Robust and high-efficient nitrogen removal from real sewage and waste activated sludge (WAS) reduction in zero-external carbon PN/A combined with in-situ fermentation-denitrification process under decreased temperatures

Despite the advantages of the combined anammox and fermentation-driven denitrification process in nitrogen removal and energy consumption, stable performance at decreased temperatures remains a challenge. In this study, a robust and high-efficient nitrogen removal efficiency (95.0-93.1 ∼ 86.8-93.4%) with desirable effluent quality (3.0-4.1 ∼ 7.9-4.9 mg/L) under long-term decreased temperatures (30 °C→25 °C→20 °C) was achieved in a zero-external carbon Partial Nitritation/Anammox combined with in-situ sludge Fermentation-Denitrification process treating sewage. Excellent sludge reduction averaged at 14.9% assuming no microbial growth. Increased hzsB mRNA (2.2-fold) and reduced Ea (80.9 kJ/mol) proved resilient anammox to lower temperature. RT-qPCR tests revealed increased NarG/NirK (5.1) and NarG/NirS (4.9) mRNA at 20 °C, suggesting higher NO3-→NO2- over NO2-→N2 pathway. Metagenomics unraveled dominant anammox bacteria (Candidatus_Brocadia, 2.27%), increased denitritation bacteria containing more NarG (Hyphomicrobium, 0.8%), fatty acid biosynthesis and CAZymes genes. Enhanced denitritation with recovered organics from sludge reserved nitrite for anammox and facilitated higher anammox contribution to N removal at 20 °C (42.4%) than 30 °C (39.5%). This study proposed an innovative low-temperature strategy for in-situ sludge fermentation, and demonstrated stability of advanced municipal wastewater treatment and sludge disposal through energy savings and carbon recovery under decreased temperatures.

Biofilm phenotypes and internal community succession determines distinct growth of anammox bacteria in functional anammox biofilms

In this study, time-series anammox functional biofilms were obtained in a lab-scale simultaneous partial nitritation/anammox process for treating high-strength ammonium. The variations in the biofilm phenotypes, community succession, and anammox bacteria abundance over time were evaluated using optical microscopy, 16S rRNA gene sequencing, and qPCR. The result revealed that biofilm has three distinct stages of the community development trajectory across a 182-day temporal scale. Anammox bacteria growth rates were 0.035 d^-1, 0.0015 d^-1, and 0.011 d^-1, respectively. The diversity and network analysis suggested that the positive priority effect of ammonia oxidizing bacteria was the primary factor for the rapid proliferation of anammox bacteria, and the species replacement triggering priority effect forfeiture and substituted functional recruitment were reasons for the slow proliferation and stable proliferation of anammox bacteria, respectively. Taken together, the higher microbial diversity and stable community composite were key prerequisites for the proliferation of the anammox bacteria.

Microbial community and performance of a partial nitritation/anammox sequencing batch reactor treating textile wastewater

Implementation of onsite bioremediation technologies is essential for textile industries due to rising concerns in terms of water resources and quality. Partial nitritation-anaerobic ammonium oxidation (PN/A) processes emerged as a valid, but unexplored, solution. In this study, the performance of a PN/A pilot-scale (9 m3) sequencing batch reactor treating digital textile printing wastewater (10-40 m3 d-1) was monitored by computing nitrogen (N) removal rate and efficiencies. Moreover, the structure of the bacterial community was assessed by next generation sequencing and quantitative polymerase chain reaction (qPCR) analyses of several genes, which are involved in the N cycle. Although anaerobic ammonium oxidation activity was inhibited and denitrification occurred, N removal rate increased from 16 to 61 mg N g VSS-1 d-1 reaching satisfactory removal efficiency (up to 70%). Ammonium (18-70 mg L-1) and nitrite (16-82 mg L-1) were detected in the effluent demonstrating an unbalance between the aerobic and anaerobic ammonia oxidation activity, while constant organic N was attributed to recalcitrant azo dyes. Ratio between nitrification and anammox genes remained stable reflecting a constant ammonia oxidation activity. A prevalence of ammonium oxidizing bacteria and denitrifiers suggested the presence of alternative pathways. PN/A resulted a promising cost-effective alternative for textile wastewater N treatment as shown by the technical-economic assessment. However, operational conditions and design need further tailoring to promote the activity of the anammox bacteria.

