rRNA Gene Expression of Abundant and Rare Activated-Sludge Microorganisms and Growth Rate Induced Micropollutant Removal

Particle size transfer of antibiotic resistance genes in typical processes of municipal wastewater treatment plant

Occurrence and transfer of antibiotic resistance genes (ARGs) was investigated concerning sludge particle size in a typical wastewater treatment plant, and the roles of vertical (VGT) and horizontal gene transfer (HGT) in the spread of ARGs were explored. Results showed that although membrane bioreactor (MBR) effectively reduced the relative abundance of ARGs in the water phase, it concurrently enriched ARGs in MBR sludge, particularly for the largest-size particles (>150 μm). A decreasing trend in the relative abundance of ARGs was observed along with the decrease of sludge particle size, and larger-size particle sludge (>106 μm) formed a relatively stable composition of ARGs, while ARGs on smaller-size particle sludge (6.5-106 μm) fluctuate rapidly. Particle size does not affect the abundance distribution patterns or assembly mechanisms of ARGs as deterministic processes. The smallest-size particles were the primary attachment site for bacterial pathogens with highest diversity. Larger-size particle sludge (>106 μm) showed higher frequency of HGT, with Proteobacteria as the dominant hosts for this process.

Read-Across of Biotransformation Potential between Activated Sludge and the Terrestrial Environment: Toward Making It Practical and Plausible

Recent emphasis on the development of safe-and-sustainable-by-design chemicals highlights the need for methods facilitating the early assessment of persistence. Activated sludge experiments have been proposed as a time- and resource-efficient way to predict half-lives in simulation studies. Here, this persistence "read-across" approach was developed to be more broadly and robustly applicable. We evaluated 21 previously used reference plant protection products (PPPs) for their broader applicability in calibrating regression and classification models for predicting half-lives in soil (DT50OECD307) and water-sediment systems (DT50OECD308) based on their half-life in sludge and the organic carbon-water partition coefficient KOC as predictors. The calibrated regression models showed satisfactory predictions of DT50OECD307 for another 22 test PPPs. Performance was less satisfying for the prediction of DT50OECD308 for 46 active pharmaceutical ingredients (APIs), suggesting a need for expanding the set of calibration substances and more experimental KOC values. The classification models mostly correctly classified persistent and non-persistent test compounds for both PPPs and APIs, which is relevant for early-stage screening of persistence. Transformation products of the reference compounds in activated sludge samples were consistent with the reported degradation pathways in soil, particularly with respect to major aerobic, enzyme-catalyzed transformation reactions. Overall, "reading across" biotransformation in environmental compartments such as soils or sediments from experiments with activated sludge outperformed three widely used in silico approaches for estimating half-lives and hence has immediate potential to support early assessment of biodegradability when aiming to develop chemicals that are safe and sustainable by design.

Phosphorus recovery and tetracycline Mitigation: The role of Bacillus cereus LB-9 in struvite biomineralization from wastewater

Struvite biomineralization is an ecologically sound technology, adept at the efficient recovery and recycling of phosphorus from wastewater. However, the biomineralization process is often perturbed by the presence of antibiotics, notably tetracycline (TC), the impact of which on the biomineralization system has not been elucidated. This study examines the efficacy of Bacillus cereus LB-9 in struvite biomineralization, focusing on the precipitates' composition, morphology, and TC content. LB-9 facilitate an alkaline environment that effectively recovering nitrogen and phosphorus. These findings indicate that TC retards the initial formation of struvite and the concurrent recovery of nitrogen and phosphorus. However, at concentrations below 10 mg/L TC concentrations, TC enhanced struvite production (0.38g) by stimulating LB-9's growth and metabolic activity. Conversely, at a concentration of 10 mg/L TC, the strain's activity was markedly suppressed within the initial four days. This data suggests that TC promotes the strain's proliferation and metabolism, potentially through cellular secretions, thereby augmenting phosphorus recovery from wastewater. Notably, the recovered struvite doesn't contain TC, aligning with regulatory standards for agricultural application. In summary, LB-9-mediated struvite recovery is an effective strategy for producing phosphorus-enriched fertilizers and mitigating TC contamination, offering significant implications for wastewater treatment and industrial process development, particularly in the context of prevalent TC in wastewater.

Biological degradation of organic micropollutants in GAC filters-temporal development and spatial variations

The capacity for organic micropollutant removal in granular activated carbon (GAC) filters for wastewater treatment changes over time. These changes are in general attributed to changes in adsorption, but may in some cases also be affected by biological degradation. Knowledge on the degradation of organic micropollutants, however, is scarce. In this work, the degradation of micropollutants in several full-scale GAC and sand filters was investigated through incubation experiments over a period of three years, using 14C-labeled organic micropollutants with different susceptibilities to biological degradation (ibuprofen, diclofenac, and carbamazepine), with parallel 16S rRNA gene sequencing. The results showed that the degradation of diclofenac and ibuprofen in GAC filters increased with increasing numbers of bed volumes when free oxygen was available in the filter, while variations over filter depth were limited. Despite relatively large differences in bacterial composition between filters, a degradation of diclofenac was consistently observed for the GAC filters that had been operated with high influent oxygen concentration (DO >8 mg/L). The results of this comprehensive experimental work provide an increased understanding of the interactions between microbial composition, filter material, and oxygen availability in the biological degradation of organic micropollutants in GAC filters.

Reducing the accumulation of cadmium and phenanthrene in rice by optimizing planting spacing: Role of low-abundance but core rhizobacterial communities

Optimizing planting spacing is a common agricultural practice for enhancing rice growth. However, its effect on the accumulation of cadmium (Cd) and phenanthrene (Phen) in soil-rice systems and the response mechanisms of rhizobacteria to co-contaminants remain unclear. This study found that reducing rice planting spacing to 5 cm and 10 cm significantly decreased the bioavailability of Cd (by 7.9 %-29.5 %) and Phen (by 12.9 %-47.6 %) in the rhizosphere soil by converting them into insoluble forms. The increased accumulation of Cd and Phen in roots and iron plaques (IPs) ultimately led to decreased Cd (by 32.2 %-39.9 %) and Phen (by 4.2 %-17.3 %) levels in brown rice, and also significantly affected the composition of rhizobacteria. Specifically, reducing rice planting spacing increased the abundance of low-abundance but core rhizobacteria in the rhizosphere soil and IPs, including Bacillus, Clostridium, Sphingomonas, Paenibacillus, and Leifsonia. These low-abundance but core rhizobacteria exhibited enhanced metabolic capacities for Cd and Phen, accompanied by increased abundances of Cd-resistance genes (e.g., czcC and czcB) and Phen-degradation genes (e.g., pahE4 and pahE1) within the rhizosphere soil and IPs. Reduced planting spacing had no noticeable impact on rice biomass. These findings provide new insights into the role of low-abundance but core rhizobacterial communities in Cd and Phen uptake by rice, highlighting the potential of reduced planting spacing as an eco-friendly strategy for ensuring the safety of rice production on contaminated paddy soils.

