Effect of organic carbon on nitrogen conversion and microbial communities in the completely autotrophic nitrogen removal process

Effect of organic matter on nitrogen removal through the anammox process

In the anaerobic ammonium oxidation (anammox) process, the anammox bacterial activity is inhibited by high chemical oxygen demand (COD) contents. In this study, the effect of the COD/total nitrogen (TN) ratio and hydraulic retention time (HRT) on the efficiency of the anammox process in a fixed-bed reactor was assessed via experiments with increasing COD/TN ratios from 0 to 7.0 and different HRTs of 6, 9, and 12 hr. Felibendy cubes were used as the biomass carrier. The presence of organic compounds affects the performance of the treatment process, and the level of influence increases with the increase in influent COD. With COD greater than 300 mg/L, the anammox process was inhibited, TN efficiency was the lowest, and effluent water quality did not yield effluent standards. The kinetic parameters U_max (the maximum substrate removal rate constant) and K_B (the saturation constant) of the Stover-Kincannon model corresponding to the different COD/TN ratios were determined in this study. The kinetic parameter values of the ammonium removal process decreased gradually from U_max  = 0.685 g/L/day and K_B  = 0.846 g/L/day, for COD/TN = 0, to U_max  = 0.314 g/L/day and K_B  = 0.498 g/L/day, for COD/TN = 6. The kinetic parameter of the TN removal process also decreased gradually as the COD/TN ratio in wastewater increased gradually. This means that the anammox process was inhibited as much as high organic compounds. PRACTITIONER POINTS: The higher COD/TN ratios the more anammox activity is affected, especially COD/TN = 7.0 (COD = 350 mg/L). The tolerance threshold of anammox process about organic matter concentration is 300 mg/L as COD (COD/TN ratio <6). With the COD/TN ratios from 1.0 to 6.0, the values of U_max and K_B constants of Stover-Kincannon model for ammonium removal by Anammox process decrease gradually from 0.685 to 0.314 and from 0.846 to 0.498, respectively.

Model-based investigation of membrane biofilm reactors coupling anammox with nitrite/nitrate-dependent anaerobic methane oxidation

An innovative process coupling anaerobic ammonium oxidation (anammox) with nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) in membrane biofilm reactors (MBfRs) has been developed to achieve high-level nitrogen removal from both sidestream (i.e., anaerobic digestion liquor) and mainstream (i.e., domestic strength) wastewater. In this study, a 1D biofilm model embedding the n-DAMO and anammox reactions was developed to facilitate further understanding of the process and its optimization. The model was calibrated and validated using comprehensive data sets from two independent MBfRs, treating sidestream- and mainstream-strength wastewater, respectively. Modelling results revealed a unique biofilm stratification. While anammox bacteria dominated throughout the biofilm, n-DAMO archaea (coupling nitrate reduction with anaerobic methane oxidation) only occurred at the inner layer and n-DAMO bacteria (coupling nitrite reduction with anaerobic methane oxidation) spread more evenly with a slightly higher fraction in the outer layer. The established MBfRs were robust against dynamic influent flowrates and nitrite/ammonium ratios. Thicker biofilms were beneficial for not only the total nitrogen (TN) removal but also the system robustness. Additionally, a positive correlation between the nitrogen removal efficiency and the residual methane emission was observed, as a result of higher methane partial pressure required. However, there was a threshold of methane partial pressure, above which the residual methane increased but nitrogen removal efficiency was stable. Meanwhile, thicker biofilms were also favorable to achieve less residual methane emission. Simulation results also suggested the feasibility of methane-based MBfRs to polish mainstream anammox effluent to meet a stringent N discharge standard (e.g., TN < 5 mg/L).

