Municipal Treatment Plant Research Articles

Impact of ozonation on the genotoxic activity of tertiary treated municipal wastewater

Ozonation is an emerging technology for the removal of micropollutants from treated wastewater. Aim of the present study was to investigate the impact of ozone treatment on genotoxic and acute toxic effects of tertiary treated municipal wastewater. It is known that DNA-damaging chemicals cause adverse effects in the environment and that exposure to humans leads to cancer and other diseases. Toxicity was tested in organisms from three trophic levels namely in bacteria ( Salmonella/microsome assays) which enable the detection of gene mutations, in a plant bioassay (micronucleus assay with root tip cells of Allium cepa) which reflects clastogenic and aneugenic effects and in single cell gel electrophoresis (SCGE) tests with mammalian cells which detect DNA migration caused by single-, double strand breaks and alkali labile sites. In the bacterial tests negative results were obtained with untreated samples but after concentration with C 18 cartridges a positive result was found in strains TA1537 and TA98 which are sensitive to frameshift mutagens while no mutations were induced in other tester strains (TA100, TA102 and YG1024). Ozone treatment led to a decrease of the mutagenic activity of the samples. In the SCGE experiments, DNA migration was detected with the unconcentrated effluent of the treatment plant and ozonation led to a substantial decrease of this effect. In the plant bioassays, negative results were obtained with the effluent and ozone treatment did not cause an alteration of the micronucleus frequencies. Also acute toxic effects were monitored in the different indicator organisms under all experimental conditions. The bacteriocidal/bacteriostatic effects which were seen with the concentrated samples were reduced by ozonation. In the experiments with the eukaryotic (plant and animal) cells no acute toxicity was seen with the effluents and ozonation had no impact on their viability. In conclusion findings of this study indicate that ozonation of tertiary effluents of a municipal treatment plant reduces the adverse effects caused by release of mutagens in aquatic ecosystems and does not decrease the viability of bacteria and eukaryotic cells. However, future research is required to find out if, and to which extent these findings can be generalized and which mechanisms account for the detoxification of the wastewater.

The Effect of Isolated Mixed Microbial Cultures on the Reduction of Carbon Dioxide Into Methane During Biogas Production

This work describes the effect of mixed culture on reduction of carbon dioxide into methane during biogas production. Various mixed cultures, such as IMS, IWT, IRP, and IMW1, were isolated from marshy soil, decaying woody tissue, rice paddy soil, and mixture of activated sludge from municipal waste treatment plant along with a cow dung-based biogas plant (IMW1), respectively, and separately grown in cow dung slurry. Combinations of these mixed inocula were used for biogas production at 35°C in a 1.5 L batch reactor. A relatively high average yield of methane (0.345 Lg−1 volatile solid), low average yield of carbon dioxide (0.131 gL−1VS), and maximum 72% methane and 23% carbon dioxide content in biogas during 25–30 days was obtained with inoculum IRP. Whereas a corresponding average yield of methane (0.313, 0.315, 0.308 Lg−1VS), carbon dioxide (0.164, 0.158, and 0.167 Lg−1VS), and average methane content (70, 68, and 68% during 25–30 d) were observed with inocula IMS, IWT, and IMW1, respectively. Experimental results found that a 21.5% reduction in the average yield of carbon dioxide along with a 12% increase in average yield of methane and 4% increase in methane content (25–30 d) in biogas was observed with IRP, whereas a 1.8 and 5.4% reduction of carbon dioxide was found with a small increase in average methane yield 2 and 2.3% and in methane content 2% and no change (25–30 d) was observed with IMS and IWT mixed cultures as compared to IMW1 inoculum. These results of the experiments indicate that mixed microbial culture (IRP) can be used for improved quality biogas (72% methane and 23% carbon dioxide) production.

Estimating changes in river faecal coliform loading using nonparametric multiplicative regression

Faecal coliform (FC) concentration was monitored weekly in the Tangipahoa River over an eight year period. Available USGS discharge and precipitation data were used to construct a nonparametric multiplicative regression (NPMR) model for both forecasting and backcasting of FC density. NPMR backcasting and forecasting of FC allowed for estimation of concentration for any flow regime. During this study a remediation effort was undertaken to improve disinfection systems of contributing municipal waste water treatment plants in the watershed. Time-series analysis of FC concentrations demonstrated a drop in FC levels coinciding with remediation efforts. The NPMR model suggested the reduction in FC levels was not due to climate variance (i.e. discharge and precipitation changes) alone. Use of the NPMR method circumvented the need for construction of a more complex physical watershed model to estimate FC loading in the river. This method can be used to detect and estimate new discharge impacts, or forecast daily FC estimates.

