High Free Ammonia Concentrations Research Articles

Operation stability and recovery performance in an Anammox EGSB reactor after pH shock

An Anaerobic ammonia oxidation (Anammox) expanded granular sludge bed (EGSB) reactor was investigated with a long-term continuous operation. The nitrogen removal performance and stability of process subjected to transient pH shock (decreased from 9.0 to 7.0 and maintaining for 24h) were evaluated. In the steady stage, the nitrogen loading rate (NLR) and nitrogen removal rate (NRR) can reach to 10 and 8.5kg-Nm−3d−1, respectively. However, the system had a low tolerance for transient pH shock. The deterioration of the granule sludge properties and the inhibition of specific Anammox activity (SAA) resulted in the destabilization of the EGSB after pH shock. The main factors governing the treatment performance of EGSB were the high concentration of free nitrous acid (FNA) and free ammonia (FA). However, the lower than 10% reduction of ammonia removal can be maintained when FA and FNA were lower than 15mgl−1 and 15μgl−1, respectively. The process was successfully recovered by controlling the FNA and FA concentration after the low pH shock.

Determine the operational boundary of a pilot-scale single-stage partial nitritation/anammox system with granular sludge.

The partial nitritation/anammox (PN/A) process has been applied to ammonium-rich wastewater treatment, but the operational boundary has not been well determined for long-term stability. This pilot-scale study was targeted at a single-stage PN/A process using a sequencing batch reactor (SBR) (volume: 53 m(3)) and granulated activated sludge. The maximum nitrogen removal rate reached 0.83 kg N/(m(3)·d). Microbial analysis suggested that ammonium oxidizing bacteria were mainly present in small sludge flocs while anammox bacteria were prone to grow in large sludge granules. The PN/A performance was enhanced when dissolved oxygen (DO) was increased from 0.25 to 0.76 mg/L, and deteriorated at DO higher than 1.15 mg/L. The PN/A was inhibited at free ammonia (FA) over 77.0 mg/L. High DO or FA concentrations inhibited anammox activity and further induced high and inhibitory nitrite concentrations. Therefore, appropriate DO and FA concentrations should be controlled to achieve single-stage PN/A in SBRs.

Rapid achievement of nitrification in CSTR and SBR treating reject water at high ammonia levels

The ability of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) to carry out nitrification in an environment with high ammonia levels was evaluated with the aim to suggest a suitable strategy for a quick start-up of nitrifying reactors treating reject water. It was proved that nitrification could be initiated even without any inoculation of nitrifying biomass. The nitritation was achieved faster (23 d after beginning of reactor operation) than nitratation (after 180 d) due to the presence of AOB and the absence of NOB in raw reject water. Free ammonia (FA) concentration reaching up to 160 mg N L−1 strongly inhibited nitrifying bacteria during the initial phase of reactor operation. Consequently, free nitrous acid (FNA) with a concentration up to 1.25 mg N L−1 supported NOB inhibition after the initiation of AOB activity. High AOB, as well as NOB, activity was observed even during the first day of the operation in a completely stirred tank reactor (CSTR, nitrogen loading rate—NLR 0.5 kg N (m3 d)−1, nitrogen oxidation rate—NOR 0.25±0.05 kg N (m3 d)−1) as well as a sequencing batch reactor (SBR, NLR 0.3 kg N (m3 d)−1, NOR 0.14±0.01 kg N (m3 d)−1) inoculated with common activated sludge at a zero initial total ammonium nitrogen concentration. It was proved that temporary inhibition of NOB activity induced by FA concentration elevated to 88 mg N L−1 may lead to the permanent transformation of standard nitrification into shortcut nitrification in SBRs, thanks to periodical exposition of NOB to high FA and FNA concentration during SBR cycle. Thus, it was demonstrated that the possibility to initiate this process very quickly is essential as a first stage for subsequent anaerobic oxidations of ammonium or shortcut denitrification.

