Indicator compounds for assessment of wastewater effluent contributions to flow and water quality

Numerous studies have reported the presence of trace (i.e., ng/L) organic chemicals in municipal wastewater effluents, but it is unclear which compounds will be useful to evaluate the contribution of effluent to overall river flow or the attenuation processes that occur in receiving streams. This paper presents a new approach that uses a suite of common trace organic chemicals as indicators to assess the degree of impact and attenuation of trace organic chemicals in receiving streams. The utility of the approach was validated by effluent monitoring at ten wastewater treatment plants and two effluent-impacted rivers with short retention times (<17 h). A total of 56 compounds were particularly well suited as potential indicators, occurring frequently in effluent samples at concentrations that were at least five times higher than their limit of quantification. Monitoring data from two effluent-impacted rivers indicated that biotransformation was not important for these two river stretches, whereas photolysis attenuation was possibly important for the shallow river. The application of this approach to receiving waters and water reclamation and reuse systems will allow for more effective allocation of resources in future monitoring programs.     

Nitrogen and phosphorus removal from municipal wastewater effluent using microalgal biofilms

Microalgal biofilms have so far received little attention as post-treatment for municipal wastewater treatment plants, with the result that the removal capacity of microalgal biofilms in post-treatment systems is unknown. This study investigates the capacity of microalgal biofilms as a post-treatment step for the effluent of municipal wastewater treatment plants. Microalgal biofilms were grown in flow cells with different nutrient loads under continuous lighting of 230 μmol/m²/s (PAR photons, 400-700 nm). It was found that the maximum uptake capacity of the microalgal biofilm was reached at loading rates of 1.0 g/m²/day nitrogen and 0.13 g/m²/day phosphorus. These maximum uptake capacities were the highest loads at which the target effluent values of 2.2 mg/L nitrogen and 0.15 mg/L phosphorus were still achieved. Microalgal biomass analysis revealed an increasing nitrogen and phosphorus content with increasing loading rates until the maximum uptake capacities. The internal nitrogen to phosphorus ratio decreased from 23:1 to 11:1 when increasing the loading rate. This combination of findings demonstrates that microalgal biofilms can be used for removing both nitrogen and phosphorus from municipal wastewater effluent.     

Persistence and prevalence of pathogenic and extended-spectrum beta-lactamase-producing Escherichia coli in municipal wastewater treatment plant receiving slaughterhouse wastewater

We compared the prevalence of pathogenic and extended-spectrum beta-lactamase (ESBL) – producing Escherichia coli in effluents of a municipal wastewater treatment plant (WWTP) receiving wastewater from a slaughterhouse. A total of 1248 isolates were screened for the presence of virulence genes associated with enterohemorrhagic E. coli (EHEC) (stx₁, stx₂, and eae) and extraintestinal pathogenic E. coli (ExPEC) (sfa/focDE, kpsMT K1, hlyA, papEF, afa/draBC, clbN, f17A and cnf). The prevalence of atypical enteropathogenic E. coli (EPEC) was 0.7%, 0.2% and 0.5% in city wastewater, slaughterhouse wastewater and in the treated effluent, respectively. One stx_1a and stx_2b-positive E. coli isolate was detected in city wastewater. The prevalence of ExPEC was significantly higher in city wastewater (8.4%), compared to slaughterhouse wastewater (1.2%). Treatment in the WWTP did not significantly impact the prevalence of ExPEC in the outlet effluent (5.0%) compared to city wastewater. Moreover, the most potentially pathogenic ExPEC were isolated from city wastewater and from the treated effluent. ESBL-producing E. coli was also mainly detected in city wastewater (1.7%), compared to slaughterhouse wastewater (0.2%), and treated effluent (0.2%). One ESBL-producing E. coli, isolated from city wastewater, was eae-β1 positive. These results showed that pathogenic and/or ESBL-producing E. coli were mainly detected in human wastewater, and at a lesser extend in animal wastewater. Treatment failed to eliminate these strains which were discharged into the river, and then these strains could be transmitted to animals and humans via the environment.     

