Activated Sludge Samples Research Articles

Assessment and characterization of the bacterial community structure in advanced activated sludge systems

The present study is aimed to assess and characterize the structure of bacterial community in advanced activated sludge systems. In particular, activated sludge samples were collected from an Integrated Fixed-film Activated Sludge – Membrane Bioreactor pilot plant under a University of Cape Town configuration with in-series anaerobic (Noair)/anoxic (Anox)/aerobic (Oxy) reactors – and further analyzed. The achieved results – based on Next Generation Sequencing (NGS) of 16S rDNA amplicons – revealed that the bacterial biofilm (bf) communities on plastic carriers of Oxy and Anox reactors had a greater diversity compared to suspended (sp) bacterial flocs of Oxy, Anox and Noair. Indeed, the Shannon diversity indices of both biofilm communities were higher than those of suspended growth samples (Oxy-bf = 4.1 and Anox-bf = 4.2 vs. Oxy-sp = 3.4, Anox-sp = 3.5 and Noair-sp = 3.4). The most striking differences have been reported in Rhodobacteraceae being more abundant in biofilm specimens than in suspended biomass samples. The vast majority of the identified bacteria differs from those obtained by culture dependent method, thus suggesting that NGS-based method is really suitable to analyze the bacterial community composition, even in advanced systems for wastewater treatment.

Capability of Nocardia nova found in activated sludge to use synthetic BTEX as sole source of carbon and energy

ABSTRACT Benzene and its derivatives, toluene, ethylbenzene and xylenes (BTEXs) are volatile monocyclic aromatic compounds which are widely distributed in nature and highly hazardous to human and animal health. The species belonging to the genus Nocardia are filamentous bacteria widely present in the flocs of activated sludge. They lead to the formation of foams and bulking sludge, which disrupt the sludge settling properties and thus reduce the quality of the purified water. The aim of our study is to isolate from these filamentous bacteria in the activated sludge, indigenous strains of Nocardia able to use the BTEX as the sole source of carbon and energy. Two isolation media were used to isolate the Nocardia species found in the activated sludge samples taken from the Ibn Ziad (Constantine-Algeria) wastewater treatment plant. The results show that five strains that morphologically resemble Nocardia have been isolated. The strain named (S) was identified by the sequencing of its 16S rRNA gene as Nocardia nova. This strain was the only isolate capable of growing under laboratory conditions, on BTE (m-o-p) X added at concentrations of 1500 mg/L each. Such a result can be used in the bioremediation of aquatic or terrestrial ecosystems contaminated by these compounds, using this bacterium as a bio-augmentation agent.

Screening and Denitrification Characteristics of Simultaneous Heterotrophic Nitrification and Aerobic Denitrification Bacteria

Two highly efficient simultaneous heterotrophic nitrification and aerobic denitrification (SND) bacteria, isolated from activated sludge samples which domesticated in laboratory, were identified by 16S rDNA as Pseudomonas sp. QU02 and Paracoccus sp. QU07. The factors influencing the denitrification of Pseudomonas sp. QU02 and Paracoccus sp. QU07. SND were investigated. For Pseudomonas sp. QU02, the optimum carbon source was sodium succinate, the optimum C/N was 10, the optimum initial pH was 8, and the optimum NaCl concentration was 5 g/L. The optimum carbon source for denitrification of Paracoccus sp. QU07 was sodium succinate, the optimum C/N was 10, and the optimum initial pH was 7, the optimum NaCl concentration was found to be 5 g/L, and salt tolerance response in two strains. The SND process was characterized under optimized conditions. The N removal rate of two strains were higher than 90% after 72 h SND. The enhanced denitrification of industrial wastewater by mixed SND strains was investigated. After 144 h SND, N removal rate is obviously better than unstrengthened. The mixed SND strains have potential practical application value in enhancing SND of industrial wastewater.

