Barrier permeation properties of EVOH thin-film membranes under aqueous and non-aqueous conditions

Ethylene vinyl alcohol (EVOH) copolymers can provide a superior barrier to hydrocarbons and are increasingly being used in co-extruded geomembranes for geoenvironmental applications. These thin-films behave differently under different humidity conditions. This study investigated the permeation properties of toluene through two EVOH thin-films (32?mol% ethylene and 44?mol% ethylene) for both non-aqueous and aqueous solutions. The results of this study are used to gain a better understanding of the behaviour of the EVOH layer used in co-extruded geomembranes. The thin-film results are compared with published values for co-extruded linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) geomembranes with an EVOH core. Permeation coefficients are presented over a range of contaminant concentrations from 25?ppm to 99% toluene based on almost five years of continuous testing and the effect of moisture is discussed. A number of EVOH thin-films were affected by humidity (i.e., where moisture diffused into the film) prior to diffusion testing under non-aqueous conditions. This observation led to an investigation of the effect of moisture uptake on the permeation of toluene under non-aqueous testing. In these cases, the 44?mol% thin-film had lower toluene permeation coefficient values than the 32?mol% thin-film. These values were similar to toluene permeation coefficient values from tests with aqueous solutions. When relative humidity was less than 60%, the 32?mol% had slightly lower permeability values than 44?mol% thin-film. However, even when affected by humidity, the permeability of both thin-films were considerably (two to three orders of magnitude) lower than previously observed in a water-saturated solution. Permeation of toluene from a 1/1 toluene/hexane solution was also examined for the 32?mol% EVOH thin-film at temperatures of 23–50?°C and results fit well with a conventional Arrhenius relationship of increasing P_g values with increasing temperature.     

Effect of prehydration,permeant, and desiccation on GCL/Geomembrane interface transmissivity

A laboratory investigation was conducted on two different conventional GCLs (one with fine granular and another one with powdered bentonite) to explore the effect of prehydration and permeant fluid; GCL desiccation on the interface transmissivity, θ, between the interfaces of a 1.5 mm-thick high-density polyethylene (HDPE) geomembrane (GMB) and a GCL. The study also aimed to assess the self-healing capacity of desiccated GCLs for three different permeant solutions under a range of applied stresses (10–150 kPa). It was found that at stresses less than 70 kPa, θ was dominated by variability in the initial contact condition between the GMB-GCL interfaces. The effect of other factors was largely masked by the contact variability. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors on θ in the absence of desiccation. GCL desiccation increased θ by up to three orders of magnitude than an intact specimen at 10–100 kPa. Even at 150 kPa, desiccated specimens had a θ ≤ 8.0 × 10⁻⁹ m²/s for all specimens tested. The chemical composition of the permeant solutions, crack width, and nature of bentonite could play an important role in healing the cracks of desiccated GCLs.     

Comparison of the equilibrium sorption of five organic compounds to HDPE, PP, and PVC geomembranes

This experimental investigation quantified the sorption uptake of five commonly encountered organic groundwater contaminants, methyl tertiary-butyl-ether (MTBE), benzene, trichloroethylene (TCE), 1,2-dichorobenzene (1,2-DCB), and trinitrotoluene (TNT), to geomembranes made from high density polyethylene (HDPE), polypropylene (PP), and polyvinylchloride (PVC). The organic compounds were chosen to span a range of aqueous solubilities and chemical properties. The geomembranes tested in this study exhibited sorption capacities that were of similar magnitude for each of the contaminants tested, with the exception of 1,2-DCB to HDPE, which exhibited strong uptake in comparison to the other solute/sorbent combinations. In general, the PVC geomembrane demonstrated the highest sorption capacities, while the HDPE geomembrane demonstrated the lowest sorption capacities. Measured partitioning coefficients for the contaminant/geomembrane combinations ranged from S_gf<1 to 160, but most commonly had values between 10 and 75.     

