Conventional Activated Sludge Reactor Research Articles

Unaerated feeding alters the fate of dissolved methane during aerobic wastewater treatment

In municipal wastewater treatment plants, some dissolved methane can enter the aerobic bioreactors. This greenhouse gas originates from sewers and return flows from anaerobic sludge treatment. In well-mixed conventional activated sludge reactors, methane emissions are largely avoided because methane oxidizing bacteria consume a large fraction, even without optimizing for this purpose. In this work, the fate of dissolved methane is studied in aerobic granular sludge reactors, as they become increasingly popular. The influence of the characteristic design and operating conditions of these reactors are studied with a mathematical model with apparent conversion kinetics and stripping: the separation of feeding and aeration in time, a higher substrate transport resistance, a high retention time of granular biomass and a taller water column. Even for a best-case scenario combining an unrealistically low intragranule substrate transport resistance, a high retention time, a tall reactor, an extremely high influent methane concentration and no oxygen limitation, the methane conversion efficiency was only 12% when feeding and aeration were separated in time, which is lower than for continuous activated sludge reactors under typical conditions. A more rigorous model was used to confirm the limited conversion, considering the multi-species and multi-substrate biofilm kinetics, anoxic methane consumers and the high substrate concentration at the bottom during upward plug flow feeding. The observed limited methane conversion is mainly due to the high concentration that accumulates during unaerated feeding phases, which favours stripping more than conversion in the subsequent aeration phase. Based on these findings, strategies were proposed to mitigate methane emissions from wastewater treatment plants with sequentially operated reactors.

A review of the adsorption-biological hybrid processes for the abatement of emerging pollutants: Removal efficiencies, physicochemical analysis, and economic evaluation

The limited efficiency of conventional wastewater treatment plants (WWTPs) in emerging pollutants (EPs) removal encourages the development of alternative technologies for the adequate treatment of wastewater, due to its adverse effects on human health and ecosystems. The biological, physical or chemical hybrid technologies to treat EPs results interesting since they can enhance the performance of WWTPs. Among them, hybrid adsorption/biological technology could offer different possibilities that are explored in this work (PAC-MBR, PACT/GAC-CAS, BAC configurations). In this way, different variations in the adsorption process have been considered: the form of the adsorbent, the feed to the system, and the type of biological process, either conventional activated sludge (CAS), membrane bioreactor (MBR) or biofilm systems. For each combination, the removal efficiency of micropollutants, classified according to their use into pharmaceuticals, personal care products (PCPs) and other micropollutants (mainly benzotriazoles) was analysed. From reported data, it was observed a beneficial synergistic effect of dipole moment and octanol-water partition coefficient on the removal efficiency of micropollutants by adsorption/biological hybrid technology. Finally, a preliminary economic evaluation of the powdered activated carbon in a conventional activated sludge reactor (PACT), powdered activated carbon-membrane bioreactor (PAC-MBR) and biological activated carbon (BAC) hybrid systems was carried out by analysing the capital expenditure (CAPEX) of plants for capacities up to 75,000 m3d-1. Likewise, estimations of adsorbent concentration for a hypothetical plant with a capacity of 10,000 m3d-1 is presented. Among these hybrid configurations, PAC-MBR achieved the highest micropollutant elimination percentages; however, it presents the highest CAPEX and activated carbon requirements.

Effects of subinhibitory quinolone concentrations on functionality, microbial community composition, and abundance of antibiotic resistant bacteria and qnrS in activated sludge

Wastewater treatment plants (WWTPs) are continuously exposed to sub-inhibitory concentrations of antibiotics that are thought to contribute to the spreading of antibiotic resistant bacteria and antibiotic resistance genes, which are eventually released to downstream environments through effluents. In order to understand the effects of sub-inhibitory concentrations of antibiotics on sludge microbiome and resistome, we spiked a conventional activated sludge (CAS) model system with ciprofloxacin, a common fluoroquinolone antibiotic, from 0.0001 mg/L (about twice the typical ciprofloxacin concentration observed in municipal wastewater) up to 0.1 mg/L (one order of magnitude below the clinical MIC for Enterobacteriaceae) for 151 days. The abundance of ciprofloxacin resistant bacteria and qnrS, a plasmid-associated gene that confers resistance to quinolones, in activated sludge and in effluents of control and spiked CAS reactors, showed no measurable effect of the antibiotic amendment. This was also true for the bacterial community structure and for indicators of WW treatment such as N removal efficiency. Surprisingly, temporal fluctuations in both reactors could explain the observed internal variability of these antibiotic resistance determinants better than the hypothesized antibiotic-driven selective pressure. Overall, this work shows that the core sludge microbiome in CAS systems is resilient to sub-inhibitory concentrations of ciprofloxacin at a functional, structural, and antibiotic resistance levels.