Monitoring the Activated Sludge Activities Affected by Industrial Toxins via an Early-Warning System Based on the Relative Oxygen Uptake Rate (ROUR) Index

Shock load from industrial wastewater is known to harm the microbial activities of the activated sludge in wastewater treatment plants (WWTPs) and disturb their performance. This study developed a system monitoring the activated sludge activities based on the relative oxygen uptake rate (ROUR) and explored the influential factors with wastewater and the activated sludge samples collected from a typical WWTP in the Taihu Lake of southern Jiangsu province, China. The ROUR was affected by the concentration of toxic substances, mixed liquid suspended solids (MLSS), hydraulic retention time (HRT) and pH. Higher toxin contents significantly decreased the ROUR and the EC50 value of Zn2+, Ni2+, Cr(VI), Cu2+, and Cd2+ was 13.40, 15.54, 97.56, 12.01, and 14.65 mg/L, respectively. The ROUR declined with the increasing HRT and MLSS above 2000 mg/L had buffering capacities for the impacts of toxic substances to some extent. The ROUR remained stable within a broad range pH (6–10), covering most of the operational pH in WWTPs and behaving as an appropriate indicator for monitoring the shock load. A toxicity model assessing and predicting the ROUR was developed and fitted well with experimental data. Coupling the ROUR monitoring system and toxicity model, an online early-warning system was assembled and successfully used for predicting the toxicity of different potential toxic metals. This study provides a new universal toxicity model and an online early-warning system for monitoring the shock load from industrial wastewater, which is useful for improving the performance of WWTPs.

Experimental Investigation of 11x11 cm2 Membrane Microbial Fuel Cell Performance

Abstract Microbial fuel cells (MFC’s) are systems that can convert chemical energy into electricity directly from biochemically commutable substances (organic substances) by using microorganisms as a catalyst in an oxygen-free environment. Electrode-growing microorganisms in the anode chamber converts water organics into hydrogen ions and electrons. In this study, performance of 11x11 cm2 membrane microbial fuel cell have been experimentally investigated. In the experimental studies, activated sludge sample have been used. In this study, current density (= I/V), power density (= P / V) values have been calculated by measuring current (I) and voltage (V) values. The performance of 11x11 cm2 Nafion 117 membrane activated sludge microbial fuel cell (#ASMFC) have been determined. The maximum values of voltage, current density and power density values from the (#ASMFC) are represented subsequently (0.616 V, 0.463 A/m3 and 75.64 W/m3).

Diversity and abundance of denitrifying bacteria in a simultaneously nitrifying and denitrifying rotating biological contactor treating real wastewater at low temperatures

Abstract Nitrification and denitrification processes occur simultaneously in aerobic wastewater biofilms. Although wide regions of the world have average temperatures of less than 15 °C for a half year, few studies have investigated the nitrogen removal by nitrification and denitrification in a single-stage aerobic biofilm reactor used for treating real wastewater under low-temperature conditions. This study showed successful wastewater treatment in a high average nitrogen removal rate of 78% at low water temperatures by simultaneous nitrification and denitrification in a rotating biological contactor (RBC) biofilm. Batch operations using the RBC to evaluate the rates of ammonium decrease at low temperatures demonstrated that the rate of ammonium decrease at 8 °C was 76% of that at 20 °C. Daily monitoring of nitrification and denitrification rates suggested that the denitrification rate was highly correlated with the nitrification rate. Next-generation sequencing (NGS) analysis revealed the presence of diverse and abundant denitrifying bacteria and aerobic bacteria in the RBC biofilm more than those in the activated sludge samples, which probably enabled the achievement of the high nitrogen removal rates at such low temperatures. Furthermore, correlation with the colony counts showed that the NGS analysis had the quantification range of three orders of magnitude (from 0.001% to 1%).

Conjugative potential of antibiotic resistance plasmids to activated sludge bacteria from wastewater treatment plants

Wastewater treatment plants (WWTPs) have been considered as hotspots to spread antibiotic resistance genes (ARGs). However, the potential of activated sludge bacteria to disseminating ARGs in WWTPs has not been clearly understood. In this study, we investigated the conjugative potential of bacteria in five activated sludge samples from different WWTPs, by using Escherichia coli as donor of broad-host range IncP-1 plasmid pKJK5. The maximum transfer frequencies were in the range of 1 × 10−3–4.3 × 10−2 per recipient. Conjugative potentials were determined as the ratio of relative abundances in transconjuants to those in recipients. In all transconjugant pools, there were fifteen shared genera, which contained pathogenic bacteria in potential. The dominant shared genera included Aeromonas, Acinetobacter and Enterobacter, accounting for 65%–96% of the total abundance, with the conjugative potential in the range of 0.4–4.9. Gram-positive bacteria, Vagococcus and Kurthia, were also found in the transconjugants. Their relative abundances were in the range of 1.5%–7.2% and conjugative potentials were much higher (ranged from 2.1 to 48), deserving more attention to estimating the risks. Conjugative potential for certain genera were found diverse in different processes, implying possible correlation with process operation. This study can be helpful for ARG risk assessment and control in WWTPs.

