Development of an integrated fixed-film activated sludge (IFAS) reactor treating landfill leachate for the biological nitrogen removal through nitritation-denitritation.

The current work investigated the performance of an Integrated Fixed-Film Activated Sludge Sequencing Batch Reactor (IFAS-SBR) for Biological Nitrogen Removal (BNR) from mature landfill leachate through the nitritation-denitritation process. During the experimental period two IFAS-SBR configurations were examined using two different biocarrier types with the same filling ratio (50%). The dissolved oxygen (DO) concentration ranged between 2 and 3 mg/L and 4-6 mg/L in the first (baseline-IFAS) and the second (S8-IFAS) setup, respectively. Baseline-IFAS operated for 542 days and demonstrated a high and stable BNR performance maintaining a removal efficiency above 90% under a Nitrogen Loading Rate (NLR) up to 0.45 kg N/m3-d, while S8-IFAS, which operated for 230 days, was characterized by a limited and unstable BNR performance being unable to operate sufficiently under an NLR higher than 0.20 kg N/m3-d. It also experienced a severe inhibition period, when the BNR process was fully deteriorated. Moreover, S8-IFAS suffered from extensive biocarrier stagnant zones and a particularly poor sludge settleability. The attached biomass cultivated in both IFAS configurations had a negligible content of nitrifying bacteria, probably attributed to the insufficient DO diffusion through the biofilm, caused by the low DO concentration in the liquid in the baseline case and the extensive stagnant zones in the S8-IFAS case. As a result of the high biocarrier filling ratio, the S8-IFAS was unstable and low. This was probably attributed to the mass transfer limitations caused by the biocarrier stagnant zones, which hinder substrate and oxygen diffusion, thus reducing the biomass activity and increasing its vulnerability to inhibitory and toxic factors. Hence, the biocarrier filling fraction is a crucial parameter for the efficient operation of the IFAS-SBR and should be carefully selected taking into consideration both the media type and the overall reactor configuration.

Inhibition of free nitrous acid and free ammonia on polyphosphate accumulating organisms: Evidence of insufficient phosphorus removal through nitritation-denitritation.

The purpose of this study is to investigate the effect of Free Nitrous Acid (FNA) and Free Ammonia (FA) on enhanced biological phosphorus removal (EBPR) and in particular on the aerobic phosphorus uptake rate (PUR). To this end, a PAO-enriched biomass was developed at a lab-scale reactor in order to fuel a series of ex-situ batch experiments to test the effect of various nitrite or ammonium concentrations on the phosphorus uptake rate at different pH values. FNA was found to be a strong inhibitor of EBPR, in agreement with other studies with PUR being inhibited by 50 % under 1.5 µg HNO2-N L-1 and 100 % at 13 µg HNO2-N L-1. FA was also found to inhibit EBPR with PUR being inhibited by 50 % under 6.4 mg NH3-N L-1. The results of this study suggest that EBPR under high nitrogen loading alongside nitritation-denitritation may not be a viable option.

The role of activated carbon and disinfection on the removal of endocrine disrupting chemicals and non-steroidal anti-inflammatory drugs from wastewater.

Endocrine disrupting chemicals and non-steroidal anti-inflammatory drugs are two important groups of emerging pollutants due to their toxicological and chemical characteristics and their persistent detection in the aquatic environment. Wastewater treatment plants are a significant pathway for their transfer to the water courses. It is well evidenced that these chemicals are only partially removed through biological treatment of wastewater and therefore being detected in secondary effluents. This work focuses on the evaluation of the efficiency of two well-established disinfection technologies (chlorination and UV irradiation) along with UV/H2O2 and powdered activated carbon (PAC) to remove these chemicals from biologically treated wastewater. Based on the results it is shown that appreciable removal efficiencies due to chlorination should be expected for most of the target compounds, whereas this was not the case for ibuprofen and ketoprofen. With the exemption of diclofenac and ketoprofen direct UV irradiation did not efficiently removed target compounds for UV doses usually applied for disinfection purposes. The application of advanced UV treatment through the addition of H2O2 although resulted in increased removal of the target compounds is not sufficient at moderate UV and H2O2 doses to achieve satisfactory removal efficiencies. PAC use resulted in sufficient removal of target compounds although high PAC doses were required for some chemicals. Comparison of Freundlich isotherms of this study with those of other studies, derived employing water samples, suggested that the water matrix along with the target compounds concentration range can significantly affect the outcome of the experiments.