A comparative study of conventional activated sludge and fixed bed hybrid biological reactor for oilfield produced water treatment: Influence of hydraulic retention time

This study focuses on the development of a hybrid biological reactor for the treatment of synthetic oilfield produced water. To face increasingly strict regulations concerning produced water discharge, a fixed bed hybrid biological reactor (FBHBR) containing a combination of free activated sludge and a fixed biofilm support was compared to a conventional activated sludge reactor (CAS). After gradual microbial acclimation, a 133-day experiment showed that both bioreactors were able to efficiently remove phenol, toluene, xylenes, and polycyclic aromatic hydrocarbons (PAHs) from a synthetic wastewater with a chemical oxygen demand (COD) removal rate above 95%, at hydraulic retention times (HRT) of 24 h and 18 h, and that only the FBHBR was able to maintain high removal efficiency at an HRT of 12 h. Ecotoxicity tests showed that outlet waters from both bioreactors were non-toxic. Assessment of the bacterial population revealed notable differences between the CAS reactor and FBHBR. In particular, wider diversity was observed in the FBHBR. The marked similarity between the bacterial composition of the free sludge and that of the biofilm in the FBHBR suggests that biofilm detachment played an important part role in bacterial development in the free sludge.

KMS platform: A complete tool for modeling chemical and biochemical reactors

Computer-based activities to study chemical kinetic principles have been widely used in engineering education for the last 60 years. These academic tools must be user-friendly for teachers and students and easy to adapt to practical cases. The Chemical and Environmental Engineering Group (GIQA) at University Rey Juan Carlos (URJC) has developed a Kinetic, Modeling and Simulation (KMS) platform for chemical and biochemical processes. The KMS platform provides different simulation applications programmed in Matlab® with a friendly interface design environment for users. In this work, we will focus on the Kinetics in Conventional Activated Sludge (KCAS) application, as part of the KMS platform. The design of kinetic models and the dynamic simulation of wastewater treatment plants are typically implemented by a variety of dedicated software, mostly developed for research and development purposes at universities, institutes or consulting companies. These programming tools have inherent weaknesses as simulators in virtual laboratories for academics, needing steep learning curves and lacking flexibility to evaluate complex problems, particularly in bachelor degrees programs that are not completely devoted to the mathematical modeling of processes. The simplicity and versatility of KCAS simulator make it very useful for students as starting educational tool to define kinetic models and to make the dynamic simulation of a conventional activated sludge reactor followed by a secondary decanter using different inlet streams or operation conditions. The students interact with the program through a friendly Graphical User Interface (GUI), and the results can be easily exported to a conventional spreadsheet in EXCEL© format files. With KCAS, the students can formulate kinetic models (potential models, diffusional models, Monod equations, etc.) to simulate the evolution from simple reaction systems, to complex biochemical models such as ASM (Activated Sludge Models) from the International Water Association (IWA). The KCAS simulator allows the students to enhance the understanding of complex interactions among the physical, chemical, and biological processes, reinforcing the acquisition of competences and learning skills in Environmental Engineering. The KCAS application was implemented as laboratory demonstration practice in subjects of different Engineering Degrees at URJC.

Minimization of excess bio-sludge and pollution load in oxic-settling-anaerobic modified activated sludge treatment for tannery wastewater

Abstract The operational capability of anoxic and anaerobic upgradations to the conventional activated sludge process was tested. A mainstream oxic-settling-anaerobic configuration coupled to the conventional aerobic activated sludge was demonstrated the first time to elucidate the mechanism of excess sludge disruption and pollution removal from real tannery wastewater feed. The given oxic-settling-anaerobic coupled conventional activated sludge configuration, on the lab scale, was operated in parallel with an anoxic modified conventional activated sludge, and an identical conventional activated sludge reactor. The operational performance with emphasis to sludge yield minimization achieved that corresponds to operational strategies followed for the tested process configurations was compared. The oxic-settling-anaerobic coupled conventional activated sludge configuration confirmed around 51% of bio-sludge reduction, and prominent treated effluent quality compared with parallel run conventional activated sludge, and 21% less volatile accumulation than anoxic modified conventional activated sludge system. Furthermore, oxic-settling-anaerobic coupled conventional activated sludge implementation yields a lucrative reduction of feed pollutants including more than 90% removal of soluble and total chemical oxygen demand, total organic carbon, and ammonia nitrogen. Moreover, oxic-settling-anaerobic coupled conventional activated sludge configuration evidenced outstanding operational sustainability toward feed shock loading of tannery influent. The anoxic modified conventional system, on the contrary, witnessed a comparable removal of total chemical oxygen demand (89.8%), and ammonia nitrogen (91.7%). The present study evidenced the key influence of sludge interchange borne hydrolysis-acidification, energy uncoupling, and maintenance metabolism that further supported by batch anaerobic and aerobic digestion of nitrate, phosphate, sulfate, alkalinity, volatile fatty acids, and sludge pool present in the mixed liquid. Importantly, the experimental sludge yield recorded were processed to perform the simulation, and also modification of the sludge retention time correlation reported in the literature.