Antimicrobial resistance of fecal indicators in municipal wastewater treatment plant

Antimicrobial resistance of fecal coliforms (n = 153) and enterococci (n = 199) isolates was investigated in municipal wastewater treatment plant (WWTP) based on activated sludge system. The number of fecal indicators (in influent and effluent as well as in the aeration chamber and in return activated sludge mixture) was determined using selective media. Susceptibility of selected strains was tested against 19 (aminoglycosides, aztreonam, carbapenems, cephalosporins, β-lactam/β-lactamase inhibitors, fluoroquinolones, penicillines, tetracycline and trimethoprim/sulfamethoxazole) and 17 (high-level aminoglycosides, ampicillin, chloramphenicol, erythromycin, fluoroquinolones, glycopeptides, linezolid, lincosamides, nitrofuration, streptogramins, tetracycline) antimicrobial agents respectively. Among enterococci the predominant species were Enterococcus faecium (60.8%) and Enterococcus faecalis (22.1%), while remaining isolates belonged to Enterococcus hirae (12.1%), Enterococcus casseliflavus/gallinarum (4.5%), and Enterococcus durans (0.5%). Resistance to nitrofuration and erythromycin was common among enterococci (53% and 44%, respectively), and followed by resistance to ciprofloxacin (29%) and tetracycline (20%). The resistance phenotypes related to glycopeptides (up to 3.2%) and high-level aminoglycosides (up to 5.4%) were also observed. Most frequently, among Escherichia coli isolates the resistance patterns were found for ampicillin (34%), piperacillin (24%) and tetracycline (23%). Extended-spectrum β-lactamase producing E. coli was detected once, in the aeration chamber. In the study the applied wastewater treatment processes considerably reduced the number of fecal indicators. Nevertheless their number in the WWTP effluent was higher than 10⁴ CFU per 100 ml and periodically contained 90% of bacteria with antimicrobial resistance patterns. The positive selection of isolates with antimicrobial resistance patterns was observed during the treatment processes. Substantial concern should be paid to the isolates resistant to 3 or more chemical classes of antimicrobials (MAR). In treated wastewater MAR E. coli and MAR enterococci constituted respectively 9% and 29% of tested isolates.     

Laccase-catalyzed oxidation of oxybenzone in municipal wastewater primary effluent

Pharmaceuticals and personal care products (PPCPs) are now routinely detected in raw and treated municipal wastewater. Since conventional wastewater treatment processes are not particularly effective for PPCP removal, treated wastewater discharges are the main entry points for PPCPs into the environment, and eventually into our drinking water. This study investigates the use of laccase-catalyzed oxidation for removing low concentrations of PPCPs from municipal wastewater primary effluent. Oxybenzone was selected as a representative PPCP. Like many other PPCPs, it is not recognized directly by the laccase enzyme. Therefore, mediators were used to expand the oxidative range of laccase, and the efficacy of this laccase-mediator system in primary effluent was evaluated. Eight potential mediators were investigated, and 2,2′-Azino-bis(3-ethylbenzthiazoline-6sulphonic acid) diammonium salt (ABTS), a synthetic mediator, and acetosyringone (ACE), a natural mediator, provided the greatest oxybenzone removal efficiencies. An environmentally relevant concentration of oxybenzone (43.8 nM, 10 μg/L) in primary effluent was completely removed (below the detection limit) after two hours of treatment with ABTS, and 95% was removed after two hours of treatment with ACE. Several mediator/oxybenzone molar ratios were investigated at two different initial oxybenzone concentrations. Higher mediator/oxybenzone molar ratios were required at the lower (environmentally relevant) oxybenzone concentration, and ACE required higher molar ratios than ABTS to achieve comparable oxybenzone removal. Oxybenzone oxidation byproducts generated by the laccase-mediator system were characterized and compared to those generated during ozonation. Enzymatic treatment generated byproducts with higher mass to charge (m/z) ratios, likely due to oxidative coupling reactions. The results of this study suggest that, with further development, the laccase-mediator system has the potential to extend the treatment range of laccase to PPCPs not directly recognized by the enzyme, even in a primary effluent matrix.     