Evaluating an integrated nano zero-valent iron column system for emerging contaminants removal from different wastewater matrices - Identification of transformation products.

The presence of micropollutants in water bodies has become a growing concern due to their persistence, bioaccumulation and potential toxicological effects on aquatic life and humans. In this study, the performance of a column system consisting of zero-valent iron nanoparticles (nZVI) incorporated into a cationic resin and synthesized from green tea extract with the addition of persulfate for the elimination of selected pharmaceuticals and endocrine disruptors from wastewater is evaluated. Ibuprofen, naproxen, diclofenac and ketoprofen were the target pharmaceuticals from non-steroidal anti-inflammatory drugs group, while bisphenol A was the target endocrine disruptor. In this context, different real wastewater effluent matrices were investigated: anaerobic membrane bioreactor (AnMBR), upflow anaerobic sludge blanket reactor (UASB) after microfiltration, tertiary treated by conventional activated sludge system and saturated vertical constructed wetland followed by a sand filtration unit effluent (hybrid). The transformation products of diclofenac and bisphenol A were also identified. The experimental results indicated that the performance of the R-nFe/PS system towards the removal efficiency of the target compounds was enhanced in the order of effluents: tertiary > AnMBR ≈ hybrid > UASB. More than 70% removal was obtained for almost all target compounds when conventional tertiary effluent was used, while the maximum removal efficiency was about 50% in the case of filtered UASB. As far as we know, this is the first time that nZVI has been assessed in combination with persulfate for the removal of micropollutants in a continuous flow system receiving various types of real wastewater with different matrix characteristics.

Investigating upflow anaerobic sludge blanket process to treat forward osmosis effluents of concentrated municipal wastewater under psychrophilic temperature.

This work investigated the stability of the Upflow Anaerobic Sludge Blanket (UASB) reactor under psychrophilic temperatures with varying feed streams, simulating typical and concentrated sewage. In Phase I, treating municipal wastewater, chemical oxygen demand (COD) removal dropped from 77 ± 6 % to 41 ± 2 % as hydraulic retention time decreased from 24 to 12 h and organic loading rate (OLR) increased from 0.6 to 1.3 gCOD/(L∙d). In Phase II, at a similar OLR (≈1.2 gCOD/(L∙d)), the UASB treated organic-rich effluents (from 1.0 to 2.1 ± 0.1 gCOD/L) resulting from the pre-treatment of the forward osmosis (FO) process. The UASB performance improved significantly, achieving 87 ± 3 % COD removal and 63 ± 4 % methane recovery, with microbial analysis confirming methanogen growth. The COD mass balance showed up to 30 % more electrical energy recovery from sewage compared to conventional wastewater treatment plants (WWTPs), indicating that the FO-UASB combination is a promising approach to achieve energy-neutral operation in WWTPs.

Comparing the performance of microbial electrochemical assisted and aerated treatment wetlands in pilot-scale: Removal of major pollutants and organic micropollutants.

The combination of treatment wetlands (TWs) with microbial electrochemical technologies (MET) is often studied in the lab to improve the performance and decrease the footprint of TWs. In this article we evaluated the long-term performance of four pilot-scale vertical sub-surface flow TWs for major pollutants' and organic micropollutants' removal from domestic wastewater. Three of them were filled with electroconductive material and operated under saturated (MET SAT), unsaturated (MET UNSAT) and unsaturated-saturated (MET HYBRID) conditions while the fourth one was a saturated intensified aerated system (AEW) filled with gravel. The MET-TWs achieved significant removals of COD (>78 %) with no clogging issues at the maximum applied OLR (249 g COD m-3 d-1) while under these loading conditions TSS removal exceeded 84 %. Among all electroactive TWs, UNSAT could remove 25 g NH4-N m-3 d-1 through nitrification when peak ammonium loading rate was applied; however this removal was significantly lower than AEW (35 g NH4-N m-3d-1). No important removal of P was observed in all systems with the exception of MET-SAT were precipitation reactions of P with iron occurred when anaerobic pretreated wastewater was used. The removal of the sum of studied organic micropollutants ranged between 70 ± 18 % (MET UNSAT) to 91 ± 4 % (AEW) and improved with feeding pulses increase. Moderate to high removal of specific microcontaminants was observed depending on the target compound, the studied system and the operational conditions. AEW and MET HYBRID systems complied with the limits set by EU for wastewater discharge to non-sensitive water bodies and for Class B water reuse. Scale-up calculations for a settlement of 500 PE showed that these systems require much less area per PE (0.51 m2 PE-1) comparing to conventional TWs while the operational cost was calculated to 0.07 € m-3 for the AEW and 0.02 € m-3 for the MET HYBRID.