Redworm elimination in an integrated fixed-film activated sludge reactor

Two pilot-scale biological nutrient removal bioreactors were operated side by side under selected flow rate conditions. One reactor was operated under an integrated fixed-film activated sludge (IFAS) reactor configuration, and the second reactor was operated under a conventional activated sludge reactor configuration. The relatively high dissolved oxygen (DO) and low food-to-microorganism ratio created a favourable environment for the proliferation of redworm identified as Aeolosoma hemprichi in the IFAS reactor from day 116, which led to significant deterioration on ammonia removal in the IFAS reactor. The removal efficiencies of chemical oxygen demand (COD) and phosphorus were not significantly affected by the redworm bloom. Several strategies were applied to eliminate the redworms from the IFAS reactor. The most effective strategy was shutting off the DO supply in the aerobic zone and stopping the influent feed for 48 h. Finally, it took about 1·5 months for the IFAS reactor to recover from performing the worm elimination strategy.

Short operational differences support granulation in a lab scale reactor in comparison to another conventional activated sludge reactor

This study explains how small operational differences support excellent granulation in aerobic granular reactors. Short settling time promoted granulation in AGS reactor. Gene expressions based on mRNA revealed much higher ammonium monooxygenase (amoA) in conventional reactor biomass than in the aerobic granular reactor (AGS) biomass during a complete cycle operation. The number of glycogen accumulating organisms in conventional was much higher than in the granular reactor. The denitrifying functional genes in the granular systems were upregulated in anaerobic and aerobic phases. The granular reactor removed 1.84 kg COD-m−3day−1, 0.09 kg NH4+-N-m−3day−1, and 0.063 kg PO43—P-m−3day−1. The conventional reactor removed 1.14 Kg-m−3day−1 COD, 0.05 kg-m−3day−1 NH4+-N, and 0.028 kg-m−3day−1 PO43−-P. The granular reactor showed faster kinetics for nutrient and organics removal compared to the conventional reactor. Flocs in the conventional reactor had a lower abundance of Candidatus accumulibacter sp. and higher relative abundance of Candidatus competibacter.

Profile of organic carbon and nitrogen removal by a continuous flowing conventional activated sludge reactor with pulse aeration

Abstract This study aimed to investigate the effect of pulse aeration (on/off time 5/10 min) on the pollutants removal efficiencies and on the evolution of the denitrifying bacteria communities of a continuous flowing completely mixed activated sludge reactor. Organic matters and nitrogen removal were evaluated and the denitrifying bacteria community structure was analyzed by MiSeq sequencing technology. Results showed that the TOC removal rates were steadily above 81.6% and the NH4+-N removal rates were 92.1% ± 0.5% when the pulse aerated activated sludge reactor was operated with the optimized pulse aeration cycle (PAC) of 5/10 min. There was no significant impact on both TOC and NH4+-N removal efficiencies, while the average TN removal rate of the pulse aerated reactor (58.4%) was significantly higher than that of the constantly aerated one (30.6%). The removal efficiencies of both organics and nitrogen were stable during the 60-day acclimation period regardless of the changing DO concentration fluctuated in pulse mode. Although the denitrifier bacterial compositions varied between the pulse aerated group and the constantly aerated group, the denitrifier community richness and diversity were similar. Boost of the TN removal was mainly due to the anoxic denitrifying environment provided by the non-aerated phase in each PAC.

Biodegradation and Metabolism of Tetrabromobisphenol A (TBBPA) in the Bioaugmented Activated Sludge Batch Bioreactor System by Heterotrophic and Nitrifying Bacteria.