Removal of tetracycline in nitrification membrane bioreactors with different ammonia loading rates: Performance, metabolic pathway, and key contributors

Membrane bioreactors (MBRs) have been widely applied for the treatment of wastewater that contains high concentrations of both ammonium and antibiotics. Nonetheless, information about tetracycline (TC) removal in nitrification MBRs with high ammonium loading rates (ALRs) is still very limited. Herein, the fate of TC at four different concentrations of 1, 5, 20, and 50 mg/L in three parallel lab-scale nitrification MBRs with different ALRs (named AN50, AN500, and AN1000) were investigated in this study. Excellent nitrification performance and high TC removal efficiency (90.46%) were achieved in AN1000 at influent TC concentration of 50 mg/L. Higher ALRs promoted the removal of TC at lower influent TC concentration (≤5 mg/L), while no significant difference was observed in TC removal efficiencies among different ALRs MBRs at higher influent TC concentration (≥20 mg/L), implying that the heterotrophic degradation could be strengthened after long-term exposure to high concentration of TC. Batch tests demonstrated that adsorption and biodegradation were the primary TC removal routes by nitrification sludge, of which both autotrophic ammonia oxidizers and heterotrophic microorganisms played an important role in the biodegradation of TC. FT-IR spectroscopy confirmed that amide groups on the sludge biomass contributed to the adsorption of TC. Mass balance analyses indicated that biodegradation (63.4-88.6% for AN50, 74.5-88.4% for AN500 and 74.4-91.4% for AN1000) was the major mechanism responsible for the removal of TC in nitrification MBRs, and its contribution increased with influent TC concentration, while only 1.1%-15.0% of TC removal was due to biosorption. TC was progressively degraded to small molecules and the presence of TC had no notable effect on membrane permeability. These jointly confirmed TC could be effectively removed via initial adsorption and subsequent biodegradation, while biodegradation was the primary mechanism in this study.

Predicting and improving the microbial removal of organic micropollutants during wastewater treatment: A review

Organic micropollutants (OMPs) consist of widely used chemicals such as pharmaceuticals and pesticides that can persist in surface and groundwaters at low concentrations (ng/L to μg/L) for a long time. The presence of OMPs in water can disrupt aquatic ecosystems and threaten the quality of drinking water sources. Wastewater treatment plants (WWTPs) rely on microorganisms to remove major nutrients from water, but their effectiveness at removing OMPs varies. Low removal efficiency might be the result of low concentrations, inherent stable chemical structures of OMPs, or suboptimal conditions in WWTPs. In this review, we discuss these factors, with special emphasis on the ongoing adaptation of microorganisms to degrade OMPs. Finally, recommendations are drawn to improve the prediction of OMP removal in WWTPs and to optimize the design of new microbial treatment strategies. OMP removal seems to be concentration-, compound-, and process-dependent, which poses a great complexity to develop accurate prediction models and effective microbial processes targeting all OMPs.

Removal of organic micropollutants from municipal wastewater by aerobic granular sludge and conventional activated sludge

Removal performances of organic micropollutants by conventional activated sludge (CAS) and aerobic granular sludge (AGS) were investigated at a full-scale wastewater treatment plant. Lab-scale kinetic experiments were performed to assess the micropollutant transformation rates under oxic and anoxic conditions. Transformation rates were used to model the micropollutant removal in the full-scale processes. Metagenomic sequencing was used to compare the microbial communities and antimicrobial resistance genes of the CAS and AGS systems. Higher transformation ability was observed for CAS compared to AGS for most compounds, both at the full-scale plant and in the complementary batch experiments. Oxic conditions supported the transformation of several micropollutants with faster and/or comparable rates compared to anoxic conditions. The estimated transformation rates from batch experiments adequately predicted the removal for most micropollutants in the full-scale processes. While the compositions in microbial communities differed between AGS and CAS, the full-scale biological reactors shared similar resistome profiles. Even though granular biomass showed lower potential for micropollutant transformation, AGS systems had somewhat higher gene cluster diversity compared to CAS, which could be related to a higher functional diversity. Micropollutant exposure to biomass or mass transfer limitations, therefore played more important roles in the observed differences in OMP removal.

Effects of Turning Frequency on Ammonia Emission during the Composting of Chicken Manure and Soybean Straw

Here, we investigated the impact of different turning frequency (TF) on dynamic changes of N fractions, NH3 emission and bacterial/archaeal community during chicken manure composting. Compared to higher TF (i.e., turning every 1 or 3 days in CMS1 or CMS3 treatments, respectively), lower TF (i.e., turning every 5 or 7 days in CMS5 or CMS7 treatments, respectively) decreased NH3 emission by 11.42-18.95%. Compared with CMS1, CMS3 and CMS7 treatments, the total nitrogen loss of CMS5 decreased by 38.03%, 17.06% and 24.76%, respectively. Ammonia oxidizing bacterial/archaeal (AOB/AOA) communities analysis revealed that the relative abundance of Nitrosospira and Nitrososphaera was higher in lower TF treatment during the thermophilic and cooling stages, which could contribute to the reduction of NH3 emission. Thus, different TF had a great influence on NH3 emission and microbial community during composting. It is practically feasible to increase the abundance of AOB/AOA through adjusting TF and reduce NH3 emission the loss of nitrogen during chicken manure composting.

Enzymatic cometabolic biotransformation of organic micropollutants in wastewater treatment plants: A review

Biotransformation of trace-level organic micropollutants (OMPs) by complex microbial communities in wastewater treatment facilities is a key process for their detoxification and environmental impact reduction. Therefore, understanding the metabolic activities and mechanisms that contribute to their biotransformation is essential when developing approaches aiming to minimize their discharge. This review addresses the relevance of cometabolic processes and discusses the main enzymatic activities currently known to take part in OMPs removal under different redox environments in the compartments of wastewater treatment plants. Furthermore, the most common methodologies to decipher such enzymes are discussed, including the use of in vitro enzyme assays, enzymatic inhibitors, the analysis of transformation products and the application of several -omic techniques. Finally, perspectives on major challenges and future research requirements to improve OMPs biotransformation are proposed.

