The inhibitory effects of free ammonia on ammonia oxidizing bacteria and nitrite oxidizing bacteria under anaerobic condition

Conversion of monocropping to intercropping promotes rhizosphere microbiome functionality and soil nitrogen cycling

Intercropping can increase soil nutrient availability and provide greater crop yields for intensive agroecosystems. Despite its multiple benefits, how intercropping influences rhizosphere microbiome assemblages, functionality, and complex soil nitrogen cycling is not fully understood. Here, a three-year field experiment was carried out on different cropping system with five fertilization treatments at the main soybean production regions. We found that soybean yields in intercropped systems were on average 17 % greater than in monocropping system, regardless of fertilization treatments. We also found that intercropping systems significant increased network modularity (by 46 %) and functional diversity (by 11 %) than monocropping systems. Metagenomics analyses further indicated intercropping promotes microbiome functional adaptation, particularly enriching core functions related to nitrogen metabolism. Cropping patterns had a stronger influence on the functional genes associated with soil nitrogen cycling (R2 = 0.499). Monocropping systems increased the abundance of functional genes related to organic nitrogen ammonification, nitrogen fixation, and denitrification, while functional guilds of nitrate assimilation (by 28 %), nitrification (by 31 %), and dissimilatory nitrate reduction (by 10.1 %) genes were enriched in intercropping systems. Furthermore, we found that abiotic factors (i.e. AP, pH, and Moisture) are important drivers in shaping soil microbial community assemblage and nitrogen cycling. The functional genes include hzsB, and nrfA, and nxrA that affected by these biotic and abiotic variables were strongly related to crop yield (R2 = 0.076 ~ R2 = 0.249), suggesting a key role for maintaining crop production. We demonstrated that land use conversion from maize monocropping to maize-soybean intercropping diversify rhizosphere microbiome and functionality signatures, and intercropping increased key gene abundance related to soil nitrogen cycling to maintain the advantage of crop yield. The results of this study significantly facilitate our understanding of the complex soil nitrogen cycling processes and lay the foundation for manipulating desired specific functional taxa for improved crop productivity under sustainable intensification.

Achieving advanced nitrogen removal from mature landfill leachate in continuous flow system involving partial nitrification-anammox and denitrification

Considering the challenges associated with nitrogen removal from mature landfill leachate, a novel combined continuous-flow process integrating denitrification and partial nitrification-Anammox (PN/A) was developed using an internal circulation (IC) system and a biological aerated filter (BAF) biofilm reactor (IBBR). In this study, IBBR successfully operated for 343 days, and when influent NH4+-N concentration of mature landfill leachate reached 1258.1 mg/L, an impressive total nitrogen removal efficiency (TNRE) of 93.3 % was achieved, along with a nitrogen removal rate (NRR) of 1.13 kg N/(m3·d). The analysis of the microbial community revealed that Candidatus Kuenenia, the dominant genus responsible for anammox, accounted for 1.7 % (day 265). Additionally, Nitrosomonas, Thauera and Truepera were identified as key contributors to the efficient removal of nitrogen from mature landfill. As a novel nitrogen removal strategy, the practical application of the IBBR system offers novel perspectives on addressing mature landfill leachate.

Difference of high-salinity-induced inhibition of ammonia-oxidising bacteria and nitrite-oxidising bacteria and its applications

The difficulty in achieving stable partial nitritation (PN) is a challenge that limits the application of mainstream anaerobic ammonium oxidation (anammox). This study proposes high-salinity treatment as a novel strategy for inactivating nitrite-oxidising bacteria (NOB). The study indicated that NOB are more sensitive to high salinity than ammonia-oxidising bacteria (AOB). The inhibitory effect on the nitrifier gradually increased with increasing salinity from 0 to 100 g NaCl/L. After 24 h and 35 g NaCl/L inhibition, the AOB and NOB activities were 36.65% and 7.15% of their original activities, respectively. After one high-salinity treatment, nitrite accumulation rate (NAR) was above 33% during nitrification. Moreover, the sludge characteristics remained almost unchanged after suppression. A novel process for achieving mainstream PN was proposed and evaluated based on the results. An energy consumption analysis showed that mainstream PN/anammox based on the ex situ high-salinity treatment can achieve higher energy self-sufficiency compared with activated sludge.

Response of partial nitritation and denitrification processes to high levels of free ammonia in a pilot mature landfill leachate treatment system: Stability and microbial community dynamics

The high levels of free ammonia (FA) challenge the application of partial nitritation (PN) and denitrification (DN) in the treatment of ammonia-rich wastewater. This study explored the impact of high levels of FA on the PN and DN stability and microbial community dynamics. By reducing reflux and increasing influent load, the concentrations of FA in PN and DN reactors increased from 28.9 mg/L and 140.0 mg/L to 1099.8 mg/L and 868.4 mg/L, respectively. During this process, the performance of PN and DN remained stable. The microbial analysis revealed that the Nitrosomonas exhibited strong tolerance to high levels of FA, and its relative abundance was positively correlated with amoABC (R2 0.984) and hao (R2 0.999) genes. The increase in microbial diversity could enhance the resistance ability of PN against the FA impact. In contrast, high levels of FA had scant influence on the microbial community and performance of DN.

Multi-chambers of pilot-scale reactor enhanced partial nitritation performance

Nowadays, applying anammox to treat high nitrogenous side-stream wastewater has taken a step forward. However, the partial nitritation process is sensitive to the ammonium concentration and the nitrogen loading rate, which significantly influences the nitrogen removal performance. This study investigated the performance of a novel nitritation pilot-scale reactor which was divided into four chambers. The nitrite accumulation efficiency reached more than 90 % in the rural wastewater treatment process. As the reactor was divided into four chambers, the comprehensive statistical results showed that the concentration of free ammonium in the front chambers had been effectively improved. The proportion of free ammonium concentration (>0.1 mg NH₃·L^-1), which could inhibit the activity of nitrite oxidizing bacteria, in first chamber (PN1) was 2 times higher than in the last chamber (PN4). Meanwhile, Nitrosomonas, responsible for ammonium oxidation to nitrite, was highly enriched in the first two chambers even though the dissolved oxygen was maintained at 1.5 ± 0.3 mg·L^-1. Compare to conventional reactor, the resistance of the novel reactor to volumetric shock loading has been enhanced. Even though the ammonium loading rate fluctuated greatly, the effluent was still stable and could meet the demand following the anammox process. This study demonstrated that the reactor with multi-chambers could effectively improve the nitrite accumulation efficiency in the partial nitritation process and thus provide a new perspective on the partial nitritation process in a single reactor and further promote the anammox performance in the wastewater treatment process.

Achieving advanced nitrogen removal from low-carbon municipal wastewater using partial-nitrification/anammox and endogenous partial-denitrification/anammox

To achieve advanced nitrogen removal from low-carbon wastewater, a partial-nitrification/anammox and endogenous partial-denitrification/ anammox (PN/A-EPD/A) process was developed in a sequential batch biofilm reactor (SBBR). Advanced nitrogen was achieved with the effluent total nitrogen (TN) of 3.29 mg/L when the influent COD/TN and the TN were 2.86 and 59.59 mg/L, respectively. This was attributed to a stable PN/A-EPD/A, which was achieved through the integration of four strategies, including treating the inoculated sludge with free nitrous acid, inoculating anammox biofilm, discharging excess activated sludge and residual ammonium at the end of oxic stage. The 16S rRNA high-throughput sequencing results demonstrated that anammox bacteria coexisted with ammonia oxidizing bacteria, nitrite oxidizing bacteria, denitrifying glycogen accumulating organisms (DGAOs) and denitrifying phosphorus accumulating organisms (DPAOs) in biofilms. The abundance of anammox bacteria in the inner layer of the biofilm is higher, while that of DGAOs and DPAOs is higher in the outer layer.