Effects of design flow and treatment level on construction and operation costs of municipal wastewater treatment plants and their implications on policy making

Construction costs of 55 municipal wastewater treatment plants in Israel (secondary, advanced secondary, and advanced treatment) were analysed in order to derive cost functions expressing the effects of design flow and treatment level on construction costs. Three equations were derived (statistically significant, p<0.01), one for each treatment level. These indicate that economy of scale may become weaker as treatment level rises. Analysis of the distribution of construction costs revealed negative correlation (p<0.05) between the proportional cost of civil engineering and design flow, positive correlation (p<0.05) between the proportional cost of elecromechanical equipment and design flow, and no correlation between the proportional cost of electricity and control and design flow. Operation costs were found to be 20–70% more sensitive than construction costs to treatment level. The share of operation costs as part of the total annual costs was found to increase both with design flow and treatment level, whereas the share of construction costs concurrently decreased. The implication of the findings on policy, and consequently on treatment plants performance is discussed in the last part of the paper.     

Fate of antibiotics during municipal water recycling treatment processes

Municipal water recycling processes are potential human and environmental exposure routes for low concentrations of persistent antibiotics. While the implications of such exposure scenarios are unknown, concerns have been raised regarding the possibility that continuous discharge of antibiotics to the environment may facilitate the development or proliferation of resistant strains of bacteria. As potable and non-potable water recycling schemes are continuously developed, it is imperative to improve our understanding of the fate of antibiotics during conventional and advanced wastewater treatment processes leading to high-quality water reclamation. This review collates existing knowledge with the aim of providing new insight to the influence of a wide range of treatment processes to the ultimate fate of antibiotics during conventional and advanced wastewater treatment. Although conventional biological wastewater treatment processes are effective for the removal of some antibiotics, many have been reported to occur at 10-1000 ng L⁻¹ concentrations in secondary treated effluents. These include β-lactams, sulfonamides, trimethoprim, macrolides, fluoroquinolones, and tetracyclines. Tertiary and advanced treatment processes may be required to fully manage environmental and human exposure to these contaminants in water recycling schemes. The effectiveness of a range of processes including tertiary media filtration, ozonation, chlorination, UV irradiation, activated carbon adsorption, and NF/RO filtration has been reviewed and, where possible, semi-quantitative estimations of antibiotics removals have been provided.     

Baffled membrane bioreactor (BMBR) for efficient nutrient removal from municipal wastewater

Submerged membrane bioreactors (MBRs) are now widely used for various types of wastewater treatment. One drawback of submerged MBRs is the difficulty in removing nitrogen because intensive aeration is usually carried out in the tank and the MBRs must therefore be operated under aerobic conditions. In this study, the feasibility of treating municipal wastewater by a baffled membrane bioreactor (BMBR), particularly in terms of nitrogen removal, was examined. Simultaneous nitrification/denitrification in a single and small reaction tank was possible by inserting baffles into a normal submerged MBR as long as wastewater was fed in the appropriate way. To examine the applicability of the BMBR, pilot-scale experiments were carried out using real municipal wastewater. Although neither external carbon addition nor mixed liquor circulation was carried out in the operation of the BMBR, average removal rates of total organic carbon (TOC), total phosphorus (T-P) and total nitrogen (T-N) reached 85%, 97% and 77%, respectively, with the hydraulic retention time (HRT) of 4.7h. Permeability of the membrane could be maintained at a high level throughout the operation. It was found that denitrification was the limiting step in removal of nitrogen in the BMBR in this study. Various types of monitoring carried out in the BMBR also demonstrated the possibility of further improvements in its performance.     

Distribution of antidepressant residues in wastewater and biosolids following different treatment processes by municipal wastewater treatment plants in Canada