Application of UV-rays in removal of polycyclic aromatic hydrocarbons from treated wastewater

This study was undertaken to investigate the removal of PAHs by UV-rays from treated wastewater. Samples of wastewater originating from a municipal treatment plant were taken twofold (series I and series II). The initial concentration of PAHs ranged: 0.8 μg/L (in series I) and 1.2 μg/L (in series II), respectively. A standard mixture of 16 compounds was added to the wastewater samples. The amount of individual hydrocarbons in the added mixture was equal to 40 μg/L, (in series I) and 50 μg/L, (in series II), respectively. The samples of wastewater without the added standard mixture were treated as a control samples. All samples of wastewater were exposed to UV-rays during 10, 20, 30 and 60 seconds, respectively. Afterwards, the PAHs concentration in both the wastewater samples containing the standard mixture and in the control samples was determined. Determinations of PAHs concentration in wastewater samples in each series were made in triplicates. A quantitative analysis of PAHs was provided by gas chromatography-mass spectrometry (GC-MS). It was found that exposition to ultraviolet rays resulted in the decrease in the concentration of PAHs in the wastewater samples without added standard mixture up to 65%. The efficiency of the removal of hydrocarbons grouped according to a number of rings was in the range of 0 (for 5- ring and 6-ring of PAHs) to 71% (for naphthalene). It was also found that exposure of wastewater samples to UV-rays resulted in a decrease of PAHs concentration in wastewater samples with the added standard mixture up to 84%. The efficiency of the removal of hydrocarbons grouped according to a number of rings differed significantly (to 94% for naphthalene).

Occupational Safety Management Aspects on Municipal Waste Water Treatment Plant

There are new municipal waste water treatment plants being constructed in Latvia in the framework of the projects implemented with the EU co-financing. New modern equipment requires certain approach to occupational safety provision. This paper discloses aspects for occupational safety increasing on newly constructed municipal waste water treatment plants within several samples of WWTP designs, actuated in Latvia. WWTP occupational issues are examined from two aspects: operational safety and designed-in safety. The paper reviews several tender requirements for WWTP construction, Latvian Cabinet of Ministers regulations, EU-OSHA materials and abstracts of some medical researches in WWTP occupational health risks. Finally, recommendations to WWTP occupational safety increasing via project owner specification to design and via project owner staff operational safety trainings are presented.

Determination of atrazine in surface waters by combination of POCIS passive sampling and ELISA detection

Polar organic compound integrative samplers (POCIS) in combination with instrumental techniques such as LC-MS-MS were previously used to monitor environmental pollutants but the performance of alternative immunochemical methods such as ELISA (enzyme-linked immunosorbent assay) has been explored less. In the present study, POCIS technology was applied to surface water sampling in the Czech Republic, and ELISA was used as a detection technique for the herbicide atrazine. In the first study, 28 samples from streams around small municipal waste water treatment plants (WWTPs) were collected using two different devices (POCISpest and POCISpharm) over the course of 21 days. Elevated atrazine concentrations (up to 25 ng per POCIS) were found in samples down-stream of WWTPs. This observation was also confirmed in another two year study (4 sampling periods) investigating 7 river sites around a major city of Brno as well as the inlet and outlet of the city's WWTP. High atrazine levels were systematically determined at the outlet from the WWTPs (120-605 ng per POCIS). A decreasing trend in the atrazine concentrations in rivers around the city of Brno has been observed, with the highest levels observed within the first sampling period in spring 2007 (100-600 ng per POCIS, with an extreme value of 2760 ng per POCIS). Results of the atrazine ELISA were closely correlated with LC-MS/MS, which confirmed good applicability of ELISA as a cost-effective screening tool.