Exposure to fine,ultrafine particles and black carbon in two preschools in nur-sultan city of kazakhstan

Children in preschools were studied as an exceptionally vulnerable group to lung diseases due to their immature immune system. Few data are available in the literature addressing the exposure of children in preschools to ultrafine (>10 nm) particles. Exposure of children to fine, ultrafine (10 nm–1 µm) particles and black carbon particles present inside and near two preschools in Nur-Sultan, Kazakhstan, during Fall 2019 was investigated. For Preschool I, the average daily (6 h) indoor (outdoor) PM₁, PM_2.5, and PM₁₀ concentrations over three-week measurements were 15.0 (SD 12.5) µg/m³, 34.6 (SD 35.1) µg/m³, and 47.2 (SD 45.2) µg/m³, respectively. Average indoor UFP concentrations (>10.0 nm) including candle burning events were 5.20 × 10³ (SD 8.80 × 10³) particles/cm³, with the background UFP concentration to be 3.30 × 10³ (SD 1.80 × 10³) particles/cm³. In Preschool II, the average UFP concentration (>30.0 nm) in the morning and afternoon was 3.94 × 10³ (SD 5.34 × 10²) and 3.36 × 10³ (SD 1.90 × 10³) particles/cm³, respectively. Indoor black carbon (BC) concentrations were correlated with the outdoor smoking activity. The major sources of the indoor particles in the preschools were dust resuspension, candle burning, and infiltrated outdoor particles.     

Degradation of HDPE,LLDPE, and blended polyethylene geomembranes in extremely low and high pH mining solutions at 85 °C

The durability of five 1.5-mm thick geomembranes (GMBs) is investigated in pH 0.5 and 13.5 synthetic mining solutions using immersion tests. Two high density polyethylene (HDPE), two linear low density polyethylene (LLDPE), and one blended polyethylene (BPO) GMBs are investigated at 85 °C for incubation durations of 4.5–6.5 years. It is shown that the degradation of all five GMBs in the high pH solution is faster than in the low pH solution. In the pH 0.5 solution, one of the HDPEs and the BPO GMBs exhibited polymer degradation before or at the time of the depletion of their antioxidants. In pH 13.5, four out of the five GMBs exhibited degradation and followed the conceptual three-stage degradation model until nominal failure. However, there is no correlation between the long-term performance of these GMBs and their resin type or their initial properties since one of the examined LLDPEs outperformed all the higher density/crystallinity GMBs with higher initial properties while the other LLDPE did not perform well. Thus, when selecting a GMB for a desired application, the relative performance of different candidate GMBs can be only assessed using immersion tests using the solutions expected in the field.     

Hydraulic conductivity of bentonite-polymer geosynthetic clay liners to coal combustion product leachates

Hydraulic conductivity of seven geosynthetic clay liners (GCLs) to synthetic coal combustion product (CCP) leachates were evaluated in this study. The leachates are chemically representative of typical and worst scenarios observed in CCP landfills. The ionic strength (I) of the synthetic CCP leachates ranged from 50 mM to 4676 mM (TCCP-50, LRMD-96, TFGDS-473, LR-2577, HI-3179 and HR-4676). One of the GCLs contained conventional sodium bentonite (Na–B) and the other six contained bentonite-polymer (B–P) mixture with polymer loadings ranging from 0.5% to 12.7%. Hydraulic conductivity tests were conducted at an effective confining stress of 20 kPa. The hydraulic conductivity of the Na–B GCLs were >1 × 10⁻¹⁰ m/s when permeated with all six CCP leachates, whereas the B–P GCLs with sufficient polymer loading maintained low hydraulic conductivity to synthetic CCP leachates. All the B–P GCLs showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) to low ionic strength leachates (TCCP-50, I = 50 mM and LRMD-96, I = 96 mM). B–P GCLs with P > 5% showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) up to HI-3179 leachates. These results suggest that B–P GCLs with sufficient polymer loading can be used to manage aggressive CCP leachates.     

Inhibition of aerobic metabolic cis-1,2-di-chloroethene biodegradation by other chloroethenes

The presence of other chloroethenes influences aerobic metabolic biodegradation of cis-1,2-dichloroethene (cDCE). A new metabolically cDCE degrading enrichment culture was identified as also being capable of degrading vinyl chloride (VC), but not 1,1-dichloroethene (1,1DCE), trans-1,2-dichloroethene (tDCE), trichloroethene (TCE), or tetrachloroethene (PCE). The fastest degradation of cDCE was observed in the absence of any other chloroethene. In the presence of a second chloroethene (40-90 μM), the rate of cDCE (60 μM) degradation decreased in the following order: cDCE (+PCE) > cDCE (+tDCE) > cDCE (+VC)> cDCE (+1,1DCE) ≈ cDCE (+TCE). With increasing concentrations of VC, ranging from 10 to 110 μM, the rate of cDCE (60 μM) degradation decreased. This study demonstrates that the inhibiting effects of chloroethene mixtures have to be considered during laboratory studies and bioremediation approaches based on metabolic cDCE degradation.     

