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Inactivation of pathogenic microorganisms and viruses and removal of antibiotic resistance genes from constructed wetlands wastewater

Kaliakatsos Andreas

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Summary

The need for efficient and integrated wastewater treatment is now becoming imperative for the protection of public health, while the increasing consumption requirements make water reuse crucial. As many areas still do not have access to clean water, reuse is of particular importance. The presence of pathogenic microorganisms in the effluents of wastewater treatment plants is of great concern, since they pose considerable risk to public health, especially if the effluent is intended for reuse. In addition to pathogenic microorganisms, in recent years an emerging problem from the spread of resistant bacteria and resistance genes is arising. The wastewater treatment plants and the conventional processes, such as the activate sludge, are not able to remove efficiently persistent pathogens like viruses or antibiotic resistance genes. Moreover, the construction, maintenance cost and the generation of harmful byproducts are limiting factors for their installation.The aim of the study was to examine the capability of economic and eco-friendly treatments to encounter the challenge to offer water safety. In this perspective and within the framework of the present study, pilot scale constructed wetlands and heterogeneous photo-catalytic processes using semiconductor oxides processes were developed as alternative wastewater treatment systems with the view to evaluate their disinfection efficiency, in terms of their potential to remove fecal bacterial indicators (E. coli, Enterococci) and bacteriophages (MS2), and the elimination of enteric viruses, namely adenoviruses and enteroviruses. In order to compare the different systems an alternative statistic approach using the Bayesian model was applied.In addition, the concertation of the antibiotic resistance genes in different types of genetic material was estimated and the alterations of antibiotic resistance profile of bacteria were examined. A multi linear regression model was developed in an effort to estimate the role of the resistance genes to population’s reduction after the exposure to the selected antibiotics.Results showed the ability of the horizontal subsurface constructed wetlands to eliminate substantially bacterial indicators and also to achieve reduction of the viruses’ concentration. Regarding the resistance genes, they continue to be present in the system’s effluent, while no clear pattern in terms of bacterial resistance profile was observed. Moreover, it was not clear which of the resistance genes are involved in the bacterial antibiotic resistance.As far as the process of photocatalysis is concerned, there was a significant removal of fecal bacterial indicators and the use of doped photocatalysts contributed in the overall acceleration of the inactivation under solar irradiation. Also, photocatalytic processes achieved satisfactory inactivation of the tested viruses. However, significant concentration of resistance genes was observed in the effluents after the aforementioned processes.Compared to conventional treatment methods, the examined treatments provided promising results taking into account their advantages. However, as shown by the quantitative microbial risk assessment, public health risks still remain, especially if the water is intended for reuse. The hazards arise mainly due to the presence of viruses in the final effluents of the systems. Particular attention should be paid on the presence of resistant bacteria and resistance genes as it is still difficult to assess the future risks that they may cause. The failure of bacterial indicators to predict the presence and risks that viruses may cause makes imperative to select carefully the water quality indicators that will be used to indicate the presence of viruses. In this trend, factors such as the kind of the treatment and the type and composition of the wastewater should always be taken into account.

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