Bioavailability and Distribution and of Ceria Nanoparticles in Simulated Aquatic Ecosystems, Quantification with a Radiotracer Technique

Although the presence of manufactured nanoparticles in the aquatic environment is still largely undocumented, their release could certainly occur in the future, particularly via municipal treatment plant effluents of cities supporting nano-industries. To get an initial estimate of the environmental behavior of nanomaterials, we investigated the distribution and accumulation of ceria nanoparticles in simulated aquatic ecosystems which included aquatic plant, shellfish, fish, water, and sediment using a radiotracer technique. Radioactive ceria (141CeO2) nanoparticles with a diameter of ca. 7 nm were synthesized by a precipitation method and added to the simulated aquatic ecosystems. The results indicate that the concentration of ceria nanoparticles in water decreased to a steady-state value after 3 days; meanwhile, the concentrations of ceria nanoparticles in the aquatic plant and sediment increased to their highest values. The distribution and accumulation characteristics of ceria nanoparticles in various aquatic organisms were different. Ceratophyllum demersum showed a high ability of accumulation of ceria nanoparticles from water.

Development and validation of a rapid and wide-scope qualitative screening method for detection and identification of organic pollutants in natural water and wastewater by gas chromatography time-of-flight mass spectrometry

In this work, a multiclass screening method for organic contaminants in natural and wastewater has been developed and validated for qualitative purposes, i.e. to ensure the reliable and sensitive identification of compounds detected in samples at a certain level of concentration. The screening is based on the use of GC-TOF MS, and the sample procedure involves solid phase extraction with C 18 cartridges. Around 150 organic contaminants from different chemical families were investigated, including PAHs, octyl/nonyl phenols, PCBs, PBDEs and a notable number of pesticides, such as insecticides (organochlorines, organophosphorus, carbamates and pyrethroids), herbicides (triazines and chloroacetanilides), fungicides and several relevant metabolites. Surface water, ground water and effluent wastewater were spiked with all target analytes at three concentration levels (0.02, 0.1 and 1 μg/L). Influent wastewater and raw leachate from a municipal solid waste treatment plant were spiked at two levels (0.1 and 1 μg/L). Up to five m/ z ions were evaluated for every compound. The identification criterion was the presence of, at least, two m/ z ions at the expected retention time, measured at their accurate mass, and the accomplishment of the Q/ q i intensity ratio within specified tolerances. The vast majority of compounds investigated were correctly identified in the samples spiked at 1 μg/L. When analyte concentration was lowered down to 0.1 μg/L the identification was more problematic, especially in complex-matrix samples like influent wastewater. On the contrary, many contaminants could be properly identified at the lowest level 0.02 μg/L in cleaner matrices. The procedure was applied to the screening of water samples of different origin and matrix composition and allowed the detection of several target contaminants. A highly reliable identification could be carried out thanks to the sensitive full-spectrum acquisition at accurate mass, the high selectivity reached with the use of narrow-mass window extracted ion chromatograms, the low mass errors observed in the positive detections and the Q/ q ratio accomplishment.

Investigation on the removal of natural and synthetic estrogens using biofilms in continuous flow biofilm reactors and batch experiments analysed by gas chromatography/mass spectrometry

The degradation of the natural estrogen 17β-estradiol and the synthetic steroid hormone 17α-ethinylestradiol, two estrogens already detected in surface waters at low concentration levels, was investigated using continuous flow biofilm reactors and batch experiments. Biofilms in continuous flow experiments were created by natural organisms from river systems of the national park Unteres Odertal, Germany, whereas batch experiments were performed with isolated bacterial strains derived from biofilms. The analytical method, including solid phase extraction, silylation of analytes and measurement with GC/MS, was optimised for the target compounds 17β-estradiol, 17α-ethinylestradiol and the possible metabolites estrone and estriol. The performance characteristics of the analytical method, namely recovery, standard deviations, method detection limits (MDL) and method quantification limits (MQL), were evaluated for accurate interpretation of degradation experiments. Continuous flow biofilm reactors were operated with two different nutrient media under dosage of estradiol and ethinylestradiol. Both estrogens were rapidly degraded within several hours; the metabolite estrone (from estradiol as well as from ethinylestradiol) was detected in significant amounts and was further decomposed. In additional batch experiments using isolated bacterial strains from the natural biofilms to decompose estradiol and ethinylestradiol, different metabolisms of isolates were explored. Five of the 15 isolated bacterial strains tested degraded estradiol and ethinylestradiol with different degradation rates. The results suggest that biofilms from national park Unteres Odertal possess a high capability to aerobically decompose natural and also synthetic estrogens so that these microorganisms could provide enhanced removal of pollutants in municipal water treatment plants.

