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Application of nano-bubbles in drinking water disinfection and the operation of bioreactors

Seridou Petroula

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Summary

A major threat to human health is considered the bacterial contamination and the subsequent infections and there is dire need to prevent the waterborne diseases to ensure water safety. Moreover, the occurrence and the fate of trace organic compounds in wastewater have attracted the attention and the concern of the scientific community since conventional wastewater treatment plants (WWTPs) have not been designed for their elimination leading to their discharge to natural water bodies and the effects of chronic exposure to low levels of these compounds are unknown. Within the context of upgrading the water and wastewater treatment processes, the development of new treatment technologies is addressed, with a view to provide high quality water at the least possible cost to the consumers. Nanobubbles (NBs) technology is an emerging solution, which is considered that has brought revolution in the field of water treatment and contaminants remediation. NBs are tiny spherical bubbles with a diameter less than 1 μm and exhibit notable characteristics in comparison to the macrobubbles (MaBs). First and foremost, the long residence time thanks to their stability is highlighted as a vital property, since it has been found that NBs remain stable in aqueous solution for a long period of time, due to their negligible buoyancy. Moreover, NBs improve the mass transfer effect and the oxidation ability, on account of the fact that the contact area of gas and water is increased. In addition, the gas solubility and chemical reactions at the gas-liquid boundary are remarkably enhanced.In terms of water disinfection processes, ozonation is widely used since ozone is a strong oxidant and highly efficient to inactivate pathogenic organisms for the prevention of waterborne diseases spread to users and the environment. However, the performance of this method is limited by the fact that ozone is unstable and short lived as the decay rate in water is high. By combining the higher gaseous ozone half-life time (3 days versus 20 min at 20 oC) and the noteworthy properties of NBs technology, the use of ozone nanobubbles (OzNBs) is proposed for water and ballast water disinfection. The main objective of this study is to compare the effect of ozone nanobubbles on the inactivation of the pathogenic microorganisms and the residual activity compared to the conventional ozonation in tap water and ballast water. In this study, four harmful types of bacteria commonly used as primary indicators of contamination in fresh water quality were selected (Escherichia coli, Bacillus cereus, Staphylococcus aureus, Enterococcus faecalis). Based on the experimental results, applying OzNBs technology had a considerable effect on inactivation and the ozone decay rate was greatly decreased, hence it can be concluded that it is a promising technology for drinking water treatment. As regards the ballast water disinfection, the survival rate of Escherichia coli (E. coli), which was used as indicator microorganism, along with the ozone consumption at different salinities (1.5, 4, 8 and 15 PSU) and bacterial concentrations (107, 106, and 105 CFU/mL) with and without supplementation of OzNBs were investigated. The results indicated a statistical difference in the residual concentration of total residual oxidants (TRO) with the presence of OzNBs at salinity level 1.5 PSU and at 4 PSU only at the lowest bacterial content. At a low salinity and high bacterial concentration, the concentration of TRO was 6-fold higher in the presence of OzNBs. The salinity of water has a strong impact on the residual concentration of ozone. When salinity is increased, ozone reacts more rapidly with the bromide and chloride ions. The use of OzNBs exhibited a greater disinfection performance and higher residual activity.In this thesis, another application of NBs technology that was investigated was the implementation of air nanobubbles (ANBs) in constructed wetlands (CWs) as it has been found that artificial aeration enhances the removal rate of conventional pollutants (COD, nitrogen and phosphorus) as well as organic compounds. The oxygen supply was conducted via nanobubble injection by a nanotube porous diffuser and in-situ electrochemical production. A higher removal rate was observed when ANBs were supplemented in wetland bed through the nanotube diffuser in phenol and toluene removal and in combination of both compared to the control. In addition, the oxygen content remained at a high level (above 7 mg/L) in all experimental cycles. Moreover, primary-treated wastewater collected from Wastewater Treatment Plant (WWTP) in Platanias (Chania) was used as substrate in wetlands along with the concentration of phenol and toluene at 100 ppm. Also in this case, the CW supplemented with ANBs by nanotube diffuser exhibited better performance in phenol and toluene removal, while the addition of wastewater enhanced the efficiency of integrated-electrolysis CW. All the wastewater quality parameters were measured, exhibiting great removal efficiencies in all CWs, however no significant difference was reported among the treatments.Finally, another field in which NBs were applied was bioremediation. In particular, the impact of irrigation with water supplemented with oxygen nanobubbles (ONBs) was also examined. In this study, soil from a shooting range was collected and spiked with an initial antimonite (Sb(III)) concentration of 50 mg/kg and a pot experiment was conducted to investigate whether Nerium oleander assisted by organic acids (OAs) and ONBs could accumulate Sb in the root and further translocate it to the aboveground tissue. The translocation of Sb for every treatment was very low, confirming that N. oleander plant cannot transfer Sb from the root to the shoots. A higher amount of Sb was accumulated in the plants that were irrigated with the ONBs. As regards the bioaccumulation of the elements Fe, Mg and Mn from soil to plant tissues, Fe and Mn were not mobilized, whereas Mg was extracted as the bioconcentration factor (BCF) was evaluated above one and significant higher with the presence of ONBs. The BCF of Mn and Mg were significantly greater when ONBs were used for irrigation, while the opposite trend was observed regarding the translocation factor. Nanobubbles can enhance the stabilization of these elements in roots and not the translocation to the upper part of the plants. Moreover, the mobilization of antimony (Sb) from soil by non-bioaugmented and bioaugmented processes coupled with nanobubble technology was investigated. ONBs enhanced the mobilization of Sb in the non-bioaugmented experiments. The bioaugmentation had a significant effect in Sb release to the aqueous phase since the percentage of Sb remaining in the soil was found to be lower in the bioaugmented experiment implying the mobilization of about 75% of the original Sb in the soil. Nanobubbles were found to have no significant effect on Sb release from the soils, since the same percentage of Sb was also found in the bioaugmented treatment with NBs water.In conclusion, the overall outcome of this study based on the experimental evidence is the significant contribution of NBs technology to various environmental fields including disinfection, wastewater treatment, and phytoremediation. In this regard, the application of NBs technology is paving the way to novel integrated and highly efficient water and soil treatment systems.

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