Reactive and microbial inhibitory mechanisms depicting the panoramic view of pH stress effect on common biological nitrification

pH is a crucial factor of microbial nitrification, which often combines with high-strength ammonium to influence nitrogen removal pathway in wastewater treatment. However, the detailed inhibitory mechanisms of pH stress are not sufficiently disclosed yet. In this study, the pH stress effect on nitrification was comprehensively studied by a set of experiments which identified the reactivity of nitrification processes and activity of nitrifiers, the time dependence of inhibition effect and the hybrid pH stress effect with ammonium. The results revealed two distinct inhibitory mechanisms dominating in alkaline and acid ranges. In alkaline range (pH > 8), pH stress causes physiological damages on microorganisms which is named as microbial inhibition. It has the features of less recoverability of nitrifiers, time-dependent inhibition effect and low pH-tolerance of nitrite oxidation bacteria. Free ammonia enhanced microbial inhibition and greatly promoted nitrite accumulation. A novel reactive inhibition mechanism dominated in acid range (pH < 7) was disclosed. It only impedes ammonia oxidation process (AOP) but not impair microbial activity obviously and the effect is time-independent. The mechanism was clarified from H⁺ transport because AOP involved H⁺ production. The H⁺ transport was impeded under acid stress owing to the decrease of pH gradient across cell membrane. The two mechanisms formed a panoramic view of pH stress effect on nitrification advancing the understanding of nitrifier adaptability and nitritation regulation in wastewater treatment processes.

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.

Feeding strategy for single-stage deammonification to treat moderate-strength ammonium under low free ammonia conditions

Single-stage deammonification (SSD) processes have been successfully operated using the step-feeding strategy to treat high-strength NH₄⁺ (>300 mg/L), but often failed to treat moderate-strength NH₄⁺ (100-300 mg/L). Because it is hard to maintain the free ammonia (FA) above 1 mg/L, which is a concentration in which the activity of NO₂^- oxidizing bacteria (NOB) can be selectively suppressed. In this study, to evaluate the effectiveness of the step-feeding strategy on the long-term stability of treating moderate-strength NH₄⁺, two SSD sequential-batch reactors (SBRs) were operated under one-step feeding and multi-step feeding strategies. The one-step feeding SBR achieved a higher nitrogen removal efficiency (86 %), nitrogen removal rate (0.61 kg/m³/d), and COD removal efficiency (95 %) than the multi-step feeding SBR (73 %, 0.39 kg/m³/d, and 95 %, respectively). This means the appropriate FA to selectively suppress NOB activity was successfully maintained in the one-step feeding SBR (FA > 1 mg/L). Therefore, it the necessary to apply a step feed strategy that can be maintained above FA (1 mg/L) from the start-up of operation to treat moderate-strength NH₄⁺.

Continuous plug-flow anammox system for mature landfill leachate treatment: Key zone for anammox pathway

This study established the one-stage partial nitrification coupled anammox and partial denitrification coupled anammox process in an anoxic/oxic continuous plug-flow system and operated for 465 days to treat mature landfill leachate. 97.9 %-98.1 % of inorganic nitrogen was removed when the nitrogen loading rate was maintained at 0.33-0.36 kg N/m³/d, and a high anammox contribution to nitrogen removal (89.8 %-92.4 %) was achieved. The long-term in-situ free ammonia (FA) anoxic treatment contributed to the stable performances of partial nitrification and in-situ fermentation. The employed integrated fixed-film activated sludge technology favored the enrichment of hzsA, hzsB, hdh, amoA, hao, narG, and napA functional genes. The oxic zone, particularly oxic biofilm, was the key zone for anammox pathway, where Candidatus_Kuenenia (from 1.6 % to 8.3 %) with high tolerance to FA and salinity stress outcompeted Candidatus_Brocadia (from 18.3 % to 0.1 %) as the dominant anammox bacteria. This study could provide guidance for anammox-mediated landfill leachate treatment in practical projects.

Chemical oxygen demand and nitrogen removal from real membrane-manufacturing wastewater by a pilot-scale internal circulation reactor integrated with partial nitritation-anammox

A pilot-scale system integrating internal circulation and partial nitritation-anammox successfully treated real high-strength membrane-manufacturing wastewater in this study. With this pilot-scale system, a high chemical oxygen demand (COD) removal efficiency of 85 % and a nitrogen removal of 90 % are achieved at an organic loading rate of 6.0 kg COD/m3/d. The nitrogenous organic matters in the internal circulation zone are degraded into ammonia nitrogen. In the partial nitrification zone, nitrite accumulation is achieved, providing a suitable NH4+-N/NO2--N ratio for anammox reaction. Partial nitritation is achieved by maintaining an operational temperature at 30-35 °C, free ammonia concentration at 5-7 mg/L and dissolved oxygen at 0.4-0.7 mg/L with a reflux ratio of 150 %. The COD to nitrogen ratio in the internal circulation effluent is maintained below 3.0 to inhibit nitrite oxidizing bacteria. This study demonstrates that a pilot-scale system can efficiently remove organic matters and nitrogen from wastewater of membrane-manufacturing industry.

Achieving PN through the selective recovery of AOB activity in inactivated nitrifying bacteria: Combined aerobic starvation and FA

A novel strategy combining aerobic starvation and free ammonia (FA) was proposed to achieve partial nitrification (PN). The impact of the combined strategy on nitrifying bacteria was explored in a 200-day experiment. The effluent concentration of ammonia was below the detection limits (0.1 mg/L), and the effluent concentration of nitrite and nitrate was 68.12 mg/L and 3.46 mg/L without adding carbon source to the artificial wastewater. The nitrite accumulation rate (NAR) was maintained at 90.15% even when the dissolved oxygen (DO) concentration was 1.50 mg/L. Further analysis showed that PN was achieved by selectively restoring the activity of ammonia-oxidizing bacteria (AOB) in nitrifying bacteria that had lost their activity after starvation. The specific ammonia oxidation rate (SAOR) was 46.25 mg N/g MLVSS/h, and the specific nitrate product rate (SNPR) was only 0.73 mg N/g MLVSS/h in the stable operation stage. The increase in AOB abundance (from 2.79% to 7.13%) and the decrease in nitrite-oxidizing bacteria (NOB) abundance (from 8.75% to 1.44%) explained this phenomenon. Finally, the analyses on the secretion of extracellular polymer substance (EPS), strategies to resist harsh environments, and physical properties of sludge explored the potential mechanism and provided references for applying the combined strategy.