Research on the large-scale application of nitrogen removal for the treatment of synthetic ammonia wastewater

The capability of anoxic–aerobic (A/O) process treatment and influencing factors of high concentration ammonia industrial wastewater were studied in this paper. Results showed that the A/O process system had achieved favorable performance and the quality of effluent could meet the direct emission standard of Discharge standard of water pollutants for ammonia industry (GB13458-2013) under the optimal conditions as follows: the perfect inflow loading of ammonia 0.12 kg/(m3 d), the mixed liquid recycle ratio (R) 200%, and sludge return ratio 80%, the carbon-to-nitrogen ratio (C/N ratio) 4.5–5.5. Besides, the results also confirmed that the activity of nitrifying bacteria in activated sludge was greatly inhibited due to the high concentration of free ammonia (FA). In addition, carbon shortage had a marked effect on the denitrification efficiency of the system.

A report on the incidence of Trichodina heterodentata from fishes of Jammu, J&K India.

The present study was aimed at screening for the presence of protozoan's among Cyprinid fishes collected from various fish ponds and farms in Jammu division of Jammu and Kashmir (J and K) state. Out of 75 fishes collected from local water bodies of Jammu division, only 35, (49.6%) were infested with Trichodina. Trichodina infestations were studied in the period from November to February. In light infestation Trichodina was usually present on gills, fins and skin of apparently healthy fish. Clinical signs of Trichodiniasis appeared on fish with heavy infections and in presence of one or more stress factors including, rough handling during transportation from ponds, over crowdedness, malnutrition, high concentration of free ammonia and low oxygen concentration. Clinical signs of Trichodiniasis in fish such as sluggish movement, loss of appetite, black colouration, necrosis and ulcer on different parts of the body, detached scales and excessive accumulation of mucous in gill pouches were also observed.

Effect of carbon source on acclimatization of nitrifying bacteria to achieve high-rate partial nitrification of wastewater with high ammonium concentration

Experiments in two laboratory-scale sequential batch reactors were carried out to investigate the effect of heterotrophic bacteria on nitrifying bacteria using external carbon sources. Partial nitrification of ammonium-rich wastewater during short-term acclimatization enriched the activity of ammonia-oxidizing bacteria in both reactors. Heterotrophic bacteria exhibited a minor effect on nitrifying bacteria, and complete removal of ammonium occurred at a rate of 41 mg L−1 h−1 in both reactors. The main strategy of this research was to carry out partial nitrification using high-activity ammonia-oxidizing bacteria with a high concentration of free ammonia (70 mg L−1). The NO2−/(NO3− + NO2−) ratio was greater than 0.9 in both reactors most of the time.

Effect of forced ventilation during composting on Agaricus bisporus substrate selectivity

The production of mushroom substrate with limited aeration in phase I composting was studied. A standard mushroom compost made in windrow piles with forced aeration was compared with the experimental compost prepared under limited aeration during part of phase I. Limited ventilation resulted in a product with chemical parameters similar to those produced with full aeration. Despite this, loss of compost selectivity was observed. When aeration was limited during part of the composting process, 50% mushroom yield losses caused by strong infestation of Trichoderma aggressivum were observed. Limited ventilation in part of phase I resulted in a reduction of redox potential, increase of pH, and concentration of free ammonia, and thus disturbed the transformation of compost into selective medium for Agaricus. Mushroom yield correlated with redox potential in the compost, and prolonged high concentration of free ammonia was the main factor in delayed but faster organic matter degradation in non-ventilated compost. The changes of redox potential and electrical conductivity in the course of composting can be useful tools for process control and evaluation of the compost quality potential and susceptibility to green mould infestation.

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>600 mg 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.

Analysis of the characteristics of short-cut nitrifying granular sludge and pollutant removal processes in a sequencing batch reactor

Aerobic granular sludge is a new type of microbe auto-immobilization technology; in this paper, short-cut nitrification and denitrification were effectively combined with the granular sludge technology. Simultaneous nitrification and denitrification granules were developed in a sequencing batch reactor (SBR) using synthetic wastewater with a high concentration of ammonia nitrogen at 25 °C with a dissolved oxygen concentration above 2.0 mg/L and a 15 days sludge retention time. The characteristics of the sludge and the removal efficiency were studied, and the removal mechanisms of the pollutants and the process of short-cut nitrification were analyzed. The average granule diameter of the granular sludge was 704.0 μm. The removal rates of pollutants and the accumulation rate of nitrite in the SBR were studied. During treatment of wastewater with a high concentration of ammonia nitrogen, simultaneous nitrification, and denitrification and the stripping process could contribute to the removal of total nitrogen. The high pH value, the high concentration of free ammonia, and the delamination of granular sludge were the main factors contributing to the short-cut nitrification property of granular sludge in the reaction process.