A review of antibiotic removal from domestic wastewater using the activated sludge process: removal routes, kinetics and operational parameters

Many advanced technologies have shown encouraging results in removing antibiotics from domestic wastewater. However, as activated sludge treatment is the most common sewage treatment system employed worldwide, improving its effect on antibiotic removal would be more desirable. Understanding the removal mechanisms, kinetics and factors that affect antibiotic removal in the activated sludge process is important as it would allow us to improve the treatment performance. Although these have been discussed in various literature covering different types of antibiotics and wastewater, a specific review on antibiotics and domestic wastewater is clearly missing. This review paper collates, discusses and analyses the removal of antibiotics from sewage in the activated sludge process along with the removal mechanisms and kinetics. The antibiotics are categorised into six classes: β-lactam, dihydrofolate reductase inhibitor, fluoroquinolone, macrolide, sulfonamides and tetracycline. Furthermore, the factors affecting the system performance with regard to antibiotic removal are examined.

Methodological Advances to Study Contaminant Biotransformation: New Prospects for Understanding and Reducing Environmental Persistence?

Complex microbial communities in environmental systems play a key role in the detoxification of chemical contaminants by transforming them into less active metabolites or by complete mineralization. Biotransformation, i.e., transformation by microbes, is well understood for a number of priority pollutants, but a similar level of understanding is lacking for many emerging contaminants encountered at low concentrations and in complex mixtures across natural and engineered systems. Any advanced approaches aiming to reduce environmental exposure to such contaminants (e.g., novel engineered biological water treatment systems, design of readily degradable chemicals, or improved regulatory assessment strategies to determine contaminant persistence a priori) will depend on understanding the causal links among contaminant removal, the key driving agents of biotransformation at low concentrations (i.e., relevant microbes and their metabolic activities), and how their presence and activity depend on environmental conditions. In this Perspective, we present the current understanding and recent methodological advances that can help to identify such links, even in complex environmental microbiomes and for contaminants present at low concentrations in complex chemical mixtures. We discuss the ensuing insights into contaminant biotransformation across varying environments and conditions and ask how much closer we have come to designing improved approaches to reducing environmental exposure to contaminants.

Effect of cathode material and charge loading on the nitrification performance and bacterial community in leachate treating Electro-MBRs

Electro-MBR technology, which combines an electrocoagulation process inside the mixed liquor of a membrane bioreactor, was studied for the treatment of a high-strength ammonia leachate (124 ± 4 mg NH₄-N L^-1). A lab-scale aerobic Electro-MBR was operated with a solid retention time of 45 days, hydraulic retention times of 24h and 12h, and charge loading ranging from 100 to 400 mAh L^-1. At 400 mAh L^-1, with a combination of a Ti/Pt cathode and a sacrificial iron anode, removal percentages for ammonia nitrogen, total organic carbon, and total phosphorus were 99.8%, 38%, and 99.0%, respectively. At 400 mAh L^-1, the estimated ferric ion dosage was 325 mg Fe³⁺ L^-1. Experiments conducted with different cathode materials showed that previously reported inhibition phenomena may result from a cathodic nitrate reduction into ammonia nitrogen. Conventional cathode materials, such as graphite, have electrochemical nitrate reduction rates of -0.03 mg NO₃-N mAh^-1. By comparison, when using Ti/Pt, the rate was -0.0045 mg NO₃-N mAh^-1(85% lower than graphite due to its low hydrogen overpotential). Charge loading tested in this study had no significant impact on both nitrification performance and microbial population diversity. However, the relative abundance of the mixed liquor's Nitrosomonas increased from 4.8% to 8.2% when the charge loading increased from 0 to 400 mAh L^-1. Results from this study are promising for future applications of the Ti/Pt - Iron Electro-MBR in various high-strength ammonia wastewater treatment applications.

Sludge retention time affects the microbial community structure: A large-scale sampling of aeration tanks throughout China

Microbial communities in activated sludge (AS) have a significant influence on the functions and stability of aeration tanks in wastewater treatment plants (WWTPs). The microbial community structure is affected by various factors, among which operational parameters outcompeted as the key factors in shaping its structure. However, as an important operational parameter of aeration tank, the mechanisms by which sludge retention time (SRT) affect community properties remain unclear. In this study, 144 AS samples from aeration tanks of 48 full-scale WWTPs operating under different SRT conditions were examined via high-throughput Illumina-MiSeq sequencing technology. The results indicated that SRT significantly affected the diversity, composition, assembly, and co-occurrence patterns of the microbial community in aeration tanks. Moreover, our results provided clear evidence that the microbial communities in aeration tanks operating under SRT of 10-20 days have the highest biodiversity, the lowest stochastic processes influence, the more stable molecular ecological network structure, the lowest risks of filamentous sludge bulking and enhanced nitrogen removal potential. The microbial communities could be more stable and resilient to disturbance when aeration tanks were operated under this SRT condition. The findings of this study provided a reference for the optimization of aeration tanks from an of microbial community perspective.

MBR treatment of leachates originating from waste management facilities: A reference study of the design parameters for efficient treatment

The main objective of the study was to define the interaction between the solid retention time (SRT) and the contaminant loading rate on a membrane bioreactor's efficacy in removing contaminants frequently detected (chemical oxygen demand (COD), NH₄⁺, total phosphorus and metals) above the discharge criteria in waste-originating leachates. The rates and coefficient calculated from this study's experimental data can be used for the design of membrane bioreactor treating wastewaters, even beyond the scope of this experiment. Over a period of 152 days, SRTs of 28 and 47 days and HRTs of 13, 25, 36 and 52 h were studied using a real leachate with a constant composition. Results showed that membrane bioreactors can efficiently treat >1850 mg COD L^-1 d^-1 of highly to moderately biodegradable COD, with the SRT having no significant impact on the removal of recalcitrant COD. Overall ammonium removal rates of >740 mg NH₄-N L^-1 d^-1 can be achieved as long as a residual alkalinity of 200 mg CaCO₃ L-1 and an adequate dissolved oxygen concentration (6-7 mg L^-1) are both maintained. Overall phosphorus removal rates are independent of the phosphorus loading rate. However, the highest overall phosphorus removal rate (39 ± 2 mg P per g of total suspended solids) was obtained at the lowest SRT (28 days) due to an increased extracellular polymeric substance production. Finally, membrane bioreactor's metal removal capacity is mostly dependent on the metals' affinity to both the leachate's recalcitrant COD as well as sludge concentrations.