Tetrabromobisphenol A [2,4'-isopropylidenebis(2,6-dibromophenol)] has been identified in wastewater samples collected from the Guelph municipal wastewater treatment plant (GWWTP). In order to assess the kinetics and metabolic mechanisms of the dissolved TBBPA, bench scale experiments were completed with batch bioreactors. The biodegradation test was conducted by taking aerobic sludge from the conventional activated sludge reactor (CAS) and membrane bioreactor (MBR), and bioaugmenting both reactors with soil based strains of Bacillus brevis and Bacillus pumilus. This novel biodegradation process showed that the CAS bioreactor had a biodegradation rate of 0.127 d-1, while the sludge from the MBR had a biodegradation rate of 0.171 d-1. This is the first reported aerobic biodegradation of TBBPA by heterotrophic and nitrifying bacteria in an activated sludge bioreactor system. A tentative biotransformation metabolic scheme for TBBPA biodegradation and metabolite formation has been proposed as well.

Fate of selected pharmaceutically active compounds in the integrated fixed film activated sludge process.

The potential for integrated fixed film activated sludge (IFAS) processes to achieve enhanced transformation of pharmaceuticals relative to conventional activated sludge (CAS) processes was assessed. Previous studies have focused on direct comparisons of parallel reactors with and without fixed film carriers and little information is available on the impacts of how varying operating parameters impact the differences in observed pharmaceutical compound (PC) transformation capabilities between CAS reactors and those equipped with both an activated sludge (AS) and fixed film carriers. The testing was carried out using bench scale sequencing batch reactors fed with authentic municipal wastewater and operated at selected combinations of temperature and solids retention time (SRT). PC transformation efficiencies were assessed in a 22 factorial design that employed the IFAS and CAS processes, operated in parallel under identical process conditions. Nitrification rate testing that was conducted to obtain insight into the biomass activity demonstrated that IFAS consistently had improved nitrification kinetics despite lower mixed liquor volatile suspended solids levels thereby demonstrating the contribution of the biofilm to nitrification. Increased transformation of atenolol (ATEN; ranging from 10-60%) and trimethoprim (TRIM; ranging from 30-50%) in the IFAS equipped reactors relative to their respective activated sludge (AS) controls was observed under all experimental conditions. Negligible transformation of carbamazepine was observed in both reactors under all conditions investigated. More than 99% of acetaminophen was transformed in both configurations under all conditions. There was no correspondence between nitrification activity and TRIM removal in the control AS while conditions that stimulated nitrification in the control AS also resulted in enhanced removal of ATEN. The results of this study indicate that the integration of biofilms in AS processes enhances transformation of some PCs.

A Comparative Study on Kinetics, Process Performance and Microbial Ecology for Aerobic Granular and Conventional Activated Sludge reactors

A Comparative Study on Kinetics, Process Performance and Microbial Ecology for Aerobic Granular and Conventional Activated Sludge reactors

Sludge reduction via biodegradation of the endogenous residue (XE): experimental verification and modeling.

The feasibility of sludge reduction via the XE biodegradation process was explored both experimentally and through modeling, where the main focus was on determining the value of the bE parameter (first order degradation of XE) from a continuous process. Two activated sludge (AS) systems (30 L) were operated in parallel with synthetic wastewater during 16 months: a conventional activated sludge (CAS) system and a modified low-sludge production activated sludge (LSP-AS) process equipped with a side-stream digester unit (DU). First, the long term data of the CAS reactor (1 year) were used to calibrate the ASM model and to estimate the heterotrophic decay constant of the cultivated sludge (bH = 0.29 d-1, death-regeneration basis). Second, pre-simulations were performed to design the LSP-AS system and to estimate the DU volume required (40 L), to avoid XE accumulation in the process. Third, the LSP-AS process was built, put in operation and monitored for more than 9 months. This allowed assessment of the actual behavior of the quasi-complete solids retention system. Once calibrated, the modified AS model estimated the value of the bE parameter to be in the range of 0.003-0.006 d-1, satisfactorily describing the overall sludge yield reduction of up to 49% observed in the experiments.

Understanding the removal mechanisms of PPCPs and the influence of main technological parameters in anaerobic UASB and aerobic CAS reactors

The removal of 16 Pharmaceutical and Personal Care Products (PPCPs) were studied in a conventional activated sludge (CAS) unit and an upflow anaerobic sludge blanket (UASB) reactor. Special attention was paid to each biomass conformation and activity as well as to operational conditions. Biodegradation was the main PPCP removal mechanism, being higher removals achieved under aerobic conditions, except in the case of sulfamethoxazole and trimetrophim. Under anaerobic conditions, PPCP biodegradation was correlated with the methanogenic rate, while in the aerobic reactor a relationship with nitrification was found. Sorption onto sludge was influenced by biomass conformation, being only significant for musk fragrances in the UASB reactor, in which an increase of the upward velocity and hydraulic retention time improved this removal. Additionally, PPCP sorption increased with time in the UASB reactor, due to the granular biomass structure which suggests the existence of intra-molecular diffusion.

