Bioaugmentation of sidestream nitrifying-denitrifying phosphorus-accumulating granules in a low-SRT activated sludge system at low temperature

A comprehensive comparison of microbial communities between aerobic granular sludge and flocculent sludge for nutrient removal in full-scale wastewater treatment plants

Understanding the microbial community structure of sludge is crucial for improving the design, operation and optimisation of full-scale wastewater treatment plants (WWTPs). This study aimed to have a comprehensive comparison of microbial communities between aerobic granular sludge and flocculent sludge from two full-scale sequential batch reactors-based WWTPs with nutrient removal for the first time. To better understand key functional bacteria such as polyphosphate accumulating bacteria (PAOs), competitive bacteria such as glycogen accumulating bacteria (GAOs) and nitrifying bacteria for both nitrogen and phosphorus removal, another two full-scale WWTPs with only carbon (C) removal and C and nitrogen (N) removal were compared too. It was found that the richness and diversity of the microbial population in sludge increased with pollutant removal from only C, C and N, to C,N, P removal. For C, N P removal, granule structure led to a more diverse and rich microbial community structure than flocculent structure. Although more abundant nitrifying bacteria were enriched in granular sludge than flocculent sludge, the abundance of total putative PAOs was equivalent. However, the most typical putative PAOs such as Tetrasphaera and Candidatus Accumulibacter seemed to be more correlated with biological phosphorus removal performance, which might be more proper to be used as an indication for P removal potential. The higher abundance of GAOs in flocculent sludge with better phosphorus removal performance might suggest that further investigation is needed to understand the functions of GAOs. In addition, the equivalent abundances of PAOs in the WWTPs with only C removal and with C, N, and P removal, respectively, indicate that many newly reported putative PAOs might not contribute to P removal. This study provides insight into the microbial communities and functional bacteria in aerobic granular sludge and flocculent sludge in full-scale SBRs, which can provide microbes-informed optimisation of reactor operation for better nutrient removal.

Utilizing anaerobic substrate distribution for growth of aerobic granular sludge in continuous-flow reactors

The development of continuous flow reactors (CFRs) employing aerobic granular sludge (AGS) for the retrofit of existing wastewater treatment plants (WWTPs) using a continuous-flow activated sludge (CFAS) system has garnered increasing interest. This follows the worldwide adoption of AGS technology in sequencing batch reactors (SBRs). The better settleability of AGS compared to AS allows for process intensification of existing wastewater treatment plants without the difficult conversion of often relatively shallow CFRs to deeper AGS-SBRs. To retrofit existing CFAS systems with AGS, achieving both increased hydraulic capacity and enhanced biological nutrient removal necessitates the formation of granular sludge based on the same selective pressures applied in AGS-SBRs. Previous efforts have focussed mainly on the selective wasting of flocculent sludge and retaining granular sludge to drive aerobic granulation. In this study a pilot-scale CFR was developed to best mimic the implementation of the granulation mechanisms of full-scale AGS-SBRs. The pilot-scale reactor was fed with pre-settled municipal wastewater. We established metrics to assess the degree to which the proposed mechanisms were implemented in the pilot-scale CFR and compared them to data from full-scale AGS-SBRs, specifically with respect to the anaerobic distribution of granule forming substrates (GFS). The selective pressures for granular sludge formation were implemented through inclusion of anaerobic upflow selectors with a water depth of 2.5 meters, which yielded a sludge with properties similar to AGS from full-scale SBRs. In comparison to the CFAS system at Harnaschpolder WWTP treating the same pre-settled wastewater, a more than twofold increase in volumetric removal capacity for both phosphorus and nitrogen was achieved. The use of a completely mixed anaerobic selector, as opposed to an anaerobic upflow selector, caused a shift in EBPR activity from the largest towards the smallest size class, while nitrification was majorly unaffected. Anaerobic selective feeding via bottom-feeding is, therefore, favorable for the long-term stability of AGS, especially for less acidified wastewater. The research underlines the potential of AGS for enhancing the hydraulic and biological treatment capacity of existing CFAS systems.

Vertical Distribution and Seasonal Patterns of Candidatus Nitrotoga in a Sub-Alpine Lake

The nitrite oxidizing bacterial genus Ca. Nitrotoga was only recently discovered to be widespread in freshwater systems; however, limited information is currently available on the environmental factors and seasonal effects that influence its distribution in lakes. In a one-year study in a dimictic lake, based on monthly sampling along a vertical profile, the droplet digital PCR quantification of Ca. Nitrotoga showed a strong spatio-temporal patchiness. A correlation ana-lysis with environmental parameters revealed that the abundance of Ca. Nitrotoga correlated with dissolved oxygen and ammonium, suggesting that the upper hypolimnion of the lake is the preferred habitat.

Enhancement of nutrient removal in an activated sludge process using aerobic granular sludge augmentation strategy with ammonium-based aeration control

To enhance nutrient removal from low-strength municipal wastewater in a continuous-flow activated sludge (CFAS) process using aerobic granular sludge (AGS) augmentation strategy, a pilot-scale demonstration was configured with a mainstream reactor (anaerobic/aerobic process) and a sidestream sequencing batch reactor for AGS production. The aeration of the mainstream reactor was controlled based on dissolved oxygen (DO) and ammonium concentrations during Phases I and II-III, respectively. During Phase III, an anoxic zone was created in the mainstream aerobic tank. Throughout the demonstration period, excellent sludge settleability in the mainstream reactor (SVI30 ≤ 80 mL g-1) under long sludge retention time conditions (≥12 d) allowed the maintenance of a high mixed liquor suspended solids concentration (≥3000 mg L-1). The total nitrogen (TN) removal ratio improved significantly during Phases II and III (49.3 ± 4.1% and 50.1 ± 10.2%, respectively) compared to Phase I (43.2 ± 5.5%). Low DO concentration (< 0.5 mg L-1) by the ammonium-based aeration tended to increase the simultaneous nitrification and denitrification efficiency (> 40%), enhancing TN removal (> 50%). The reduction of DO and nitrate concentrations in the returning sludge liquor can stabilize phosphorus removal (approximately 80% of the 25th percentile). In addition, the aeration efficiency during Phase III decreased by 26-29% compared to Phase I. These results suggest that the introduction of ammonium-based aeration control to the CFAS using the AGS augmentation strategy could contribute to superior sewerage treatment, including nutrient removal and a low carbon footprint.