Identifying the link between MBRs' key operating parameters and bacterial community: A step towards optimized leachate treatment

A MBR treating compost leachate was studied in order to link the operating parameters (solid and hydraulic retention time) to contaminant's specific bacterial catabolic activity. In this context, a lab-scale aerobic membrane bioreactor was operated for 200 days, at solid retention times (SRT) of 30 and 45 days and four different contaminant load rates. Results showed that increasing the food to microorganism ratio (F/M) by increasing the contaminant load rates lessened the selectivity pressure, which allowed the proliferation of subdominant operational taxonomic units (OTU) (relative abundance >3%) that were otherwise inhibited by highly adapted dominant OTUs (relative abundance >10%). Subsequently, increasing the SRT resulted in a lower species richness and the selection of two dominant types of bacteria: 1) genera with low growth rates that feed on non-limiting substrates or substrates with few competitors, and 2) genera with metabolisms that are highly specific to the available substrates and that can outcompete the other genera by using the substrate more efficiently. The bacterial population evolution observed during this study suggests that the mixed liquor population diversity and structure can be modulated with the operating conditions for the bioenhancement of contaminant specific catabolic activity. Identified dominant and subdominant genera were linked to the MBR's NH₄⁺ and COD removal performances. Interestingly, nitrification performances were unaffected by the organic load rate and the Nitrosomonas relative abundance.

Distribution and Removal of Pharmaceuticals in Liquid and Solid Phases in the Unit Processes of Sewage Treatment Plants

In this study, we analyzed 27 pharmaceuticals in liquid and solid phase samples collected from the unit processes of four different sewage treatment plants (STPs) to evaluate their distribution and behavior of the pharmaceuticals. The examination of the relative distributions of various categories of pharmaceuticals in the influent showed that non-steroidal anti-inflammatory drugs (NSAIDs) were the most dominant. While the relative distribution of antibiotics in the influent was not high (i.e., 3%–5%), it increased to 14%–30% in the effluent. In the four STPs, the mass load of the target pharmaceuticals was reduced by 88%–95% mainly in the biological treatment process, whereas the ratio of pharmaceuticals in waste sludge to those in the influent (w/w) was only 2%. In all the STPs, the removal efficiencies for the stimulant caffeine, NSAIDs (acetaminophen, naproxen, and acetylsalicylic acid), and the antibiotic cefradine were high; they were removed mainly by biological processes. Certain compounds, such as the NSAID ketoprofen, contrast agent iopromide, lipid regulator gemfibrozil, and antibiotic sulfamethoxazole, showed varying removal efficiencies depending on the contribution of biodegradation and sludge sorption. In addition, a quantitative meta-analysis was performed to compare the pharmaceutical removal efficiencies of the biological treatment processes in the four STPs, which were a membrane bioreactor (MBR) process, sequencing batch reactor (SBR) process, anaerobic–anoxic–oxic (A2O) process, and moving-bed biofilm reactor (MBBR) process. Among the biological processes, the removal efficiency was in the order of MBR > SBR > A2O > MBBR. Among the tertiary treatment processes investigated, powdered activated carbon showed the highest removal efficiency of 18%–63% for gemfibrozil, ibuprofen, ketoprofen, atenolol, cimetidine, and trimethoprim.

Nexus of Stochastic and Deterministic Processes on Microbial Community Assembly in Biological Systems

Microbial community assembly in engineered biological systems is often simultaneously influenced by stochastic and deterministic processes, and the nexus of these two mechanisms remains to be further investigated. Here, three lab-scale activated sludge reactors were seeded with identical inoculum and operated in parallel under eight different sludge retention time (SRT) by sequentially reducing the SRT from 15 days to 1 day. Using 16S rRNA gene amplicon sequencing data, the microbial populations at the start-up (15-day SRT) and SRT-driven (≤10-day SRT) phases were observed to be noticeably different. Clustering results demonstrated ecological succession at the start-up phase with no consistent successional steps among the three reactors, suggesting that stochastic processes played an important role in the community assembly during primary succession. At the SRT-driven phase, the three reactors shared 31 core operational taxonomic units (OTUs). Putative primary acetate utilizers and secondary metabolizers were proposed based on K-means clustering, network and synchrony analysis. The shared core populations accounted for 65% of the total abundance, indicating that the microbial communities at the SRT-driven phase were shaped predominantly by deterministic processes. Sloan’s Neutral model and a null model analysis were performed to disentangle and quantify the relative influence of stochastic and deterministic processes on community assembly. The increased estimated migration rate in the neutral community model and the higher percentage of stochasticity in the null model implied that stochastic community assembly was intensified by strong deterministic factors. This was confirmed by the significantly different α- and β-diversity indices at SRTs shorter than 2 days and the observation that over half of the core OTUs were unshared or unsynchronized. Overall, this study provided quantitative insights into the nexus of stochastic and deterministic processes on microbial community assembly in a biological process.

A quantitative structure-biodegradation relationship (QSBR) approach to predict biodegradation rates of aromatic chemicals

The objective of this work was to develop a QSBR model for the prioritization of organic pollutants based on biodegradation rates from a database containing globally harmonized biodegradation tests using relevant molecular descriptors. To do this, we first categorized the chemicals into three groups (Group 1: simple aromatic chemicals with a single ring, Group 2: aromatic chemicals with multiple rings and Group3: Group 1 plus Group 2) based on molecular descriptors, estimated the first order biodegradation rate of the chemicals using rating values derived from the BIOWIN3 model, and finally developed, validated and defined the applicability domain of models for each group using a multiple linear regression approach. All the developed QSBR models complied with OECD principles for QSAR validation. The biodegradation rate in the models for the two groups (Group 2 and 3 chemicals) are associated with abstract molecular descriptors that provide little relevant practical information towards understanding the relationship between chemical structure and biodegradation rates. However, molecular descriptors associated with the QSBR model for Group 1 chemicals (R² = 0.89, Q²_loo = 0.87) provided information on properties that can readily be scrutinised and interpreted in relation to biodegradation processes. In combination, these results lead to the conclusion that QSBRs can be an alternative tool to estimate the persistence of chemicals, some of which can provide further insights into those factors affecting biodegradation.