Radiation dose and antioxidant depletion in a HDPE geomembrane

The impact of α and β radiation on antioxidant depletion in smooth high-density polyethylene (HDPE) geomembranes (GMs) is described. Smooth HDPE GMs having different thickness (0.04-mm, 0.1-mm, 0.2-mm) were created by mechanically pulverizing sections of 2-mm-thick smooth HDPE GM and extruding the polymer at different thicknesses using a film blowing machine. The 2-mm-thick smooth HDPE GM was also used in the experiments. HDPE GM specimens were exposed to sealed sources of ²⁴¹Am and ⁹⁹Tc for 1–50?h to simulate the impact of α and β radiation from U and ⁹⁹Tc in low-level radioactive waste (LLW) leachate. Standard oxidative induction time (OIT) tests were conducted to determine antioxidant depletion. No change in OIT occurred in the 2-mm-thick HDPE GM after exposure to sealed sources of ²⁴¹Am and ⁹⁹Tc for 50?h. In much thinner GMs (e.g., 0.04?mm), however, significant antioxidant depletion occurred after exposure most likely due to penetration of α and β particles. Penetration depth of α and β particles and dose deposition in HDPE GMs were estimated with the GEometry ANd Tracking (GEANT4) program. Predictions from GEANT4 show that maximum dose deposition occurs at the surface of the HDPE GM and decreases with depth. A multilayer model is used to estimate antioxidant depletion in HDPE GMs for depth-dependent doses. These estimates suggest that radiation from LLW leachate has an insignificant effect on antioxidant depletion in HDPE GMs due to the low dose deposition (e.g., 2.42?Gy) expected over a 1000-yr service life, even if the level of activity in LLW leachate increases 10x to 100x the level typical of today.     

Indoor air quality assessment in dwellings with different ventilation strategies in Nunavik and impacts on bacterial and fungal microbiota

Indoor air quality is a major issue for public health, particularly in northern communities. In this extreme environment, adequate ventilation is crucial to provide a healthier indoor environment, especially in airtight dwellings. The main objective of the study is to assess the impact of ventilation systems and their optimization on microbial communities in bioaerosols and dust in 54 dwellings in Nunavik. Dwellings with three ventilation strategies (without mechanical ventilators, with heat recovery ventilators, and with energy recovery ventilators) were investigated before and after optimization of the ventilation systems. Indoor environmental conditions (temperature, relative humidity) and microbiological parameters (total bacteria, Aspergillus/Penicillium, endotoxin, and microbial biodiversity) were measured. Dust samples were collected in closed face cassettes with a polycarbonate filter using a micro-vacuum while a volume of 20 m³ of bioaerosols were collected on filters using a SASS3100 (airflow of 300 L/min). In bioaerosols, the median number of copies was 4.01 × 10³ copies/m³ of air for total bacteria and 1.45 × 10¹ copies/m³ for Aspergillus/Penicillium. Median concentrations were 5.13 × 10⁴ copies/mg of dust, 5.07 × 10¹ copies/mg, 9.98 EU/mg for total bacteria, Aspergillus/Penicillium and endotoxin concentrations, respectively. The main microorganisms were associated with human occupancy such as skin-related bacteria or yeasts, regardless of the type of ventilation.     

Probable aerosol transmission of SARS-CoV-2 in a poorly ventilated courtroom

There is increasing evidence of SARS-CoV-2 transmission via aerosol; the number of cases of transmission via this route reported in the literature remains however limited. This study examines a case of clustering that occurred in a courtroom, in which 5 of the 10 participants were tested positive within days of the hearing. Ventilation loss rates and dispersion of fine aerosols were measured through CO₂ injections and lactose aerosol generation. Emission rate and influencing parameters were then computed using a well-mixed dispersion model. The emission rate from the index case was estimated at 130 quanta h⁻¹ (interquartile (97–155 quanta h⁻¹). Measured lactose concentrations in the room were found relatively homogenous (n = 8, mean 336 µg m⁻³, SD = 39 µg m⁻³). Air renewal was found to play an important role for event durations greater than 0.5 h and loss rate below 2–3 h⁻¹. The estimated emission rate suggests a high viral load in the index case and/or a high SARS-CoV-2 infection coefficient. High probabilities of infection in similar indoor situations are related to unfavorable conditions of ventilation, emission rate, and event durations. Source emission control appears essential to reduce aerosolized infection in events lasting longer than 0.5 h.     