Feasibility of a return-sludge nursery concept for mainstream anammox biostimulation: creating optimal conditions for anammox to recover and grow in a parallel tank

To overcome limiting anammox activity, a return-sludge nursery concept is proposed. This concept blends reject water treated with partial nitritation with mainstream effluent to increase the temperature, N levels, and EC of the anammox nursery reactor, which sludge periodically passes through the return sludge line of the mainstream system. Various nursery frequencies were tested in two 2.5 L reactors, including 0.5-2 days of nursery treatment per 3.5-14 days of the total operation. Bioreactor experiments showed that nursery increased nitrogen removal rates during mainstream operation by 33-38%. The increased anammox activity can be partly (35-60%) explained by higher temperatures. Elevated EC, higher nitrogen concentrations, and a putative synergy and/or unknown factor were responsible for 15-16%, 12-14%, and 10-36%, respectively. A relatively stable microbial community dominated by "Candidatus Brocadia" was observed. This new concept boosted activity and sludge growth, which may facilitate mainstream anammox implementations based on partial nitritation/anammox or partial nitrification/denitratation/anammox.

Stress response and signalling of a low-temperature bioaugmentation system in decentralized wastewater treatment: Degradation characteristics, community structure, and bioaugmented mechanisms

Low temperatures present challenges for stable wastewater treatment operations in cold regions. Low-temperature effective microorganisms (LTEM) were added as a bioaugmentation strategy at a decentralized treatment facility to improve performance. The effects of a low-temperature bioaugmentation system (LTBS) with LTEM at low temperatures (4 °C) on organic pollutant performance, microbial community changes, and the metabolic pathways of functional genes and functional enzymes were studied. To explore the bioaugmentation mechanism of LTBS based on stress response and signalling. The results showed that the start-up time of the LTBS (S2) with LTEM was shorter (8 days) and that it removed COD and NH₄⁺-N at higher rates (87 % and 72 %, respectively) at 4 °C. LTEM effectively degraded complex macromolecular organics into small molecular organics, and decomposing sludge flocs and the changing the extracellular polymeric substances (EPS) structure removed more organics and nitrogen. LTEM and local microbial communities (nitrifying and denitrifying bacteria) improved the ability of organic matter degradation and denitrification of the LTBS and formed a core microbial community dominated by LTEM (Bacillus and Pseudomonas). Finally, based on the functional enzymes and metabolic pathways of the LTBS, a low-temperature strengthening mechanism consisting of 6 cold stress responses and signal pathways under low temperatures was formed. This study demonstrated that the LTEM-dominated LTBS could provide an engineering alternative for future decentralized wastewater treatment in cold regions.

Life cycle assessment and techno-economic analysis of nitrogen recovery by ammonia air-stripping from wastewater treatment

Wastewater treatment plants (WWTPs) with anaerobic digestion of biosolids produce an ammonia-rich sidestream out of which nitrogen can be recovered through air stripping. Recovered ammonia can be used to produce ammonium sulfate (AS) for agricultural use, enabling the circular return of nitrogen as fertilizer to the food system. We investigate the cost and life cycle environmental impact of recovering ammonia from the sidestream of WWTPs for conversion to AS and compare it to AS production from the Haber Bosch process. We perform life cycle assessment (LCA) to investigate the environmental impact of AS fertilizer production by air-stripping ammonia from WWTP sidestreams at varying sidestream nitrogen concentrations. Techno-economic analysis (TEA) is performed to assess the break-even selling price of sidestream AS production at a WWTP in the City of Philadelphia. Greenhouse gas emissions for air-stripping technology range between 0.2 and 0.5 kg CO₂e/kg AS, about six times lower than the hydrocarbon-based Haber-Bosch process, estimated at 2.5 kg CO₂e/kg AS. Further reduction of greenhouse gas emissions is feasible by replacing fossil-based energy use in air-stripping process (82-98 % of net energy demand) with renewable sources. Also, a significant reduction in mineral depletion and improvement in human and ecosystem health are observed for the air-stripping approach. Furthermore, the break-even selling price for installing sidestream-based AS production at the Philadelphia's WWTP, considering capital and operating costs, is estimated at $0.046/kg AS (100 %), which is 92 % lower than the 2014 estimate of AS's average selling price at farms in the United States. We conclude that even with varying ammonia concentrations and high sidestream volume, air-stripping technology offers an environmentally and economically favorable option for implementing nitrogen recovery and simultaneous production of AS at WWTPs.

Simultaneous nitrification and denitrification in membrane bioreactor: Effect of dissolved oxygen

Membrane bioreactor with the floc activated sludge (mixed liquor suspended solids (MLSS) = 7500 mg/L) was constructed in this work for simultaneously nitrification and denitrification (SND). The effect of dissolved oxygen (DO) on SND process and the nitrogen pathways were investigated. The average TN removal efficiencies were 63.05%, 91.17%, 87.04% and 70.02% for DO 0.5, 1, 2 and 3 mg/L systems, respectively. The effluent ammonia concentration was continuously lower than 5.0 mg/L when the DO was higher than 1 mg/L. Nitrogen in DO 1 and DO 2 mg/L systems was mainly removed via the SND process. The rise of DO concentration increased the abundance of nitrite oxidizing bacteria (NOB) and Nitrospira was the predominant NOB in all the four MBRs. Dechloromonas and Azoarcus were the dominant denitrifying bacteria (DNB) in DO 1 systems responsible for nitrite denitrification. The dominant aerobic DNB Pseudomonas also contributed SND via nitrate denitrification and was little affected by DO changes. Nitrate reductase was the main enzyme for the reduction of NO₃^--N to NO₂^--N, and narG was the main responsible gene. Nitrite oxidoreductase was the main enzyme for the oxidation of NO₂^--N to NO₃^--N, and nxrA was the main responsible gene in all the four MBR systems.