Investigation of long-term operation and biomass activity in a membrane bioreactor system.

The aim of this work was to evaluate the long-term performance of a Membrane Bioreactor (MBR) that operated continuously for 2.5 years and to assess membrane fouling and biomass activity under various operating conditions. Furthermore, a method for the characterisation of influent wastewater was developed based on its separation into various fractions. The MBR system operated at the solids retention times (SRT) of 10, 15, 20 and 33 days. The increase of SRT resulted in a decrease of the fouling rate associated with the reduction of extracellular polymeric substances. Moreover, the SRT increase resulted in a significant reduction of the Oxygen Uptake Rate (OUR) due to the lower availability of substrate and in a notable decrease of the maximum OUR since high SRT allowed the development of slower growing microorganisms. Biomass consisted of small flocs due to extensive deflocculation caused by intense aeration. Finally, the method developed for wastewater characterisation is straightforward and less time consuming than the usual method that is employed.

Model-based evidence for the relevance of microbial community variability to the efficiency of the anaerobic reductive dechlorination of TCE.

The composition of mixed dechlorinating communities varies considerably in field and laboratory conditions. Dechlorinators thrive alongside with distinctive populations that help or hinder dechlorination. The variability of the composition of dechlorinating communities inevitably precludes a firm consensus regarding the optimal strategies for biostimulation. This lack of consensus motivated a model-based approach for the investigation of how the variability of the composition of a microbial community impacts the electron donor supply strategies for accelerating chloroethene removal. To this end, a kinetic model accounting for dechlorination in conjunction with cooperative and competing processes was developed. Model parameters were estimated using a multi-experiment, multi-start algorithm and data from research previously performed with two generations of a methane-producing, Dehalococcoides mccartyi-dominated consortium. The two generations of the consortium functioned comparably under maintenance conditions but performed divergently under high electron donor surpluses. The multi-experiment, multi-start algorithm overcame the hurdles of poor parameter identifiability and offered a probable cause for the different behaviors exhibited by each of the two generations of the chloroethene-degrading consortium: modest differences in the make-up of non-dechlorinators, which were minority populations, significantly influenced the fate of the offered electron donor.

Fate of Emerging Contaminants in High-Rate Activated Sludge Systems.

High-rate activated sludge (HRAS) systems are designed to shift the energy-intensive processes to energy-saving and sustainable technologies for wastewater treatment. The high food-to-microorganism (F/M) ratios and low solid retention times (SRTs) and hydraulic retention times (HRTs) applied in HRAS systems result in the maximization of organic matter diversion to the sludge which can produce large amounts of biogas during anaerobic digestion, thus moving toward energy-neutral (or positive) treatment processes. However, in addition to the energy optimization, the removal of emerging contaminants (ECs) is the new challenge in wastewater treatment. In the context of this study, the removal efficiencies and the fates of selected ECs (three endocrine disruptors (endocrine disrupting chemicals (EDCs))-nonylphenol, bisphenol A and triclosan, and four pharmaceuticals (PhACs)-ibuprofen, naproxen, diclofenac and ketoprofen) in HRAS systems have been studied. According to the results, EDCs occurred in raw wastewater and secondary sludge at higher concentrations compared to PhACs. In HRAS operating schemes, all compounds were poorly (<40%) to moderately (<60%) removed. Regarding removal mechanisms, biotransformation was found to be the dominant process for PhACs, while for EDCs sorption onto sludge is the most significant removal mechanism affecting their fates and their presence in excess sludge.

Diuron biodegradation in activated sludge batch reactors under aerobic and anoxic conditions.

Diuron biodegradation was studied in activated sludge reactors and the impacts of aerobic and anoxic conditions, presence of supplemental substrate and biomass acclimatization on its removal were investigated. Diuron and three known metabolites, namely DCPMU (1-(3,4-dichlorophenyl)-3-methylurea), DCPU (1-3,4-dichlorophenylurea) and DCA (3,4-dichloroaniline), were extracted by solid-phase extraction (dissolved phase) or sonication (particulate phase) and determined using High Performance Liquid Chromatography-Diode Array Detector (HPLC-DAD). During the experiments only a minor part of these compounds was associated with the suspended solids. Under aerobic conditions, almost 60% of Diuron was biodegraded, while its major metabolite was DCA. The existence of anoxic conditions increased Diuron biodegradation to more than 95%, while the major metabolite was DCPU. Mass balance calculation showed that a significant fraction of Diuron is mineralized or biotransformed to other unknown metabolites. The presence of low concentrations of supplemental substrate did not affect Diuron biodegradation, whereas the acclimatization of biomass slightly accelerated its elimination under anoxic conditions. Calculation of half-lives showed that under aerobic conditions DCPMU, DCPU and DCA are biodegraded much faster than the parent compound. In the future, the sequential use of anoxic and aerobic conditions could provide sufficient removal of Diuron and its metabolites from runoff waters.