Evaluation of the accuracy of two simple methods for microscopic activated sludge analysis.

Biological microscopic analysis is a popular method employed in wastewater treatment plants worldwide for evaluating activated sludge condition. However, many operators still have reservations regarding its reliability. In this study, we evaluated and compared two methods of microscopic sludge investigation: the sludge index (SI) and the Eikelboom-van Buijsen method (EB). We investigated 79 activated sludge samples from nine treatment plants located in southern Poland over a 1-year period. For each sample, sludge volume index values were calculated and compared with the results of evaluation made on the basis of microscopic analysis. Additionally, the effluent quality was analysed in 45 of 79 cases, including investigation of suspended solids, biochemical oxygen demand, chemical oxygen demand, total nitrogen and total phosphorous. The sign test and Wilcoxon matched pairs test showed that a significant difference existed between the two investigated methods. General conclusions from both methods do not provide reliable information concerning nitrogen and phosphorus removal. The EB method had a tendency to be more conservative in its general conclusions than the SI method. Both are highly reliable for estimating activated sludge quality and solid separation properties.

Influence of nalidixic acid on tandem heterotrophic-autotrophic kinetics in a “NIPHO” activated sludge reactor

This work analyzes the effect of nalidixic acid (NAL) on the kinetics of the heterotrophic and autotrophic biomass growth within a “NIPHO” activated sludge reactor treating municipal wastewater. Thus, the effect of this chemical in the degradation rates of carbon and nitrogen sources and net biomass growth rate is evaluated. Activated sludge samples were taken at three different operation conditions, changing the values of hydraulic retention time (2.8–3.8 h), biomass concentration (1400–1700 mgVSS L−1), temperature (12.6–14.8 °C), and sludge retention time (11.0–12.6 day). A respirometric method was applied to model the kinetic performance of heterotrophic and autotrophic biomass in absence and presence of NAL, and a multivariable statistical analysis was carried out to characterize the influence of the operation variables on the kinetic response of the system, which was finally optimized. The results showed that there was no inhibitory effect of NAL on heterotrophic biomass, with an increase of net heterotrophic biomass growth rate from 1.70 to 6.73 mgVSS L−1 h−1 at the most favorable period. By contrast, the autotrophic biomass was negatively affected by NAL, reducing the value of net autotrophic biomass growth rate from 25.37 to 10.29 mgVSS L−1 h−1 at the best operation conditions. In general, biomass concentration and temperature had the highest influence on the degradation rate of carbon and nitrogen sources, whereas hydraulic retention time and sludge retention time were the most influential on net heterotrophic and autotrophic biomass growth rates.

Temporal heterogeneity and temperature response of active ammonia-oxidizing microorganisms in winter in full-scale wastewater treatment plants

Ammonia-oxidizing microorganisms (AOMs) play important roles in nitrogen removal, but their metabolic activity in winter months in wastewater treatment plants (WWTPs) remains unclear. Here, two full-scale WWTPs were selected, and a total of 48 DNA-based stable isotope probing (DNA-SIP) microcosms of activated sludge samples at different winter months (November-January) and temperatures (13 °C, 25 °C and 37 °C) were constructed to evaluate the temporal heterogeneity and temperature response of active AOMs. The results provided direct evidence that the temporal heterogeneity of AOMs’ activity was not obvious in winter months. In general, ammonia-oxidizing archaea (AOA) and bacteria (AOB) co-participated in the active ammonia oxidation in three winter months at in situ temperature (13 °C) in two WWTPs, and AOA were the dominant active AOMs, even though AOB outnumbered AOA in seed sludge. In contrast, higher peaks of AOA in “heavy” fractions at 25 °C and 37 °C also provided compelling evidence for the dominant contribution of AOA to active ammonia oxidation under higher temperatures. The Illumina sequencing of 13C-DNA obtained from the DNA-SIP microcosms at 13 °C in three winter months further suggest that Candidatus Nitrososphaera evergladensis and Candidatus Nitrosocosmicus exaquare were the dominant active AOA in two WWTPs, while the active AOB were dominated by Nitrosomonas oligotropha. At higher temperatures, active AOA shifted to Ca. N. evergladensis and Candidatus Nitrososphaera gargensis, and Nitrosomonas marina, Nitrosomonas europaea and Nitrosomonas communis dominated in the active AOB. Overall, AOA rather than AOB dominated the active ammonia oxidation in winter months and at higher temperatures in the full-scale WWTPs tested.