Arsenate, Cadmium and Lead affect the plant detoxifi cation cascade for organic pollutants

Multiple sites of moderate to low pollution with heavy metals or organic xenobiotics have been identified throughout Europe, and those where no immediate pressure exists for further use, may well be remediated by plant based technologies. Hence, phytoremediation has become a green sustainable technology that has gained increasing attention during the last decade. Municipal and industrial waste water treatment plants can be optimized by the addition of lagoons with aquatic macrophytes. However, it is obvious, that quite a number of remediation projects failed due to insufficient plant performance. In this paper, we present evidence that Typha and Phragmites, two widely used plant species in phytoremediation, are capable of removal and metabolism of heavy metals and organic pollutants from water and sediments to some extent, but will fail at higher concentrations. We show also that pollution with heavy metals will interfere with both, the oxidative stress defence in plants, and with their ability to conjugate and detoxify organic xenobiotics. Despite plant species dependent differences, the general reactions seem to include oxidative stress and an induction of antioxidative enzymes. Both species respond to trace elements with oxidative stress and decay of central physiological functions. In non-hyperaccumulators like the chosen species, photosynthesis, growth, but also defense will cease at higher concentrations or prolonged exposure to heavy metals. Interestingly, defense reactions against organic xenobiotics, so far considered unrelated to heavy metal stress will also be impaired by mixed pollution, especially cadmium. Here, direct interactions with enzyme proteins are possible, but influence on transcription is frequently observed. For the practical use of plants in phytoremediation, it is discussed that only species with proven stress resistance or mixed plant consortia with differential resistances and uptake can be recommended.

Photodegradation and toxicity changes of antibiotics in UV and UV/H 2O 2 process

The photodegradation of three antibiotics, oxytetracycline (OTC), doxycycline (DTC), and ciprofloxacin (CIP) in UV and UV/H 2O 2 process was investigated with a low-pressure UV lamp system. Experiments were performed in buffered ultrapure water (UW), local surface water (SW), and treated water from local municipal drinking water treatment plant (DW) and wastewater treatment plant (WW). The efficiency of UV/H 2O 2 process was affected by water quality. For all of the three selected antibiotics, the fastest degradation was observed in DW, and the slowest degradation occurred in WW. This phenomenon can be explained by R OH,UV, defined as the experimentally determined OH radical exposure per UV fluence. The R OH,UV values represent the background OH radical scavenging in water matrix, obtained by the degradation of para-chlorobenzoic acid (pCBA), a probe compound. In natural water, the indirect degradation of CIP did not significantly increase with the addition of H 2O 2 due to its effective degradation by UV direct photolysis. Moreover, the formation of several photoproducts and oxidation products of antibiotics in UV/H 2O 2 process was identified using GC–MS. Toxicity assessed by Vibrio fischer (V. fischer), was increased in UV photolysis, for the photoproducts still preserving the characteristic structure of the parent compounds. While in UV/H 2O 2 process, toxicity increased first, and then decreased; nontoxic products were formed by the oxidation of OH radical. In this process, detoxification was much easier than mineralization for the tested antibiotics, and the optimal time for the degradation of pollutants in UV/H 2O 2 process would be determined by parent compound degradation and toxicity changes.

Cost benefit analysis of measures to reduce industrial effluent discharges

In an environmentally harmonized society the most cost effective measures to reduce the total effluent discharges should be taken into account. Generalised discharge values are presented for organic material and nutrients in this paper. Depending on conditions in the receiving water any of these parameters can be the determining factor for the eutrophication and oxygen demand. These parameters can be generalized into equivalent loads of TOC, nitrogen or phosphorus by recalculation according to the Redfield ratio. The cost for reduction of organic material and nutrients from a pulp and paper mill is calculated as a cost per unit pollutant (cost equivalent). This cost equivalent is compared with alternative costs, expressed in the same way, for reduction of organic material and nutrients in adjacent industries, municipal treatment plants, impact from transportation, farming, air deposits etc. In order to find where the most cost efficient measures for the society should be taken the cost equivalent for the mill is compared with the alternative measures and their equivalent costs.