Importance of thickness reduction and squeeze-out Std-OIT loss for HDPE geomembrane fusion seams

The difference in seam squeeze-out antioxidant loss (in terms of standard oxidative induction time, Std-OIT loss) and thickness reduction are evaluated for three different 1.5 mm-thick high density polyethylene (HDPE) geomembranes (GMBs) seamed using a variety of welding parameters and two different wedge welders. Partial squeeze-out antioxidant loss was detected in seams created from each of the three materials examined, with the majority off loss occurring when seam thickness reductions fell between 0.4 mm and 0.8 mm. Seams with thickness reduction exceeding 0.8 mm were found to exhibit greater squeeze-out Std-OIT loss, with near full Std-OIT depletion for one material. Wedge welder size was found to influence this relationship, some seams created with the large wedge welder exhibiting a near full Std-OIT depletion from squeeze-out at approximately 0.6 mm thickness reduction. Variation in seaming pressure and high load melt index (HLMI) were found to shift the degree of thickness reduction a seam may experience for a given welding speed and temperature, with higher seaming pressure and HLMI values generally resulting in greater thickness reduction. Although, for a given welding speed, wedge temperature, and sheet temperature combination, changes in seaming pressure had a limited effect on squeeze-out Std-OIT loss. This paper provides a rational basis into defining a practical 1.5 mm fusion seam thickness reduction criteria based on limiting antioxidant loss within a seam's squeeze-out and also provides a framework for identifying potentially higher risk fusion seams for future research.     

Building and environmental factors that influence bacterial and fungal loading on air conditioning cooling coils

We investigated bacterial and fungal concentrations on cooling coils of commercial AC units and quantified associations between microbial loads and AC unit or building operational parameters. A field campaign was conducted to sample 25 AC units in the humid, subtropical climate of Southern CT, USA and 15 AC units in the hot‐summer Mediterranean climate of Sacramento, CA, USA. Median concentrations (with interquartile range) of bacteria and fungi on the cooling coils were 1.2 × 10⁷ (5.1 × 10⁶‐3.9 × 10⁷) cells/m² and 7.6 × 10⁵ (5.6 × 10⁴‐4.4 × 10⁶) spore equivalents (SE)/m², respectively. Concentrations varied among units with median unit concentrations ranging three orders of magnitude for bacteria and seven orders of magnitude for fungi. Controlled comparisons and multivariable regressions indicate that dominant factors associated with AC coil loading include the nominal efficiency of upstream filters (P = .008 for bacteria and P < .001 for fungi) and coil moisture, which was reflected in fungal loading differences between top and bottom halves of the AC coils in Southern CT (P = .05) and the dew points of the two climates considered (P = .04). Environmental and building characteristics explained 42% (P < .001) of bacterial concentration variability and 66% (P < .001) of fungal concentration variability among samples.     

Aerodynamic modeling of the Yigong gigantic rock slide-debris avalanche,Tibet, China

On the April 9 of 2000, a gigantic rapid rock slide-debris avalanche occurred in Bomi, Tibet. Some 280–300 × 10⁶ m³ of material travelled 10 km within 10 min and dammed the Yigong River, forming a 2.9 × 10⁹ m3 barrier lake. Wind tunnel testing was undertaken to determine the aerodynamic parameters involved, from the initial rock slide-fall to its point of impact. The paper reports the numerical analyses undertaken and discusses the significance of the air-cushion effect. It is concluded that Yigong rapid rock slide-debris avalanche “flew” in the air for about 12.9 s from its detachment from the source to its collision with the floor of the Zamunong gully at 3,317 m asl. The velocity of the avalanche mass at the point of detachment was some 81.8 m/s and the collision velocity approximately 117 m/s.     