생물막 반응조에서 아질산염 축적에 미치는 운전인자 영향과 하수슬러지 가용화에 의한 탈질반응의 외부탄소원 공급에 관한 연구

A combined process consisted of a biofilm reactor and a continuously stirred-tank reactor (CSTR) was investigated for highly loaded ammonium wastewater treatment via nitrite accumulation. To enhance ammonium oxidizing bacteria over nitrite oxidizing bacteria on the surface of carriers, the biofilm reactor was operated at temperature of 35°C for more than three months but the influent ammonium (500 mg-N/L) was partially oxidized to nitrite (240 mg-N/L). As pH was increased from 7.5 to 8.0, nitrite accumulation was fully achieved due to the inhibition of nitrite oxidizing bacteria under high free ammonia concentration. The biofilm reactor performance was severely deteriorated at the hydraulic retention time of 12 hr, at which incomplete nitrification of ammonia was observed. Various solubilization methods were applied to sewage sludge for enhancing its biodegradability and the combined method, alkaline followed by ultrasonic, gave the highest solubilization efficiency (58%); the solubilized solution was used as the external carbon source for denitrification reaction in CSTR. FISH analysis showed that the dominant microorganisms on the carriers were ammonium oxidizing bacteria such as Nitrosomonas spp. and Nitrospirar spp. but low amounts of nitrite oxidizing bacteria as Nitrobacter spp. was also detected.

The kinetics of process dependent ammonia inhibition of methanogenesis from acetic acid

Advanced anaerobic digestion processes aimed at improving the methanization of sewage sludge may be potentially impaired by the production of inhibitory compounds (e.g. free ammonia). The result of methanogenic inhibition is relatively high effluent concentrations of acetic acid and other soluble organics, as well as reduced methane yields. An extreme example of such an advanced process is the thermal hydrolytic pretreatment of sludge prior to high solids digestion (THD). Compared to a conventional mesophilic anaerobic digestion process (MAD), THD operates in a state of constant inhibition driven by high free ammonia concentrations, and elevated pH values. As such, previous investigations of the kinetics of methanogenesis from acetic acid under uninhibited conditions do not necessarily apply well to the modeling of extreme processes such as THD. By conducting batch ammonia toxicity assays using biomass from THD and MAD reactors, we compared the response of these communities over a broad range of ammonia inhibition. For both processes, increased inhibitor concentrations resulted in a reduction of biomass growth rate (rmax = μmax∙X) and a resulting decrease in the substrate half saturation coefficient (KS). These two parameters exhibited a high degree of correlation, suggesting that for a constant transport limited system, the KS was mostly a linear function of the growth rate. After correcting for reactor pH and temperature, we found that the THD and MAD biomass were both able to perform methanogenesis from acetate at high free ammonia concentrations (equivalent to 3–5 g/L total ammonia nitrogen), albeit at less than 30% of their respective maximum rates. The reduction in methane production was slightly less pronounced for the THD biomass than for MAD, suggesting that the long term exposure to ammonia had selected for a methanogenic pathway less dependent on those organisms most sensitive to ammonia inhibition (i.e. aceticlastic methanogens).