Carrier type induces anammox biofilm structure and the nitrogen removal pathway: Demonstration in a full-scale partial nitritation/anammox process

In this study, two typical carrier types, microporous and macroporous carriers, were collected from a full-scale partial nitritation/anammox reactor for analysis and comparison of the biofilm structure characteristics, performance and removal nitrogen pathway. For microporous carriers, a thicker biofilm (>5 mm) was obtained with higher biomass and abundance of anammox bacteria as well as a higher nitrogen removal efficiency due to the integration of denitrifying and anammox bacteria. In addition, higher microbial community stability can be expected under varying environmental conditions. In comparison, macroporous carrier biofilm exhibited a lower thickness (0.4-2.3 mm) and lower microbial richness, with a strong network correlation among genera. Analysis showed that the mainly positive correlation between anammox bacteria and ammonium oxidizing bacteria, enhancing coupling partial nitritation and anammox. These findings help further our understanding of the mechanisms of anammox biofilm nitrogen removal and provide a baseline for optimization of the design of carrier structures.

Unravelling adaptation of nitrite-oxidizing bacteria in mainstream PN/A process: Mechanisms and counter-strategies

Stable suppression of nitrite-oxidizing bacteria (NOB) is still a major challenge for the implementation of partial nitritation and anammox (PN/A) in mainstream treatment. Despite numerous suppression strategies demonstrated, it is increasingly recognized that NOB could develop resistance to these strategies, threatening the long-term stability of the mainstream PN/A process. This study aims to understand adaption mechanisms and develop counter-strategies to overcome the adaptation. To this end, three previously-demonstrated suppression strategies, including NOB inactivation via side stream sludge treatment with free ammonia (FA), the use of low dissolved oxygen (DO), and the use of anammox to scavenge nitrite, were stepwise applied, over a period of 800 days, to a laboratory-scale reactor treating effluent from a high-rate activated sludge (HRAS) plant. FA sludge treatment alone sustained nitrite accumulation for about two months, after which NOB adaptation occurred causing PN to fail. The FA adaptation was induced by a shift in the NOB community from Nitrospira to Ca. Nitrotoga. The latter was found to have higher resistance to FA and also a higher maximum specific growth rate. Low DO at 0.2-0.4 mg O₂ L^-1 was then applied, in conjunction with FA treatment, which successfully eliminated Ca. Nitrotoga and re-established PN. However, new and unidentified NOB with a higher apparent oxygen affinity emerged in three months, again leading to PN failure. Lastly, as the third strategy for NOB suppression, anammox was introduced as an in-situ nitrite-scavenger. The combo-strategy delivered reliable NOB suppression with no further adaptation in the remaining experimental period (eight months). The resulted one-stage PN/A reactor achieved a nitrogen removal efficiency of 84.2 ± 5.37%. A control reactor, operated in parallel under the same conditions but without FA treatment, only achieved 10.4 ± 4.6% nitrogen removal, with anammox completely outcompeted by NOB in the last phase.

Full-scale partial nitritation/anammox (PN/A) process for treating sludge dewatering liquor from anaerobic digestion after thermal hydrolysis

In this study, a full-scale partial nitritation/anammox (PN/A) process was successfully established to treat dewatering liquor (filtrate) from the activated sludge after thermal hydrolysis (THP) - anaerobic digestion (AD). The filtrate had an average ammonium of 1407 mg/L with a COD/N ratio of 1.43 ± 0.3. Under limited anammox biofilm inoculation, PN/A was started-up in an integrated fixed - biofilm activated sludge (IFAS) reactor. During the stable period, 2500 m³ of THP - AD sludge filtrate was treated daily and an average nitrogen removal rate of 0.21 kg N/(m³·d) was maintained with a removal efficiency over 85%. The application of PN/A reduced mainstream total inorganic nitrogen in effluent by 4.4 mg/L, saving $3.5 million in operational costs annually due to the reduction of organics addition. Overall, IFAS - PN/A process can be an efficient and economical method to treat THP - AD sludge filtrate and improve mainstream nitrogen removal performance.