The inhibitory effects and underlying mechanism of high ammonia stress on sulfide-driven denitrification process

Sulfide-driven denitrification (SD) process has been widely studied for treating wastewater containing sulfate and ammonia in recent years. But influence of high ammonia stress on the SD process and microbial community remained unclear. In this work, a series of tests were conducted to investigate effects of different ammonia stress (200-3000 mg-total ammonia nitrogen (TAN)/L) on denitrification efficiency, byproduct accumulation and microbial community of the SD process. According to our results, the SD process was severely inhibited, and 32.67 ± 5.15 mg/L NO₂^--N was accumulated when ammonia stress reached 3000 mg TAN/L. But the inhibited SD process could recover in about 40 days when ammonia stress was decreased to 200 mg TAN/L. After analyzing the microbial community, Thiobacillus sp. (Thiobacillus sp. 65-29, Thiobacillus sp. SCN 64-317, Thiobacillus sp. 63-78 and Thiobacillus denitrificans) was confirmed as dominant bacteria responsible for the SD process. Further, expression of narG, napA, nirK and nirS were inhibited under high ammonia stress, thus making the SD process stuck in NO₃^- and NO₂^- reduction step. This study reveals the inhibitory effects of high ammonia stress on the SD process and its possible underlying mechanism with discussion in gene level.

Exploiting Biofilm Characteristics to Enhance Biological Nutrient Removal in Wastewater Treatment Plants

Biological treatments are integral processes in wastewater treatment plants (WWTPs). They can be carried out using sludge or biofilm processes. Although the sludge process is effective for biological wastewater systems, it has some drawbacks that make it undesirable. Hence, biofilm processes have gained popularity, since they address the drawbacks of sludge treatments, such as the high rates of sludge production. Although biofilms have been reported to be essential for wastewater, few studies have reviewed the different ways in which the biofilm properties can be explored, especially for the benefit of wastewater treatment. Thus, this review explores the properties of biofilms that can be exploited to enhance biological wastewater systems. In this review, it is revealed that various biofilm properties, such as the extracellular polymeric substances (EPS), quorum sensing (Qs), and acylated homoserine lactones (AHLs), can be enhanced as a sustainable and cost-effective strategy to enhance the biofilm. Moreover, the exploitation of other biofilm properties such as the SOS, which is only reported in the medical field, with no literature reporting it in the context of wastewater treatment, is also recommended to improve the biofilm technology for wastewater treatment processes. Additionally, this review further elaborates on ways that these properties can be exploited to advance biofilm wastewater treatment systems. A special emphasis is placed on exploiting these properties in simultaneous nitrification and denitrification and biological phosphorus removal processes, which have been reported to be the most sensitive processes in biological wastewater treatment.

Meta-analysis of the prevalence of dissolved organic nitrogen (DON) in water and wastewater and review of DON removal and recovery strategies

Most wastewater removal and recovery processes primarily target dissolved inorganic nitrogen (DIN) species, leaving the untreated non-reactive dissolved organic nitrogen (DON) in the effluent. This DON fraction can account for a substantial part of the total nitrogen (N) load. We analyzed large datasets of N species and concentrations (with a focus on quantifying the fraction of DON) in surface water, ground water, and wastewater effluent across the United States. We then reviewed strategies to remove and recover DON based on results of a range of treatment technologies reported in the literature, including laboratory-scale up to full-scale operation in wastewater treatment plants. Our meta-analysis showed that DON concentrations are greatest in wastewater effluent followed by surface water and groundwater. The concentration of DON in wastewater effluent varied from 0.01 to 10.9 mg N/L (number of data points, n = 163), where the range in surface water was 0.002 to 14.3 mg N/L (n = 11,803). Organic N accounted for the majority of total N in 12.3% of wastewater effluent samples and 49.1% of surface waters. Our literature review showed that currently available wastewater treatment processes do not efficiently target DON removal nor recovery of the DON as a valuable product. One potential DON removal and recovery strategy is transforming DON into DIN, which is generally more easily removed and recovered. Transformation strategies reported in the literature include ozonation, UV/H₂O₂, and electrooxidation. However, as advanced oxidation processes are often energy- and cost-intensive, further research is needed to improve DON removal and recovery.

Characterization of Alcaligenes aquatilis as a novel member of heterotrophic nitrifier-aerobic denitrifier and its performance in treating piggery wastewater

A novel strain AS1 with heterotrophic nitrifying-aerobic denitrifying capacity in the species of Alcaligenes aquatilis was isolated from the aerobic activated sludge. It showed a great capability of ammonia removal, and the aerobic metabolic pathways to yield gaseous-nitrogen by hydroxylamine oxidation and nitrite denitrification were proposed. AS1 could efficiently remove ammonia under a wide range of environmental conditions, including the ratio of chemical oxygen demand to total nitrogen: 15-30, pH: 6-10, NaCl: 0-60 g/L, shaking speed of 0-180 rpm, and succinate, acetate, or citrate as carbon source. In the treatment of actual piggery wastewater, 95.3%, 95.1% and 84.9% of NH₄⁺-N was removed by AS1 when the initial ammonia concentration was 500, 1300, and 2000 mg/L, respectively, with the maximum NH₄⁺-N removal rate of 30.5 mg/L/h and 569.7 mg/L/d. Furthermore, plate colony-counting showed that AS1 achieved an efficient proliferation. These results imply the application potential of AS1 in treating high-ammonia wastewater.

Advanced light-tolerant microalgae-nitrifying bacteria consortia for stable ammonia removal under strong light irradiation using light-shielding hydrogel

The consortium of microalgae and nitrifying bacteria has attracted attention owing to its advantages, such as energy- and cost-efficiency in terms of using only light irradiation without aeration. However, high light intensity can easily cause photoinhibition of nitrifying bacteria, resulting in process breakdown of the consortium. This challenge limits its practical application in outdoor environment. In a previous study, we developed a "light-shielding hydrogel" which entrapped nitrifying bacteria in carbon black-added alginate hydrogel beads and confirmed its effectiveness of protecting the nitrifying bacteria from intense light up to 1600 μmol photons m^-2 s^-1. However, the applicability of the light-shielding hydrogel to microalgae-nitrifying bacteria consortia under strong light irradiation has not yet been clarified. In this study, we aimed to establish consortia of Chlorella sorokiniana and nitrifying bacteria immobilised in light-shielding hydrogel and evaluate their nitrification performance under strong light. Three nitrifying bacteria conditions were used: light-shielding hydrogel, hydrogel containing only nitrifying bacteria without carbon black ('hydrogel'), and dispersed nitrifier without immobilisation ('dispersion') as a control. At 1600 μmol photons m^-2 s^-1, the dispersion afforded a significant decrease in nitrification activity and subsequent process breakdown. In contrast, light-shielding hydrogel achieved complete nitrification without nitrite accumulation and had nitrification rates of approximately nine and two times higher than those for the dispersion and hydrogel conditions, respectively. Based on the overall evaluation, a possible sequence of process breakdown under strong light was also proposed. This study demonstrated for the first time that the light-shielding hydrogel/consortia combination had potential for applications, which require mitigation of photoinhibition under strong light irradiation. Further, it is expected that the proposed method will contribute to realise the practical application of microalgae-nitrifying bacteria consortia in various countries that experience high sunlight intensity due to their location in the sunbelt areas.

Efficient nitrogen removal from mature landfill leachate in a step feed continuous plug-flow system based on one-stage anammox process

A continuous plug-flow multistage anoxic/oxic (A/O) system based on one-stage partial nitrification coupled anammox (PNA) process with integrated fixed-film activated sludge (IFAS) was established and operated over 400 days. A step feed strategy effectively controlled free ammonia concentration and alleviated impacts on ammonia oxidizing bacteria (AOB) and anammox bacteria (AnAOB). During day 301-405, 98.1% of total inorganic nitrogen was removed from mature landfill leachate, whereas chemical oxygen demand (COD) removal efficiency was 52.9%. With the enrichment of AnAOB in oxic biofilm, nitrogen removal via the anammox pathway reached 94.3%-95.0%. During system operation, the dominant anammox genus shifted from Candidatus_Brocadia to Candidatus_Kuenenia. Fluorescent in situ hybridization (FISH) indicated AnAOB encapsulated by AOB colonies were mainly distributed inside of the biofilm, which promoted nitrite utilization by the anammox process. This innovative system and the results are of great value to practical applications.

