSS·h)) reached the peak, suggesting superior DPR performance with higher contribution of denitrifying PAOs (DPAOs) (70%) than denitrifying GAOs (DGAOs) (30%) in the anoxic stage. Microbial community analysis showed that Accumulibacter (27.66-30.01%) was more enriched than Competibacter (13.41-14.34%) and was responsible for the improved C, N, P removals and DPR characteristics. For optimizing operation, the combined effect of nitrate recycling ratio with other process parameters especially economic evaluation should be considered.

Enhancing the nitrogen removal of anammox by treating municipal wastewater with sludge fermentation products in a continuous flow reactor

In this study, a novel process was developed to treat real sewage with a low chemical oxygen demand/total nitrogen ratio (COD/TN = 3.2) and to obtain enhanced nitrogen removal through Anaerobic ammonia oxidation (anammox). Anaerobic/aerobic/anoxic/aerobic (AOAO) reactor processes were amended with a fixed anammox biofilm in the anoxic zone. During an operational period of 212 days, an average effluent TN of 13.7 mg/L with a removal efficiency of 72.0% was obtained with an influent of 47.0 mg/L ammonium. Mass balance analysis suggested that the anammox resulted in removal of 33.6% of the TN. Besides, by adding sludge fermentation products, nitrite accumulation occurred via nitration while in the aerobic zone for the anammox process. This study demonstrated an alternative way to apply a sewage anammox process via excess sludge fermentation products triggering nitrite production in a continuous flow reactor.

Bioaugmentation of low C/N ratio wastewater: Effect of acetate and propionate on nutrient removal, substrate transformation, and microbial community behavior

The effect of various acetate/propionate ratios (1:0, 2:1, 1:1, 1:2, and 0:1) in a two-sludge A2/O - MBBR process was investigated. Results showed that the increased propionic/acetic ratios exerted indistinctive impact on COD (91.21-93.44%) and P (92.23-93.87%) removals, but high P content (7.42%) accelerated sludge granulation proved by SEM and EDS. Acetate favored N removal (79.52%-82.92%) with higher PURA (3.53-4.06 mgP/(gVSS·h)), while the removal declined (75.14%) due to lower PHB/PHA ratio (52.3-57.8%) with propionate as sole carbon source. Based on the stoichiometry-based quantifications, PAOs were the major contributors to nutrient removal although certain GAOs and OHO participated. The mixture ratio of 1:1 facilitated microbial diversity (995 OTUs), Rhodobacteraceae (25.63%) was responsible for high-efficient denitrifying phosphorus removal, while Defluviicoccus (15.23%) contributed to nitrite accumulation was the main competitiveness with PAOs. Nitrospira, Nitrosomonas, and Nitrosomonadaceae responsible for nitrification accounted for 7.73%, 27.11%, and 38.76% in MBBR, but the biodiversity decreased owing to the enrichment and purification.

Elucidating sludge characteristic, substrate transformation and microbial evolution in a two-sludge denitrifying phosphorus removal system under the impact of HRT

Granule formation has been recognized as a promising biotechnology in denitrifying phosphorus removal (DPR) systems by facilitating phosphorus accumulation organisms (PAOs) especially denitrifying PAOs (DPAOs), and hydraulic selection made this a more difficult task in continuous operation. This study aimed at exploring the microscopic mechanism and putting forward an effective strategy for DPR granulation under the impact of hydraulic retention time (HRT) (12 h, 10 h, 8 h) in a novel Anaerobic Anoxic Oxic - Moving Bed Biofilm Reactor (A²/O - MBBR) system. With the reduction of intracellular carbon storage (COD_intra) efficiency (88.58%-78.53%), nitrogen (N) (85.45%-79.11%) and phosphorus (P) (96.55%-92.47%) removals both dropped, but it exhibited a growth of anoxic phosphorus uptake rate (PUR_A) (3.79-5.68 mg P/(gMLVSS·h)). The batch tests associating with substrate transformation of poly-β-hydroxyalkanoates (PHA), glycogen (Gly) agreed well with the corresponding stoichiometry of phosphorus release rate (PRR) (4.83-7.53 mg P/(gMLVSS·h)), PUR_A (3.55-5.43 mg P/(gMLVSS·h)), oxic phosphorus uptake rate (PUR_O) (6.08-6.21 mg P/(gMLVSS·h)), and DPAOs/PAOs ratios (57.17%-89.31%), indicating a shift of microbial community. DPR granules gradually stabilized with low sludge volume index (SVI₅/SVI₃₀ ratio = 1.1-1.2), dense and compact structure, higher P content (11.63%), more extracted extracellular polymeric substances (EPS) (111.40-160.31 mg/gMLVSS) as proteins/polysaccharides (PN/PS) ratios (1.70-3.47) increased, leading to better sludge settleability and cell hydrophobicity. Fluorescence in situ hybridization (FISH) results showed that PAOs (mainly Cluster I: 20.20%) were the dominant bacteria in the A²/O reactor although a small amount of Defluviicoccus (3.18-3.48%) was responsible for nitrite accumulation, while ammonium-oxidizing bacteria (AOB) (mainly Nitrosomonas: 10.75%) and nitrite-oxidizing bacteria (NOB) (mainly Nitrospira: 15.06%) were enriched in the MBBR.