Impact of nitrite and oxygen on nitrous oxide emissions from a granular sludge sequencing batch reactor

Maximizing nutrient removal and minimizing greenhouse gas (GHG) emissions is imperative for the future of wastewater treatment. As municipalities focus on minimizing their carbon footprints, future permits could regulate GHG emissions from wastewater treatment plants. This study investigates how nitrous oxide (N₂O) emissions are affected by dissolved oxygen and nitrite concentrations, providing potential strategies to meet possible gaseous emission permits. A lab-scale sequencing batch reactor (SBR) was enriched with aerobic granular sludge (AGS) capable of phosphate removal and simultaneous nitrification-denitrification (SND). N₂O emissions were tracked at varying dissolved oxygen (DO) and nitrite (NO₂^-) concentrations, with >99% SND efficiency and 93%-100% phosphate removal efficiency. Higher DO and NO₂^- concentrations were associated with higher N₂O emissions. Emissions were minimized at a DO concentration of 1 mg L^-1, with an average emission factor of 0.18% of oxidized NH₃-N emitted as N₂O-N, which is lower than factors from many full-scale treatment plants (Vasilaki et al., 2019) and similar to a Nereda® full-scale AGS SBR (van Dijk et al., 2021). This challenges assertions that AGS emits more N₂O than conventional activated sludge, although more research at full-scale with influent quality variations is required to confirm this trend. Molecular analyses revealed that the efficient SND was likely achieved with shortcut nitrogen removal facilitated by a low presence of nitrite oxidizing bacteria and a large population of denitrifying phosphate accumulating organisms, which far outnumbered denitrifying glycogen accumulating organisms.

Cultivation of Nitrifying and Nitrifying-Denitrifying Aerobic Granular Sludge for Sidestream Treatment of Anaerobically Digested Sludge Centrate

In this study, three 1.2-L aerobic granular sludge sequencing batch reactors (AGS-SBRs) were used to cultivate nitrifying and nitrifying-denitrifying granules (w/supplemental carbon) and investigate sidestream treatment of synthetic-centrate and real-centrate samples from Ashbridges Bay Treatment Plant (ABTP) in Toronto, Ontario, Canada. Results showed that although the cultivation of distinct granules was not observed in the nitrifying reactors, sludge volume index (SVI30) values achieved while treating real and synthetic centrate were 72 ± 12 mL/g and 59 ± 11 mL/g (after day 14), respectively. Ammonia-nitrogen (NH3-N) removal in the nitrifying SBRs were 93 ± 19% and 94 ± 16% for real and synthetic centrate, respectively. Granules with a distinct round structure were successfully formed in the nitrifying-denitrifying SBR, resulting in an SVI30 of 52 ± 23 mL/g. NH3-N, chemical oxygen demand (COD) and phosphorus (P) removal in the nitrifying-denitrifying SBR were 92 ± 9%, 94 ± 5%, and 81 ± 14% (7th to 114th day), respectively with a low nitrite (NO2-N) and nitrate (NO3-N) concentration in the effluent indicating simultaneous nitrification-denitrification (SND) activity. High nutrient removal efficiencies via the nitrification and SND pathways shows that AGS technology is a viable process for treating sidestreams generated in a WWTP.

Response of nitritation granules to anaerobically pre-treated municipal wastewater at low temperatures in a continuous-flow reactor

Achieving mainstream nitritation with aerobic granules is attractive based on increasing evidence but generally treating artificial low-ammonium wastewater. Real municipal wastewater is much more complex in composition, the behavior of the nitritation granules would be different when treating real municipal wastewater. Herein, the response of nitritation granules to influent shift from artificial low-ammonium (35-40 mg/L) wastewater to anaerobically pre-treated municipal wastewater (MWW_pre-treated) was investigated at low temperatures. Results showed that MWW_pre-treated caused the outgrowth of filamentous bacteria on the granule surface and developed into finger-like structures, which in turn resulted in the decrease of the overall granular sludge settleability. Batch-tests and microbial analysis indicated the functional and microbial differentiation between the newly formed fluffy exterior and the original compact granule. The fluffy exterior was dominated by genus Flavobacterium (66.6%) and primarily functioned as COD removal, whereas the nitrifiers (mainly Nitrosomonas) were still located in the compact core and performed nitritation. Moreover, the heterotrophs-dominated fluffy exterior hindered the oxygen transfer towards nitrifiers located in the compact granule and thereby facilitated the stable NOB repression in the granule particularly at low temperatures (<10 °C). Finally, gradual recovery of the granular sludge morphology and settleability occurred after the influent reverted to synthetic low-ammonium wastewater. Overall, this work demonstrated that the feeding of MWW_pre-treated only caused morphological changes of the nitritation granules, but its structural and functional stability could be maintained stably.

Towards mainstream partial nitritation/anammox in four seasons: Feasibility of bioaugmentation with stored summer sludge for winter anammox assistance

The strong effect of low temperatures on anammox challenges its mainstream application over the winter in temperate climates. Winter bioaugmentation with stored summer surplus sludge is a potential solution to guarantee sufficient nitrogen removal in winter. Firstly, the systems for which nitrogen removal deteriorated by the temperature decrease (25 °C → 20 °C) could be fully restored bioaugmenting with granules resp. flocs stored for 6 months at 118 resp. 220% of the initial biomass levels. Secondly, the reactivation of these stored sludges was tested in lower temperature systems (15.3 ± 0.4/10.4 ± 0.4 °C). Compared to the activity before storage, between 56% and 41% of the activity of granules was restored within one month, and 41%-32% for flocs. Additionally, 85-87% of granules and 50-53% of flocs were retained in the systems. After reactivation (15.3 ± 0.4/10.4 ± 0.4 °C), a more specialized community was formed (diversity decreased) with Candidatus Brocadia still dominant in terms of relative abundance. Capital and operating expenditures (CAPEX, OPEX) were negligible, representing only 0.19-0.36% of sewage treatment costs.