Occurrence of anammox on suspended sediment (SPS) in oxic river water: Effect of the SPS particle size

Anammox is a newly discovered nitrogen transformation process. However, its role in nitrogen removal in fresh water is far from understood. Here, we hypothesized that anammox could occur on suspended sediment in oxic river water. To test this hypothesis, simulation experiments with a nitrogen stable (¹⁵N) isotopic tracer technique were conducted to study the occurrence of anammox on suspended sediment (SPS) in oxic river water, and the effects of the SPS particle size, including <20 μm, 20-63 μm, 63-100 μm, 100-200 μm, and <200 μm (original SPS) size fractions, were investigated. The results showed that anammox occurred in oxic water with SPS due to the existence of low oxygen microsites around/on SPS, and the anammox rate was even higher than the denitrification rate. The anammox rate increased with the SPS concentration, and it was negatively correlated with the particle size and was positively correlated with the organic carbon content of SPS (p < 0.05). The ²⁹N₂ produced by anammox in a system containing 1.0 g L^-1 SPS with a particle size below 20 μm was 0.27 mg-N/m³·d, which was 5.3 times higher than that produced with a particle size of 100-200 μm. The anammox rate was significantly positively correlated with the anammox bacterial abundance (p < 0.01), and Ca. Brocadia was the dominant species. This study suggests that the SPS in oxic water may be a 'hotspot' for the anammox process and that its role in nitrogen removal should be considered in future studies.

Evaluation of Partial Nitritation/Anammox (PN/A) Process Performance and Microorganisms Community Composition under Different C/N Ratio

A one-stage partial nitritation/anammox (PN/A) process with intermittent aeration is possible under sidestream conditions, but implementation in a mainstream is a challenge due to increased Carbon/Nitrogen (C/N) ratios in domestic wastewater. This study investigated the effect of C/N ratios on process efficiency and the effect of narrowing non-aeration time on process improvement at high chemical oxygen demand (COD) load. An increase in TN removal efficiency was achieved in both series with gradual change of C/N ratio from 1 to 3, from 65.1% to 83.4% and 63.5% to 78% in 1st and 2nd series, respectively. However, at the same time, the ammonium utilization rate (AUR) value decreased with the increase in C/N ratio. At a high COD (C/N = 3) concentration, the process broke down and regained productivity after narrowing the non-aeration time in both series. Shifts in the system performance were also connected to adaptive changes in microbial community revealed by data obtained from 16S rRNA NGS (next-generation sequencing), which showed intensive growth of the bacteria with dominant heterotrophic metabolism and the decreasing ratio of autotrophic bacteria. The study shows that deammonification is applicable to the mainstream provided that the C/N ratio and the aeration/non-aeration time are optimized.

Enhancing mainstream nitrogen removal by employing nitrate/nitrite-dependent anaerobic methane oxidation processes

Due to serious eutrophication in water bodies, nitrogen removal has become a critical stage for wastewater treatment plants (WWTPs) over past decades. Conventional biological nitrogen removal processes are based on nitrification and denitrification (N/DN), and are suffering from several major drawbacks, including substantial aeration consumption, high fugitive greenhouse gas emissions, a requirement for external carbon sources, excessive sludge production and low energy recovery efficiency, and thus unable to satisfy the escalating public needs. Recently, the discovery of anaerobic ammonium oxidation (anammox) bacteria has promoted an update of conventional N/DN-based processes to autotrophic nitrogen removal. However, the application of anammox to treat domestic wastewater has been hindered mainly by unsatisfactory effluent quality with nitrogen removal efficiency below 80%. The discovery of nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) during the last decade has provided new opportunities to remove this barrier and to achieve a robust system with high-level nitrogen removal from municipal wastewater, by utilizing methane as an alternative carbon source. In the present review, opportunities and challenges for nitrate/nitrite-dependent anaerobic methane oxidation are discussed. Particularly, the prospective technologies driven by the cooperation of anammox and n-DAMO microorganisms are put forward based on previous experimental and modeling studies. Finally, a novel WWTP system acting as an energy exporter is delineated.