Impacts of solids retention time and antibiotic loading in activated sludge systems on secondary effluent water quality and microbial community structure

Solids retention time (SRT) is one of the most important factors in designing and operating activated sludge systems for biological wastewater treatment. Longer SRTs have been shown to alter the structure and function of microbial communities, thereby leading to improved treatment efficacy with respect to bulk and trace organics, nutrient removal, and membrane fouling. Research has also shown that longer SRTs and/or higher influent antibiotic concentrations may lead to increased prevalence of antibiotic resistance. However, it is unclear whether elevated, yet subclinical, concentrations of antibiotics also impact the overall microbial community. The purpose of this study was to characterize changes in microbial community structure in a laboratory-scale activated sludge system as a function of SRT (2-20 days) and influent concentrations (1×-100× ambient) of ampicillin, sulfamethoxazole, tetracycline, trimethoprim, and vancomycin. Changes in microbial community structure were evaluated based on 16S rRNA gene sequencing, and microbial community function was evaluated based on changes in effluent water quality, including attenuation of bulk and trace organics. The results confirmed that longer SRTs-but not antibiotic loadings-had a significant impact on microbial community structure and effluent water quality. Therefore, moderate spikes in influent antibiotic concentrations are not expected to adversely impact biological wastewater treatment. PRACTITIONER POINTS: Longer SRTs lead to changes in microbial community structure, including alpha and beta diversity and relative abundance of various taxa. Enhanced TOrC attenuation at longer SRTs may be linked to biomass abundance rather than changes in microbial community structure. Moderate spikes in influent antibiotic concentrations do not impact activated sludge performance or microbial community structure. The phyla Proteobacteria and Bacteroidetes comprise a majority of the microbial community in primary effluent and mixed liquor.

Biodegradation aspects of ibuprofen and identification of ibuprofen-degrading microbiota in an immobilized cell bioreactor

An enrichment process was employed by applying high ibuprofen concentration in an immobilized cell bioreactor in order to favor the ibuprofen-degrading community present in activated sludge. Experimental data showed the ability of the immobilized cell bioreactor to achieve high ibuprofen removal efficiencies (98.4 ± 0.3%), the tendency of the enriched biomass to acidify the treated liquor, and the inhibition of the nitrification process. Illumina sequencing revealed a massive increase in the relative abundance of Alphaproteobacteria and Gammaproteobacteria (from 29.1 to 80.8%) and a dramatic decrease in the proportion of Bacteroidetes, Planctomycetes, and Verrucomicrobia (from 42.7 to 2.1%) when pure ibuprofen served as the sole carbonaceous feeding substrate. This shift in the feeding conditions resulted in the predominance of Novosphingobium and Rhodanobacter (25.5 ± 10.8% and 25.2 ± 3.0%, respectively) and demonstrated a specialized ibuprofen-degrading bacterial community in activated sludge, which possessed the selective advantage to cope with its degradation. To the best of our knowledge, this bioreactor system was capable of effectively treating the highest ibuprofen concentration applied in wastewater treatment plants.

Microbial residence time is a controlling parameter of the taxonomic composition and functional profile of microbial communities

A remaining challenge within microbial ecology is to understand the determinants of richness and diversity observed in environmental microbial communities. In a range of systems, including activated sludge bioreactors, the microbial residence time (MRT) has been previously shown to shape the microbial community composition. However, the physiological and ecological mechanisms driving this influence have remained unclear. Here, this relationship is explored by analyzing an activated sludge system fed with municipal wastewater. Using a model designed in this study based on Monod-growth kinetics, longer MRTs were shown to increase the range of growth parameters that enable persistence, resulting in increased richness and diversity in the modeled community. In laboratory experiments, six sequencing batch reactors treating domestic wastewater were operated in parallel at MRTs between 1 and 15 days. The communities were characterized using both 16S ribosomal RNA and non-target messenger RNA sequencing (metatranscriptomic analysis), and model-predicted monotonic increases in richness were confirmed in both profiles. Accordingly, taxonomic Shannon diversity also increased with MRT. In contrast, the diversity in enzyme class annotations resulting from the metatranscriptomic analysis displayed a non-monotonic trend over the MRT gradient. Disproportionately high abundances of transcripts encoding for rarer enzymes occur at longer MRTs and lead to the disconnect between taxonomic and functional diversity profiles.

Rare Taxa Exhibit Disproportionate Cell-Level Metabolic Activity in Enriched Anaerobic Digestion Microbial Communities

Microbial communities are composed of populations with vastly different abundances and levels of metabolic and replicative activity, ranging from actively metabolizing and dividing to dormant or nonviable. The 16S rRNA/rDNA ratio is an emerging tool for evaluating cell-level metabolic activity independent of abundance. In this study, we used five long-term enriched model anaerobic digestion (AD) communities to investigate community composition, diversity, structure, and in particular activity based on the rRNA/rDNA ratio. We cross-validated the 16S amplicon-based results using two alternative operational taxonomic unit (OTU) formation methods (conventional 97% sequence similarity and 100% sequence similar zero-radius OTUs by UNOISE3) and compared these to metagenome-derived population genomes and metatranscriptomes. Significant positive correlations were observed between microbial total activity and abundance with both the amplicon- and omic-based methods. All three methods revealed disproportionately high transcription/abundance ratios for some rare taxa but lower ratios for most abundant taxa for all the communities, which was further corroborated by the high replication rate (iRep) of most low-abundance population genomes. IMPORTANCE Variation in microbial activity levels is increasingly being recognized as both an important dimension in community function and a complicating factor in sequencing-based survey methods. This study extends previous reports that rare taxa may contribute disproportionately to community activity in some natural environments, showing that this may also hold in artificially maintained model communities with well-described inputs, outputs, and biochemical functions. These results demonstrate that assessment of activity levels using the rRNA/rDNA ratio is robust across taxonomic unit formation methods and is independently corroborated by omics methods. The results also provide insight into the comparative advantages and disadvantages of different taxonomic unit formation methods in amplicon sequencing studies, showing that UNOISE3 provides comparable microbial diversity, structure, and activity information as the 97% sequence similarity method but potentially loses some phylogenetic diversity and creates more "phantom taxa" (which are present in the RNA pool but not the corresponding DNA pool).

rDNA- and rRNA-derived communities present divergent assemblage patterns and functional traits throughout full-scale landfill leachate treatment process trains