Nitrogen-associated niche characteristics and bacterial community estimated by 15N-DNA-stable isotope probing in one-stage partial nitritation/anammox process with different ammonium loading

Anaerobic ammonium oxidation coupled with partial nitritation is critical for cleaner production in sewage treatment. The long-term effects of high- and low-strength influent ammonium (NH₄⁺-N) on the anammox activity, ecological niche characteristics and active microbial community were investigated in a one-stage partial nitritation/anammox (PN/A) process. The total nitrogen (TN) removal efficiency was up to 90% with influent NH₄⁺-N of 192 mg/L. The ¹⁵N-isotope pairing technique illustrated that the potential anammox rate could reach to 3507.8 nmoL/g-sludge/h, accounting for 73.2% of dinitrogen production. As the influent NH₄⁺-N decreased to 63 mg/L, the anammox population significantly decreased and the Nitrospira became the dominant specialized species in the PN/A system. The Nitrobacter had the smallest niche overlap value and the furthest ecological distance to the anammox bacteria among the seven investigated nitrogen conversion-related genes along the influent NH₄⁺-N concentration gradient, indicating different ecological similarities. The redundancy analysis showed that the rise of dissolved oxygen caused by low NH₄⁺-N might be the main cause of the excessive proliferation of the Nitrospira. The ¹⁵N-DNA-stable isotope probing illustrated that both the class Anaerolineae and Proteobacteria had closely symbiotic relations with the Planctomycetacia in this in situ surveys. This study provides a deep understanding of PN/A process treating low-ammonium mainstream wastewater from the viewpoint of microecology.

Autotrophic nitrogen removal in an integrated fixed-biofilm activated sludge (IFAS) reactor: Anammox bacteria enriched in the flocs have been overlooked

In this study, an autotrophic nitrogen removal process was established using an integrated fixed-biofilm activated sludge (IFAS) reactor treated with high ammonium wastewater. A nitrogen removal rate (NRR) of 2.78 kg N/(m³·d) was obtained during the 206-day operation. Moreover, during the stable period, the large flocs (D > 0.2 mm) had a significantly higher abundance of anammox bacteria than the small flocs (D < 0.2 mm) and biofilm, resulting in 51% of the anammox bacteria being located in the flocs. The result indicates that anammox bacteria can be enriched in the flocs and in the biofilm, which has been rarely reported for IFAS reactors. In addition, the large flocs are likely formed through biofilm detachment since the microbial community was similar for the two kinds of biomass. Overall, the role of flocs in IFAS reactors are complicated and their contribution to the anammox reaction have been overlooked thus far.

Biomass segregation between biofilm and flocs improves the control of nitrite-oxidizing bacteria in mainstream partial nitritation and anammox processes

The control of nitrite-oxidizing bacteria (NOB) challenges the implementation of partial nitritation and anammox (PN/A) processes under mainstream conditions. The aim of the present study was to understand how operating conditions impact microbial competition and the control of NOB in hybrid PN/A systems, where biofilm and flocs coexist. A hybrid PN/A moving-bed biofilm reactor (MBBR; also referred to as integrated fixed film activated sludge or IFAS) was operated at 15 °C on aerobically pre-treated municipal wastewater (23 mg_NH4-N L^-1). Ammonium-oxidizing bacteria (AOB) and NOB were enriched primarily in the flocs, and anammox bacteria (AMX) in the biofilm. After decreasing the dissolved oxygen concentration (DO) from 1.2 to 0.17 mg_O2 L^-1 - with all other operating conditions unchanged - washout of NOB from the flocs was observed. The activity of the minor NOB fraction remaining in the biofilm was suppressed at low DO. As a result, low effluent NO₃^- concentrations (0.5 mg_N L^-1) were consistently achieved at aerobic nitrogen removal rates (80 mg_N L^-1 d^-1) comparable to those of conventional treatment plants. A simple dynamic mathematical model, assuming perfect biomass segregation with AOB and NOB in the flocs and AMX in the biofilm, was able to qualitatively reproduce the selective washout of NOB from the flocs in response to the decrease in DO-setpoint. Similarly, numerical simulations indicated that flocs removal is an effective operational strategy to achieve the selective washout of NOB. The direct competition for NO₂^- between NOB and AMX - the latter retained in the biofilm and acting as a "NO₂-sink" - was identified by the model as key mechanism leading to a difference in the actual growth rates of AOB and NOB (i.e., μ_NOB < μ_AOB in flocs) and allowing for the selective NOB washout over a broad range of simulated sludge retention times (SRT = 6.8-24.5 d). Experimental results and model predictions demonstrate the increased operational flexibility, in terms of variables that can be easily controlled by operators, offered by hybrid systems as compared to solely biofilm systems for the control of NOB in mainstream PN/A applications.