The link between nitrous oxide emissions, microbial community profile and function from three full-scale WWTPs

Few attempts have been made in previous studies to link the microbial community structure and function with nitrous oxide (N2O) emissions at full-scale wastewater treatment plants (WWTPs). In this work, high-throughput sequencing and reverse transcriptase-qPCR (RT-qPCR) was applied to activated sludge samples from three WWTPs for two seasonal periods (winter and summer) and linked with the N2O emissions and wastewater characteristics. The total N2O emissions ranged from 7.2 to 937.0 g N-N2O/day, which corresponds to an emission factor of 0.001 to 0.280% of the influent NH4-N being emitted as N2O. Those emissions were related to the abundance of Nitrotoga, Candidatus Microthrix and Rhodobacter genera, which were favored by higher dissolved oxygen (DO) and nitrate (NO3−) concentrations in the activated sludge tanks. Furthermore, a relationship between the nirK gene expression and N2O emissions was verified. Detected N2O emission peaks were associated with different process events, related to aeration transition periods, that occurred during the regular operation of the plants, which could be potentially associated to increased emissions of the WWTP. The design of mitigation strategies, such as optimizing the aeration regime, is therefore important to avoid process events that lead to those N2O emissions peaks. Furthermore, this study also demonstrates the importance of assessing the gene expression of nosZ clade II, since its high abundance in WWTPs could be an important key to reduce the N2O emissions.

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.

Towards an eDNA metabarcode-based performance indicator for full-scale municipal wastewater treatment plants

Knowledge-driven management for wastewater treatment plant (WWTP) performance becomes increasingly important considering the globally growing production of wastewater and the rising demand of clean water supply. Even though the potential of microbial organisms (bacteria and protists) as bioindicators for WWTP performance is well known, it is far from being fully exploited for routine monitoring programs. Therefore, we here used massive sequencing of environmental (e)DNA metabarcodes from bacterial (V3-V4 region of the SSU rRNA gene) and eukaryote (V9 region of the SSU rRNA gene) communities in 21 activated sludge samples obtained from full-scale municipal WWTPs in Germany. Microbial community patterns were compared to standard WWTP operating parameters and two traditionally used WWTP performance indicators (Sludge Biotic Index and Sludge Index). Both indices showed low concordance and hardly correlated with chemical WWTP performance parameters nor did they correlate with microbial community structures. In contrast, microbial community profiles significantly correlated with WWTP performance parameters and operating conditions of the plants under study. Therefore, eDNA metabarcode profiles of whole microbial communities indicate the performance of WWTP and can provide useful information for management strategies. We here suggest a strategy for the development of an eDNA metabarcode based bioindicator system, which can be implemented in future standard monitoring programs for WWTP performance and effluent quality.

Synchronic Biotransformation of 1,4-Dioxane and 1,1-Dichloroethylene by a Gram-Negative Propanotroph Azoarcus sp. DD4

Bioremediation of 1,4-dioxane is strongly challenged by the concurrent contamination of chlorinated solvents, particularly 1,1-dichloroethylene (1,1-DCE), a potent inhibitor of archetypic 1,4-dioxane degraders (e.g., Pseudonocardia and Mycobacterium). Vegetative growth and aggregation behavior of these Gram-positive Actinomycetes also hinder their field application. A new Gram-negative bacterium Azoarcus sp. DD4 was isolated from an activated sludge sample. Notably, DD4 can sustain the concurrent co-oxidation of 1,4-dioxane and 1,1-DCE using propane as the primary substrate without observable formation of clumps. Microcosm assays prepared with source zone groundwater samples from a contaminated site indicated DD4 can efficiently remove dioxane, with the concentration decreasing from an initial value of 10.4 mg/L to <0.4 μg/L within 14 days of incubation. Removal of 1,4-dioxane was partially inhibited when an excessive amount of 1,1-DCE (3.28 ± 0.19 mg/L) was artificially spiked into the microcosms but sig...