Microthrix parvicella and Gordona amarae in mesophilic and thermophilic anaerobic digestion systems.

The scope of the study presented in this paper is to determine the fate of the filamentous bacteria Gordona amarae and Microthrix parvicella in anaerobic digestion operating under mesophilic and thermophilic conditions. In order to detect and quantify foaming bacteria in the anaerobic digesters, a fluorescent in situ hybridization (FISH) method was developed and applied. This paper presents the results of a laboratory-scale study that involved the operation of four lab-scale anaerobic digestion systems operating in the mesophilic (35 degrees C) and thermophilic (55 degrees C) temperature ranges at 20 days' detention time. According to the FISH counts of G. amarae and M. parvicella, it appears that thermophilic conditions resulted in a higher destruction of both filamentous bacteria, averaging approximately 97% and 94% for the single thermophilic digester and the dual thermophilic/mesophilic system, respectively. Within the context of this study, the overall performance of the four different anaerobic digestion systems was evaluated in terms of biogas production per mass of volatile solids destroyed, COD destruction, sludge dewaterability and foaming characteristics. The dual stage systems used in this study outperformed the single stage digesters.

Reductive dechlorination of trichloroethene under different sulfate-reducing and electron donor conditions.

The effect of sulfate presence on reductive dechlorination of chlorinated ethenes has been a matter of conflict among the limited reports found in literature. This paper aims to clarify the misconceptions regarding the performance of trichloroethene biotransformation under sulfate reducing conditions by evaluating the effect of different sulfate concentrations on reductive dechlorination and to assess the influence of electron donor dose on dechlorination rate. To this end, batch experiments containing different sulfate and butyrate concentrations were conducted using trichloroethene-dechlorinating and sulfate-reducing parent cultures. Results demonstrated that if sufficient time and electron donor is provided, complete dechlorination can be achieved, even at up to 400 mg/L initial sulfate concentration. However, the rate of dichloroethene and vinyl chloride degradation is reduced as sulfide concentration increases. Moreover, the excess electron donor dose induced a slightly slower dechlorination rate. The findings of this paper present an explanatory framework for the dechlorination of TCE under sulfate reducing conditions and can contribute to the state-of-art bioremediation of contaminated sites.

Analytical and mathematical assessment of emerging pollutants fate in a river system.

The fate of several emerging pollutants in a Greek river system was assessed through analytical measurements and mathematical modelling. Target compounds selected in this study consist of five endocrine disrupting chemicals and four non-steroidal anti-inflammatory drugs. Two sampling campaigns were implemented to assess target compounds concentrations along the river system during dry period. Furthermore a mathematical model was developed in order to simulate the spatial distribution of target compounds concentration. The mathematical model describes several abiotic and biotic processes (sorption, photodegradation, biodegradation, biotransformation) in order to account for the removal of target compounds. Following sensitivity analysis, the model was calibrated and validated against measured values. Environmental risk assessment was performed based on both analytical measurements and simulation results. Uncertainty analysis was also conducted by applying Monte Carlo technique. According to the results the simulation data matched very satisfactorily with the analytical measurements, thus confirming the main experimental observations showing that the primary removal mechanism for the photo-sensitive chemicals is photodegradation, the latter being mostly influenced by weather conditions and river general quality characteristics (e.g. chlorophyll, turbidity). Model results demonstrate a gradual increase of uncertainty from the upstream to the downstream of the river system for all target compounds.

Occurrence and fate of endocrine disrupters in Greek sewage treatment plants.