Determination of molybdenum in extracts of soil and sewage sludge CRMs after fractionation by means of BCR modified sequential extraction procedure

A modified three-step sequential extraction procedure proposed by the Commission of European Communities Bureau of Reference (BCR) was applied to certified reference materials of three different soil groups (rendzina, luvisol, and cambisol) and sewage sludge of different compositions originating from a municipal water treatment plant in order to assess potential mobility and the distribution of molybdenum in the resulting fractions. In the soils examined, molybdenum was present almost entirely in the mineral lattice, the content of molybdenum in the fractions of the studied reference materials of sludges was predominant in the fraction, represents Mo bound to organic matter and sulphide. The internal check of accuracy was performed on the results of the sequential extraction by comparing of the extractable amounts of molybdenum in the sequential procedure with the results of the pseudototal digestion of original samples. The recovery ranged from 96 to 101% and the precision (RSD) in the extracts was below 10%.

Oxidation kinetics of cyclophosphamide and methotrexate by ozone in drinking water

This study investigates the aqueous degradation by ozone of two target cytostatic drugs, cyclophosphamide and methotrexate. A column switching technique for on-line solid phase extraction (SPE) coupled to electro-spray ionization-tandem mass spectrometry (LC–ESI-MS/MS) was used for the simultaneous detection of the trace contaminants. The second-order kinetic rate constants for the reaction of cyclophosphamide with molecular ozone and hydroxyl radicals were determined in bench-scale experiments at pH 8.10. The molecular ozone oxidation kinetics was studied in buffered ultrapure water and compared to the oxidation kinetics in natural water from a municipal drinking water treatment plant in the province of Quebec (Canada). For cyclophosphamide, the degradation rate constant with molecular ozone in ultrapure water was low ( k O 3 = 3.3 ± 0.2 M −1 s −1) and the extent of oxidation was linearly correlated to the ozone exposure. The impact of water quality matrix on oxidation efficacy was not significant during direct ozone reaction ( k O 3 = 2.9 ± 0.3 M −1 s −1). The rate constant with hydroxyl radicals was higher at 2.0 × 10 9 M −1 s −1. Methotrexate reacted quickly with molecular ozone at dosages typically applied in drinking water treatment ( k O 3 > 3.6 × 10 3 M −1 s −1). Overall, the results confirmed that organic compounds reactivity with ozone was dependent of their chemical structure. Ozone was very effective against methotrexate but high oxidant concentration × contact time (CT) values were required to completely remove cyclophosphamide from drinking water. Further studies should be conducted in order to identify the ozonation by-products and explore the impact of ozone on their degradation and toxicity.

Monitoring the organic matter properties in a combined anaerobic/aerobic full-scale municipal source-separated waste treatment plant

Respiration indices (dynamic and cumulative) and the anaerobic biogasification potential are applied to the quantitative calculation of the biodegradation efficiency in a combined anaerobic/aerobic treatment for the organic fraction of municipal solid waste (OFMSW). They also permit to observe possible deficiencies in some parts of the entire sequence of organic matter decomposition. On the contrary, chemical methods presented a limited utility. Dynamic respiration indices highlighted that anaerobic digestion was the most efficient step to reduce the respiration activity of the waste (61% calculated on a DRI24h basis). Respirometric activity of final compost was 93% lower than initial OFMSW confirming the overall efficiency of the plant studied and the stability of the final product (0.3g O2 kg TS−1h−1). Finally, the use of an advanced methodology such as the Diffuse Reflectance Infrared Fourier Transformed (DRIFT) allows the determination of the main functional groups of organic matter, which significantly change during the biological treatment of organic matter.

Perfluoroalkyl acids in Lake Superior water: Trends and sources

Perfluoroalkyl acids (PFAs) are a family of highly persistent compounds which are present in the environment as a result of degradation of polyfluorinated precursors, from use as processing aids for production of fluoropolymers, and use in fire fighting foams. The purpose of this study was to investigate prevailing concentrations and possible sources of PFAs in Lake Superior, as well as in Siskiwit Lake on Isle Royale. Between 2001 and 2005, replicate water samples were taken from lake surface waters, and from depth profiles, as well as from major tributaries including municipal waste water treatment plants (WWTPs) at three major population centers. Perfluorooctanoate (PFOA) was the predominant PFA in Lake Superior, with concentrations ranging from 0.07 to 1.2 ng/L in surface waters. PFOA concentrations were generally 1.5 to 2-fold greater than perfluorooctanesulfonate (PFOS) levels. WWTPs were found to contribute up to 20 fold higher concentrations of PFOA (22 ng/L) relative to the intake water from Lake Superior, while most tributaries contained lower concentrations of perfluorocarboxylates (PFCA) and perfluoroalkylsulfonates (PFSs) (< 0.1 ng/L). Overall tributaries and precipitation were estimated to be the major sources of PFCAs and PFSs to Lake Superior. Tributaries were estimated to be the largest source contributing 59% of PFOA and 57% of PFOS inputs to the lake. Profiles conducted over the deepest points in the lake showed that PFAs were found throughout the water column, however, there was no distinctive trend with depth.