Effect of suspended solids on peracetic acid decay and bacterial inactivation kinetics: Experimental assessment and definition of predictive models

The work addresses the effect of total suspended solids (TSS) on disinfection by peracetic acid (PAA) concerning both PAA decay and bacterial inactivation kinetics. The effect of TSS on PAA decay was evaluated at five TSS concentrations (5, 40, 80, 120 and 160 mg/L), obtained from stock TSS solutions prepared from activated sludge samples. The influence of the soluble matter associated to the suspended solids on PAA decay was evaluated separately, using the same stock TSS solution after the removal of solids by filtration. The contributions of suspended and soluble fractions were found to be independent, and a predictive model formed by two additive sub-models was proposed to describe the overall PAA decay kinetics. Moreover, an uncertainty analysis was performed by a series of Monte Carlo simulations to propagate the uncertainties associated to the coefficients of the model. Then, the disinfectant dose (mg/L min) was highlighted as the main parameter determining disinfection efficiency on a pure culture of E. coli and an inactivation kinetic model was developed based on the response of E. coli to various PAA doses. Finally, the effect of TSS (40 and 160 mg/L) on the inactivation of free-swimming E. coli was investigated at two PAA doses (5 and 20 mg/L min). TSS reduced inactivation extent an average of 0.4 logs at 5 mg/L min and 1.5 logs at 20 mg/L min. It was hypothesized that this might be due to the formation of bacteria aggregates as defense mechanism against disinfection, enhanced by the presence of solids.

Exploring abundance, diversity and variation of a widespread antibiotic resistance gene in wastewater treatment plants

An updated sul1 gene sequence database was constructed and new degenerate primers were designed to better investigate the abundance, diversity, and variation of a ubiquitous antibiotic resistance gene, sul1, with PCR-based methods in activated sludge from wastewater treatment plants (WWTPs). The newly designed degenerate primers showed high specificity and higher coverage in both in-silico evaluations and activated sludge samples compared to previous sul1 primers. Using the new primers, the abundance and diversity of sul1 gene, together with 16S rRNA gene, in activated sludge from five WWTPs in summer and winter were determined by quantitative PCR and MiSeq sequencing. The sul1 gene was found to be prevalent and displayed a comparable abundance (0.081 copies per bacterial cell in average) to the total bacteria across all samples. However, compared to the significant seasonal and geographical divergences in the quantity and diversity of bacterial communities in WWTPs, there were no significant seasonal or geographical variations of representative clusters of sul1 gene in most cases. Additionally, the representative sul1 clusters showed fairly close phylogeny and there was no obvious correlation between sul1 gene and the dominant bacterial genera, as well as the int1 gene, suggesting that bacterial hosts of sul1 gene is not stable, the sul1 gene may be carried by mobile genetic elements, sometimes integrated with class 1 integrons and sometimes not. Thus mobile genetic elements likely play a greater role than specific microbial taxa in determining the composition of sul1 gene in WWTPs.

Two identical copies of the hydrazine synthase gene clusters found in the genomes of anammox bacteria

Hydrazine synthase (HZS), encoded by the hzsABC gene cluster, is the unique and irreplaceable enzyme for the anammox reaction in this group of bacteria. This gene is widely used as a functional gene biomarker to retrieve the diversity, abundance and activity of anammox bacteria in different ecological niches. Genomes of anammox bacteria are constantly acquired, but the copy number of hzsABC gene cluster in the genome of anammox bacteria remains unsettled. In this study, three high quality anammox bacterial genomes were acquired from metagenomic datasets of activated sludge samples containing anammox bacteria. Interestingly, the coverage of hzsABC gene cluster in all of the three genomes was approximately two folds, comparing to the coverage of the corresponding genomic bin. However, only one assembled scaffold contained the full length of hzsABC in each genome, therefore it is likely that two identical copies of the hzsABC gene cluster are present in each of the three genomes. To confirm this, specific PCR primers were designed based on the assembled scaffolds containing the full-length hzsABC and partial hzsC genes. Two complete sequences connecting with the 5’ terminal of hzsC were successfully amplified using genomic DNA as templates, which further supported our hypothesis of the two identical copies of the gene. Moreover, the twofold coverage relationships of hzs genes and genomes were also found in the published genomes of“Ca. Brocadia sinica”, “Ca. Brocadia_caroliniensis”, “Ca. Brocadia_fulgida”. Overall, we would conclude that the genome of anammox bacteria contains two identical copies of hzsABC gene cluster, but functionality of the two identical genes needs further confirmations.