Some like it cold: The cellular organization and physiological limits of cold-tolerant nitrite-oxidizing Nitrotoga

Chemolithoautotrophic production of nitrate is accomplished by the polyphyletic functional group of nitrite-oxidizing bacteria (NOB). A widely distributed and important NOB clade in nitrogen removal processes at low temperatures is Nitrotoga, which however remains understudied due to the scarcity of cultivated representatives. Here, we present physiological, ultrastructural and genomic features of Nitrotoga strains from various habitats, including the first marine species enriched from an aquaculture system. Immunocytochemical analyses localized the nitrite-oxidizing enzyme machinery in the wide irregularly shaped periplasm, apparently without contact to the cytoplasmic membrane, confirming previous genomic data suggesting a soluble nature. Interestingly, in two strains we also observed multicellular complexes with a shared periplasmic space, which seem to form through incomplete cell division and might enhance fitness or survival. Physiological tests revealed differing tolerance limits towards dissolved inorganic nitrogen concentrations and confirmed the generally psychrotolerant nature of the genus was. Moreover, comparative analysis of 15 Nitrotoga genomes showed, e.g., a unique gene repertoire of the marine strain that could be advantageous in its natural habitat and confirmed the lack of genes for assimilatory nitrite reduction in a strain found to require ammonium for growth. Overall, these novel insights largely broaden our knowledge of Nitrotoga and elucidate the metabolic variability, physiological limits and thus potential ecological roles of this group of nitrite oxidizers. This article is protected by copyright. All rights reserved.

Can nitrifiers from the sidestream deammonification process be a remedy for the N-overload of the mainstream reactor?

The combination of sidestream deammonification and bioaugmentation of the mainstream reactor using ammonia oxidizers from partial nitritation (PN) was not achieved before. This novel solution not only enables the efficient sidestream nitrogen removal, but also improves mainstream resistance to stress situations such as biomass washout or nitrogen overload. This feature is important for wastewater treatment plants (WWTPs) equipped with reject water deammonification as its implementation leads to lower nitrifier mass in the mainstream reactor and therefore diminish ability to cope with rapid increase in the loading rate (i.e. due to sidestream process failure). The proposed approach presents the use of the excess sludge from a modified PN process to boost the mainstream nitrification in unfavourable conditions. In a long-term laboratory experiment, the operation of an existing WWTP at low temperature was simulated in two reactors using real wastewater fluxes. One of them was augmented with the excess sludge from a PN reactor that treats reject water containing 20% of the WWTP N-load. The treatment efficiency in both reactors was tested under different nitrogen loading rates, as well as in the case of the of biomass loss. The bioaugmentation intensity was set according to the actual nitrogen load balance of the modelled WWTP, resulting in a daily seed volume only equal to 0.28% of the reactors' influent. Two incidents were simulated, where the nitrogen load increased by about 24.5% and 34%. In both cases, the nitrification efficiency in the non-augmented reactor dropped by about 45%, while the augmented reactor maintained efficient ammonium removal. The bioaugmentation effect was also noticeable during biomass washout - only in the non-augmented reactor nitrification was insufficient for over 60 days. These results undoubtedly show the possibility of combining two different approaches for sidestream nitrogen removal into one technology demonstrating the advantages of both component solutions.

Relevance of Candidatus Nitrotoga for nitrite oxidation in technical nitrogen removal systems

Abstract

Many biotechnological applications deal with nitrification, one of the main steps of the global nitrogen cycle. The biological oxidation of ammonia to nitrite and further to nitrate is critical to avoid environmental damage and its functioning has to be retained even under adverse conditions. Bacteria performing the second reaction, oxidation of nitrite to nitrate, are fastidious microorganisms that are highly sensitive against disturbances. One important finding with relevance for nitrogen removal systems was the discovery of the mainly cold-adapted Cand. Nitrotoga, whose activity seems to be essential for the recovery of nitrite oxidation in wastewater treatment plants at low temperatures, e.g., during cold seasons. Several new strains of this genus have been recently described and ecophysiologically characterized including genome analyses. With increasing diversity, also mesophilic Cand. Nitrotoga representatives have been detected in activated sludge. This review summarizes the natural distribution and driving forces defining niche separation in artificial nitrification systems. Further critical aspects for the competition with Nitrospira and Nitrobacter are discussed. Knowledge about the physiological capacities and limits of Cand. Nitrotoga can help to define physico-chemical parameters for example in reactor systems that need to be run at low temperatures.

Key points

• Characterization of the psychrotolerant nitrite oxidizer Cand. Nitrotoga

• Comparison of the physiological features of Cand. Nitrotoga with those of other NOB

• Identification of beneficial environmental/operational parameters for proliferation

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11487-5.

Rationale for constant flow to optimize wastewater treatment and advanced water treatment performance for potable reuse applications

Population growth, the impacts of climate change, and the need for greater water security have made the reuse of wastewater, including potable use, increasingly desirable. As interest in potable reuse of wastewater increases, a variety of processes have been proposed for advanced water treatment following conventional wastewater treatment. In all cases, the operation and performance of advanced water treatment facilities (AWTFs) is improved when the treated wastewater feed is of the highest quality that can be achieved and the advanced water treatment (AWT) processes are operated at a constant flow. One proven method of optimizing the performance of wastewater treatment facilities (WWTFs) is constant flow operation with no extraneous return flows other than internal process recycle flows, such as return settled solids. A number of approaches can be used to achieve constant flow including flow equalization, divided treatment trains, and satellite treatment. The ways in which constant flow wastewater treatment benefits both WWTFs as well as the AWTFs are considered with special emphasis on the ability to achieve predictable log removal credits (LRCs) for specific microorganisms. Actual performance data from constant flow WWTFs are used to illustrate how LRCs are determined. PRACTITIONER POINTS: Constant flow WWTFs should be considered to produce the highest quality secondary effluent for AWT. Flow equalization, divided treatment trains, and satellite treatment can be used to achieve constant flow to optimize wastewater treatment in small and medium size WWTFs. Flow equalization can be used to maximize the amount of wastewater that can be recovered for potable reuse. Important benefits of constant flow for wastewater treatment facilities include economic and operational savings, stable and predictable treatment performance, energy savings, ability to optimize performance for the removal of specific constituents, and the ability to assign pathogen log removal credits (LRCs). Important benefits of constant flow and optimized WWT for AWTFs include economic and operational savings; less pretreatment needed, including energy and chemical usage; elimination of the need to cycle treatment processes; and added factor of safety with respect to the required pathogen LRCs. In large WWTFs, constant flow for AWTFs will typically be achieved by effluent diversion; depending on the effluent quality additional pretreatment may be needed. The design and implementation of WWTFs and AWTFs for potable reuse should be integrated for optimal performance and protection of public health.

