Microbial activity of suspended biomass from a nitritation–anammox SBR in dependence of operational condition and size fraction

Looking deeper into the effects of scouring and aeration on membrane aerated biofilms: Analysis of nitrogen conversion, oxygen profiles and nitrous oxide emissions

This study investigated the effects of aeration and scouring strategies on the performance of Membrane Aerated Biofilm Reactors (MABRs) and the distribution of oxygen and nitrous oxide in the biofilm. Four flat sheet MABRs were operated with synthetic feed under different conditions: two with intermittent aeration (iMABR) and two with continuous aeration (cMABR). Scouring was induced by bubbling dinitrogen gas through the reactor bulk at low and high frequencies (LF and HF). In the iMABRs, a partial nitritation biofilm initially developed, but the biofilm adapted to the aeration strategy over time and became nitrifying. The cMABRs directly developed a nitrifying biofilm without a significant phase of partial nitritation. Limiting oxygen availability improved the overall performance with regards to total nitrogen (TN) removal by providing a better environment for anaerobic ammonium oxidation (Anammox) while limiting complete nitrification. Oxygen profiles were measured in the iMABR over time at different biofilms depths, showing that intermittent aeration led to various oxygen concentrations and temporal variations in the oxygen availabilities at different depths of the biofilm. Also, N2O emissions from the MABRs differed greatly between the different systems, but still remained lower compared to other reactor configurations for nitrogen removal, making the MABR technology a worthy alternative. The results showed large differences between the operating strategies of the MABRs and can help to gain more insight into the specific properties of MABRs for nitrogen removal.

Dual inner circulation and multi-partition driving single-stage autotrophic nitrogen removal in a bioreactor

The single-stage autotrophic nitrogen removal (ANR) process is impeded by a long start-up cycle and unstable operation performance. In this study, an airlift inner-circulation partition bioreactor (AIPBR) was operated continuously for 215 days to explore methods of strengthening the performance and stable operation of the single-stage ANR system. AIPBR start-up period took around 38 days, the total nitrogen removal efficiency was > 85% on day 35. With the decrease of hydraulic retention time and the increase of aeration rate, the nitrogen removal rate increased to 0.85 ± 0.02 kg-N/m³/day. The sludge morphology gradually changed into dark-red floc-coupled granular sludge. Nitrosomonas (9.95%) and Candidatus Brocadia (6.41%) were dominant in the sludge. During long-term operation, AIPBR achieved the dual inner circulation of sewage and sludge and then formed effective dissolved oxygen and sludge partitions to provide a suitable growth environment for various functional bacteria, promote synergy between them, and strengthen the ANR performance.

Comparing nitrite-limited and ammonium-limited anammox processes treating low-strength wastewater: Functional and population heterogeneity

Biomass segregation between granules/biofilm and flocs is widespread in anammox-based processes. The segregation of biomass allows for easy control of processes stability. The goal of this study is to understand the biomass segregation in two anoxic anammox reactors respectively operated in nitrite-limited (R_NO2) and ammonium-limited (R_NH4) modes treating low-strength wastewater at 20 °C. Results showed that size-based biomass segregation was developed in both reactors. But the functional and population heterogeneity was more significant in the ammonium-limited anammox reactor. The activity and abundance of anammox bacteria in large granules were significantly higher than that in flocs under the ammonium-limited conditions. The large granules played a major role in nitrogen removal in R_NH4. By contrast, both large granules and small flocs contributed significantly to the nitrogen loss in the nitrite-limited anammox reactor, since a large number of anammox bacteria existed in both granules and flocs. Besides, a number of Nitrospira-like NOB were also detected in both anoxic anammox reactors, which primarily inhabited in flocs seemingly droved by the availability of oxygen. But the abundance of Nitrospira in R_NH4 was much higher than that in R_NO2. All these results suggested that selective flocs removal would be necessary for R_NH4 to improve its anammox performance but non-essential for R_NO2. The two anammox reactors shared the predominant anammox species with the closest relative to Ca. Brocadia sp. 40 (98%). Unexpectedly, the anammox species grew faster in R_NH4. But the microbial diversity and evenness was much greater in R_NO2, suggesting its higher functional stability.

Is Anoxic Operation Effective to Control Nitrate Build-Up and Sludge Loss for the Combined Partial Nitritation and Anammox (CPNA) Process?

There were three main issues of long start-up period, nitrate build-up and sludge loss during the operation of combined partial-nitritation anammox (CPNA). To fully start up the CPNA reactor, the fast achievement of partial-nitritation (PN) was the first step. Firstly, the PN process was successfully achieved within 22 days by 2 mg·L−1 hydroxylamine (NH2OH) addition and online intermittent aeration control at 0.2~0.3 mg·L−1 dissolved oxygen (DO). Then, a novel strategy of adding anoxic stirring phase between feeding and aeration period during CPNA operation was applied. It was shown effective to control nitrate build-up since the mole ratio of NO3−-N production and NH4+-N removed (MNRR) was mostly below 15%. Also, the procedure adjustment was proven useful to alleviate sludge loss by sustaining filamentous bacteria that could act as biomass framework and reduce nitrate substrate. The filamentous denitrifying bacteria could cause sludge bulking. The total nitrogen removal rate (TNRR) varied from 0.20 to 0.45 kg·m−3·d−1 during CPNA operation. In Stage III, after adding anoxic stirring phase, the abundance of nitrogen transformation functional microorganism’s nitrite oxidizing bacteria (NOB) was below 1.6%, which was one order of magnitude lower than Anammox and ammonia oxidizing bacteria (AOB).