High diversity of potential nitrate-reducing Fe(II)-oxidizing bacteria enriched from activated sludge.

Nitrate-dependent Fe(II) oxidation (NDFO) has been discovered in various environments including activated sludge and can potentially be used to remove nitrate from wastewater. In this study, NDFO sludge was successfully enriched from activated sludge under high Fe(II) concentrations over 100days and the denitrification rate achieved 1.37mmolN/(gVSSday). High-throughput sequencing of the bacterial 16S rRNA gene was used to investigate the microbial community structure dynamics during the enrichment process. The results showed that the microbial community changed significantly and high diversity of potential Fe(II)-oxidizing bacteria (FeOB) was observed in the enriched sludge. Thermomonas and Gallionella were the dominant bacterial genera in the enriched sludge and their relative abundances accounted for 9.49 and 4.08%, respectively. Furthermore, it was found that potential FeOB were also abundantly present in activated sludge samples of common municipal wastewater treatment plants. Collectively, this study demonstrated that NDFO could be successfully performed by enriched activated sludge and high diversity of bacteria is involved in this process, and the results also provide baseline information for future research and engineering application of NDFO process.

Examining relationships between total silver concentration and Sil silver resistance genes in domestic wastewater treatment plants.

The overarching aims of this study were to determine if microbes found in wastewater treatment plants (WWTPs) carry silver resistance genes and ascertain whether a relationship exists between silver loading and sil silver resistance gene copy numbers. Activated sludge samples were collected from 17 WWTPs across the United States and characterized for chemical oxygen demand (COD) and total silver concentration. Gene copy numbers were quantified using qPCR for four common silver resistance genes (silC, silP, silR and silS). Spearman correlation analyses were performed to examine associations between three WWTP characteristics (i.e. size, COD and total silver concentration) and sil gene copy numbers. sil genes were found in all activated sludge samples, however the quantity of gene copy numbers was inconsistent. Of the sil genes screened, silS was the most abundant, followed by silR. Gene copy numbers for silC and silP were generally similar and overall lower than silS and silR. Neither individual or combined sil gene copy numbers correlated significantly with total silver concentration, suggesting either the presence of alternate mechanisms of silver resistance or that a critical silver concentration threshold within WWTPs was not met that would require specific resistance against silver. To the authors' knowledge, this is one of the first studies to measure the prevalence of sil genes in engineered systems. Results from this study suggest that at least four silver resistance genes are commonly found within WWTP-activated sludge, however it is unclear if these genes are being expressed under current total silver loadings.

Linking nitrous oxide emissions to population dynamics of nitrifying and denitrifying prokaryotes in four full-scale wastewater treatment plants

Ammonia-oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA) and N2O-reducing denitrifiers were measured by quantitative real-time PCR (qPCR) in activated sludge samples from four full-scale wastewater treatment plants (WWTPs) in South Spain, and their abundances were linked to the generation of N2O in the samples using multivariate analysis (Non-metric multidimensional scaling, MDS, and BIO-ENV). The average abundances of AOA remained in similar orders of magnitude in all WWTPs (106 copies amoA/L activated sludge mixed liquor), while significant differences were detected for AOB (105-109copies amoA/L) and N2O-reducers (107-1010copies nosZ/L). Average N2O emissions measured in activated sludge samples ranged from 0.10 ± 0.05 to 6.49 ± 8.89 mg N2O-N/h/L activated sludge, and were strongly correlated with increased abundances of AOB and lower counts of N2O-reducers. A significant contribution of AOA to N2O generation was unlikely, since their abundance correlated negatively to N2O emissions. AOB abundance was favoured by higher NO3− and NO2−concentrations in the activated sludge.