Realization of partial nitrification and in-situ anammox in continuous-flow anaerobic/aerobic/anoxic process with side-stream sludge fermentation for real sewage

A continuous-flow anaerobic/aerobic/anoxic reactor with complete suspended activated sludge using sludge alkaline fermentation products as carbon source was utilized to strengthen nitrogen removal performance for low C/N ratio (<4) wastewater. Long-time performance indicated that the nitrite accumulation rate reached 60.40%, which strengthened the contribution of anammox. The average total inorganic nitrogen removal efficiency improved 19.40%. The abundance of ammonia oxidizing bacteria has not changed, but the abundance of nitrite oxidizing bacteria reduced from 5.79% to 0.69%. Quantitative PCR results demonstrated that the abundance of anammox bacteria has raised by 80.5 times. These results indicated that side-stream sludge alkaline fermentation promoted the mainstream partial nitrification, consequently accelerating the in-situ enrichment of anammox bacteria. No external carbon source dosing and short oxic hydraulic retention time (5.3 h) save energy and reduce consumption significantly in this system.

Achieving Partial Nitritation in Anammox Start-Up Environment

Removing ammonium via the partial nitritation anammox (PNA) process has been widely applied because of its cost and energy effectiveness. However, the first stage of PNA, partial nitritation, is hard to implement practically due to the challenging suppression of nitrate oxidizing bacteria (NOB) and should be achieved in the anammox environment to extend it to one stage PNA. Hence, this article evaluates different techniques, such as the combination of low dissolve oxygen (DO) and high free ammonia (FA), and the intermittent aeration cycle to achieve partial nitritation in an anammox start-up environment. For this purpose, a 10.5 L lab-scale moving bed biofilm reactor was set up and fed with synthetic wastewater and the transformation of influent ammonium into nitrate and nitrite was measured. The results showed that, despite applying low DO and higher free ammonia than the inhibition range of NOB, the nitrate production rate (NPR) was consistently higher than the nitrite accumulation rate (NAR), signifying no sufficient NOB suppression, partial nitritation under continuous aeration and up to a 0.27 gN/m2·d surface ammonium loading rate (SALR). Higher SALR than 0.27 gN/m2·d could result in partial nitritation since nitrogen compounds transformation was closer to partial nitritation when the reactor was subjected to 0.27 gN/m2·d rather than 0.14 gN/m2·d. Lifting up the SALR, on the other hand, results in a bad anammox environment and cannot prolong it to one-stage PNA. An intermittent aeration cycle with four different cycle lengths sets, obtained by monitoring nitrogen compound transformation, was, therefore, applied to the reactor. The relatively shorter aerobic length of 10 min ON and 30 OFF intermittent aeration cycle with 0.5 mg/L aerated DO was successful in achieving the partial nitritation with NPR, NAR, and ammonium removal efficiency (ARE) values of 17%, 78%, and 37%, respectively, showing that shorter aerated length suppresses NOB to a high degree due to less available time for NOB after oxygen starvation.

Non-submerged attached growth process for domestic wastewater treatment: Influence of media types and internal recirculation ratios

This study is aimed to comprehend the treatment of non-submerged attached growth systems using bio-sponge, bio-cord, and bio-cloth media. Three reactors were set up with internal recirculation ratio of 1 (IR = 1) and similar media surface area. Bio-sponge and bio-cloth reactors showed removal of COD (79 vs. 76%) and NH₄⁺-N (78 vs. 73%). While bio-cord treatment was deteriorated due to time-dependent process. Multiple linear regression revealed that alkalinity governed the formation degree of the anaerobic zone in bio-sponges, partially affecting nitrification. Increasing IR from 1 to 3 caused sloughing of the attached biomass and was positively correlated with effluent nitrite nitrogen concentration, indicating the sensitivity of nitrification to spatial distribution effects. In addition, bio-sponge system obtained superior performance at IR of 2 while bio-cloth one might be also an effective media for wastewater treatment if having good durability.

Insight into the mechanism of nitritation establishment through external fermented sludge addition

In this study, a novel strategy for nitritation establishment through external fermented sludge addition is proposed. The nitrifying activities under various fermentation times were assessed; batch tests results revealed that the decay rate of nitrite oxidizing bacteria (0.53 ± 0.08 d^-1) was much higher than that of ammonia oxidizing bacteria (0.15 ± 0.04 d^-1), during alkaline fermentation with high endogenous free ammonia concentrations (0.57-138.79 mg·L^-1). The long-term effects of fermented sludge addition were constantly monitored in an anaerobic/oxic/anoxic sequencing batch reactor. During 60 days of operation, the nitrite accumulation rate reached above 90% and could be maintained stably. Total inorganic nitrogen removal achieved 94.7%, with an average external sludge reduction rate of 61.3%, after incorporation of the sludge addition. Illumina MiSeq sequencing demonstrated that nitrite oxidizing bacteria were successfully eliminated and the relative abundance of ammonia oxidizing bacteria remained at 0.39%, resulting in nitrite accumulation.

Microbial community analysis of biopiles in Antarctica provides evidence of successful hydrocarbon biodegradation and initial soil ecosystem recovery

Microorganisms comprise the bulk of biodiversity and biomass in Antarctic terrestrial ecosystems. To effectively protect and manage the Antarctic environment from anthropogenic impacts including contamination, the response and recovery of microbial communities should be included in soil remediation efficacy and environmental risk assessments. This is the first investigation into the microbial dynamics associated with large scale bioremediation of hydrocarbon contaminated soil in Antarctica. Over five years of active management, two significant shifts in the microbial community were observed. The initial shift at 12-24 months was significantly correlated with the highest hydrocarbon degradation rates, increased microbial loads, and significant increases in alkB gene abundances. ANCOM analysis identified bacterial genera most likely responsible for the bulk of degradation including Alkanindiges, Arthrobacter, Dietzia and Rhodococcus. The second microbial community shift occurring from 36 to 60 months was associated with further reductions in hydrocarbons and a recovery of amoA nitrification genes, but also increasing pH, accumulation of nitrite and a reduction of oligotrophic bacterial species. Over time, the addition of inorganic fertilisers altered the soil chemistry and led to a disruption of the nitrogen cycle, most likely decoupling ammonia oxidisers from nitrite oxidisers, resulting in nitrite accumulation. The results from this study provide key insights to the long-term management of hydrocarbon bioremediation in Antarctic soils.

Achieving stable and long-term partial nitrification of domestic wastewater by side-stream sludge treatment using a novel nitrite oxidation inhibitor chloroxylenol

Using inhibitors to selectively suppress the activity of nitrite-oxidizing bacteria (NOB) was an emerging way to rapidly achieve partial nitrification (PN). This study explored the feasibility of inactivating NOB by a novel inhibitor chloroxylenol (PCMX) in real domestic wastewater. Different frequencies (periodic strategy and concentrative time strategy) of PCMX side-stream sludge treatment were used to achieve and maintain PN during 250 days. PN was realized by PCMX treatment once a day about 20 days, due to the inhibition of Nitrospira. PN was completely destroyed after 212 days by periodic strategy, caused by the increase of Candidatus Nitrotoga. PN maintained without PCMX in following 201 days by concentrative time strategy. The risks of PCMX were assessed and almost no PCMX was detected in the effluent of mainstream sequencing batch reactors. These results meant PN realized by PCMX side-stream sludge treatment was feasible and concentrative time strategy was a better operating strategy.