An investigation into the optimal granular sludge size for simultaneous nitrogen and phosphate removal

An aerobic granular sludge (AGS) pilot plant fed with a mixture of acetate amended centrate and secondary effluent was used to investigate the optimal granule size range for simultaneous nitrification and denitrification (SND) and ortho-phosphate removal. The anaerobic phase was mixed to understand how AGS will perform if integrated with a continuous flow activated sludge system that cannot feed the influent through the settled sludge bed. Five different granule size fractions were taken from the pilot (operated at DO setpoint of 2mgO₂/L) and each size was subjected to activity tests in a well-controlled lab-scale AGS reactor at four dissolved oxygen (DO) concentrations of 1, 2, 3, and 4 mgO₂/L. The size fractions were: 212 - 600 µm, 600 - 1000 µm, 1000 - 1400 µm, 1400 - 2000 µm, and >2000 µm. The smallest size range (212 - 600 µm) had the highest nitrification and phosphate removal rates at DO setpoints from 1 - 3 mgO₂/L, which was attributed to the higher aerobic volume fraction in small granules and hence a higher abundance of phosphorus accumulating organisms (PAO) and ammonia oxidizing bacteria (AOB). In comparison, large granules (>1000 µm) had 1.4 - 4.7 times lower ammonia oxidation rates than the smallest size range, which aligned with their lower AOB abundance relative to granule biomass. The granules with the highest anoxic volume fraction had the highest abundance of nitrite reductase genes (nir gene) but did not show the highest specific nitrogen removal rate. Instead, smaller granules (212 - 600 and 600 - 1000 µm), which had a lower nir gene abundance, had the highest specific nitrogen removal rates (1.2 - 3.1 times higher than larger granules) across all DO values except at 4 mgO₂/L. At a DO setpoint of 4 mgO₂/L, nitrite production by ammonia oxidation (ammonia monooxygenase) exceeded nitrite reduction by nitrite reductase in granules smaller than 1000 µm, in addition, some denitrifying heterotrophs switched to oxygen utilization in deeper layers hence suppressing denitrification activity. At the DO range of 2 - 4 mg/L, granular size had a greater effect on nutrient removal than DO. Therefore, for AGS developed at an average DO setpoint of 2 mgO₂/L, selecting for size fractions in the range of 212 - 1000 µm and avoiding DO values higher than 3 mgO₂/L can achieve both a higher nitrogen removal capacity and energy savings. This study is the first to investigate the influence of different DO values on SND and biological phosphorus removal performance of different aerobic granular sludge sizes.

Application of aerobic kenaf granules for biological nutrient removal in a full-scale continuous flow activated sludge system

Aerobic granular sludge (AGS) is a biofilm technology that offers more treatment capacity in comparison to activated sludge. The integration of AGS into existing continuous-flow activated sludge systems is of great interest as process intensification can be achieved without the use of plastic-based biofilm carriers. Such integration should allow good separation of granules/flocs and ideally with minor retrofitting, making it an ongoing challenge. This study utilized an all-organic media carrier made of porous kenaf plant stalks with high surface areas to facilitate biofilm attachment and granule development. A 5-stage Bardenpho plant was upgraded with the addition of kenaf media and a rotary drum screen to retain the larger particles from the secondary clarifier underflow whereas flocs were selectively wasted. Startup took 5 months with a sludge volume index (SVI) reduction from >200 to 50 mL g^-1. Most of the kenaf granules fell in the size range of 600-1400 μm and had a clear biofilm layer. The wet biomass density, SVI₃₀, and SVI₃₀/SVI₅ of the kenaf granules were 1035 g L^-1, 30.6 mL g^-1, and 1.0, respectively, which met the standards of aerobic granules. Improved stability of biological phosphorus removal performance enabled a 25% reduction in sodium aluminate usage. Microbial activities of kenaf granules were compared with aerobic granules, showing comparable N and P removal rates and presence of ammonium-oxidizing bacteria and polyphosphate-accumulating organisms in the outer 50-60 μm layer of the granule. This work is the first viable example for integrating fully organic biofilm particles in existing continuous-flow systems.

Quantifying the electron-donating and -accepting capacities of wastewater for evaluating and optimizing biological wastewater treatment processes

Biological processes have been widely used for the treatment of both domestic and industrial wastewaters. In such biological processes, pollutants are converted into pollution-free substances by microorganisms through oxidation-reduction reactions. Thus, how to quantify the internal oxidation-reduction properties wastewaters and seek out targeted countermeasures is essential to understand, operate, and optimize biological wastewater treatment systems. So far, no such approach is available yet. In this work, a novel concept of electron neutralization-based evaluation is proposed to describe the internal oxidation-reduction properties of wastewater. Pollutants in wastewater are defined as electron donor substances (EDSs) or electron acceptor substances (EASs), which could give or accept electrons, respectively. With such an electron neutralization concept, several parameters, i.e., electron residual concentration (R), economy-related index (E and E_r), and economical evaluation index (Y and Y_r), are defined. Then, these parameters are used to evaluate the performance and economic aspects of currently applied wastewater treatment processes and even optimize systems. Three case studies demonstrate that the proposed concept could be effectively used to reduce wastewater treatment costs, assess energy recovery, and evaluate process performance. Therefore, a new, simple, and reliable methodology is established to describe the oxidation-reduction properties of wastewater and assess the biological wastewater treatment processes.

