Proceedings

The main objective of this study is to explore possible synergistic or additive effects of combinations of chemical disinfectants (sodium hypochlorite, peracetic acid, hydrogen peroxide, chlorine dioxide) and UV in their efficacy in inactivating free-living bacteria and removing biofilms. In contrast to most studies, this study examines disinfection of municipal water in a pilot-scale system using a mixed bacterial suspension, which enables a better simulation of the conditions encountered in actual industrial environments. It was shown that the combination of either hypochlorite, hydrogen peroxide, peracetic acid, or chlorine dioxide with UV yielded additive effects on the inactivation of free-living bacteria. Actual synergy was observed for the combination of UV and 5 ppm hydrogen peroxide. Regarding biofilm treatment, additive effects were observed using the combination of hydrogen peroxide and UV. The promising results obtained in this study, indicate that the combination of UV and chemical disinfectants can considerably reduce the amount of chemicals required for the effective disinfection and treatment of biofilms.

Introduction

In both industrial and non-industrial applications, water is circulated and reused. However, continuous water reuse leads to a considerable decrease in water quality due to the proliferation of microorganisms and biofouling. Biofilm formation frequently results in operational problems, such as increased fluid frictional resistance, reduced heat transfer in heat exchangers, and unexpected corrosion of stainless steel (Ludensky, 2003; Coetser and Cloete, 2005). Moreover, biofilms are difficult to remove because they provide a niche where resident microbes are protected against environmental stress or disinfectants. Furthermore, biofilms may be regarded as reservoirs of potential pathogens, because they release potential pathogens in the planktonic environment on a rather constant basis (Lehtola et al., 2007). So it is clear that biofilm formation can have a considerable economic impact and pose a threat to human health as well. Hence, an effective, cost-efficient, and environmental-friendly water treatment is required to reduce biofilm-associated problems.

Currently, a variety of water disinfection techniques are commonly used, including biocide application and ultraviolet light (UV) (Pozos et al., 2004). UV penetrates the bacterial cell membrane and blocks DNA replication, which eventually results in lethal damage. Disadvantages of UV treatment are its limited efficacy in highly light scattering or absorbing solutions, and possible photoreactivation or dark repair of UV-damaged microorganisms, enabling regrowth of the microbial population under certain conditions (Jungfer et al., 2007).  For many years the preferred biocide has been hypochlorite, because of its effectiveness, ease of use, and relatively low cost (Grant and Bott, 2005).  However, the use of hypochlorite is increasingly debated because of the formation of toxic, mutagenic and/or carcinogenic disinfection by-products, such as trihalomethanes, haloacetic acids, and chlorine residuals (Gopal et al., 2007). Environmental concern has thus led to the awareness that the use of toxic biocides, like hypochlorite, should be replaced or reduced. This can be achieved either by (i) finding an alternative biocide that does not generate toxic by-products; or by (ii) reducing the hypochlorite concentration. Chlorine dioxide, with its strong oxidation capacity and its high effectiveness against a wide variety of microorganisms, is often used as an alternative to hypochlorite because it doesn’t produce signicant amounts of chlorinated toxic by-products (Gomez-Lopez et al., 2009).  Hydrogen peroxide and peracetic acid (PAA) are alternative biocides that do not generate significant amounts of toxic or mutagenic by-products, or chemical residues in effluents (De Luca et al., 2008).

Apart from using biocides that are less hazardous for the environment, the reduction of toxic by-products may be achieved by combining hypochlorite treatment with a physical disinfection technique, such as UV irradiation Therefore, the main objective of this study was to explore possible synergistic or additive effects of UV and chemical disinfectants on the inactivation of free-living bacteria and treatment of biofilms in a pilot-scale system.