The occurrence of five endocrine-disrupting compounds (EDCs), namely 4-n-nonylphenol (4-n-NP), nonylphenol monoethoxylate (NP1EO), nonylphenol diethoxylate (NP2EO), triclosan (TCS) and bisphenol A (BPA), was assessed in the raw, treated wastewater and sewage sludge of eight sewage treatment plants (STPs) in Greece. The analytes were extracted by solid-phase extraction (dissolved phase) or sonication (solid phase). Qualitative and quantitative analyses were performed by gas chromatography-mass spectrometry (GC-MS). The average concentrations in the raw and treated wastewater ranged from 0.23 (4-n-NP) to 5.76microgL(-1) (NP1EO) and from 0.15 (BPA) to 1.84microgL(-1) (NP2EO), respectively. A great part of the detected EDCs was sorbed on suspended solids. In sewage sludge, the average concentrations ranged between 0.17 (4-n-NP) and 12.3microgg(-1)dw (NP1EO). Analysis of daily mass flows in STP of Athens showed that, with the exception of 4-n-NP, all other EDCs were significantly removed (>85%) during wastewater treatment. Regarding the fate of these compounds, a significant part ranging from 45% (for TCS) to more than 70% (for NP1EO, NP2EO and BPA) was transformed by abiotic or biotic mechanisms, while the rest was accumulated in sewage sludge or disposed to the environment via the effluents. Calculation of risk quotients showed the existence of possible threat due to the presence of certain EDCs in treated wastewater and sludge.

Application of the OECD 301F respirometric test for the biodegradability assessment of various potential endocrine disrupting chemicals.

The biodegradability of several potential endocrine disrupting compounds, namely 4-n-nonylphenol (4-n-NP), nonylphenol monoethoxylate (NP1EO), nonylphenol diethoxylate (NP2EO), bisphenol A (BPA), triclosan (TCS), di-(2-ethylhexyl)-phthalate (DEHP), perfluorooctanoate (PFOA) and perfluorononanoate (PFNA) was evaluated in this study, using OECD method 301F (manometric respirometry test) and activated sludge as inoculum. According to the results, 4-n-NP and BPA meet the strict definition of ready biodegradability and they are not expected to be persistent during the activated sludge process. Partial biodegradation was observed for DEHP (58.7+/-5.7%, n=3), TCS (52.1+/-8.5%, n=3) and NP1EO (25.9+/-8.1%, n=3), indicating their possible biodegradation in wastewater treatment systems, while no biodegradation was observed for NP2EO, PFOA and PFNA. Experiments in the co-presence of a readily biodegradable compound showed the absence of co-metabolic phenomena during 4-n-NP, BPA and TCS biodegradation. Using first order kinetics to describe biodegradation of the target compounds, half-lives of 4.3+/-0.6, 1.3+/-0.2, 1.8+/-0.5, 6.9+/-2.6 days were calculated for 4-n-NP, BPA, TCS and DEHP, respectively. Toxicity tests using marine bacterium Vibrio fischeri showed that biodegradation of 4-n-NP, NP1EO, BPA and TCS is a simultaneous detoxification process, while possible abiotic or biotic transformations of NP2EO, DEHP, PFOA and PFNA during respirometric test resulted to significant increase of their toxicities.

Inhibitory effect of triclosan and nonylphenol on respiration rates and ammonia removal in activated sludge systems.

The toxic effects of triclosan (TCS) and nonylphenol (4-n-NP) on activated sludge heterotrophic and autotrophic microorganisms were evaluated. Toxicity experiments with specific oxygen uptake rate (SOUR) and ammonia uptake rate (AUR) revealed that TCS was much more toxic to heterotrophic and autotrophic microorganisms than 4-n-NP. In experiments with heterotrophic biomass, increase of sludge age (theta(c)) from 5 to 15 days resulted in a decrease of median effective concentrations (EC(50)) of TCS from 38.2 to 9.97 mg l(-1) and in an increase of EC(50) values of 4-n-NP from 441 to 649 mg l(-1). In experiments with autotrophic biomass and sludge age of 15 days, significantly lower EC(50) values were obtained for both compounds, indicating the higher sensitivity of nitrifiers to TCS and 4-n-NP. To compare toxicity of TCS and 4-n-NP towards single species and mixed wastewater cultures, experiments were performed using marine bacterium Vibrio fischeri. EC(50) values of 0.22 and 3.51 mg l(-1) were estimated for TCS and 4-n-NP, respectively, indicating the higher sensitivity of this bioassay to toxicants. According to the levels of tested compounds commonly found in influent wastewater and the results of this study, there is a possible risk for deterioration of nitrification in activated sludge systems due to the presence of TCS.

Comparison of bioluminescence and nitrification inhibition methods for assessing toxicity to municipal activated sludge.