Stable nitritation of mature landfill leachate via in-situ selective inhibition by free nitrous acid

In-situ free nitrous acid (FNA) and free ammonia (FA) treatments are more feasible than side-stream methods to achieve nitritation. To assess the optimum conditions and long-term performance of in-situ inhibition by FNA, batch tests and a sequencing batch reactor (SBR) treating mature landfill leachate were conducted and established. As a result, the selective inhibition characteristic by FNA was more conspicuous than FA, and FNA (0.175 mg N/L, 6 h) treatment are more biocidal to nitrite oxidizing bacteria (NOB). Moreover, ammonia oxidizing bacteria (AOB) were more sensitive to the FA environment but its activity recovered preferentially compared to NOB. The SBR achieved a sustained nitrite accumulation rate above 90% for 200 days, with a significant decrease of NOB activity and microbial abundance according to qPCR and 16S rRNA gene sequencing results. In-situ selective inhibition by FNA (0.175 mg N/L, 6 h) has been proved to be effective to maintain stable nitritation.

Constraining nitrification by intermittent aeration to achieve methane-driven ammonia recovery of the mainstream anaerobic effluent

Mainstream anaerobic treatment has the potential to capture organic energy, and represents a sustainable development trend, but with the problems of low biogas quality and dissolved methane emissions. In this study, methane-driven ammonia recovery of anaerobic effluent was proposed. A 380-day long-term experiment, which was divided into four phases according to different aeration modes, was conducted. The ammonia conversion and microbial characteristics shows that ammonia oxidizing bacteria (AOB) were constrained during Phases 2 (DO: <0.2 mg L^-1) and 4 (DO: 0.1-1.6 mg L^-1), and were active during Phase 3 (DO: 2-4 mg L^-1). During phase 4, when the intermittent aeration was used, the total nitrogen removal rate was higher than during Phases 2 and 3, and nearly 100% ammonia was removed. Methylomonas, a genus of methane oxidizing bacteria (MOB), was enriched during Phase 4. The serum bottle experiment confirmed that the ammonia removal occurred through the MOB assimilation. The protein content in the CH₄-added group was 35.5%, which was higher than in the group without CH₄ (23.3%). The powerful ammonia assimilation and protein synthesis capabilities of MOB give a meaning to the anaerobic effluent for ammonia recovery and protein production. Intermittent aeration could be used to constrain AOB and improve ammonia recovery efficiency.

Understanding the effect of free ammonia on microbial nitrification mechanisms in suspended activated sludge bioreactors

During nitrification, the varieties of microbial structures, metabolic pathways and functional profiles in four parallel laboratory-scale sequencing batch reactors (SBRs) with 0.5, 5, 10 and 15 mg/L of free ammonia (FA) concentrations were analyzed by high-throughput sequencing of the 16S rRNA gene. The SBRs were named S_0.5, S₅, S₁₀ and S₁₅, respectively. Ammonia removal via the nitrate pathway was achieved in S_0.5 and S₅ throughout the whole experimental period, while ammonia removal via the nitrite pathway was established in S₁₀ and S₁₅ after 89 and 146 day, respectively. The key finding of this study is that both the microbial diversity and richness were significantly affected (p < 0.05) by the FA concentration at different taxonomic levels. The most dominant taxa of S₅, S₁₀ and S₁₅ were same, and mainly included Thauera while S_0.5 was mainly composed of Zoogloea. Linear discriminant analysis (LDA) effect size (LEfSe) analysis was used to identify unique biomarkers in SBR activated sludge (AS) sample. The functional genera and enzyme in the four SBRs are similar but different in abundance and they are responsible for the removal of organics and nitrogen. Moreover, metabolic pathways are similar by PICRUSt analysis. The relative proportions of pathway-specific genes involved in some metabolic pathways differed to some extent. The ammonia oxidation rate was positively linked to Nitrosomonas and amo (both Spearman correlation coefficients (ρ) = 0.777) while the nitrite oxidation rate was positively linked to Nitrospira (ρ = 0.777) by co-occurrence network analysis. This work deciphered the response of microbial characteristics to different FA constraints in AS process and could provide helpful information for revealing the biological mechanism of FA inhibition on nitrogen removal.

Development of Strategies for AOB and NOB Competition Supported by Mathematical Modeling in Terms of Successful Deammonification Implementation for Energy-Efficient WWTPs

Novel technologies such as partial nitritation (PN) and partial denitritation (PDN) could be combined with the anammox-based process in order to alleviate energy input. The former combination, also noted as deammonification, has been intensively studied in a frame of lab and full-scale wastewater treatment in order to optimize operational costs and process efficiency. For the deammonification process, key functional microbes include ammonia-oxidizing bacteria (AOB) and anaerobic ammonia oxidation bacteria (AnAOB), which coexisting and interact with heterotrophs and nitrite oxidizing bacteria (NOB). The aim of the presented review was to summarize current knowledge about deammonification process principles, related to microbial interactions responsible for the process maintenance under varying operational conditions. Particular attention was paid to the factors influencing the targeted selection of AOB/AnAOB over the NOB and application of the mathematical modeling as a powerful tool enabling accelerated process optimization and characterization. Another reviewed aspect was the potential energetic and resources savings connected with deammonification application in relation to the technologies based on the conventional nitrification/denitrification processes.

Microbial analysis for the ammonium removal from landfill leachate in an aerobic granular sludge sequencing batch reactor

In this study, a laboratory-scale sequencing batch reactor (SBR) equipped with aerobic granular sludge (AGS) technology was continuously operated for 220 days to remove ammonium from an existing landfill leachate. The ammonium removal was characterized by polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) technology. This method helped to analyze the long-term community structural stability of ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB) and denitrifying bacteria (DB) throughout the experiment. Simultaneously, 16S rRNA gene cloning and sequencing analysis identified the dominant species of different microbial species. Experimental results confirmed that ammonium removal was inhibited at the high nitrogen loading rate (NLR) stage while the low NLR stage achieved satisfactory ammonium removal. Moreover, the findings demonstrated that functionally stable wastewater treatment bioreactors facilitated the occurrence of stable microbial community structures.

Long-term stability of a non-adapted aerobic granular sludge process treating fish canning wastewater associated to EPS producers in the core microbiome

The tolerance of aerobic granular sludge (AGS) to variable wastewater composition is perceived as one of its greatest advantages compared to other aerobic processes. However, research studies select optimal operational conditions for evaluating AGS performance, such as the use of pre-adapted biomass and the control of wastewater composition. In this study, non-adapted granular sludge was used to treat fish canning wastewater presenting highly variable organic, nutrient and salt levels over a period of ca. 8 months. Despite salt levels up to 14 g NaCl L^-1, the organic loading rate (OLR) was found to be the main factor driving AGS performance. Throughout the first months of operation, the OLR was generally lower than 1.2 kg COD m^-3 day^-1, resulting in stable nitrification and low COD and phosphorous levels at the outlet. An increase in OLR up to 2.3 kg COD m^-3 day^-1 disturbed nitrification and COD and phosphate removal, but a decrease to average values between 1 and 1.6 kg COD m^-3 day^-1 led to resuming of those processes. Most of the bacteria present in the AGS core microbiome were associated to extracellular polymeric substances (EPS) production, such as Thauera and Paracoccus, which increased during the higher OLR period. Ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) species were detected in AGS biomass; while AOB were identified throughout the operation, NOB were no further identified after the period of increased OLR. Different polyphosphate-accumulating organisms (PAOs) were detected along the process: CandidatusAccumulibacter, Tetrasphaera and Gemmatimonas. A non-adapted granular sludge was able to treat the fish canning wastewater and to tolerate salinity fluctuations up to 14 g L^-1. Overall, a high microbial diversity associated to EPS producers allowed to preserve bacterial groups responsible for nutrients removal, contributing to the adaptation and long-term stability of the AGS system.

