Nitrifying moving bed biofilm reactor: Performance at low temperatures and response to cold-shock

Response and recovery of nitrifying moving bed biofilm reactor systems exposed to 1°C with varying levels of ammonia starvation

This study investigated the impact of varying total ammonia nitrogen (TAN) feed levels along with water temperature decreases on the performance of nitrifying moving bed biofilm reactor (MBBR) at 1 °C and its recovery at 3 °C. Five MBBR reactors were operated with different TAN concentrations as water temperature decreased from 20 to 3 °C: reactor R1 at 30 mg N/L, reactor R2 at 20 mg N/L, reactor R3 at 15 mg N/L, reactor R4 at 10 mg N/L and reactor R5 at 0 mg N/L. The corresponding biofilm characteristics were also analyzed to understand further nitrifying MBBR under different TAN feeding scenarios. The findings revealed that the higher TAN levels were before reaching 1 °C, the better nitrification performance and the more biomass grew. However, the highest TAN concentration (30 mg N/L) might negatively affect the nitrification performance, the activity of nitrifiers, and the growth of biofilms at 1 °C because of the toxic effects of un-ionized or free ammonia (FA). It was observed that the activities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were affected by FA concentrations ranging from 0.2 to 0.7 mg N/L at 1 °C, but they could gradually be adapted to such inhibitory environment, with NOB recovering more quickly and robustly than AOB. The study identified 20 mg N/L (67 % of maximum influent TAN at 1 °C in R2 as the optimal TAN feeding concentration, achieving over 90 % TAN removal and a surface area removal rate (SARR) of 0.78 ± 0.02 g N/m2·d at 1 °C. Meanwhile, R2 also exhibited the highest biofilm mass, with total solids at 13.3 mg/carrier and volatile solids at 11.3 mg/carrier. As TAN was removed, nitrite accumulation was observed at 1 °C, and higher influent TAN concentrations prior to 1 °C appeared to delay the accumulation. When water temperature increased from 1 °C to 3 °C, nitrification performance improved significantly in all reactors without nitrite accumulation, and the higher TAN feeding in the previous stage led to faster recovery. Compared with 20 °C, biofilm became thinner and denser at 1 °C and 3 °C. Furthermore, this study revealed significant shifts in microbial community composition and nitrifier abundances in response to changes in water temperature and influent TAN levels. The dominant nitrifiers were identified as Nitrosomonadaceae (AOB) and Nitrospiraceae (NOB). At 1 °C, the nitrifier abundances were significantly correlated with SARRs, FA, and biofilm density. R2, which exhibited the best nitrification performance, maintained higher nitrifier abundances at 1 °C.

Nitrogen removal in moving-bed biofilm reactor plants at low temperatures: experiences from Norway

Cold and diluted wastewater is a major challenge for nitrogen removal at wastewater treatment plants. The moving-bed biofilm reactor (MBBR) process has proven suitable for nitrogen removal under these conditions and several full-scale plants have been in operation for more than 20 years. These plants are still performing well, even at above design loads. Recently some of these plants have been evaluated for their low-temperature performance. In general, the results showed that temperatures down to 5 °C only had a minor impact on observed nitrification and denitrification rates. Higher dissolved oxygen concentrations can boost nitrification rates and are used as a tool to increase rates at low temperatures, thus partially compensating for the temperature effect. Post-denitrification rates were boosted by a controlled increase in carbon-to-nitrogen ratios at low temperatures. MBBR processes with combined pre- and post-denitrification are recommended for nitrogen-removal plants operating at low temperatures. Design recommendations and examples of flowsheets are given.

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.

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.

Performance and recovery of nitrifying biofilm after exposure to prolonged starvation

Achieving consistent ammonia removal in post-lagoon processes faces two major challenges impacting nitrifiers due to the unique seasonal variation of lagoon-based systems: summer to winter temperature drop and summer to fall ammonia starvation period while lagoon is removing ammonia. The objective of this study was to follow microbial diversity and define conditions that could overcome these challenges in a post-lagoon moving bed biofilm reactor (MBBR) operated at an initial surface area loading rate (SALR) of 0.3 g-NH4-N m^-2d^-1 from mesophilic (20 °C) to psychrophilic (4 °C). Initially the temperature was maintained at 20 °C and decreased to 10 °C until steady state was achieved. During starvation conditions (i.e., continuous, intermittent and no aeration without inflow; decanted media; and intermittent and continuous ammonia supplement) the temperature was decreased by 2 °C per week until 4 °C. The results indicated that operational procedures, such as intermittent ammonia supplement with SALR of 0.15 g-NH₄-N m^-2d^-1 could improve performance with 80% ammonia removal achieved immediately after starvation period. Intermittent ammonia supplement had produced the greatest biofilm preservation comparable to the initial load with the highest specific and surface area removal rates. In the recovery phase (initial load restoration) 10 days were required to reestablish performance above 95% ammonia removal. When temperature was decreased from mesophilic to psychrophilic, the microbial diversity was found higher when starving biofilm compared to the control operated at the initial load while it converged to a similar population over recovery. The main actors associated to nitrification enriched at psychrophilic conditions were Proteobacteria and Bacteriodotes at phyla level. Ammonia oxidation to nitrite was mainly driven by the order Burkholderiales and nitrite oxidation to nitrate by Pseudomonadales. This procedure should be considered in the implementation of full-scale post-lagoon MBBR technologies to ensure reliable, robust, and consistent performance despite the inherent seasonal variability of lagoon-based processes.