Recalcitrant organic matter removal from textile wastewater by an aerobic cell-immobilized pellet column

The treatment of textile wastewater is difficult because of its recalcitrant organic content. The biological removal of recalcitrant organics requires a long retention time for microbial growth. Activated sludge was immobilized in a polyethylene glycol pellet to allow for sufficient sludge retention time. The pellets were filled in an aerobic cell-immobilized pellet column (CIPC) reactor in order to investigate the removal of recalcitrant organics from textile wastewater. A textile wastewater effluent treated by a conventional activated sludge reactor was used as a target wastewater. The chemical oxygen demand (COD) removal efficiency of the aerobic CIPC reactor at various empty bed contact times was in the range of 42.2-60.5%. Half of the input COD was removed in the lower part (bottom 25% of the reactor volume) of the reactor when the organic loading rate was less than 1.5 kg COD/(m(3)•d). About 15-30% of the input COD was removed in the remaining part of the column reactor. The COD removed in this region was limitedly biodegradable. The biodegradation of recalcitrant organics could be carried out by the interactional functions of the various bacteria consortia by using a cell-immobilization process. The CIPC process could effectively treat textile wastewater using a short retention time because the microorganisms that degrade limitedly biodegradable organics were dominant in the reactor.

Biodegradation kinetic constants and sorption coefficients of micropollutants in membrane bioreactors

In order to elucidate the capability of biomass developed in membrane bioreactors (MBR) to degrade and sorb emerging micropollutants, biodegradation (kbiol) and sorption (ksor) kinetic constants as well as solid-liquid partition coefficients (Kd) of 13 selected pharmaceutical and personal care products (PPCPs) were determined with MBR heterotrophic biomass adding a pulse (100ppb of each compound) and following the liquid and solid phase concentrations over time. The results obtained were compared to literature data referring to conventional activated sludge (CAS) systems. Two experiments were performed: one in the MBR itself and the second one in a batch reactor with the same type and concentration of biomass as in the MBR. Overall, both biodegradation and sorption coefficients were in the same range as previously reported by other studies in CAS systems, indicating that MBR biomass does not show better capabilities for the biological degradation and/or sorption of PPCPs compared to the biomass developed in CAS reactors. Therefore, the higher PPCPs removal efficiencies found in MBRs are explained by the high biomass concentrations obtained at the long sludge retention times at which this type of reactors are usually operated.

Membrane bioreactor (MBR) sludge inoculation in a hybrid process scheme concept to assist overloaded conventional activated sludge (CAS) process operations

This study analyzes the effect of inoculating membrane bioreactor (MBR) sludge in a parallel-operated overloaded conventional activated sludge (CAS) system. Modelling studies that showed the beneficial effect of this inoculation were confirmed though full scale tests. Total nitrogen (TN) removal in the CAS increased and higher nitrate formation rates were achieved. During MBR sludge inoculation, the TN removal in the CAS was proven to be dependent on MBR sludge loading. Special attention was given to the effect of inoculation on sludge quality. The MBR flocs, grown without selection pressure, were clearly distinct from the more compact flocs in the CAS system and also contained more filamentous bacteria. After inoculation the MBR flocs did not evolve into good-settling compact flocs, resulting in a decreasing sludge quality. During high flow conditions the effluent CAS contained more suspended solids. Sludge volume index, however, did not increase. Laboratory tests were held to determine the threshold volume of MBR sludge to be seeded into the CAS reactor. Above 16-30%, supernatant turbidity and scum formation increased markedly.