Comammox Nitrospira Species Dominate in an Efficient Partial Nitrification-Anammox Bioreactor for Treating Ammonium at Low Loadings

Bacteria capable of complete ammonia oxidation (comammox) are widespread and contribute to nitrification in wastewater treatment facilities. However, their roles in partial nitrification-anaerobic ammonium oxidation (anammox) systems remain unclear. In this study, a bench-scale bioreactor with continuous stirring was operated for more than 1000 days with limited oxygen supply to achieve efficient nitrogen removal (70.1 ± 2.7%) at a low ammonium loading of 35.2 mg-N/L/day. High-throughput amplicon sequencing analysis of the comammox ammonia monooxygenase subunit A (amoA) gene revealed seven sequence types from two clusters in clade A of comammox Nitrospira. Quantitative polymerase chain reaction analyses suggested that the comammox species dominated the ammonia-oxidizing community, with an abundance as high as 89.2 ± 7.9% in total prokaryotic amoA copies. Multiple linear regression further revealed the substantial contribution of the comammox Nitrospira to ammonia oxidation in the bioreactor. The investigation with bioreactor and batch experiments consistently showed that activities of comammox Nitrospira were inhibited by free ammonia far more severely than other ammonia-oxidizing microbes. Overall, this study provided new insight into the ecology of comammox Nitrospira under hypoxic conditions and suggested comammox-associated partial nitrification-anammox as a potential method for treating low-strength ammonium-containing wastewater.

Urine nitrification robustness for application in space: Effect of high salinity and the response to extreme free ammonia concentrations

Urine nitrification is one of the possibilities for the future recovery of water and elements for soilless crop production in space systems. The start-up of artificial urine nitrification was conducted for over 85 days in a sequencing batch reactor (SBR). Two free ammonia (FA) incidents occurred, which gave the opportunity to demonstrate the impressive ability of nitrifiers to resist temporary inhibition by FA without long lasting adverse effects. The failures led to very high FA concentrations of 280 and 84 gN-NH₃/m³, respectively. Sludge was exposed to FA for 19 and 27 h. It was possible to restore nitrification with simple remedy actions (dilution and pH restoration). No inhibitory effects on the nitrification rate were seen after implementation of the remedy actions and the nitrification rate increased considerably (up to 300% of the pre-failure value) due to decrease in salinity. After a few days of normal operation and salt concentration, the nitrification rate returned to the pre-failure values in both cases.

Achieving Partial Nitrification via Intermittent Aeration in SBR and Short-Term Effects of Different C/N Ratios on Reactor Performance and Microbial Community Structure

A sequencing batch reactor (SBR) with an intermittent aeration mode was established to achieve partial nitrification (PN) and the short-term effects of C/N ratios were investigated. Stable nitrite accumulation was achieved after 107 cycles, about 56d, with the average ammonia nitrogen removal efficiency (ARE) and nitrite accumulation rate (NAR) of 96.92% and 82.49%, respectively. When the C/N ratios decreased from 4.64 to 3.87 and 2.32, ARE and NAR still kept a stable and high level. However, when the C/N ratio further decreased to 0.77, nitrite accumulation became fluctuation, and ARE, total nitrogen (TN), and chemical oxygen demand (COD) removal performance declined obviously. Except for four common phyla (Proteobacteria, Bacteroidetes, Chloroflexi, and Actinobacteria) in the wastewater treatment system, Patescibacteria, the newly defined superphylum, was found and became the most dominant phylum in the PN sludge for their ultra-small cell size. The only ammonia oxidation bacteria (AOB), Nitrosomonas, and nitrite oxidation bacteria (NOB), Nitrospira, were detected. The relative abundance of NOB was low at different C/N ratios, showing the stable and effective inhibition effects of intermittent aeration on NOB growth.

Return-Sludge Treatment with Endogenous Free Nitrous Acid Limits Nitrate Production and N2O Emission for Mainstream Partial Nitritation/Anammox

Nitrite oxidizing bacteria (NOB) and nitrous oxide (N₂O) hinder the development of mainstream partial nitritation/anammox. To overcome these, endogenous free ammonia (FA) and free nitrous acid (FNA), which can be produced in the sidestream, were used for return-sludge treatment for two integrated-film activated sludge reactors containing biomass in flocs and on carriers. The repeated exposure of biomass from one reactor to FA shocks had a limited impact on NOB suppression but inhibited anammox bacteria (AnAOB). In the other reactor, repeated FNA shocks to the separated flocs failed to limit the system's nitrate production since NOB activity was still high on the biofilms attached to the unexposed carriers. In contrast, the repeated FNA treatment of flocs and carriers favored aerobic ammonium-oxidizing bacteria (AerAOB) over NOB activity with AnAOB negligibly affected. It was further revealed that return-sludge treatment with higher FNA levels led to lower N₂O emissions under similar effluent nitrite concentrations. On this basis, weekly 4 h FNA shocks of 2.0 mg of HNO₂-N/L were identified as an optimal and realistic treatment, which not only enabled nitrogen removal efficiencies of ∼65% at nitrogen removal rates of ∼130 mg of N/L/d (20 °C) but also yielded the lowest cost and carbon footprint.

The effect of dissolved oxygen concentration on long-term stability of partial nitrification process

Dissolved oxygen (DO) concentration is regarded as one of the crucial factors to influence partial nitrification process. However, achieving and keeping stable partial nitrification under different DO concentrations were widely reported. The mechanism of DO concentration influencing partial nitrification is still unclear. Therefore, in this study two same sequencing batch reactors (SBRs) cultivated same seeding sludge were built up with real-time control strategy. Different DO concentrations were controlled in SBRs to explore the effect of DO concentration on the long-term stability of partial nitrification process at room temperature. It was discovered that ammonium oxidation rate (AOR) was inhibited when DO concentration decreased from 2.5 to 0.5 mg/L. The abundance of Nitrospira increased from 10^11.5 to 10^13.7 copies/g DNA, and its relative percentage increased from 0.056% to 3.2% during 190 operational cycles, causing partial nitrification gradually turning into complete nitrification process. However, when DO was 2.5 mg/L the abundance of Nitrospira was stable and AOB was always kept at 10^10.7 copies/g DNA. High AOR was maintained, and stable partial nitrification process was kept. Ammonia oxidizing bacteria (AOB) activity was significantly higher than nitrite oxidizing bacteria (NOB) activity at DO of 2.5 mg/L, which was crucial to maintain excellent nitrite accumulation performance.