The fate of 14 antidepressants along with their respective N-desmethyl metabolites and the anticonvulsive drug carbamazepine (CBZ) was studied in 5 different sewage treatment plants (STPs) across Canada. Using two validated LC-MS/MS analytical methods, the concentrations of the different compounds were determined in raw influent, final effluent and treated biosolids samples. Out of the 15 compounds investigated, 13 were positively detected in most 24-h composite raw influent samples. Analysis showed that venlafaxine (VEN), its metabolite O-desmethylvenlafaxine (DVEN), citalopram (CIT), and CBZ were detected at the highest concentrations in raw influent (up to 4.3 μg L⁻¹ for DVEN). Cumulated results showed strong evidence that primary treatment and trickling filter/solids contact has limited capacity to remove antidepressants from sewage, while activated sludge, biological aerated filter, and biological nutrient removal processes yielded moderate results (mean removal rates: 30%). The more recalcitrant compounds to be eliminated from secondary STPs were VEN, DVEN and CBZ with mean removal rates close to 12%. Parent compounds were removed to a greater degree than their metabolites. The highest mean concentrations in treated biosolids samples were found for CIT (1033 ng g⁻¹), amitriptyline (768 ng g⁻¹), and VEN (833 ng g⁻¹). Experimental sorption coefficients (K_d) were also determined. The lowest K_d values were obtained with VEN, DVEN, and CBZ (67-490 L kg⁻¹). Sorption of these compounds on solids was assumed negligible (log K_d ≤ 2). However, important sorption on solids was observed for sertraline, desmethylsertraline, paroxetine and fluoxetine (log K_d > 4).     

Occurrence, partition and removal of pharmaceuticals in sewage water and sludge during wastewater treatment

During 8 sampling campaigns carried out over a period of two years, 72 samples, including influent and effluent wastewater, and sludge samples from three conventional wastewater treatment plants (WWTPs), were analyzed to assess the occurrence and fate of 43 pharmaceutical compounds. The selected pharmaceuticals belong to different therapeutic classes, i.e. non-steroidal anti-inflammatory drugs, lipid modifying agents (fibrates and statins), psychiatric drugs (benzodiazepine derivative drugs and antiepileptics), histamine H2-receptor antagonists, antibacterials for systemic use, beta blocking agents, beta-agonists, diuretics, angiotensin converting enzyme (ACE) inhibitors and anti-diabetics. The obtained results showed the presence of 32 target compounds in wastewater influent and 29 in effluent, in concentrations ranging from low ng/L to a few μg/L (e.g. NSAIDs). The analysis of sludge samples showed that 21 pharmaceuticals accumulated in sewage sludge from all three WWTPs in concentrations up to 100 ng/g. This indicates that even good removal rates obtained in aqueous phase (i.e. comparison of influent and effluent wastewater concentrations) do not imply degradation to the same extent. For this reason, the overall removal was estimated as a sum of all the losses of a parent compound produces by different mechanisms of chemical and physical transformation, biodegradation and sorption to solid matter. The target compounds showed very different removal rates and no logical pattern in behaviour even if they belong to the same therapeutic groups. What is clear is that the elimination of most of the substances is incomplete and improvements of the wastewater treatment and subsequent treatments of the produced sludge are required to prevent the introduction of these micro-pollutants in the environment.     

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 C₁₈ 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.     

城市污水厂二级处理出水内电解深度处理技术研究

采用内电解技术对城市污水处理厂出水进行深度处理,研究了pH值、废水在反应器中的停留时间等因素对处理效果的影响,并采用增加催化剂的措施提高了工艺的处理效果,解决了板结问题.试验结果表明,污水处理厂二级出水经内电解法处理后,脱色率可达90%以上,TP去除率可达90%左右,CODcr去除率也可达60%左右,内电解技术是一种经济高效的城市污水二级处理出水深度处理技术.     

Perfluoroalkyl sulfonates and perfluorocarboxylates in two wastewater treatment facilities in Kentucky and Georgia