High-resolution mapping and modeling of anammox recovery from recurrent oxygen exposure

Oxygen inhibits anammox, a bioconversion executed by anoxic ammonium oxidizing bacteria (AnAOB). Nonetheless, oxygen is mostly found in the proximity of AnAOB in nitrogen removal applications, being a substrate for nitritation. The experiments performed to date were mostly limited to batch activity tests where AnAOB activity is estimated during oxygen exposure. However, little attention has been paid to the recovery and reversibility of activity following aerobic conditions, of direct relevance for bioreactor operation. In this work, anoxic and autotrophic reactor cultivation at 20 °C yielded an enriched microbial community in AnAOB, consisting for 75% of a member of the genus Brocadia. High-resolution kinetic data were obtained with online ammonium measurements and further processed with a newly developed Python data pipeline. The experimentally obtained AnAOB response showed complete inhibition until micro-aerobic conditions were reached again (<0.02 mg O₂ L^-1). After oxygen inhibition, AnAOB recovered gradually, with recovery times of 5-37 h to reach a steady-state activity, dependent on the perceived inhibition. The recovery immediately after inhibition was lowest when exposed to higher oxygen concentrations (range: 0.5-8 mg O₂ L^-1) with long contact times (range: 9-24 h). The experimental data did not fit well with a conventional 'instant recovery' Monod-type inhibition model. Yet, the fit greatly improved by incorporating a dynamic growth rate formula accurately describing gradual activity recovery. With the upgraded model, long-term kinetic simulations for partial nitritation/anammox (PN/A) with intermittent aeration showed a decrease in growth rate compared to the instant recovery mode. These results indicate that recovery of AnAOB after oxygen exposure was previously overlooked. It is recommended to account for this effect in the intensification of partial nitritation/anammox.

Pinpointing wastewater and process parameters controlling the AOB to NOB activity ratio in sewage treatment plants

Even though nitrification/denitrification is a robust technology to remove nitrogen from sewage, economic incentives drive its future replacement by shortcut nitrogen removal processes. The latter necessitates high potential activity ratios of ammonia oxidizing to nitrite oxidizing bacteria (rAOB/rNOB). The goal of this study was to identify which wastewater and process parameters can govern this in reality. Two sewage treatment plants (STP) were chosen based on their inverse rAOB/rNOB values (at 20 °C): 0.6 for Blue Plains (BP, Washington DC, US) and 1.6 for Nieuwveer (NV, Breda, NL). Disproportional and dissimilar relationships between AOB or NOB relative abundances and respective activities pointed towards differences in community and growth/activity limiting parameters. The AOB communities showed to be particularly different. Temperature had no discriminatory effect on the nitrifiers' activities, with similar Arrhenius temperature dependences (Θ_AOB = 1.10, Θ_NOB = 1.06-1.07). To uncouple the temperature effect from potential limitations like inorganic carbon, phosphorus and nitrogen, an add-on mechanistic methodology based on kinetic modelling was developed. Results suggest that BP's AOB activity was limited by the concentration of inorganic carbon (not by residual N and P), while NOB experienced less limitation from this. For NV, the sludge-specific nitrogen loading rate seemed to be the most prevalent factor limiting AOB and NOB activities. Altogether, this study shows that bottom-up mechanistic modelling can identify parameters that influence the nitrification performance. Increasing inorganic carbon in BP could invert its rAOB/rNOB value, facilitating its transition to shortcut nitrogen removal.

Microbial community evolution in partial nitritation/anammox process: From sidestream to mainstream

This study investigated the microbial evolution in a mainstream partial nitritation/anammox (PN/A) reactor started by inoculation from sidestream PN/A. The reactor was fed with pre-treated sewage and operated for 120 days at room temperature (24-26 °C). It was found that for both sidestream and mainstream PN/A, anammox bacteria preferentially grew in granular sludge while ammonium-oxidizing bacteria (AOB) were mainly resided in flocculent sludge. After 120 days operation, the abundance of anammox bacteria in the reactor decreased from 6.6 × 10¹¹ to 3.2 × 10¹¹ copies/L. Besides, a shift of dominant anammox genera from Ca. Brocadia to Ca. Kuenenia was observed. In contrast, the dominant genera of AOB was Nitrosomonas throughout the operation. Furthermore, high-throughput sequencing revealed that heterotrophs constitute the majority of microorganisms in PN/A reactor. Especially, Chloroflexi, which can utilize cell decay materials from autotrophs, were enriched under mainstream conditions. This study provided a better understanding of the microorganisms in mainstream PN/A process.