Effect of ethanol pre-fermentation on organic load rate and stability of semi-continuous anaerobic digestion of food waste

This study investigated the effects of ethanol pre-fermentation on the organic load rate (OLR) and stability of single-phase semi-continuous anaerobic digestion system of food waste. Results revealed that the pre-fermentation group tolerated higher OLR, with a maximum OLR of 5.5 g-VS·L^-1·d^-1. Moreover, the abundance of microbial community in relation to amino acid degradation in the pre-fermentation group was significantly higher than that in the control group, which made a higher concentration of ammonia nitrogen and caused a stronger system stability and higher alkalinity. Redundancy analysis was performed to explore the factors affecting system instability under high OLR. The instability of control group was found to be significantly influenced by the total volatile fatty acids (TVFA), whereas the pre-fermentation group was affected by total ammonia nitrogen and TVFA. Therefore, if the OLR of the pre-fermentation group is to be further increased, ammonia nitrogen inhibition should also be considered.

Hydroxylamine addition and real-time aeration control in sewage nitritation system for reduced start-up period and improved process stability

Sewage nitritation is a promising process for nitrogen removal, but its practical application is limited by long start-up period and unstable operation. In this study, hydroxylamine (NH₂OH) addition and real-time aeration control strategies were adopted for the promotion of sewage nitritation in a sequencing batch reactor. Initially, 4.5 mg/L NH₂OH was added into reactor every 24 h to establish nitritation, increasing the nitrite accumulation ratio (NAR) from 1.7% to 93% in 19 d. In the following period, NH₂OH addition was stopped and nitritation remained stable over 55 d, with NAR of 97% by real-time aeration control. The aeration duration was determined by characteristic points on pH curve. The main genera of nitrite oxidizing bacteria, Nitrobacter and Nitrospira, were both eliminated from the system, supporting the long-term stability of nitritation. Overall, NH₂OH addition and real-time aeration control is an excellent strategy for establishing and maintaining effective sewage nitritation.

Improving nitrogen removal in biological aeration filter for domestic sewage treatment via adjusting microbial community structure

The rapid growth of nitrite-oxidizing bacteria (NOB) in reactor prevents the application of anaerobic ammonium oxidation (anammox) technology to main-stream wastewater treatment. How to eliminate NOB and reserve anaerobic ammonium oxidation bacteria (AnAOB) simultaneously becomes the biggest challenge. In this study two coupled biological aeration filters (BAFs) were built up to treat domestic sewage. In BAF1 nitrogen removal concentration was 21.4 mg/L via heterotrophic denitrification pathway. Backwash was conducted to BAF2 to improve nitrogen removal performance. After backwash Nitrospira proportion declined from 10.8% to 2.1%, while Candidatus Kuenenia percentage increased from 5.6% to 10.2%. Nitrogen removal concentration improved from 8.6 mg/L to 22.8 mg/L via anammox pathway in BAF2, and total nitrogen removal concentration reached to 44.2 mg/L in two coupled BAFs during aeration process. These findings could provide a new strategy for the application of anammox technology to main-stream wastewater treatment.

High-rate partial-nitritation and efficient nitrifying bacteria enrichment/out-selection via pH-DO controls: Efficiency, kinetics, and microbial community dynamics

Conventional nitrification/denitrification process is gradually being replaced with partial-nitritation/anammox (PN/A) processes due to its installation and running cost. However, high ammonia-oxidizing bacteria (AOB) and anaerobic ammonia-oxidizing (anammox) bacteria activity as well as optimum out-selection of nitrite-oxidizing bacteria (NOB) are necessary to achieving efficient PN/A process. Consequently, to enhance PN process via nitrifying bacteria enrichment/out-selection within psychrophilic environment, a novel pH-DO (dissolved oxygen) control strategy was proposed and the response of PN, kinetics, AOB enrichment, and NOB out-selection efficiency was investigated during start-up and long-term operation. With DO of 0.7 mg/L and pH of 7.5-7.9, quick start-up of the PN process was established within 34d as NO₂^--N accumulation ratio (NAR) reached 90.08 ± 1.4%. Again, when NLR was elevated to 0.8 kg/m³·d (400mgNH₄⁺-N/L), DO curtailed to 0.2 mg/L, pH maintained at 7.7 and free ammonium at 6.5 mg/L, NAR and NH₄⁺-N removal rate could still reach 97.04 ± 2.4% and 97.84 ± 1.5%, respectively. After optimum control factors had been established, real nitrogen-rich-mine-wastewater was fed (DO, 0.2 mg/L, pH, 8.9, and free ammonia, 6.5 mg/L) and NAR and NH₄⁺-N removal rate reached was 97.33 ± 0.5% and 97.76 ± 1.1%, respectively. Estimated kinetic parameters including maximum degradation rate (Vmax = 1.58/d), half-rate constant (Km = 33.8 mg/L), and inhibition constant (Ki = 201.6 mg/L) suggested that inhibition on NH₄⁺-N oxidation was most feasible at higher concentration of NH₄⁺-N. To elucidate biological mechanisms, 16S rRNA high-throughput revealed that AOB (Nitrosomonas) enrichment had increased from 0.08% to 49% whereas NOB (Nitrospira) abundance reduced from 1% to 0.034%, indicating pH-DO control efficiently enriched AOB and out-selected NOB. Conversely, when influent NH₄⁺-N was curtailed to about 200 mg/L and free ammonia concentration maintained at 6.5 mg/L, the population of AOB was observably reduced by 6% within a period of 14 days, indicating control strategies including pH-DO control and substrate availability were the key factors which substantially influenced and promoted the activities and growth of AOBs in the present SBR.

Free nitrous acid pretreatment of sludge to achieve nitritation: The effect of sludge concentration

This study investigated the effect of sludge concentration (expressed by mixed liquor volatile suspended solids, MLVSS) on free nitrous acid (FNA) pretreatment strategy to achieve nitritation. Results showed when FNA was 0.308 mgHNO₂-N/L, nitrite oxidizing bacteria (NOB) activity increased by 70.2% as MLVSS increased from 8.4 to 16.8 g/L. Nitrite accumulation ceased as MLVSS increased to 12.6 g/L, indicating that FNA inhibition of NOB gradually weakened with increasing MLVSS. When FNA was higher than 0.770 mgHNO₂-N/L, NOB activity was completely inhibited and the effect of MLVSS on FNA inhibition was negligible, with nitrite accumulation potential (NAP) varying from 88.1% to 90.0%. Mechanism investigation demonstrated flocs sizes distinctly declined, with more extracellular polymeric substances (EPS) released to resist FNA inactivation. Linear fitting showed NAP increased with FNA/MLVSS increment. Therefore, MLVSS affected FNA pretreatment performance, with FNA/MLVSS proposed as a more valuable criterion in FNA pretreatment strategy development, than the solely FNA.

Ammonium influences kinetics and structure of methanotrophic consortia

The literature is conflicted on the influence of ammonium on the kinetics and microbial ecology of methanotrophy. In this study, methanotrophic cultures were enriched, under ammonium concentrations ranging from 0 to 200 mM, from an inoculum comprising leachate and top-cover soil from a landfill. Specific CH₄ biodegradation rates were highest (7.8 × 10^-4 ± 6.0 × 10^-5 gCH₄ g_X^-1 h^-1) in cultures enriched at 4 mM NH₄⁺, which were mainly dominated by type II methanotrophs belonging to Methylocystis spp. Lower specific CH₄ oxidation rates (average values of 1.8-3.6 × 10^-4 gCH₄ g_X^-1 h^-1) were achieved by cultures enriched at higher NH₄⁺ concentrations (20 and 80 mM), and had higher affinity for CH₄ compared to 4 mM enrichments. These lower affinities were attributed to lower diversity dominated by type I methanotrophs, of the Methylosarcina, Methylobacter and Methylomicrobium genera, encountered with increasing concentrations of NH₄⁺. The study indicates that CH₄ oxidation biotechnologies applied at low NH₄⁺ concentrations can support efficient abatement of CH₄ and high diversity of methanotrophic consortia, whilst enriching type II methanotrophs.