Investigation of COD and COD/N ratio for the dominance of anammox pathway for nitrogen removal via isotope labelling technique and the relevant bacteria

This study aimed to investigate the importance of COD (chemical oxygen demand) and ratio of COD and nitrogen (COD/N) in influencing the dominance of anammox pathway to N-removal in anammox systems, which had been widely researched and results were not yet conclusive. Results showed that N-removal efficiency increased with increasing organic substrate, while the anammox contribution to N-removal decreased as confirmed by isotope labelling technique. Excessively high TN (total nitrogen) concentrations were detrimental to N-removal, and TN of 600 mg L^-1 was optimized. Specific COD of 300 mg L^-1 (a threshold value above which anammox was less active) was synergistic for N-removal. Moreover, Illumina sequencing and qPCR techniques uncovered that while the microbial community composition was relatively stable for all treatments, abundances of denitrifier were positively correlated with increase of COD, which was counter-productive for anammox abundance. Structure equation model indicated that COD was more important with respect to maintain the anammox stability than the COD/N ratio. Furthermore, experiment and model fittings revealed that anammox contributed more than 80% of N-removal when COD was below 55.7 mg L^-1, and approximately 50% at 220-300 mg L^-1 COD, respectively. These data formed a reference for regulation of anammox systems in real-world applications.

Effects of carbon sources, COD/NO2--N ratios and temperature on the nitrogen removal performance of the simultaneous partial nitrification, anammox and denitrification (SNAD) biofilm

The aim of the present work was to evaluate the effects of carbon sources and chemical oxygen demand (COD)/NO₂^--N ratios on the anammox-denitrification coupling process of the simultaneous partial nitrification, anammox and denitrification (SNAD) biofilm. Also, the anammox activities of the SNAD biofilm were investigated under different temperature. Kaldnes rings taken from the SNAD biofilm reactor were operated in batch tests to determine the nitrogen removal rates. As a result, with the carbon source of sodium acetate, the appropriate COD/NO₂^--N ratios for the anammox-denitrification coupling process were 1 and 2. With the COD/NO₂^--N ratios of 1, 2, 3, 4 and 5, the corresponding NO₂^--N consumption via anammox was 87.1%, 52.2%, 29.3%, 23.7% and 16.3%, respectively. However, with the carbon source of sodium propionate and glucose, the anammox bacteria was found to perform higher nitrite competitive ability than denitrifiers at the COD/NO₂^--N ratio of 5. Also, the SNAD biofilm could perform anammox activity at 15 °C with the nitrogen removal rate of 0.071 kg total inorganic nitrogen per kg volatile suspended solids per day. These results indicated that the SNAD biofilm process might be feasible for the treatment of municipal wastewater at normal temperature.

Pathways of N removal and N2O emission from a one-stage autotrophic N removal process under anaerobic conditions

To investigate the pathways of nitrogen (N) removal and N₂O emission in a one-stage autotrophic N removal process during the non-aeration phase, biofilm from an intermittent aeration sequencing batch biofilm reactor (SBBR) and organic carbon-free synthetic wastewater were applied to two groups of lab-scale batch experiments in anaerobic conditions using a ¹⁵N isotopic tracer and specific inhibitors, respectively. Then, the microbial composition of the biofilm was analysed using high-throughput sequencing. The results of the ¹⁵N isotopic experiments showed that anaerobic ammonium oxidation (Anammox) was the main pathway of N transformation under anaerobic conditions and was responsible for 83–92% of N₂ production within 24 h. Furthermore, experiments using specific inhibitors revealed that when nitrite was the main N source under anaerobic conditions, N₂O emissions from heterotrophic denitrification (HD) and ammonia-oxidizing bacteria (AOB) denitrification were 64% and 36%, respectively. Finally, analysing the microbial composition demonstrated that Proteobacteria, Planctomycetes, and Nitrospirae were the dominant microbes, corresponding to 21%, 13%, and 7% of the microbial community, respectively, and were probably responsible for HD, Anammox, and AOB denitrification, respectively.

Reactor performance and microbial characteristics of CANON process with step-wise increasing of C/N ratio

In this study, the nitrogen removal performance and microbial characteristics of completely autotrophic nitrogen removal over nitrite (CANON) process was investigated with a step-wise increasing of C/N ratio (0.5, 1, 2 and 4) in a membrane bioreactor. The microbial distribution of aerobic ammonia-oxidizing bacteria (AOB) and anaerobic AOB (AAOB) was analysed by fluorescence in situ hybridization (FISH). Results showed that the denitrification ratio rose up correspondingly with the increase of influent C/N, and nitrogen removal rate (NRR) reached the maximum when C/N was 1 due to the harmonious work of denitrification and CANON. However, NRR decreased when influent C/N was more than 2. The threshold C/N ratio of CANON process was 2.2; so the sewage with a high C/N ratio should be pretreated by combining with pre-oxidation of organics or anaerobic-energy-producing process. FISH results showed decreasing numbers of both AOB and AAOB with the addition of organics.