Optimization of PMA‐qPCR for Staphylococcus aureus and determination of viable bacteria in indoor air

Staphylococcus aureus may cause infections in humans from mild skin disorders to lethal pneumonia. Rapid and accurate monitoring of viable S. aureus is essential to characterize human exposure. This study evaluated quantitative PCR (qPCR) with propidium monoazide (PMA) to quantify S. aureus. The results showed comparable S. aureus counts between exclusively live cells and mixtures of live/dead cells by qPCR with 1.5 or 2.3 μg/mL PMA (P>.05), illustrating the ability of PMA‐qPCR to detect DNA exclusively from viable cells. Moreover, qPCR with 1.5 or 2.3 μg/mL PMA performed optimally with linearity over 10³‐10⁸ CFU/mL (R²≥0.9), whereas qPCR with 10, 23 or 46 μg/mL PMA significantly underestimated viable counts. Staphylococcus aureus and total viable bacteria were further determined with PMA‐qPCR (1.5 μg/mL) from 48 samples from a public library and two university dormitories and four from outside. Viable bacteria averaged 1.9×10⁴ cells/m³, and S. aureus were detected in 22 (42%) samples with a mean of 4.4×10³ cells/m³. The number of S. aureus and viable bacteria were positively correlated (r=.61, P<.005), and percentages of S. aureus relative to viable bacteria averaged 12‐44%. The results of field samples suggest that PMA‐qPCR can be used to quantify viable S. aureus cells.     

Methanogenic community development in anaerobic granular bioreactors treating trichloroethylene (TCE)-contaminated wastewater at 37 °C and 15 °C

Four expanded granular sludge bed (EGSB) bioreactors were seeded with a mesophilically-grown granular sludge and operated in duplicate for mesophilic (37 °C; R1 & R2) and low- (15°; R3 & R4) temperature treatment of a synthetic volatile fatty acid (VFA) based wastewater (3 kg COD m⁻³ d⁻¹) with one of each pair (R1 & R3) supplemented with increasing concentrations of trichloroethylene (TCE; 10, 20, 40, 60 mg l⁻¹) and one acting as a control. Bioreactor performance was evaluated by % COD removal efficiency and % biogas methane (CH₄) content. Quantitative Polymerase Chain Reaction (qPCR) was used to investigate the methanogenic community composition and dynamics in the bioreactors during the trial, while specific methanogenic activity (SMA) and toxicity assays were utilized to investigate the activity and TCE/dichloroethylene (DCE) toxicity thresholds of key trophic groups, respectively. At both 37 °C and 15 °C, TCE levels of 60 mg l⁻¹ resulted in the decline of % COD removal efficiencies to 29% (Day 235) and 37% (Day 238), respectively, and in % biogas CH₄ to 54% (Day 235) and 5% (Day 238), respectively. Despite the inhibitory effect of TCE on the anaerobic digestion process, the main drivers influencing methanogenic community development, as determined by qPCR and Non-metric multidimensional scaling analysis, were (i) wastewater composition and (ii) operating temperature. At the apical TCE concentration both SMA and qPCR of methanogenic archaea suggested that acetoclastic methanogens were somewhat inhibited by the presence of TCE and/or its degradation derivatives, while competition by dechlorinating organisms may have limited the availability of H₂ for hydrogenotrophic methanogenesis. In addition, there appeared to be an inverse correlation between SMA levels and TCE tolerance, a finding that was supported by the analysis of the inhibitory effect of TCE on two additional biomass sources. The results indicate that low-temperature anaerobic digestion is a feasible approach for the treatment of TCE-containing wastewater.     

Synergistic effect of nickel ions on the coupled dechlorination of trichloroethylene and 2,4-dichlorophenol by Fe/TiO₂ nanocomposites in the presence of UV light under anoxic conditions

The coupled removal of priority pollutants by nanocomposite materials has recently been receiving much attention. In this study, trichloroethylene (TCE) and 2,4-dichlorophenol (DCP) in aqueous solutions were simultaneously removed by Fe/TiO₂ nanocomposites under anoxic conditions in the presence of nickel ions and UV light at 365 nm. Both TCE and DCP were effectively dechlorinated by Fe/TiO₂ nanocomposites, and the pseudo-first-order rate constants (k_obs) for TCE and DCP dechlorination were (1.39 ± 0.05)×10⁻² and (1.08 ± 0.05)×10⁻² h⁻¹, respectively, which were higher than that by nanoscale zerovalent iron alone. In addition, the k_obs for DCP dechlorination was enhanced by a factor of 77 when Fe/TiO₂ was illuminated with UV light for 2 h. Hydrodechlorination was found to be the major reaction pathway for TCE dechlorination, while DCP could undergo reductive dechlorination or react with hydroxyl radicals to produce 1,4-benzoquinone and phenol. TCE was a stronger electron acceptor than DCP, which could inhibit the dechlorination efficiency and rate of DCP during simultaneous removal processes. The addition of nickel ions significantly enhanced the simultaneous photodechlorination efficiency of TCE and DCP under the illumination of UV light. The k_obs values for DCP and TCE photodechlorination by Fe/TiO₂ in the presence of 20-100 μM Ni(II) were 30.4-136 and 13.2-192 times greater, respectively, when compared with those in the dark. Electron spin resonance analysis showed that the photo-generated electron-hole pairs could be effectively separated through Ni ions cycling, leading to the improvement of electron transfer efficiency of TCE and DCP by Fe/TiO₂.     