Understanding the influences of microbial interactions and niche heterogeneities on microbial communities and functional traits is critical for determining its engineering and ecological significance. However, little is known about microbial community assemblage and functional gene expression throughout full-scale landfill leachate treatment plants. Here, we applied a combination of 16S rRNA and rDNA amplicon sequencing, shotgun metagenomic, and qPCR approaches to unveil the ecological associations between distinct communities, functional gene expression and nitrogen cycling processes. By comparing the rDNA and rRNA-derived communities, the rRNA/rDNA ratios suggested that 57.2% of rare taxa were active, and their abundance decreased as increasing of potential activities. In particular, rDNA- and rRNA-based communities exhibited divergent assemblage patterns, and stronger intra-associations among core taxa in the rRNA-based communities than in rDNA-based communities. Furthermore, results regarding both bacterial assemblage and functional traits indicated that the habitat filtering and niche differentiation (treatment units) exerted selection on microbial communities based on functional traits, particular for key ecological functions related to nitrogen cycling. Collectively, our findings provide insights into structure-function associations at the local level and shed light on ecological rules guiding rDNA- and rRNA-based community assembly in landfill leachate treatment systems.

Applications of membrane bioreactors for water reclamation: Micropollutant removal, mechanisms and perspectives

Membrane bioreactors (MBRs) have attracted attention in water reclamation as a result of the recent technical advances and cost reduction in membranes. However, the increasing occurrence of micropollutants in wastewaters has posed new challenges. Therefore, we reviewed the current state of research to identify the outstanding needs in this field. In general, the fate of micropollutants in MBRs relates to sorption, biodegradation and membrane separation processes. Hydrophobic, nonionized micropollutants are favorable in sorption, and the biological degradation shows higher efficiency at relatively long SRTs (30-40 days) and HRTs (20-30 h), as a result of co-metabolism, metabolism and/or ion trapping. Although the membrane rejection rates for micropollutants are generally minor, final water quality can be improved via combination with other technologies. This review highlights the challenges and perspectives that should be addressed to facilitate the extended use of MBRs for the removal of micropollutants in water reclamation.

Elucidating the impact of microbial community biodiversity on pharmaceutical biotransformation during wastewater treatment

In addition to removing organics and other nutrients, the microorganisms in wastewater treatment plants (WWTPs) biotransform many pharmaceuticals present in wastewater. The objective of this study was to examine the relationship between pharmaceutical biotransformation and biodiversity in WWTP bioreactor microbial communities and identify taxa and functional genes that were strongly associated with biotransformation. Dilution-to-extinction of an activated sludge microbial community was performed to establish cultures with a gradient of microbial biodiversity. Batch experiments were performed using the dilution cultures to determine biotransformation extents of several environmentally relevant pharmaceuticals. With this approach, because the communities were all established from the same original community, and using sequencing of the 16S rRNA and metatranscriptome, we identified candidate taxa and genes whose activity and transcript abundances associated with the extent of individual pharmaceutical biotransformation and were lost across the biodiversity gradient. Metabolic genes such as dehydrogenases, amidases and monooxygenases were significantly associated with pharmaceutical biotransformation, and five genera were identified whose activity significantly associated with pharmaceutical biotransformation. Understanding how biotransformation relates to biodiversity will inform the design of biological WWTPs for enhanced removal of chemicals that negatively impact environmental health.

Aerobic biotransformation of the antibiotic ciprofloxacin by Bradyrhizobium sp. isolated from activated sludge

Ciprofloxacin (CIP) is an antibiotic that is widely used to treat bacterial infections and is poorly biodegraded during wastewater treatment. In this study, a CIP-degrading bacterial strain (GLC_01) was successfully retrieved from activated sludge by enrichment and isolation. The obtained bacterial strain shares over 99% nucleotide identity of the 16S rRNA gene with Bradyrhizobium spp. Results show that Bradyrhizobium sp. GLC_01 degraded CIP via cometabolism with another carbon substrate following a first-order kinetics degradation reaction. CIP degradation by Bradyrhizobium sp. GLC_01 increased when the concentration of the primary carbon source increased. The biodegradability of the primary carbon source also affected CIP degradation. The use of glucose and sodium acetate (i.e. readily biodegradable), respectively, as a primary carbon source enhanced CIP biotransformation, compared to starch (i.e. relatively slowly biodegradable). CIP degradation decreased with the increase of the initial CIP concentration. Over 70% CIP biotransformation was achieved at 0.05 mg L^-1 whereas CIP degradation decreased to 26% at 10 mg L^-1. The phylogenetic identification and experimental verification of this CIP-degrading bacterium can lead to a bioengineering approach to manage antibiotics and possibly other persistent organic contaminants during wastewater treatment.

Composition of heterotrophic specialized sub-guilds defined by a positive RNA and polyhydroxyalkanoate correlation in activated sludge

Microbial heterotrophic guilds in activated sludge wastewater treatment systems have complex population structures and functions. A previously proposed heterotrophic-specialist model states that heterotrophs consist of sub-guilds specialized in consuming specific classes of compounds, either readily degradable substrate (RDS) or slowly degradable substrate (SDS) according to current mathematical modeling practices for wastewater treatment processes. It follows from metabolic considerations that the levels of RNA and polyhydroxyalkanoate (PHA) are correlated for strains of the same species growing in different environments; a conjecture previously tested. The proposed classification of heterotrophs into RDS or SDS consumers predicts that the same correlation would also be found across heterotrophic species in conventional activated sludge systems; this prediction was tested in the current study. The positive correlation between the RNA and PHA levels was observed in 9 conventional activated sludge plants in two independent sampling times and it was also found stable over a 6-month regular sampling period at one of these plants. Together, these results imply that the levels of RNA and PHA can be used to define heterotrophic-specialist sub-guilds. In order to gain insight in the species composition of the defined sub-guilds, flow cytometry cell sorting was used to further analyze one of the activated sludge samples. Four sorted sub-samples were obtained (high-RNA/high-PHA, low-RNA/high-PHA, high-RNA/low-PHA, and low-RNA/low-PHA), and the phylogenetic composition of each was determined using 16S rRNA gene amplicon pyrosequencing. Heterotrophic genera were identified across 12 phyla, and their representation in each sorted sub-sample showed that the high-RNA/high-PHA and low-RNA/low-PHA groups were most dissimilar. The enriched genera in these sorted sub-samples are suggested to represent the composition of heterotrophic-specialized sub-guilds defined by the kinetics of substrate consumption.