Discharge of effluents from municipal wastewater treatment plants (WWTPs) is a route for the introduction of certain organic contaminants into aquatic environments. Earlier studies have reported the occurrence of perfluorochemicals in effluents from WWTPs. In this study, contamination profiles of perfluorinated compounds (PFCs), including perfluoroalkyl sulfonates (PFASs; PFOS, PFOSA, PFHxS) and perfluoroalkyl carboxylates (PFACs; PFOA, PFNA, PFDA, PFDoDA, PFUnDA), were determined in samples collected at various stages of wastewater treatment during different seasons. The two WWTPs selected for this study represent rural (Plant A, Kentucky) and urban (Plant B, Georgia) areas. PFOS was a major contaminant in samples from Plant A (8.2–990 ng/g dry wt in solid samples and 7.0–149 ng/L in aqueous samples), followed by PFOA (8.3–219 ng/g dry wt in solid samples and 22–334 ng/L in aqueous samples). PFOA was the predominant contaminant in samples from Plant B (7.0–130 ng/g dry wt in solid samples and 1–227 ng/L in aqueous samples), followed by PFOS (<2.5–77 ng/g dry wt in solid samples and 1.8–22 ng/L in aqueous samples). PFHxS, PFNA, PFDA, and PFOSA were detected in most of the samples, whereas PFUnDA and PFDoDA were detected in very few samples. Concentrations of some PFCs, particularly PFOA, were slightly higher in effluent than in influent, suggesting that biodegradation of some precursors contributes to the increase in PFOA concentrations in wastewater treatment processes. No large-magnitude seasonal variations in concentrations were found, although mass flow of PFCs was higher in winter than in summer. In general, samples from the rural plant in Kentucky contained greater concentrations of PFCs than did those from the urban plant in Georgia. Incineration of sludge reduced the PFC levels significantly. The mass flows of PFCs in these two plants were several hundreds of mg/day, comparable to flow values reported earlier.     

The role of free water surface constructed wetlands as polishing step in municipal wastewater reclamation and reuse

In Europe, the last two decades witnessed growing water stress, both in terms of water scarcity and quality deterioration, which prompted many municipalities for a more efficient use of the water resources, including a more widespread acceptance of water reuse practices. Treatment technology encompasses a vast variety of options. Constructed wetlands are regarded as key elements in polishing conventionally treated wastewater for recreational and environmental applications. A survey was conducted to assess the performance of tertiary free water surface constructed wetlands in treating both key and emerging contaminant categories in the perspective of water reuse. A database was created with information concerning systems with emerging and free-floating macrophytes. The database includes results from both full- and pilot-scale systems, and considers a broad variety of operating conditions. This paper provides an overview of the treatment performances of the constructed wetlands in the database and discusses their significance in the optic of water reclamation and reuse practices.     

The implications of household greywater treatment and reuse for municipal wastewater flows and micropollutant loads

An increasing worldwide interest in water recycling technologies such as greywater treatment and reuse suggests that additional research to elucidate the fate of xenobiotics during such practices would be beneficial. In this paper, scenario analyses supported by empirical data are used for highlighting the potential fate of a selection of xenobiotic micropollutants in decentralised greywater treatment systems, and for investigation of the possible implications of greywater recycling for the wider urban water cycle. Potential potable water savings of up to 43% are predicted for greywater recycling based on Danish water use statistics and priority substance monitoring at a greywater treatment plant in Denmark. Adsorption represents an important mechanism for the removal of cadmium, nickel, lead and nonylphenol from influent greywater and therefore the disposal route adopted for the generated sludge can exert a major impact on the overall efficiency and environmental sustainability of greywater treatment.     

Application of ceramic membranes with pre-ozonation for treatment of secondary wastewater effluent

Membrane fouling is an inevitable problem when microfiltration (MF) and ultrafiltraion (UF) are used to treat wastewater treatment plant (WWTP) effluent. While historically the use of MF/UF for water and wastewater treatment has been almost exclusively focused on polymeric membranes, new generation ceramic membranes were recently introduced in the market and they possess unique advantages over currently available polymeric membranes. Ceramic membranes are mechanically superior and are more resistant to severe chemical and thermal environments. Due to the robustness of ceramic membranes, strong oxidants such as ozone can be used as pretreatment to reduce the membrane fouling. This paper presents results of a pilot study designed to investigate the application of new generation ceramic membranes for WWTP effluent treatment. Ozonation and coagulation pretreatment were evaluated to optimize the membrane operation. The ceramic membrane demonstrated stable performance at a filtration flux of 100 gfd (170 LMH) at 20 °C with pretreatment using PACl (1 mg/L as Al) and ozone (4 mg/L). To understand the effects of ozone and coagulation pretreatment on organic foulants, natural organic matter (NOM) in four waters - raw, ozone treated, coagulation treated, and ozone followed by coagulation treated wastewaters - were characterized using high performance size exclusion chromatography (HPSEC). The HPSEC analysis demonstrated that ozone treatment is effective at degrading colloidal NOMs which are likely responsible for the majority of membrane fouling.     