Perfluorooctanoic Acid and Perfluorooctane Sulfonate in Two Fish Species Collected from the Roter Main River, Bayreuth, Germany

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are widely distributed in the environment. In this study the accumulation potential of PFOA and PFOS in two fish species with different feeding strategies, i.e. chub (Leuciscus cephalus) and river goby (Gobio gobio) inhabiting a river receiving treated waste waters from a municipal waste water treatment plant, were estimated. PFOS was detected in chub (7-250 microg kg(-1) wet weight) and river goby (70-400 microg kg(-1) wet weight) with bioaccumulation factors (BAFs) of 4600 (liver) and 11,000 (organs). PFOA concentrations in both fish were low and in chub mostly below detection limit.

Simulation study with minimised additional data requirements to analyse control and operation of WWTP Dorsten, Germany

This paper describes a method to decrease the additional measurement effort during a model calibration/verification to create reliable models for plant optimisation. The method is based on the deterministic processes creating the dry weather inflow to municipal treatment plants, which can be described by a simple influent composition model. This method was applied to two wastewater treatment plants of Lippeverband and the usability is indicated by the good correlation between routine online effluent measurements and simulation results. This paper presents one of these applications, the case study WWTP Dorsten. The resulting model is used to analyse the current plant performance, to analyse bottlenecks and to develop future improvements of operation and control of this plant. A selection of control options is demonstrated.

Updated Activated Sludge Model n°1 Parameter Values for Improved Prediction of Nitrogen Removal in Activated Sludge Processes: Validation at 13 Full-scale Plants

The Activated Sludge Model number 1 (ASM1) is the main model used in simulation projects focusing on nitrogen removal. Recent laboratory-scale studies have found that the default values given 20 years ago for the decay rate of nitrifiers and for the heterotrophic biomass yield in anoxic conditions were inadequate. To verify the relevance of the revised parameter values at full scale, a series of simulations were carried out with ASM1 using the original and updated set of parameters at 20 degrees C and 10 degrees C. The simulation results were compared with data collected at 13 full-scale nitrifying-denitrifying municipal treatment plants. This work shows that simulations using the original ASM1 default parameters tend to overpredict the nitrification rate and underpredict the denitrification rate. The updated set of parameters allows more realistic predictions over a wide range of operating conditions.

Occurrence of motile Aeromonas in municipal drinking water and distribution of genes encoding virulence factors

Aeromonas-associated cases of gastroenteritis are generally considered waterborne. The purpose of this study was to evaluate the potential microbiological risk associated with the presence of these bacteria in public drinking water. Over a period of one year, 132 drinking-water samples were monitored in León (NW of Spain, 137,000 inhabitants) for mandatory drinking-water standards and the occurrence of Aeromonas spp. Samples were taken at the municipal water treatment plant, one storage facility, and two public artesian drinking-water fountains. Because of low numbers of coliforms or Clostridium perfringens, the non-compliance rate with microbial standards was 3.8% whereas the percentage of positive samples for motile mesophilic Aeromonas was 26.5%. For all but two samples, Aeromonas was recovered between October and early March when the temperature was below 14 °C and the residual chlorine ranged from 0.21 to 0.72 mg/l. An apparent relationship was observed between rainfall and the incidence of Aeromonas. The 35 selected Aeromonas isolates were identified as A. caviae and A. media. The alt and laf genes were present in all isolates, the aerA gene was present in six isolates, and the four remaining genes investigated ( hlyA, ast, stx1 and stx2) were absent. The combinations of putative virulence genes were: aerA − /hlyA −/ alt +/ ast −/ laf +/ stx1 −/ stx2 − (82.9%) and aerA + /hlyA −/ alt +/ ast −/ laf +/ stx1 −/ stx2 − (17.1%). None of the isolates bore plasmids. As Aeromonas strains harbouring two or more virulence-associated genes have the potential to cause disease by direct transmission via drinking water or by water use in food preparation, it would be advisable to control excessive numbers of these bacteria in drinking-water supplies.