Achieve efficient nitrogen removal from real sewage in a plug-flow integrated fixed-film activated sludge (IFAS) reactor via partial nitritation/anammox pathway

This study tested the feasibility of plug-flow integrated fixed-film activated sludge (IFAS) reactor in applying sewage partial nitritation/anammox (PN/A) process. The IFAS reactor was fed with real pre-treated sewage (C/N ratio=1.3) and operated for 200days. High nitrogen removal efficiency of 82% was achieved with nitrogen removal rates of 0.097±0.019kgN/(m³·d). Therefore, plug-flow IFAS reactor could be an alternative to applying sewage PN/A process. Besides, it was found that the stability of sewage PN/A process was significantly affected by residual ammonium. Nitrate accumulated in effluent and PN/A performance deteriorated when residual ammonium was below 1mg/L. On the contrary, long-term stable PN/A operation was achieved when residual ammonium was over 3mg/L.

Uncoupling the solids retention times of flocs and granules in mainstream deammonification: A screen as effective out-selection tool for nitrite oxidizing bacteria

This study focused on a physical separator in the form of a screen to out-select nitrite oxidizing bacteria (NOB) for mainstream sewage treatment. This separation relied on the principle that the NOB prefer to grow in flocs, while anammox bacteria (AnAOB) reside in granules. Two types of screens (vacuum and vibrating) were tested for separating these fractions. The vibrating screen was preferred due to more moderate normal forces and additional tangential forces, better balancing retention efficiency of AnAOB granules (41% of the AnAOB activity) and washout of NOB (92% activity washout). This operation resulted in increased NOB out-selection (AerAOB/NOB ratio of 2.3) and a total nitrogen removal efficiency of 70% at influent COD/N ratio of 1.4. An effluent total nitrogen concentration <10mgN/L was achieved using this novel approach combining biological selection with physical separation, opening up the path towards energy positive sewage treatment.

Mainstream partial nitritation and anammox: long-term process stability and effluent quality at low temperatures

The implementation of autotrophic anaerobic ammonium oxidation processes for the removal of nitrogen from municipal wastewater (known as "mainstream anammox") bears the potential to bring wastewater treatment plants close to energy autarky. The aim of the present work was to assess the long-term stability of partial nitritation/anammox (PN/A) processes operating at low temperatures and their reliability in meeting nitrogen concentrations in the range of typical discharge limits below 2  [Formula: see text] and 10 mgNtot·L(-1). Two main 12-L sequencing batch reactors were operated in parallel for PN/A on aerobically pre-treated municipal wastewater (21 ± 5 [Formula: see text] and residual 69 ± 19 mgCODtot·L(-1)) for more than one year, including over 5 months at 15 °C. The two systems consisted of a moving bed biofilm reactor (MBBR) and a hybrid MBBR (H-MBBR) with flocculent biomass. Operation at limiting oxygen concentrations (0.15-0.18 [Formula: see text] ) allowed stable suppression of the activity of nitrite-oxidizing bacteria at 15 °C with a production of nitrate over ammonium consumed as low as 16% in the MBBR. Promising nitrogen removal rates of 20-40 mgN·L(-1)·d(-1) were maintained at hydraulic retention times of 14 h. Stable ammonium and total nitrogen removal efficiencies over 90% and 70% respectively were achieved. Both reactors reached average concentrations of total nitrogen below 10 mgN·L(-1) in their effluents, even down to 6 mgN·L(-1) for the MBBR, with an ammonium concentration of 2 mgN·L(-1) (set as operational threshold to stop aeration). Furthermore, the two PN/A systems performed almost identically with respect to the biological removal of organic micropollutants and, importantly, to a similar extent as conventional treatments. A sudden temperature drop to 11 °C resulted in significant suppression of anammox activity, although this was rapidly recovered after the temperature was increased back to 15 °C. Analyses of 16S rRNA gene-targeted amplicon sequencing revealed that the anammox guild of the bacterial communities of the two systems was composed of the genus "Candidatus Brocadia". The potential of PN/A systems to compete with conventional treatments for biological nutrients removal both in terms of removal rates and overall effluent quality was proven.