The role of residual quantities of suspended sludge on nitrogen removal efficiency in a deammonifying moving bed biofilm reactor

In a moving bed biofilm reactor (MBBR) system, the vast majority of biomass is immobilized as biofilm besides small amounts of suspension. In this study, the influence of the individual biomass components of a deammonifying MBBR, the biofilm on carriers (BC), residual suspended biomass (SB) with a volatile suspended solids concentration of 0.09±0.03g/L, and its combination (BC+SB) on nitrogen removal efficiency was investigated. While the performance was highest for BC+SB (0.42kgN/(m(3)·d)), it was reduced by a factor of 3.5 for BC solely. SB itself was only capable of nitrite accumulation. This suggests a high abundance of AOBs within suspension besides the coexistence of AOBs and anammox bacteria in the biofilm, which could be supported by results using fluorescence in situ hybridization(FISH). Thus, small amounts of suspended microorganisms can positively influence the deammonification's efficiency. If this fraction is partially washed out, the system recovers nevertheless within hours.

Microbial activity balance in size fractionated suspended growth biomass from full-scale sidestream combined nitritation-anammox reactors

The purpose of this study was to determine the abundance, distribution and activity of aerobic ammonia-oxidizing bacteria (AOB) and anammox in size fractionated aggregates from full-scale suspended growth combined nitritation-anammox sidestream reactors. Plants with or without a cyclone device were also studied to assess a purported enrichment of anammox granules. Specific aerobic ammonium oxidation rates (p=0.01) and specific oxygen uptake rates (p=0.02) were significantly greater in flocs than in granules. AOB abundance measured using quantitative FISH was significantly higher in flocs than in granules (p=0.01). Conversely, anammox abundance was significantly greater in granules (p=0.03). The average ratio of anammox/AOB in systems employing hydrocyclone separation devices was 2.4, significantly higher (p=0.02) than the average ratio (0.5) in a system without a hydrocyclone. Our results demonstrate substantial functional and population-level segregation between floccular and granular fractions, and provide a key corroboration that cyclone separation devices can increase anammox levels in such systems.

Impact of carbon to nitrogen ratio and aeration regime on mainstream deammonification

While deammonification of high-strength wastewater in the sludge line of sewage treatment plants has become well established, the potential cost savings spur the development of this technology for mainstream applications. This study aimed at identifying the effect of aeration and organic carbon on the deammonification process. Two 10 L sequencing bath reactors with different aeration frequencies were operated at 25°C. Real wastewater effluents from chemically enhanced primary treatment and high-rate activated sludge process were fed into the reactors with biodegradable chemical oxygen demand/nitrogen (bCOD/N) of 2.0 and 0.6, respectively. It was found that shorter aerobic solids retention time (SRT) and higher aeration frequency gave more advantages for aerobic ammonium-oxidizing bacteria (AerAOB) than nitrite oxidizing bacteria (NOB) in the system. From the kinetics study, it is shown that the affinity for oxygen is higher for NOB than for AerAOB, and higher dissolved oxygen set-point could decrease the affinity of both AerAOB and NOB communities. After 514 days of operation, it was concluded that lower organic carbon levels enhanced the activity of anoxic ammonium-oxidizing bacteria (AnAOB) over denitrifiers. As a result, the contribution of AnAOB to nitrogen removal increased from 40 to 70%. Overall, a reasonably good total removal efficiency of 66% was reached under a low bCOD/N ratio of 2.0 after adaptation.

Isotopic evidence for nitrous oxide production pathways in a partial nitritation-anammox reactor

Nitrous oxide (N2O) production pathways in a single stage, continuously fed partial nitritation-anammox reactor were investigated using online isotopic analysis of offgas N2O with quantum cascade laser absorption spectroscopy (QCLAS). N2O emissions increased when reactor operating conditions were not optimal, for example, high dissolved oxygen concentration. SP measurements indicated that the increase in N2O was due to enhanced nitrifier denitrification, generally related to nitrite build-up in the reactor. The results of this study confirm that process control via online N2O monitoring is an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N2O emissions. Under normal operating conditions, the N2O isotopic site preference (SP) was much higher than expected - up to 40‰ - which could not be explained within the current understanding of N2O production pathways. Various targeted experiments were conducted to investigate the characteristics of N2O formation in the reactor. The high SP measurements during both normal operating and experimental conditions could potentially be explained by a number of hypotheses: i) unexpectedly strong heterotrophic N2O reduction, ii) unknown inorganic or anammox-associated N2O production pathway, iii) previous underestimation of SP fractionation during N2O production from NH2OH, or strong variations in SP from this pathway depending on reactor conditions. The second hypothesis - an unknown or incompletely characterised production pathway - was most consistent with results, however the other possibilities cannot be discounted. Further experiments are needed to distinguish between these hypotheses and fully resolve N2O production pathways in PN-anammox systems.