The aim of this study was to compare two alternative toxicity assessment methods to determine wastewater toxicity and predict treatment plant process upsets. The toxicity of two synthetic organic compounds (triclosan and 4-n-nonylphenol), which are commonly detected in municipal wastewater, and municipal and industrial wastewaters with different heavy metals content were evaluated by the nitrification inhibition assay and bioluminescence toxicity test. Comparison between both assays confirmed that Vibrio fischeri is generally more sensitive than autotrophic bacteria, and, if not calibrated, the bioluminescence method tends to overestimate toxic effects on activated sludge biomass. The nitrification inhibition assay appears to predict plant process upsets more accurately. Both methods showed a significant toxicity decrease through treatment that could be partially attributed to the significant heavy metals removal obtained by primary and secondary treatment. A good correlation for the two assays was obtained, as indicated by a high correlation coefficient (r2 = 0.80).

Assessment of the environmental fate of endocrine disrupting chemicals in rivers.

Laboratory tests were conducted with five endocrine disruptors (bishenol A, triclosan. nonylphenol, nonylphenol monoethoxylate and nonylphenol diethoxylate) under different redox conditions (aerobic, anoxic, anaerobic and sulfate-reducing conditions) to assess abiotic and biotic degradation in a river water/sediment system. The river water sample was collected from Spercheios River while the sediment was collected from the banks of a tributary of the river at the point where the discharge point of a wastewater treatment plant is located. To describe quantitatively elimination kinetics of the target compounds, pseudo first-order kinetics were adopted. According to the results from the microcosms studies, it can be stated that the substances are eliminated from the aqueous phase with relatively high rates under aerobic conditions due to both sorption and biotransformation processes. However, when reduced oxygen conditions were established in the microcosms incubations, biotransformation decreased, indicating the almost complete cease of the EDCs microbial degradation, while substances' sorption onto sediments showed no significant differences. All compounds were found to be biodegradable under aerobic conditions, and the low to high order of the calculated dissipation rate constants was 0.064±0.004d-1 (TCS)→0.067±0.006d-1 (NP)→0.076±0.009d-1 (NP2EO)→0.081±0.007d-1 (NP1EO)→0.103±0.011d-1 (BPA). Finally, regarding the biotransformation experiments, the elimination of the compounds limited in the absence of oxygen as compared to aerobic.

Investigation of triclosan fate and toxicity in continuous-flow activated sludge systems.

The purpose of this research was to study the fate and toxicity of triclosan (TCS) in activated sludge systems and to investigate the role of biodegradation and sorption on its removal. Two continuous-flow activated sludge systems were used; one system was used as a control, while the other received TCS concentrations equal to 0.5 and 2mgl(-1). At the end of the experiment, 1mgl(-1) TCS was added in the control system to investigate TCS behaviour and effects on non-acclimatized biomass. For all concentrations tested, more than 90% of the added TCS was removed during the activated sludge process. Determination of TCS in the dissolved and particulate phase and calculation of its mass flux revealed that TCS was mainly biodegraded. Activated sludge ability to biodegrade TCS depended on biomass acclimatization and resulted in a mean biodegradation of 97%. Experiments with batch and continuous-flow systems revealed that TCS is rapidly sorbed on the suspended solids and afterwards, direct biodegradation of sorbed TCS is performed. Regarding TCS effects on activated sludge process, addition of 0.5mgl(-1) TCS on non-acclimatized biomass initially deteriorated ammonia removal and nitrification capacity. After acclimatization of biomass, nitrification was fully recovered and further increase of TCS to 2mgl(-1) did not affect the performance of activated sludge system. The effect of TCS on organic substrate removal was minor for concentrations up to 2mgl(-1), indicating that heterotrophic microorganisms are less sensitive to TCS than nitrifiers.

Dechlorinating ability of TCE-fed microcosms with different electron donors.

The main objective of the work presented herein is to assess the effect of different electron donors (butyric acid and methanol) on the dechlorinating activity of two microbial cultures where active methanogenic populations are present, in an effort to evaluate the importance of the electron donor selection process. The ability of each anaerobic culture to dechlorinate TCE, when enriched with either butyric acid or methanol, was verified based on the results of gas chromatography. In addition, the fluorescent in situ hybridization (FISH) and the polymerase chain reaction (PCR) methods gave positive results for the presence of Dehalococcoides spp. According to results of the batch tests conducted in this study, it appears that the selection of the electron donor for stimulating TCE dechlorination depends on microbial culture composition; therefore, the decision on the appropriate electron donor should be based on site-specific microcosm studies.