Evaluation of the Physiological Bacterial Groups in a Tropical Biosecured, Zero-Exchange System Growing Whiteleg Shrimp, Litopenaeus vannamei

To elucidate the individual and multiple roles of physiological bacterial groups involved in biogeochemical cycles of carbon, nitrogen, phosphorus and sulfur, the changes in the abundance of aerobic bacteria (heterotrophs, methane oxidizers, ammonia oxidizers, sulfur oxidizers, phosphate solubilizers, phosphate accumulators) and anaerobic bacteria (total anaerobes, nitrate reducers, denitrifiers and sulfate reducers) were investigated in a biosecured, zero-exchange system stocked with whiteleg shrimp, Litopenaeus vannamei for one production cycle. Key water quality parameters during the 96-day production cycle fell within the normal range for L. vannamei culture. Results of Spearman's correlation matrix revealed that different sets of variables correlated at varying levels of significance of the interrelationships between bacterial abundances and water quality parameters. The three nitrogenous species (ammonia, nitrite and nitrate) strongly influenced the physiological bacterial groups' abundance. The strong relationship of bacterial groups with phytoplankton biomass and abundance clearly showed the trophic interconnections in nutrient exchange/recycling. Canonical correspondence analysis performed to assess the total variation revealed that the three dissolved nitrogen species followed by salinity, temperature, phytoplankton biomass and pH collectively accounted for as much as 82% of the total variation. In conclusion, the results of the study revealed that the major drivers that interweaved biogeochemical cycles are the three dissolved nitrogen species, which microbially mediated various aerobic-anaerobic assimilation/dissimilation processes in the pond ecosystem. Considering the pond microbial ecology becoming an important management tool where applied research could improve the economic and environmental sustainability of the aquaculture industry, the findings of the present study are practically relevant.

Aerobic granular sludge: Impact of size distribution on nitrification capacity

The relationship between ammonia oxidation rate, nitrifiers population, and modelled aerobic zone volume in different granule sizes was investigated using aerobic granular sludge from a pilot-scale reactor. The pilot was fed with centrate and secondary effluent amended with acetate as the main carbon source. The maximum specific ammonia oxidation rates and community composition of different aerobic granular sludge size fractions were evaluated by batch tests, quantitative PCR, and genomic analysis. Small (331µm) granules had a 4.72 ± 0.09 times higher maximum specific ammonia oxidizing rate per 1 gVSS, and a 4.05 ± 0.17 times higher specific amoA gene copy number than large (2225µm) granules per 1 gram of wet biomass. However, when related to surface area, small granules had 1.43 ± 0.01 times lower maximum specific ammonia oxidation rate and a 1.66 ± 0.04 times lower specific amoA gene copy number per unit surface than large granules. Experimental results aligned with modeling results in which smaller granules had a higher specific aerobic zone volume to biomass and lower specific aerobic zone volume to surface area. Aerobic granular sludge reactors having the same average diameter of granules may have very different proportions of granule size fractions and hence possess different nitrification rates. Therefore, instead of the commonly reported average granule diameter, a new method was proposed to determine the aerobic volume density per sample, which correlated well with the nitrification rate. This work provides a roadmap to control nitrification capacity by two methods: (a) crushing larger granules into smaller fractions, or (b) increasing the mixed liquor suspended solid concentration to increase the total aerobic zone volume of the system.

Correlation of extracellular polymeric substances and microbial community structure in denitrification biofilm exposed to adverse conditions

Microbial community may respond to different adverse conditions and result in the variation of extracellular polymeric substances (EPS) in denitrification biofilm; this study discovered the role of EPS in accordance with the analysis of cyclic diguanylate (c-di-GMP) and electron equilibrium (EE) under low organic loading rate, shock organic loading rate and low temperature conditions. Good nitrate removal performance could be achieved under shock organic loading rate and low temperature conditions; however, owing to the low organic loading rate, the carbon source was preferentially utilized for biomass growth. Tightly bound EPS (TB-EPS) contents progressively increased and facilitated cell adhesion and biofilm formation. The stable TB protein (TB-PN) content in TB-EPS built a cross-linked network to maintain internal biofilm structure and led to the rapid biosynthesis of polysaccharides, which could further enhance microbial adhesion and improve nitrate removal. C-di-GMP played an important role in biomass retention and biofilm formation, based on the correlation analysis of c-di-GMP and EPS. TB polysaccharide (TB-PS) contents presented a significant positive correlation with c-di-GMP content, microbial adhesion and biofilm stabilization was further enhanced through c-di-GMP regulation. In addition, a remarkable negative correlation between electron deletion rate (EDR) and TB-PN and TB-PS was discovered, and TB-PS was required to serve as energy source to enhance denitrification according to EE analysis. Surprisingly, dynamic microbial community was observed due to the drastic community succession under low temperature conditions, and the discrepancy between the dominant species for denitrification was found under shock organic loading rate and low temperature conditions. The notable increase in bacterial strains Simlicispira, Pseudomonas and Chryseobacterium was conducive to biofilm formation and denitrification under shock organic loading rate, while Dechloromonas and Zoogloea dramatically enriched for nitrate removal under low temperature conditions. The high abundance of Dechloromonas improved the secretion of EPS through the downstream signal transduction, and the c-di-GMP conserved in Pseudomonas concurrently facilitated to enhance exopolysaccharide production to shock organic loading rate and low temperature conditions.

Enhanced aerobic granulation at low temperature by stepwise increasing of salinity

High salinity and low temperature are generally considered to have negative effects on the formation, stability and performance of aerobic granular sludge (AGS). This study investigated whether and how salinity acclimation strategies can enhance aerobic granulation at low temperature (12 °C) in three sequencing batch reactors (SBRs). Stepwise increased concentrations of NaCl (2-10 and 4-20 g/L) were added to the influent of R1 and R2 with steps of 1 and 2 g/L per week respectively, while R0 was set as a control (salt-free). The granulation processes in R1 and R2 were rapidly started up within 9 days, and were completed within 21 and 18 days, respectively. By contrast, R0 took 25 days and 49 days to start and complete granulation. The salinity acclimation strategies improved sludge hydrophobicity, reduced repulsion barrier between cells, and stimulated EPS production during granulation processes, which simultaneously promoted the formation of AGS. When the influent salinity reached 14 g/L on day 35, granule hydrophobicity, density and size in R2 sharply decreased and granules began to disintegrate afterwards. When operated under salt-free condition, sludge bulking occurred in R0 since day 60. The treatment performance was thus impaired in these two reactors, especially in R2 with significant biomass loss. Conversely, the AGS developed in R1 maintained stable structure with high biomass concentration (8.0 gSS/L) and excellent treatment performance for COD (90%), ammonium (95%) and total nitrogen (70%). Genera Thauera, Azoarcus, and Nitrosomonas were more enriched, while Flavobacterium and Meganema were more suppressed in R1, which would have contributed to granule stability and treatment performance. In conclusion, great care has to be taken for cultivating and operating AGS at low temperature for treating saline wastewater. Increasing salinity with a lower salt gradient provides a possibility for rapid granulation of AGS with excellent treatment performance under such conditions.