Influence of effluent recirculation on the performance of Anammox process

A study employing two parallel, laboratory-scale upflow anaerobic sludge bed (UASB) reactors (R1 without and R2 with effluent recirculation) was performed to explore the impact of effluent recirculation with various recirculation ratios (r) on the performance of the anaerobic ammonium oxidation (Anammox) process. When the influent total nitrogen (TN) level was at 700mgL−1, the nitrogen removal rate of R2 (r=0.5,4,5) was stable at approximately 3.5kgm−3day−1, which is higher than the value of 2.9kgm−3day−1 for R1. This performance showed that the recirculation of effluent promoted the efficiency and stability of the Anammox process. The specific Anammox activity of the biomass in R2 reached 13.3mg-TNg−1-VSSh−1, which is 34.3% higher than that in R1. The dilution effect and the hydraulic shear force enhanced the performance of the Anammox reactor. However, effluent recirculation was prone to causing variation in the influent qualities. The variation in the substrate ratio did not significantly affect the stoichiometry of the Anammox reaction or the substrate removal capacity, but the high pH resulting from effluent recirculation, with the corresponding high free ammonia concentration, appeared to hinder the further improvement of Anammox performance.

Aerobic granulation and nitrogen removal with the effluent of internal circulation reactor in start-up of a pilot-scale sequencing batch reactor

Aerobic granular sludge was successfully cultivated with the effluent of internal circulation (IC) reactor in a pilot-scale sequencing batch reactor (SBR) using activated sludge as seeding sludge. N removal was investigated in the start-up of aerobic granulation process. Initially, the phenomenon of partial nitrification was observed and nitrite accumulation rates (NO(2) (-)-N/NO (x) (-) -N) were between 84.6 and 99.1 %. It was potentially caused by ammonium oxidizing bacteria (AOB) in the seeding activated sludge, high external environmental temperature (~32 °C) and free ammonia (FA) concentration. After 50 days' running, the aerobic granules-based bioreactor demonstrated perfect performance in simultaneous removal of organic matter and ammonia nitrogen, and average removal efficiencies were maintained above 93 and 96 %, respectively. The maximum nitrogen removal efficiency of 83.1 % was achieved after the formation of aerobic granules. The average diameter of mature aerobic granular sludge mostly ranged from 0.5 to 1.0 mm. Furthermore, one typical cyclic test indicated that pH and DO profiles could be used as effective parameters for biological reactions occurring in the aerobic/anoxic process. The obtained results could provide further information on the cultivation of aerobic granular sludge with practical wastewater, especially with regard to nitrogen-rich industrial wastewater.

Granulation of Nitrifying Bacteria in a Sequencing Batch Reactor for Biological Stabilisation of Source-Separated Urine

Biological stabilisation of human urine highly depends on the abundance and activities of nitrifying bacteria. However, it is quite difficult to enrich nitrifiers as bio-aggregation by self-immobilized biomass. In this study, granulation of nitrifying bacteria involving inoculation strategy was developed. Two sequencing batch reactors, the one inoculated with nitrifying bacteria and the other inoculated with aerobic granules, were operated in laboratory side by side with a feeding of urine solution. Aerobic nitrifying granules (ANG), with compact morphological structure and good nitrifying activity, were achieved in the reactor inoculated with aerobic granules. Enrichment of nitrifying bacteria favors the nutrient uptake, and hence, to obtain a high ammonia oxidation efficiency. Nonetheless, nitrite accumulation gradually dominated in reactor, partly attributes to a high concentration of free nitrous acid and free ammonia in bulk. The matured ANG had a rather stable microbial profile, as that a number of activated bacteria occupied the surface of granule. It was also found that ANG were much more impermeable than aerobic granules and activated sludge, which was demonstrated as smaller porosities, and therewith an excellent settleability. The results herein reveal that granulation of nitrifying bacteria could enrich the biomass to implement stabilisation of urine in biological way.

Full-scale experience with the deammonification process to treat high strength sludge water – a case study