Impact of ozonation on ecotoxicity and endocrine activity of tertiary treated wastewater effluent

Tertiary wastewater treatment plant effluent before and after ozonation (0.6-1.1 g O₃/g DOC) was tested for aquatic ecotoxicity in a battery of standardised microbioassays with green algae, daphnids, and zebrafish eggs. In addition, unconjugated estrogen and 17β-hydroxyandrogen immunoreactive substances were quantified by means of enzyme immunoassays, and endocrine effects were analysed in a 21-day fish screening assay with adult male and female medaka (Oryzias latipes). Ozonation decreased estrogen-immunoreactivity by 97.7 ± 1.2% and, to a lesser extent, androgen-immunoreactivity by 56.3 ± 16.5%. None of the short-term exposure ecotoxicity tests revealed any adverse effects of the tertiary effluent, neither before nor after the ozonation step. Similarly in the fish screening assay, reproductive fitness parameters showed no effects attributed to micropollutants, and no detrimental effects of the effluents were observed. Based on the presented screening, ozonation effectively reduced steroid hormone levels in the wastewater treatment plant effluent without increasing the effluent's ecotoxicity.     

A suitability evaluation tool for siting wastewater treatment facilities in new urban developments

This paper describes the development of a spatial decision-support tool based on soft computing that assists the optimal siting of wastewater treatment technologies within the context of new urban developments, through the creation of suitability maps. The tool is based on multi-criteria decision analysis and fuzzy logic, allowing the inclusion of both uncertainty and of the decision-makers' attitude towards risk in the decision-making process. The paper describes the attributes influencing the siting of wastewater treatment infrastructure and presents a method for using these to generate composite siting suitability maps for a given development site and propose specific locations maximizing the combined suitability index. Extensive sensitivity analysis has been undertaken and the results discussed. The proposed system architecture integrates three widely used software platforms (ArcView GIS, Matlab and MS Excel) into a flexible and user-friendly decision-support tool that can easily be adapted to different spatial decision environments.     

A regulatory robust conditional approach to measuring the efficiency of wholesale water supply and wastewater treatment services

A benchmarking evaluation instrument was designed with a European Union country regulatory authority for water supply and wastewater treatment services to determine the efficient operating cost of its service providers that operated in the wholesale market segment in the 2017–2021 period. To this end, the non-parametric Data Envelopment Analysis technique was adapted to a robust and conditional approach. The results point to similar mean efficiency scores between water supply and wastewater services in the five-year period, despite the greater heterogeneity in the latter. Furthermore, the estimated potential cost savings for both services ranged from about 2% to 3%.     

Effect of the aeration system on the levels of airborne microorganisms generated at wastewater treatment plants

Six different wastewater treatment plants were monitored in order to identify the main bioaerosol sources and to evaluate the effect of the aeration system used in the biological treatment (air diffusion, horizontal rotors and surface turbine aerators) on the airborne microorganism levels to which workers may be exposed. Air samples were collected by using a single stage impactor. Total count of mesophilic bacteria was used as the monitoring parameter to compare the impact of the aeration system on generic bacterial bioaerosols rather than a quantitative estimation for pathogens or fecal indicator microbes. In this study, pre-treatment, biological treatment and sludge thickening were the processes that generated the highest amount of bioaerosols. Aeration systems involving mechanical agitation of the wastewater, such as horizontal rotors and surface turbines, generated a larger amount of bioaerosols (between 450 and 4580CFU/m(3)) than air diffuser aerators (between 22 and 57CFU/m(3)). The levels of airborne bacteria generated by air diffusers were very similar to those registered at the background locations (lower than 50CFU/m(3)), unaffected by the activities taking place in the wastewater treatment plant. The use of air diffusers as an aeration system for the biological treatment would significantly minimise the potential biological hazard that wastewater treatment plant workers may be exposed to.