Rapid start-up and stable maintenance of domestic wastewater nitritation through short-term hydroxylamine addition

This study investigated the nitritation of domestic wastewater through the short-term addition of hydroxylamine (NH₂OH). Sequencing batch reactor (SBR) was used and the NH₂OH solution with an initial concentration of 5.0 mg/L was injected at each cycle. With NH₂OH addition, the nitritation was quickly established in 5 d with nitrite accumulation ratio above 95%. Further, stable nitritation could be maintained without NH₂OH addition in the following 53 days, even under unfavorable conditions (DO = 3 mg/L). According to qPCR results, NH₂OH significantly reduced and continuously suppressed nitrite oxidizing bacteria (NOB), while the abundance of ammonia oxidizing bacteria was stable, induced the start-up and maintenance of nitritation. Among NOB, NH₂OH significantly suppressed Nitrospira, while did not affect Nitrobactor. Nitrobactor gradually increased during the operation, which could induce the final deterioration of nitritation. Overall, this research provided the fundamental knowledge required to optimize the NH₂OH addition strategy for operating a stable nitritation in domestic wastewater.

The roles of free ammonia (FA) in biological wastewater treatment processes: A review

Free ammonia (FA) can pose inhibitory and/or biocidal effects on a variety of microorganisms involved in different biological wastewater treatment process, which is widely presented in wastewater treatment plants (WWTPs) due to the high levels of ammonium in the systems. This review article gives the up-to-date status on several essential roles of FA in biological wastewater treatment processes: the impacts of FA, mechanisms of FA roles, modeling of FA impacts, and implications of FA for wastewater treatment. Specifically, the impacts of FA on both wastewater and sludge treatment lines were firstly summarized, including nitrification, denitrification, anaerobic ammonium oxidation (Anammox), enhanced biological phosphorus removal and anaerobic processes. The involved mechanisms were then analyzed, which indicated FA inhibition can slow specific microbial activities or even reconfigure the microbial community structure, likely due to negative impacts of FA on intracellular pH, specific enzymes and extracellular polymeric substances (EPS), thus causing cell inactivation/lysis. Mathematical models describing the impact of FA on both wastewater and sludge treatment processes were also explored to facilitate process optimization. Finally, the key implications of FA were identified, that is FA can be leveraged to substantially enhance the biodegradability of secondary sludge, which would further improve biological nutrient removal and enhance renewable energy production.

The cotreatment of landfill leachate and high-nitrate wastewater using SBRs: evaluation of denitrification performance and microbial analysis

Resourceful disposal of landfill leachate has always been an intractable worldwide problem. This study was conducted to investigate the feasibility of biologically treating a combined waste stream of landfill leachate and high-concentration nitrate nitrogen (high-nitrate) wastewater. Raw landfill leachate was pretreated using anaerobic fermentation and ammonia stripping to improve biodegradability. The control sequencing batch reactor (SBR, named R0) was fed only with synthetic high-nitrate wastewater with sodium acetate as the carbon source, whereas the other experimental SBR (named R1) was loaded with mixtures containing leachates. Excessive increase in leachate adversely affected the cotreatment, and it was concluded that the landfill leachate volume ratio should never exceed 7.5% of the total wastewater (14% of the initial COD) based on further batch experiments. The maximum specific denitrification rate of 58.05 mg NO₃⁻-N (gVSS h)⁻¹ was attained in R1, while that of 32.32 mg NO₃⁻-N (gVSS h)⁻¹ was obtained in R0. Illumina MiSeq sequencing revealed that adding landfill leachate did not change the fact that Pseudomonas, Thauera, and Pannonibacter dominant in the sodium acetate supported the denitrification systems, but led to the adjustment of their relative abundance. Moreover, the narG, nirK, nirS, and norB denitrifying genes exhibited increased abundance by 138–980% in the cotreated system, which was confirmed by q-PCR analyses. These findings reveal that the denitrification efficiency of activated sludge in SBR cotreated with landfill leachate and high-nitrate wastewater significantly improved, and this may contribute toward the understanding of the molecular mechanisms of biological denitrification under the blending treatment of leachate and high-nitrate wastewater.

Resourceful disposal of landfill leachate has always been an intractable worldwide problem.

Achieving nitritation by treating sludge with free nitrous acid: The effect of starvation

Side-stream sludge treatment using free nitrous acid (FNA) is a novel strategy to achieve nitritation in mainstream of wastewater treatment plants (WWTPs). To optimize nitritation, the effect of starvation on this strategy was investigated in this study. The results showed that pre-starvation, which is the starvation before FNA treatment, enhanced the resistance of sludge to FNA. This led to a decrease in the nitrite accumulation rate (NAR), which dropped from 70% to 27% after aerobic pre-starvation. This was further confirmed in the FNA treatment using the sludge collected from the secondary settling tank (anoxic pre-starvation) and the aerobic zone (without starvation) of an Anaerobic-Anoxic-Oxic system. The post-starvation, which was the starvation after FNA treatment, decreased NAR from 63% to 14%. To obtain a higher NAR, the sludge used for FNA treatment should be collected from aerobic zone, and be returned to aerobic zone after treatment to avoid pre-starvation and post-starvation.

Nitritation, nitrous oxide emission pathways and in situ microbial community in a modified University of Cape Town process

Achieving nitritation is a prerequisite to promote nutrients removal and save energy, but emission of nitrous oxide as a greenhouse gas cannot be ignored. This study established the nitritation in a continuous-flow MUCT process and investigated the mechanism of N₂O generation. The nitrite accumulation ratio (NAR) reached 95% by controlling the low DO of 0.3-0.5 mg/L and short HRT of 8 h. The ¹⁵N-isotope tracer experiment indicated that the percentage of nitrifier-denitrification (ND) pathway increased by 12.7% under the limited-aeration mode, improving the stable operating of nitritation. Meanwhile, the autotrophic anammox pathway increased with the contribution ratio of 14.7% to N₂ emission under the nitritation mode. The ¹⁵N-DNA-SIP revealed that the Nitrosomonas executed the ND pathway and the Planctomycetes conducted the anammox process, respectively. The integration of autotrophic and heterotrophic process based on nitritation technique has potential to solve the carbon-limited issue for total nitrogen removal in mainstream WWTPs.

Control of partial nitrification using pulse aeration for treating digested effluent of swine wastewater

Three sequencing batch reactors (SBRs) were used to investigate the influence of pulse frequencies on the partial nitrification (PN) process in this study. At a total aeration time of 6 min each hour, the aerated frequencies of R1, R2 and R3 were 6, 3 and 2 time h^-1. During the steady period (117-143d), the nitrite accumulation rates (NARs) were 90.80%, 90.71% and 90.23% in R1, R2 and R3, respectively, indicating a steady nitritation was acquired. Activity measurements of the sludge samples taken at day 138 showed the activity of nitrite oxidating bacteria (NOB) was 0, indicating NOBs were successfully suppressed. The ratio of NO₂^--N to NH₄⁺-N in the effluent of R3 was 1.35, which most closely matched the influent of Anammox process. However, the energy efficiency evaluation showed that R1 had the highest actual oxygen transfer efficiency (AOTE) and dynamical efficiency (DE).