More stringent effluent criteria with regard to nitrogen calls for improved nutrient removal techniques in wastewater treatment plants (WWTPs). Besides optimisation of the liquid treatment train of the plants, attention has increasingly centred on the problem of return flows from sludge treatment. One of the most recent developments aimed at the reduction of this nitrogen load is deammonification which has been used at one of Ruhrverband's plants since 2002 by applying a moving bed system. To gain additional experience in operating this process, another full scale plant was modified in 2007 by integration of deammonification, using a SBR system with suspended biomass based on the DEMON(®) control scheme. By using seeding sludge from Strass WWTP in Austria, start-up has been achieved within only 1 day. After stable operation for several months, increasing nitrate concentrations were observed in the effluent of the system indicating growing activity of nitrite oxidising bacteria (NOB). Following severe process deterioration, it was decided to re-start the system again but the same behaviour, i.e. increasing levels of nitrate, was observed once again. Several approaches were used to suppress NOB organisms in full-scale without success, e.g. low oxygen levels and high free ammonia concentrations. Finally, the reduction of the aerobic cycle length during intermittent aeration down to 8 min, followed by an anoxic mixing period of only 18 min was successful in inhibiting the activity of NOB organisms, most probably due to their elevated lag-phase compared with ammonium oxidising bacteria. Today, nitrogen elimination that has been stabilised at more than 80% at a daily volumetric loading rate of 0.5 kg N/(m³ d). The total costs amount to €2.3/kg N(eli).

Molecular indicators of Nitrobacter spp. population and growth activity during an induced inhibition event in a bench scale nitrification reactor

The Nitrobacter spp. ribosomal RNA gene (rDNA) and transcript (rRNAt) abundance were quantified in a bench scale nitrification reactor during baseline periods of high nitrification efficiency and an intervening staged inhibition event. The transcript to gene ratio (rRNAt/rDNA) was highly sensitive to changes in the reactor nitrite oxidation rate. During high nitrification efficiency, the rRNAt/rDNA metric displayed a range from 0.68 to 2.01 with one-sided (α=0.10) lower and upper prediction intervals of 0.70 and 1.78, respectively. When nitrification was inhibited by disabling the reactor pH control system, this activity metric declined an order of magnitude to ∼0.05, well below the lower prediction interval reflecting high nitrification efficiency. The decline was rapid (2h) and preceded a significant drop in reactor nitrification performance, which occurred as ammonia accumulated. The rRNAt/rDNA ratio remained low (∼0.05) for several days after the pH control system was re-enabled at a setpoint of 8.0, which otherwise induced rapid oxidation of accumulated ammonia and produced high free ammonia concentrations. The timing of a subsequent increase in the rRNAt/rDNA ratio, which transiently exceeded the upper prediction interval established during the baseline period of high nitrification efficiency, was not coincidental with resumption of pH control at 7.2 that lowered free ammonia concentrations to non-inhibitory levels. Rather, nitrite oxidation resumed and the rRNAt/rDNA ratio increased only after oxidation of accumulated ammonia was complete, which was coincidental with reduced reactor oxygen demand. In summary, the Nitrobacter rRNAt/rDNA activity metric reflected timely and easily recognizable changes in nitrite oxidation activity, illustrating that molecular data can be used to diagnose poor biological wastewater treatment performance.

Impact of nitrogen loading rates on nitrogen transformation for anaerobic effluent from latex processing industry

This study evaluated the influence of nitrogen loading rates on fate of nitrogen treating anaerobic effluent from latex processing industry using a novel acryl-fibre biomass carrier-bio-fringe (BF) in a swim-bed reactor. At NLR of 0.5 kg N/m3.day, 60% to 70% TKN was converted and the nitrogen ratio N–NO2/ (N–NO2 + N–NO3) reached 80% to 90%. At NLR of 1 kg N/m3.day, conversion efficiency of TKN increases by 80% to 95% and the ratio reached 80% to 95%. However, this ratio decreased by 40% to 60% due to the influent alkalinity reduction. At increased NLR of 1.5–2 kg N/m3.day, nitrite and nitrate reduced less than 20 mg N/L. This is due to an increase in NLR and amount of alkalinity available. Reactor pH as high as 8.2–8.6 produced high free ammonia (FA) concentration restricting nitrite production. The COD removal efficiencies were 80–90% and 85–95% at NLR of 0.5–1 kg N/m3.day, respectively. This reduced 75– 88% at an increased NLR of 1.5–2 kg N/m3.day.