Effect of trace elements on the development of co-cultured nitrite-dependent anaerobic methane oxidation and methanogenic bacteria consortium

The aim of this work was to study the effects of key trace elements (i.e., iron, copper and molybdenum) on the development of co-cultured n-damo and methanogenic bacteria consortium, which could realize in situ CH₄ production and utilization. The results showed that rational dosage, which was 50 mg/L of Fe, 1 mg/L of Cu and 5 mg/L of Mo, significantly stimulated the removal of NO₂^-. However, the activity of microbes was noticeably inhibited at 5 mg/L of Cu and 1 mg/L of Mo. Microbial community analysis indicated that the abundances of n-damo bacteria and methanogens showed a positive response to the rational dosage. Furthermore, the expression of key functional genes was enhanced under the rational dosage condition.

Trends in Micropollutant Biotransformation along a Solids Retention Time Gradient

For many polar organic micropollutants, biotransformation by activated sludge microorganisms is a major removal process during wastewater treatment. However, our current understanding of how wastewater treatment operations influence microbial communities and their micropollutant biotransformation potential is limited, leaving major parts of observed variability in biotransformation rates across treatment facilities unexplained. Here, we present biotransformation rate constants for 42 micropollutants belonging to different chemical classes along a gradient of solids retention time (SRT). The geometric mean of biomass-normalized first-order rate constants shows a clear increase between 3 and 15 d SRT by 160% and 87%, respectively, in two experiments. However, individual micropollutants show a variety of trends. Rate constants of oxidative biotransformation reactions mostly increased with SRT. Yet, nitrifying activity could be excluded as primary driver. For substances undergoing other than oxidative reactions, i.e., mostly substitution-type reactions, more diverse dependencies on SRT were observed. Most remarkably, characteristic trends were observed for groups of substances undergoing similar types of initial transformation reaction, suggesting that shared enzymes or enzyme systems that are conjointly regulated catalyze biotransformation reactions within such groups. These findings open up opportunities for correlating rate constants with measures of enzyme abundance such as genes or gene products, which in turn should help to identify enzymes associated with the respective biotransformation reactions.

Insights into the variability of microbial community composition and micropollutant degradation in diverse biological wastewater treatment systems

The biological potential of conventional wastewater treatment plants to remove micropollutants mainly depends on process conditions and the predominant microbial community. To explore this dependence and to connect the occurrence of genera with operating conditions, five pilot-scale reactors with different process conditions were combined into two reactor cascades and fed with the effluent of the primary clarifier of a municipal WWTP. All reactors and the WWTP were analyzed for the removal of 33 micropollutants by LC-MS/MS and the presence of the microbial community using 16S rRNA gene sequencing. The overall removal of the micropollutants was slightly improved (ca. 20%) by the reactor cascades in comparison to the WWTP while certain compounds such as diatrizoate, venlafaxine or diclofenac showed an enhanced removal (ca. 70% in one or both cascades). To explore the diverse bacteria in more detail, the general community was divided into a core and a specialized community. Despite their profoundly different operating parameters (especially redox conditions), the different treatments share a core community consisted of 143 genera (9% of the overall community). Furthermore, the alpha- and beta-biodiversity as well as the occurrence of several genera belonging to the specialized microbial community could be linked to the prevalent process conditions of the individual treatments. Members of the specialized community also correlated with the removal of certain groups of micropollutants. Hence, the comparison of the specialized community with micropollutant removal and operating conditions via correlation analysis is a valuable tool for an extended evaluation of prevalent process conditions. Based on an extended data set this approach could also be used to identify organisms as indicators for operating conditions which are beneficial for an improved removal of specific micropollutants.

Deciphering the factors influencing the discrepant fate of antibiotic resistance genes in sludge and water phases during municipal wastewater treatment

The discrepant fate of antibiotic resistance genes (ARGs) in sludge and water phases was investigated in a municipal wastewater treatment plant, and a lab-scale A²O-MBR was operated to provide background value of ARGs. The influencing factors of ARGs including microbial community, co-selection from heavy metals, biomass and horizontal gene transfer were concerned. Results showed that iA²O (inversed A²O) showed better ARGs reduction, and longer SRT (sludge retention time) increased ARGs relative abundance while reduced the gene copies of ARGs in the effluent, but significantly increased the ARGs in sludge phase. Compared to background value, the most enriched ARG was tetX in water phase, while it was intI1 in sludge phase. There existed higher abundance of multi-resistant bacteria in sludge phase, and microbial community determined the fate of ARGs in both water and sludge phase, while the direct effects from horizontal gene transfer should not be overlooked especially in water phase.

Transformation, CO2 formation and uptake of four organic micropollutants by carrier-attached microorganisms

A tiered process was developed to assess the transformation, CO₂ formation and uptake of four organic micropollutants by carrier-attached microorganisms from two municipal wastewater treatment plants. At the first tier, primary transformation of ibuprofen, naproxen, diclofenac, and mecoprop by carrier-attached microorganisms was shown by the dissipation of the target compounds and the formation of five transformation products using LC-tandem MS. At the second tier, the microbial cleavage of the four organic micropollutants was confirmed with ¹⁴C-labeled micropollutants through liquid scintillation counting of the ¹⁴CO₂ formed. At the third tier, microautoradiography coupled with fluorescence in situ hybridization (MAR-FISH) was used to screen carrier-attached microorganisms for uptake of the four radiolabeled micropollutants. Results from the MAR-FISH screening indicated that only a small fraction of the microbial community (≤1‰) was involved in the uptake of the radiolabeled micropollutants and that the responsible microorganisms differed between the compounds. At the fourth tier, the microbial community structure of the carrier-attached biofilms was analyzed by 16S rRNA gene amplicon sequencing. The sequencing results showed that the MAR-FISH screening targeted ∼80% of the microbial community and that several taxonomic families within the FISH-probed populations with MAR-positive signals (i.e. Firmicutes, Gammaproteobacteria, and Deltaproteobacteria) were present in both biofilms. From the broader perspective of organic micropollutant removal in biological wastewater treatment, the MAR-FISH results of this study indicate a high degree of microbial substrate specialization that could explain differences in transformation rates and patterns between micropollutants and microbial communities.