Effect of COD/N ratio on nitrogen removal and microbial communities of CANON process in membrane bioreactors

In this study, the effect of COD/N ratio on completely autotrophic nitrogen removal over nitrite (CANON) process was investigated in five identical membrane bioreactors. The five reactors were simultaneously seeded for 1L CANON sludge and be operated for more than two months under same conditions, with influent COD/N ratio of 0, 0.5, 1, 2 and 4, respectively. DGGE was used to analyze the microbial communities of aerobic ammonia-oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing bacteria (AAOB) in five reactors. Results revealed the harmonious work of CANON and denitrification with low COD concentration, whereas too high COD concentration suppressed both AOB and AAOB. AOB and AAOB biodiversity both decreased with COD increasing, which then led to worse nitrogen removal. The suppressing threshold of COD/N ratio for CANON was 1.7. CANON was feasible for treating low COD/N sewage, while the high sewage should be converted by anaerobic biogas producing process in advance.

Bacteria in non-woven textile filters for domestic wastewater treatment

The objective of this study was preliminary identification of heterotrophic and ammonia oxidizing bacteria (AOB) cell concentration in the cross-sectional profile of geotextile filters for wastewater treatment. Filters of thicknesses 3.6 and 7.2 mm, made of non-woven textile TS20, were supplied with septic tank effluent and intermittently dosed and filtered under hydrostatic pressure. The cumulative loads of chemical oxygen demand (COD) and total solids were about 1.36 and 1.06 kg/cm2, respectively. The filters under analysis reached a relatively high removal efficiency for organic pollution 70-90% for biochemical oxygen demand (BOD5) and 60-85% for COD. The ammonia nitrogen removal efficiency level proved to be unstable (15-55%). Biomass samples for dry mass identification were taken from two regions: continuously flooded with wastewater and intermittently flooded with wastewater. The culturable heterotrophic bacteria were determined as colony-forming units (CFUs) on microbiological-selective media by means of the plate method. AOB and nitrite oxidizing bacteria (NOB) were examined using the FISH technique. A relatively wide range of heterotrophic bacteria was observed from 7.4×10(5)/cm2 to 3.8×10(6)/cm2 in geotextile layers. The highest concentration of heterotrophic bacteria (3.8×10(6)/cm2) was observed in the first layer of the textile filter. AOB were identified occasionally--about 8-15% of all bacteria colonizing the last filter layer, but occasionally much higher concentrations and ammonia nitrogen efficiency were achieved. Bacteria oxidizing nitrite to nitrate were not observed. The relation of total and organic fraction of biomass to culturable heterotrophic bacteria was also found.

Nitritating-anammox biomass tolerant to high dissolved oxygen concentration and C/N ratio in treatment of yeast factory wastewater

Maintaining stability of low concentration (< 1 g L(-1)) floccular biomass in the nitritation-anaerobic ammonium oxidation (anammox) process in the sequencing batch reactor (SBR) system for the treatment of high COD (> 15,000 mg O2 L(-1)) to N (1680 mg N L(-1)) ratio real wastewater streams coming from the food industry is challenging. The anammox process was suitable for the treatment of yeast factory wastewater containing relatively high and abruptly increased organic C/N ratio and dissolved oxygen (DO) concentrations. Maximum specific total inorganic nitrogen (TIN) loading and removal rates applied were 600 and 280 mg N g(-1) VSS d(-1), respectively. Average TIN removal efficiency over the operation period of 270 days was 70%. Prior to simultaneous reduction of high organics (total organic carbon > 600mg L(-1)) and N concentrations > 400 mg L(-1), hydraulic retention time of 15 h and DO concentrations of 3.18 (+/- 1.73) mg O2 L(-1) were applied. Surprisingly, higher DO concentrations did not inhibit the anammox process efficiency demonstrating a wider application of cultivated anammox biomass. The SBR was fed rapidly over 5% of the cycle time at 50% volumetric exchange ratio. It maintained high free ammonia concentration, suppressing growth of nitrite-oxidizing bacteria. Partial least squares and response surface modelling revealed two periods of SBR operation and the SBR performances change at different periods with different total nitrogen (TN) loadings. Anammox activity tests showed yeast factory-specific organic N compound-betaine and inorganic N simultaneous biodegradation. Among other microorganisms determined by pyrosequencing, anammox microorganism (uncultured Planctomycetales bacterium clone P4) was determined by polymerase chain reaction also after applying high TN loading rates.