Effect of free ammonia concentration on monochloramine penetration within a nitrifying biofilm and its effect on activity, viability, and recovery

Chloramine has replaced free chorine for secondary disinfection at many water utilities because of disinfection by-product (DBP) regulations. Because chloramination provides a source of ammonia, there is a potential for nitrification when using chloramines. Nitrification in drinking water distribution systems is undesirable and may result in degradation of water quality and subsequent non-compliance with existing regulations. Thus, nitrification control is a major issue and likely to become increasingly important as chloramine use increases. In this study, monochloramine penetration and its effect on nitrifying biofilm activity, viability, and recovery was investigated and evaluated using microelectrodes and confocal laser scanning microscopy (CLSM). Monochloramine was applied to nitrifying biofilm for 24 h at two different chlorine to nitrogen (Cl 2:N) mass ratios (4:1 [4.4 mg Cl 2/L] or 1:1 Cl 2:N [5.3 mg Cl 2/L]), resulting in either a low (0.23 mg N/L) or high (4.2 mg N/L) free ammonia concentration. Subsequently, these biofilm samples were allowed to recover without monochloramine and receiving 4.2 mg N/L free ammonia. Under both monochloramine application conditions, monochloramine fully penetrated into the nitrifying biofilm within 24 h. Despite this complete monochloramine penetration, complete viability loss did not occur, and both biofilm samples subsequently recovered aerobic activity when fed only free ammonia. When monochloramine was applied with a low free ammonia concentration, dissolved oxygen (DO) fully penetrated, but with a high free ammonia concentration, complete cessation of aerobic activity (i.e., oxygen utilization) did not occur and subsequent analysis indicated that oxygen consumption still remained near the substratum. During the ammonia only recovery phase, different spatial recoveries were seen in each of the samples, based on oxygen utilization. It appears that the presence of higher free ammonia concentration allowed a larger biomass to remain active during monochloramine application, particularly the organisms deeper within the biofilm, leading to faster recovery in oxygen utilization when monochloramine was removed. These results suggest that limiting the free ammonia concentration during monochloramine application will slow the onset of nitrification episodes by maintaining the biofilm biomass at a state of lower activity.

Characterization of the Microbial Community in a Partial Nitrifying Sequencing Batch Biofilm Reactor

A lab-scale partial nitrifying sequencing batch biofilm reactor was a successful start-up. Denaturing gradient gel electrophoresis (DGGE) was used to investigate the bacterial community dynamics in three periods together with inocula sludge at ambient temperature. The DGGE profiles of bacteria and Shannon-Wiener index (H') results showed that high free ammonia (FA) concentration referred to lower diversity in the bioreactor system. Cluster analysis indicated that microorganism in period III was similar with inocula sludge and was different from that in periods I and II. Similar results also appeared in ammonia-oxidizing bacteria (AOB) community structure and nitrite-oxidizing bacteria (NOB) community structure, and at least four AOB species and two NOB species were present in period III, respectively. Phylogenetic analysis of amoA gene sequences showed that Nitrosomonas eutropha cluster was predominant in all the three periods. With lower ammonium loads, three new operational taxonomic units formed and consisted Nitrosomonas sp. Cluster. This article demonstrated that microbial community, AOB, and NOB diversity were related with FA concentration closely at ambient temperature.

Development of long-term stable partial nitrification and subsequent anammox process

The partial nitrification reactor was successfully started up and operated stably for more than 250 days with a maximum nitrite production rate of 1.12 kg-N m −3 day −1. The important factors for successful partial nitrification were high ammonium loading rate (>1.0 kg-N m −3 day −1) and relatively high pH (ca. 8.0), giving high free ammonia concentrations (>10 mg NH 3-N L −1). In addition, the air flow rate must be controlled at the ratio of air flow rate to ammonium loading rate below 0.1 ( m air 3 day - 1 ) /(kg-N m −3 day −1). After the establishment of stable partial nitrification, the effluent NO 2 - -N / NH 4 + -N ratio and effluent NO 3 - -N concentration were 1.20 ± 0.33 and 1.2 ± 1.0 mg-N L −1, respectively, which was then fed into an granular-sludge anammox reactor. Consistent nitrogen removal was achieved for more than 250 days with a maximum nitrogen removal rate of 15.0 kg-TN m −3 day −1.