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).

Biological fixed-film systems

The technical papers published in 2019 regarding wastewater treatment and microbial films were classified into two categories: biofilm and biofilm reactors. The biofilm category includes biofilm formation, biofilm consortia, bacterial signals, biofouling, extracellular polymeric substances, and biofilm membrane bioreactors. The biofilm reactors category provides recent information on rotating biological contactors, fluidized-bed biofilm reactors, integrated fixed-film activated sludge, moving-bed biofilm reactors, packed-bed biofilm reactors, sequencing biofilm batch reactors, and trickling filters.

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.

Ecological insights into the underlying evolutionary patterns of biofilm formation from biological wastewater treatment systems: Red or Black Queen Hypothesis?

Interspecies interactions and phylogenetic distances were studied to reveal the underlying evolutionary adaptations of biofilms sourced from wastewater treatment processes. Based on 380 pairwise cocultures of 40 strains from two microbial aggregates (surface-attached and mobile aggregates [flocs]) at two substrate concentrations (LB broth and 0.1× LB broth), interspecies interactions were explored using biofilm classification schemes. There was a strong source-dependence of biofilm development formed by the monocultures, that is, a higher biofilm formation potential for strains from attached aggregates than for those from sludge flocs at both substrate concentrations. Interestingly, the results showed that total biofilm reduction was dominant in the dual-species biofilm sourced from flocs in both LB broth (67.37%) and 0.1× LB broth (64.21%), indicating high interspecific competition in mobile aggregates and the independence of substrate concentrations. However, biofilm reduction was higher (33.68%) than induction (19.37%) for the biofilms formed by surface-attached aggregates in LB broth, while the opposite trend was apparent in 0.1× LB broth, suggesting the occurrence of indeterministic processes for biofilm formation and important roles of substrate concentrations. In addition, the more closely related phylogenetic relationships of cocultures from mobile aggregates were consistent with higher competition compared with those from surface-attached aggregates. Overall, the underlying evolutionary patterns of biofilms formed from mobile aggregates consistently followed the essence of the "Red Queen Hypothesis," while biofilms developed from surface-attached aggregates were not deterministic. This study advanced our understanding of biofilm-related treatment processes using the principles of microbial ecology.

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

A two-sludge system consisting of A²/O (Anaerobic Anoxic Oxic) and NMBBR (Nitrification Moving Bed Biofilm Reactor) was developed. Stable and efficient denitrifying phosphorus removal can be realized by high-efficiency utilization of carbon sources in A²/O reactor with the electron acceptors of NO_x^--N in a three-stage NMBBR (consisting of N₁, N₂, N₃). The three-stage NMBBR was successfully started within 18 days without additional inoculation sludge. Then a long-term operation (22-120 d) for the optimization of nitrifying performance, microbial community, and kinetic parameters was investigated. The biofilm characteristics (MLSS and biofilm thickness) and real-time control parameters (DO and pH) initially revealed the differences of three stages, while FISH results confirmed the optimizing nitrifying bacteria populations including AOB, Nitrobacteria and Nitrospira (N₁: 5.94 ± 0.12%; N₂: 8.26 ± 0.42%; N₃: 10.06 ± 0.27% on day 50), basically consisting with the qPCR results (N₁: 4.05%; N₂: 8.04%; N₃: 14.14%). The specific ammonium oxidation rate (SAOR: 3.24-10.02 mg/(gMLSS·h)) and temperature coefficient (θ: 1.008-1.011) based on temperature variation (15-35 °C) exhibited a strong resistant ability to low temperature operation. Moreover, half-saturation constants (K_N,AOB, K_N,NOB, K_O,AOB and K_O,NOB) fitted by Monod equation proved that DO diffusion played a significant role than substrate utilization (NH₄⁺-N and NO₂^--N), but the diffusion resistance was negligible for flocs size smaller than 70 μm. Additionally, the dominant NOB (mainly Nitrospira) due to a higher K_N,NOB and K_O,NOB was more sensitive to mass transfer and diffusion resistance, which was helpful to understand the microbial competition for short-cut nitrification between AOB and NOB. Based on the above mechanism analysis, the MBBR optimization for the design and operation was put forward.