Successful application of nitritation/anammox to wastewater with elevated organic carbon to ammonia ratios

The nitritation/anammox process has been mainly applied to high-strength nitrogenous wastewaters with very low biodegradable organic carbon content (<0.5 g COD∙g N(-1)). However, several wastewaters have biodegradable organic carbon to nitrogen (COD/N) ratios between 0.5 and 1.7 g COD∙g N(-1) and thus, contain elevated amounts of organic carbon but not enough for heterotrophic denitrification. In this study, the influence of elevated COD/N ratios was studied on a nitritation/anammox process with suspended sludge. In a step-wise manner, the influent COD/N ratio was increased to 1.4 g COD∙g N(-1) by supplementing digester supernatant with acetate. The increasing availability of COD led to an increase of the nitrogen removal efficiency from around 85% with pure digester supernatant to >95% with added acetate while the nitrogen elimination rate stayed constant (275 ± 40 mg N∙L(-1)∙d(-1)). Anammox activity and abundance of anammox bacteria (AMX) were strongly correlated, and with increasing influent COD/N ratio both decreased steadily. At the same time, heterotrophic denitrification with nitrite and the activity of ammonia oxidising bacteria (AOB) gradually increased. Simultaneously, the sludge retention time (SRT) decreased significantly with increasing COD loading to about 15 d and reached critical values for the slowly growing AMX. When the SRT was increased by reducing biomass loss with the effluent, AMX activity and abundance started to rise again, while the AOB activity remained unaltered. Fluorescent in-situ hybridisation (FISH) showed that the initial AMX community shifted within only 40 d from a mixed AMX community to "Candidatus Brocadia fulgida" as the dominant AMX type with an influent COD/N ratio of 0.8 g COD∙g N(-1) and higher. "Ca. Brocadia fulgida" is known to oxidise acetate, and its ability to outcompete other types of AMX indicates that AMX participated in acetate oxidation. In a later phase, glucose was added to the influent instead of acetate. The new substrate composition did not significantly influence the nitrogen removal nor the AMX activity, and "Ca. Brocadia fulgida" remained the dominant type of AMX. Overall, this study showed that AMX can coexist with heterotrophic bacteria at elevated influent COD/N ratios if a sufficiently high SRT is maintained.

Comparing the performance and operation stability of an SBR and MBBR for single-stage nitritation-anammox treating wastewater with high organic load

Single stage nitritation-anammox reactors have gained increasing attention for their application in municipal and industrial wastewater treatment. The most commonly used system in municipal reject water treatment is at present the sequencing batch reactor (SBR), the moving-bed biofilm reactor (MBBR) is the second most common. However, little is known about their applicability to industrial wastewaters with high C/N ratios. This study presents a comparative approach to evaluate the performance of these two systems by changing the influent from reject water (C:N ratio 1:1) stepwise to an industrial wastewater (C:N ratio 3:1). An intentionally induced temperature drop that led to nitrite accumulation was also tested. The results showed that the MBBR (1.9 kg-N m(-3) d(-1)) was superior to the SBR (0.5 kg-N m(-3) d(-1)) with at maximum up to four times higher volumetric nitrogen removal rates. Both systems accumulated nitrite (> 100 mg-N L(-1)) during the temperature drop from 30 degrees C to as low as 18 degrees C (MBBR) and 20 degrees C (SBR), which subsequently resulted in almost complete loss in the removal capacities. However, the previous removal rates could be re-established in both systems within approximately 40 days. In comparison, the MBBR showed the more stable and higher performance even though higher nitrite concentrations (up to 500 mg-N L(-1)) were encountered. Overall, MBBR operation and handling was also easier and the system was more robust to disturbances compared to the SBR.