Candidates of quorum sensing bacteria in activated sludge associated with N-acyl homoserine lactones

This study aimed to explore candidates of microbial groups which is associated with quorum sensing in activated sludge. Activated sludge samples were collected from three wastewater treatment plants (WWTP) to analyze N-acyl homoserine lactone (AHL) by Fourier-transform mass spectrometry (FTMS) and 16S rRNA-based microbial community. Among activated sludge samples taken at 3 WWTPs in different seasons, 2 AHL species of N-3-hydroxyoctanoyl-l-homoserine lactone and N-3-hydroxydecanoyl-l-homoserine lactone were detected in the range of ranged of 0.1 ng/L to 1.6 ng/L. The detected AHL species were not dependent on treatment systems nor seasons. From microbial community analysis, population abundance of one strain in Verrucomicrobia and two strains in Holophagaceae had high correlation with AHL concentration in activated sludge. Comamonadaceae had also moderately correlated population with AHL concentrations among quorum sensing bacteria reported previously.

Effect of ferric hydroxide on membrane fouling in membrane bioreactor treating pharmaceutical wastewater

Membrane fouling is considered as a main drawback for MBR technology especially treating industrial wastewater. Therefore, this study aimed to investigate the effect of fouling in membrane bioreactor (MBR) treating pharmaceutical wastewater with the addition of ferric hydroxide. Two identical lab-scale MBRs, namely, a control MBR (Co-MBR) and an enhanced MBR dosed with ferric hydroxide (Fe-MBR), were operated in parallel. The results demonstrate membrane fouling was retarded by 35% with the addition of iron. Further exploration of membrane fouling mechanisms showed iron addition resulted in increase in biomass floc size, enhancement of bacteria activity and reduction of dissolved organic concentration, especially carbohydrate, biopolymer and low molecular weight compounds concentrations in mixed liquor. There was also lower abundance of bacterial associated with biofilm formation in the Fe-MBR compared with the Co-MBR. These findings collectively contributed to the positive impacts on membrane fouling mitigation.

Roles of Phosphorus Sources in Microbial Community Assembly for the Removal of Organic Matters and Ammonia in Activated Sludge

Various phosphorus sources are utilized by microbes in WWTPs, eventually affecting microbial assembly and functions. This study identified the effects of phosphorus source on microbial communities and functions in the activated sludge. By cultivation with 59 phosphorus sources, including inorganic phosphates (IP), nucleoside-monophosphates (NMP), cyclic-nucleoside-monophosphates (cNMP), and other organophosphates (OP), we evaluated the change in removal efficiencies of total organic carbon (TOC) and ammonia, microbial biomass, alkaline phosphatase (AKP) activity, microbial community structure, and AKP-associated genes. TOC and ammonia removal efficiency was highest in IP (64.8%) and cNMP (52.3%) treatments. Microbial community structure changed significantly across phosphorus sources that IP and cNMP encouraged Enterobacter and Aeromonas, respectively. The abundance of phoA and phoU genes was higher in IP treatments, whereas phoD and phoX genes dominated OP treatments. Our findings suggested that the performance of WWTPs was dependent on phosphorus sources and provided new insights into effective WWTP management.

Low Temperature and Neutral pH Define "Candidatus Nitrotoga sp." as a Competitive Nitrite Oxidizer in Coculture with Nitrospira defluvii

Nitrification is an essential process for N removal in activated sludge to avoid toxicity of ammonium and nitrite. Besides Nitrospira, "Candidatus Nitrotoga" has been identified as a key nitrite-oxidizing bacterium (NOB) performing the second step of nitrification, nitrite oxidation to nitrate, in wastewater treatment plants (WWTPs). However, the driving forces for the dominance of Nitrotoga in certain plants have often remained unclear and could not be explained solely by temperature effects. In this study, we characterized the physiology of the ammonium-dependent Nitrotoga sp. BS with regard to temperature and pH variations and evaluated its competitiveness against Nitrospira defluvii Both NOB originated from the same WWTP and shared a comparable pH optimum of 7.3. Based on these results, coculturing experiments with these NOB were performed in batch reactors operated at either 17°C or 22°C to compare their abundances under optimal (pH 7.4) or suboptimal (pH 6.4) conditions using 1 mM nitrite. As revealed by quantitative PCR (qPCR), fluorescence in situ hybridization (FISH), and 16S amplicon sequencing, Nitrotoga sp. BS was clearly favored by its optimal growth parameters and dominated over Ns. defluvii at pH 7.4 and 17°C, whereas a pH of 6.4 was more selective for Ns. defluvii Our synthetic communities revealed that niche differentiation of NOB is influenced by a complex interaction of environmental parameters and has to be evaluated for single species.IMPORTANCE "Ca. Nitrotoga" is a NOB of high environmental relevance, but physiological data exist for only a few representatives. Initially, it was detected in specialized niches of low temperature and low nitrite concentrations, but later on, its ubiquitous distribution revealed its critical role for N removal in engineered systems like WWTPs. In this study, we analyzed the competition between Nitrotoga and Nitrospira in bioreactors and identified conditions where the K strategist Ns. defluvii was almost replaced by Nitrotoga sp. BS. We show that the pH value is an important factor that regulates the composition of the nitrite-oxidizing enrichment with a dominance of Nitrotoga sp. BS versus Ns. defluvii at a neutral pH of 7.4 in combination with a temperature of 17°C. The physiological diversity of novel Nitrotoga cultures improves our knowledge about niche differentiation of NOB with regard to functional nitrification under suboptimal conditions.

A strategy of high-efficient nitrogen removal by an ammonia-oxidizing bacterium consortium

An ammonia-oxidizing bacterium consortium showed approximately 100% removal of NH₄⁺-N with an initial concentration of 262.28 ± 8.21 mg·L^-1 within 10 days, and only 16.54 ± 0.52% of NH₄⁺-N was converted to NO₂^--N in this study. The consortium removed ammonium by heterotrophic nitrification and aerobic denitrification (HNAD) without N₂O emission. The activity of AOB was not affected by low concentrations of FA or FNA, but completely inhibited by 0.04 mg HNO₂·L^-1. In a bioaugmentation treatment of eutrophic wastewater using the consortium, the removal efficiency of NH₄⁺-N reached 90.85 ± 0.8% and 77.88 ± 1.86% at initial concentrations of 1.80 ± 0.04 mg·L^-1 and 40.31 ± 0.57 mg·L^-1, respectively, and the dissolved oxygen level had a significant impact on the consortium activity. No significant changes in the bacterial community structure were observed after the consortium addition, and local functional bacteria were enriched by aeration and contributed to ammonium nitrogen removal with AOB.