Deciphering the core fouling-causing microbiota in a membrane bioreactor: Low abundance but important roles

Currently, membrane biofouling in membrane bioreactors (MBRs) is normally attributed to the occurrence of abundant bacterial species on membranes, whereas the roles of low-abundance bacteria have not been paid sufficient attention. In this study, the linear discriminant analysis (LDA) effect size (LEfSe) algorithm was used to identify active biomarkers, determining 67 different phylotypes among Bulk sludge, low-fouling Bio-cake (10 kPa), high-fouling Bio-cake (25 kPa) and Membrane pore in a membrane bioreactor with NaOCl backwash. Interestingly, a large proportion of the active biomarkers in bio-cake samples, such as Methylophilaceae, Burkholderiaceae, Paucibacter and Pseudoxanthomonas, did not fall within the abundant taxa (i.e., <0.05% relative abundance), indicating the preferential growth of these low-abundance bacteria on the membrane surface. Furthermore, the characterization of microbial interactions using a random matrix theory (RMT)-based network approach obtained a network consisting of 120 nodes and 228 edges. Specifically, network analysis showed the presence of an intense competition among bacterial species in the fouling-related communities, suggesting that negative interactions have an important effect on determining the microbial community structure. More importantly, the LEfSe algorithm and network analysis showed that most of the core species of the bio-cake, such as Burkholderiaceae, Bacillus and Rhodothermaceae, merely amounted to a very low relative abundance (<1%), suggesting their unrecognized and over-proportional ecological role in triggering the initial biofilm formation and subsequent biofilm maturation during MBR operation. Overall, this work should improve our understanding of the bacterial community structure on the fouled membranes in MBRs.

Fate of tetracycline in enhanced biological nutrient removal process

This study investigated the fate of tetracycline at four different concentrations of 20 μg L^-1, 50 μg L^-1, 2 and 5 mg L^-1 in the enhanced biological nutrient removal processes. At the tetracycline concentration below 50 μg L^-1, no obvious inhibition on the biological N&P removal was observed, while the inhibition appeared after the tetracycline concentration was increased to 2 and 5 mg L^-1. It was found that about 44%-87% of tetracycline was removed through biodegradation, while only 3%-6% of removal was due to biosorption. These results clearly suggested that a substantial amount of tetracycline eventually ended up in sludge with the tetracycline content of 23 mg to 4.5 g kg^-1 sludge depending on the tetracycline concentration. Obviously, this could pose an emerging challenge to the post sludge disposal and reuse. Furthermore, phthalic anhydride was detected as a biodegradation byproduct of tetracycline, which has been known to be more toxic than tetracycline to aquatic organisms. Consequently, this study offers in-depth insights into the fate of tetracycline in the enhanced biological nutrient removal process, highlighting on the emerging ecological risks associated with sludge disposal and effluent discharge.

Impacts of solids retention time on trace organic compound attenuation and bacterial resistance to trimethoprim and sulfamethoxazole

Bacteria can grow in the presence of trimethoprim and sulfamethoxazole by expressing antibiotic resistance genes or by acquiring thymine or thymidine from environmental reservoirs to facilitate DNA synthesis. The purpose of this study was to evaluate whether activated sludge serves as a reservoir for thymine or thymidine, potentially impacting the quantification of antibiotic resistant bacteria. This study also assessed the impacts of varying solids retention time (SRT) on trimethoprim and sulfamethoxazole removal during wastewater treatment and single and multi-drug resistance. When assayed in the presence of the antibiotics at standard clinical concentrations, up to 40% increases in the relative prevalence of resistant bacteria were observed with (1) samples manually augmented with reagent-grade thymidine, (2) samples manually augmented with sonicated biomass (i.e., cell lysate), (3) samples manually augmented with activated sludge filtrate, and (4) activated sludge samples collected from reactors with longer SRTs. These observations suggest that longer SRTs may select for antibiotic resistant bacteria and/or result in false positives for antibiotic resistance due to higher concentrations of free thymine, thymidine, or other extracellular constituents.

Effect of bio-electrochemical system on the fate and proliferation of chloramphenicol resistance genes during the treatment of chloramphenicol wastewater

Bioelectrochemical systems can effectively degrade antibiotics, but there is the need to better understand the fate of antibiotic resistance bacteria and antibiotic resistance genes during the bioelectrochemical degradation of antibiotics. In this study, a BES was developed as a platform to investigate the fate of chloramphenicol resistance bacteria (CRB) and the expression of chloramphenicol resistance genes (CRGs) under different operating conditions during chloramphenicol biodegradation. The results indicated that chloramphenicol was effectively removed and chloramphenicol removal efficiency could be improved under less chloramphenicol concentration and more negative cathode potential. Higher chloramphenicol concentration enhanced the enrichment of CRB and expression of CRGs. Furthermore, the abundances of CRB were enhanced under more negative cathode potential, the expression of CRGs under less negative cathode potential were induced. However, both the enrichment of CRB and expression of CRGs could be moderated under a medium cathode potential. This result could provide the scientific reference for research about the fate of antibiotic resistance genes in bioelectrochemical systems.

Impact of microbial physiology and microbial community structure on pharmaceutical fate driven by dissolved oxygen concentration in nitrifying bioreactors

Operation at low dissolved oxygen (DO) concentrations (<1 mg/L) in wastewater treatment could save utilities significantly by reducing aeration energy costs. However, few studies have evaluated the impact of low DO on pharmaceutical biotransformations during treatment. DO concentration can impact pharmaceutical biotransformation rates during wastewater treatment both directly and indirectly: directly by acting as a limiting substrate that slows the activity of the microorganisms involved in biotransformation; and indirectly by shaping the microbial community and selecting for a community that performs pharmaceutical biotransformation faster (or slower). In this study, nitrifying bioreactors were operated at low (∼0.3 mg/L) and high (>4 mg/L) DO concentrations to understand how DO growth conditions impacted microbial community structure. Short-term batch experiments using the biomass from the parent reactors were performed under low and high DO conditions to understand how DO concentration impacts microbial physiology. Although the low DO parent biomass had a lower specific activity with respect to ammonia oxidation than the high DO parent reactor biomass, it had faster biotransformation rates of ibuprofen, sulfamethoxazole, 17α-ethinylestradiol, acetaminophen, and atenolol in high DO batch conditions. This was likely because the low DO reactor had a 2x higher biomass concentration, was enriched for ammonia oxidizers (4x higher concentration), and harbored a more diverse microbial community (3x more unique taxa) as compared to the high DO parent reactor. Overall, the results show that there can be indirect benefits from low DO operation over high DO operation that support pharmaceutical biotransformation during wastewater treatment.