Comparing different reactor configurations for Partial Nitritation/Anammox at low temperatures

Partial Nitritation/Anammox (PN/A) is a well-established technology for side-stream nitrogen removal from highly concentrated, warm wastewaters. The focus has now shifted to weakly concentrated municipal wastewaters with much lower concentrations and temperatures. The major challenge is the temperature, which ranges from moderate 20 °C in summer to cold 10 °C in winter. For this study, the most frequently used configurations for side-stream applications were exposed to a slow temperature reduction from 20 °C to 10 °C to simulate a realistic temperature gradient. To evaluate the behavior of the different biomasses based on their properties, four lab reactors were operated in two different configurations. Synthetic wastewater was used to avoid side effects of heterotrophic growth. Differences in the response of the different reactor systems to this temperature gradient clearly indicated, that the geometry of the biomass has a major impact on the overall PN/A performance at low temperatures: While anammox activity in suspended biomass suffered already at 15 °C, it persevered in granular biomass as well as in biofilms on carriers for temperatures down to <13 °C. Further, anammox activity in thicker biofilms was less affected than in thinner biofilms and even adaption to low temperatures was observed.

Start-up of a full-scale deammonification SBR-treating effluent from digested sludge dewatering

This study shows the start-up and operation of a full-scale sequencing batch reactor (SBR) with a volume of 550 m³ for deammonification of reject water from sludge dewatering over the first 650 days of operation. The SBR was operated with discontinuous aeration and achieved an optimum of around 85% of ammonium removal at a load of 0.17 kg m⁻³ d⁻¹. The application of batch tests for the activity measurement of aerobic ammonium and nitrite oxidizing bacteria and anaerobic ammonium oxidizing bacteria were proven to support the identification of setbacks in reactor operation. Furthermore, the calculation of the oxygen uptake rates from online oxygen measurements helped to explain the overall reactor performance. The aeration regime is a key parameter for stable operation of such an SBR for deammonification. At aeration/non-aeration time ranges from 6-9 min, the best results with respect to turnover rates and low nitrate production were achieved. Compared with the nitrification/denitrification SBR operated in parallel with methanol as the carbon source, a significant reduction in costs for energy and chemicals was achieved. The costs for maintenance slightly increased.

Response of different nitrospira species to anoxic periods depends on operational do

The exploitation of a lag phase in nitrate production after anoxic periods is a promising approach to suppress nitrite oxidizing bacteria, which is crucial for implementation of the combined partial nitritation-anammox process. An in-depth study of the actual lag phase in nitrate production after short anoxic periods was performed with varied temperatures and air flow rates. In monitored batch experiments, biomass from four different full-scale partial nitritation-anammox plants was subjected to anoxic periods of 5-60 min. Ammonium and the nitrite that was produced were present to reproduce reactor conditions and enable ammonium and nitrite oxidation at the same time. The lag phase observed in nitrite oxidation exceeded the lag phase in ammonium oxidation after anoxic periods of more than 15-20 min. Lower temperatures slowed down the conversion rates but did not affect the lag phases. The operational oxygen concentration in the originating full scale plants strongly affected the length of the lag phase, which could be attributed to different species of Nitrospira spp. detected by DGGE and sequencing analysis.

Low temperature partial nitritation/anammox in a moving bed biofilm reactor treating low strength wastewater

Municipal wastewater collected in areas with moderate climate is subjected to a gradual temperature decrease from around 20 °C in summer to about 10 °C in winter. A lab-scale moving bed biofilm reactor (MBBR) with carrier material (K3 from AnoxKaldnes) was used to test the tolerance of the overall partial nitritation/anammox process to this temperature gradient. A synthetic influent, containing only ammonium and no organic carbon was used to minimize denitrification effects. After stable reactor operation at 20 °C, the temperature was slowly reduced by 2 °C per month and afterward held constant at 10 °C. Along the temperature decrease, the ammonium conversion dropped from an average of 40 gN m(-3) d(-1) (0.2 gN kgTSS h(-1)) at 20 °C to about 15 gN m(-3) d(-1) (0.07 gN kg TSS h(-1)) at 10 °C, while the effluent concentration was kept <8 mg NH4-N l(-1) during the whole operation. This also resulted in doubling of the hydraulic retention time over the temperature ramp. The MBBR with its biofilm on 10 mm thick carriers proved to sufficiently sustain enough biomass to allow anammox activity even at 10 °C. Even though there was a minor nitrite-build up when the temperature dropped below 12.5 °C, reactor performance recovered as the temperature decrease continued. Microbial community analysis by 16S rRNA amplicon analysis revealed a relatively stable community composition over the entire experimental period.