Understanding of aerobic sludge granulation enhanced by sludge retention time in the aspect of quorum sensing

Aerobic granular sludge (AGS) reactors with different sludge retention times (SRTs) were established for enhanced functional microorganism enrichment and granular formation. Results showed that higher total nitrogen (TN) removal efficiency and compact granules were achieved in the 6-day-SRT reactor. Also, Xanthomonadaceae, Rhodobacteraceae and Hyphomonadaceae with AHL-producing and EPS-secreting functions also enriched under 6-day SRT. For investigating the enhanced mechanism of sludge granulation, typical quorum sensing signals of acylated-homoserine-lactones (AHLs) and extracellular polymeric substances (EPS) were analyzed. Tryptophan-and-protein-like substances were major EPS components in granules formed at 6-day SRT. Meanwhile, most detected AHLs, i.e. C8-HSL and 3OHC8-HSL, were correlated positively with contents of tryptophan-and-protein-like substances. According to AHLs add-back test, AHLs especially those with 8-carbon sidechains, played important roles in aerobic sludge granulation via secreting special extracellular proteins by functional microbes enrichment.

Hydrodynamic shear force shaped the microbial community and function in the aerobic granular sequencing batch reactors for low carbon to nitrogen (C/N) municipal wastewater treatment

The lab-scale aerobic granules process was applied for low carbon to nitrogen (C/N < 4) wastewater treatment under different hydrodynamic shear forces. Results revealed that aerobic granules exhibited strong adaptability and stability. The aerobic granules might adopt an extracellular polymeric substances (EPS) regulating mechanism to address the changes in operational conditions, especially through growing secretion of fluorescence protein. The hydrodynamic shear force determinedly shaped and regulated the diversity and structure of dominant microbial community, briefly, reduced aeration intensity with increased time led to higher microbial richness, lower diversity and evenness, and shifts of predominant microorganisms. Phylogenetic classification of the key functional groups including bacteria related to carbon and nutrients removal, EPS production and quorum sensing (QS) presented much more differences among the reactors subject to different conditions. Therefore, the present work adds insight into the comprehensive understanding of the effect of aeration induced hydrodynamic shear force on aerobic granules.

Bioaugmented soil aquifer treatment for P-nitrophenol removal in wastewater unique for cold regions

P-nitrophenol (PNP) is a toxic and recalcitrant organic pollutant and a usual intermediate in the production of fine chemicals, which has posed a significant threat to subsurface environment safety. Soil aquifer treatment (SAT) is a promising method to remove and remediate contamination in vadose zone with low cost and high efficiency. However, there are still research gaps for the treatment of recalcitrant contaminants by SAT in cold regions, such as un-robust indigenous microbes and low temperature constraint in vadose zone. The bioaugmentation technology was first introduced into SAT in order to enhance the removal ability of PNP by SAT operated in cold regions in this study. A high-efficiency PNP-degrading bacterium was successfully isolated, which can efficiently degrade PNP below 200 mg L^-1 with a degradation rate above 99% at 15 °C close to the real subsurface temperature in cold regions, and added into SAT for bioaugmentation. The feasibility of bioaugmented SAT and associated PNP removal process were investigated by laboratory sand columns, along with effects of the SAT operative parameters (namely PNP loading concentration, flow rate and soil saturation level of SAT). Within the range of PNP loading stresses tested (1-200 mg L^-1), PNP removal efficiency was optimal at constant flow rate of 219 mL d^-1 in unsaturated operating condition of SAT under 15 °C among all the investigated experimental conditions. Longer hydraulic residence time increased the PNP removal rate, although the accumulated mass removed reduced and the removal efficiencies remained constant in unsaturated operating condition of SAT. It is found from the comparison between the PNP removals via both unsaturated and saturated columns that slight difference only in the removal rate of PNP was observed and the highly efficient bioaugmented SAT can completely degrade PNP of 10 mg L^-1 within 5 wetting/drying cycles under both scenarios.

Cultivation and stable operation of aerobic granular sludge at low temperature by sieving out the batt-like sludge

Aerobic granules were successfully cultivated at 10 °C with relatively low strength substrate. Stable granules coexisted with the batt-like sludge (BLS) were obtained in 60 days. After removing the BLS, nutrient removal performance was greatly improved and stable removal efficiencies of 99% phosphorous, 98% ammonia and 60% TN were achieved. The bacterial community structure revealed that it was an unclassified-Comamonadaceae genus dominant in the BLS, which represented for low relative abundance in mature granules. Overgrowth of unclassified-Comamonadaceae genus was considered to be the key factor for inhibiting the performance of granules. The final configuration of granules was dominated by DPAO genus Flavobacterium and polysaccharide nutritional genus Chryseolinea. This study showed that stable aerobic granules with superior performance under low temperature could be successfully cultivated by sieving out the BLS.

Approaches and processes for ammonia removal from side-streams of municipal effluent treatment plants

The main objective of this review article is to provide a comprehensive view on various conventional and emerging side-stream ammonia removal treatment options for municipal wastewater treatment plants (WWTPs). Optimization of wastewater treatment facilities from an energy and emissions stand-point necessitates consideration of the impact of the various internal side-streams. Side-streams from anaerobic sludge digesters in particular have the potential to be a significant ammonium load to the mainstream treatment process. However, the literature suggests that managing side-streams through their treatment in the mainstream process is not the most energy efficient approach, nor does it allow for practical recovery of nutrients. Furthermore, as effluent criteria become more stringent in some jurisdictions and sludge hydrolysis pre-treatment for digesters more common, an understanding of treatment options for ammonia in digester supernatant becomes more important. Given these considerations, a variety of side-stream treatment processes described in the literature are reviewed.