Hybrid activated sludge/biofilm process for the treatment of municipal wastewater in a cold climate region: a case study

A hybrid activated sludge/biofilm process was investigated for wastewater treatment in a cold climate region. This process, which contains both suspended biomass and biofilm, usually referred as IFAS process, is created by introducing plastic elements as biofilm carrier media into a conventional activated sludge reactor. In the present study, a hybrid process, composed of an activated sludge and a moving bed biofilm reactor was used. The aim of this paper has been to investigate the performances of a hybrid process, and in particular to gain insight the nitrification process, when operated at relatively low MLSS SRT and low temperatures. The results of a pilot-scale study carried out at the Department of Hydraulic and Environmental Engineering at the Norwegian University of Science and Technology in Trondheim are presented. The experimental campaign was divided into two periods. The pilot plant was first operated with a constant HRT of 4.5 hours, while in the second period the influent flow was increased so that HRT was 3.5 hours. The average temperature was near 11.5°C in the overall experimental campaign. The average mixed liquor SRT was 5.7 days. Batch tests on both carriers and suspended biomass were performed in order to evaluate the nitrification rate of the two different biomasses. The results demonstrated that this kind of reactor can efficiently be used for the upgrading of conventional activated sludge plant for achieving year-round nitrification, also in presence of low temperatures, and without the need of additional volumes.

Influence of the employment of adsorption and coprecipitation agents for the removal of PPCPs in conventional activated sludge (CAS) systems

Three activated sludge reactors were operated to improve the removal of organic micropollutants such as Pharmaceutical and Personal Care Products (PPCPs). Reactor 1 (R1) was operated as a Conventional Activated Sludge reactor (CAS), Reactor 2 (R2) consisted of a CAS unit that was continuously fed with FeCl3 whereas granular activated carbon (GAC) was fed directly into the mixed liquor of Reactor 3 (R3) in order to attain concentrations in the range 100-1,000 mg/L. PPCPs removal rates varied depending on the compound present in each reactor during the entire 220 days of operation. Some substances showed the same behaviour in all reactors, such as the acidic pharmaceuticals naproxen and ibuprofen, which were almost completely removed (> 90%). More hydrophobic organic substances, like musk fragrances, were about 90% removed after 40 days of operation in all of the reactors. The main difference between the three reactors was obtained in R3 when the GAC concentrations in the aeration tank were around 500-1,000 mg/L. Under these conditions, the more recalcitrant compounds like diazepam and carbamazepine could be removed by up to 40%, and diclofenac up to 85%. Adsorption isotherms for PPCPs were obtained with activated carbon, and the results were successfully fitted to the Freundlinch equation. The more recalcitrant compounds (carbamazepine, diazepam and diclofenac) had the highest adsorption capacities onto GAC, which is consistent with the behaviour observed in R3 and helps to identify the removal mechanism (adsorption for these compounds, whereas absorption for fragrances).

Removal of Pharmaceutical and Personal Care Products (PPCPs) under nitrifying and denitrifying conditions

The contribution of volatilization, sorption and transformation to the removal of 16 Pharmaceutical and Personal Care Products (PPCPs) in two lab-scale conventional activated sludge reactors, working under nitrifying (aerobic) and denitrifying (anoxic) conditions for more than 1.5 years, have been assessed. Pseudo-first order biological degradation rate constants ( k biol) were calculated for the selected compounds in both reactors. Faster degradation kinetics were measured in the nitrifying reactor compared to the denitrifying system for the majority of PPCPs. Compounds could be classified according to their k biol into very highly ( k biol > 5 L g SS −1 d −1), highly (1 < k biol < 5 L g SS −1 d −1), moderately (0.5 < k biol < 1 L g SS −1 d −1) and hardly ( k biol < 0.5 L g SS −1 d −1) biodegradable. Results indicated that fluoxetine (FLX), natural estrogens (E1 + E2) and musk fragrances (HHCB, AHTN and ADBI) were transformed to a large extent under aerobic (>75%) and anoxic (>65%) conditions, whereas naproxen (NPX), ethinylestradiol (EE2), roxithromycin (ROX) and erythromycin (ERY) were only significantly transformed in the aerobic reactor (>80%). The anti-depressant citalopram (CTL) was moderately biotransformed under both, aerobic and anoxic conditions (>60% and >40%, respectively). Some compounds, as carbamazepine (CBZ), diazepam (DZP), sulfamethoxazole (SMX) and trimethoprim (TMP), manifested high resistance to biological transformation. Solids Retention Time (SRT aerobic >50 d and <50 d; SRT anoxic >20 d and <20 d) had a slightly positive effect on the removal of FLX, NPX, CTL, EE2 and natural estrogens (increase in removal efficiencies <10%). Removal of diclofenac (DCF) in the aerobic reactor was positively affected by the development of nitrifying biomass and increased from 0% up to 74%. Similarly, efficient anoxic transformation of ibuprofen (75%) was observed after an adaptation period of 340 d. Temperature (16–26 °C) only had a slight effect on the removal of CTL which increased in 4%.