Methane release rate and methanogenic bacterial populations in lake sediments

Methane release rates from the sediment in the lake were estimated by a batch culture. The methane release rates were decreased from 6.1 ± 0.7 mgCH₄-m⁻²-h⁻¹ at top 5 cm depth to 2.6 mgCH₄-m⁻²-h⁻¹ at 20–30 cm depth of sediments. Overall methane release rate from sediments was 19.9 mgCH₄-m⁻²-h⁻¹. The maximum specific acetate- and H₂-utilization rates, ν_m, were 5.2 × 10⁻⁵ to 7.96 × 10⁻⁶ and 0.9 × 10⁻⁵ to 1.0 × 10⁻⁴ gCOD-gVSS⁻¹-h⁻¹ for the sediments collected in the depths of sediments, respectively. The half-velocity constant, K_s, were from 21 to 65.5 and 27 to 194 mgCOD-1⁻¹ for acetate-and H₂-utilizing methanogens in the depths of sediments, respectively. The acetate-utilizing methanogens were enumerated by the most probable number (MPN) technique, and showed the number of acetate-utilizing methanogens increases as the ν_m increases. The populational distributions were 5.27 × 10⁸ to 2.8 × 10⁸, and 1.2 × 10⁸ MPN-gVSS⁻¹ at the sediments of top 20 and 20–30 cm, respectively. The specific methane production rates of sediments ranged from 1.2 × 10⁻¹¹ to 3.3 × 10⁻¹¹ mgCH₄-MPN⁻¹-h⁻¹ (average = 2.1 × 10⁻¹¹ mgCH₄-MPN⁻¹-h⁻¹) and are reasonably close to values reported in anaerobic treatment reactors and marine sediments. The sediment of top 20 cm had high microbial activity compared with the deeper section at 20–30 cm depth. In addition, the number of H₂-utilizing methanogens was smaller than that of acetate-utilizing methanogens in the sediments.     

Low-temperature (7 °C) anaerobic treatment of a trichloroethylene-contaminated wastewater: microbial community development

The feasibility of low-temperature (7 °C) anaerobic digestion for the treatment of a trichloroethylene (TCE) contaminated wastewater was investigated. Two expanded granular sludge bed (EGSB) bioreactors (R1 and R2) were employed for the mineralisation of a synthetic volatile fatty acid based wastewater at an initial organic loading rate (OLR) of 3 kg COD m⁻³ d⁻¹, and an operating temperature of 15 °C. Successive reductions in OLR to 0.75 kg COD m⁻³ d⁻¹, and operational temperature to 7 °C, resulted in stable bioreactor operation by day 417, with COD removal efficiency and biogas CH₄ content ≥74%, for both bioreactors. Subsequently, the influent to R1 was supplemented with increasing concentrations (10, 20, 30 mg l⁻¹) of TCE, while R2 acted as a control. At an influent TCE concentration of 30 mg l⁻¹, although phase average TCE removal rates of 79% were recorded, a sustained decrease in R1 performance was observed, with COD removal of 6%, and % biogas CH₄ of 3% recorded on days 595 and 607, respectively. Specific methanogenic activity (SMA) assays identified a general shift from acetate- to hydrogen-mediated methanogenesis in both R1 and R2 biomass, while toxicity assays confirmed an increased sensitivity of the acetoclastic community in R1 to TCE and dichloroethylene (DCE), which contributed to acetate accumulation. Quantitative Polymerase Chain Reaction (qPCR) analysis of the methanogenic community confirmed the dominance of hydrogenotrophic methanogens in both R1 and R2, representing 71-89% of the total methanogenic population, however acetoclastic Methanosaeta were the dominant organisms, based on 16S rRNA gene clone library analysis of reactor biomass. The greatest change in the bacterial community, as demonstrated by UPGMA analysis of DGGE banding profiles, was observed in R1 biomass between days 417 and 609, although 88% similarity was retained between these sampling points.