**4. Sequential disinfection using Solar-driven Fenton-like processes**

As proposed earlier, AOPs by themselves may not be capable enough to reach the desired level of pathogen deactivation as well as to assure quality of treated water because of their lack of disinfectant residual. In the past, different authors have suggested that the application of a strong oxidant (i.e. ozone) followed by a weaker oxidant (i.e. free or combined chlorine) could produce important synergistic effects on the deactivating kinetics of strong pathogens such as *Cryptosporidium parvum* oocysts (Rennecker et al., 2000; Driedger et al., 2000; 2001). The high efficiency demonstrated by sequential disinfection using the ozone-chlorine pair could be related to the generation of different reactive oxygen species (i.e. hydroxyl radicals) by the use of ozone which could synergically enhance the oxidative properties of chlorine, improving the overall inactivation rate. Considering this hypothesis, it could be possible that other methods producing hydroxyl radicals would be able to produce similar synergistic effects in similar sequential processes.

In a recent work, the efficiency of the sequential application of AOPs followed by free chlorine processes was investigated in order to deactivate helminth eggs, another highly resistant waterborne pathogen which is commonly found in surface waters contaminated with untreated domestic wastewater. Detailed information on the experimental matrix and methods used to perform these tests and to evaluate microorganism viability are described elsewhere (Bandala et al., 2011b). Figure 4 depicts the experimental results of sequential deactivation of *Ascaris suum* eggs using solar photocatalysis followed by free chlorine. For comparison purposes, notice that the inactivation results of *A. suum* eggs using free chlorine alone are also shown. It is important to mention that the reaction time, rather than the common *CT* value, is shown on the horizontal axis since the initial concentration of free chlorine (7.0 mg/L) was found to remain constant throughout the duration of the experiments. It is also important to mention that all deactivating tests were conducted in synthetic water at pH 7.0, at which hypochlorous acid (HOCl) was the predominant free chlorine species.

As shown in the experimental results of Figure 4, practically no effect on egg viability was observed when free chlorine alone was used as a single disinfectant. On the other hand, egg deactivation in the range of 25-30% was achieved when free chlorine was applied for 20 minutes after primary treatment with solar photocatalysis was applied. Experimental runs labeled as "Sequential 1" and "Sequential 2" are two different experiments carried out separately under the same reaction conditions ([Fe(II)]= 5mM; [H2O2]= 140 mM after 60 minutes of photo-assisted treatment). As mentioned, the effect of chlorine by itself on egg

Spore deactivation using photo-Fenton reaction is considerably affected by the ionic strength and natural organic matter, mainly by delaying the beginning of actual deactivation process. As shown in Figure 2, an increase in the concentration of natural organic matter has an important slowing effect on spore deactivation since it increases the

The experimental results presented in Figures 2 and 3 suggest that the efficiency of photo-Fenton processes used for disinfection depend largely on the quality of background water. Therefore, if these processes are to be cost-effective, they should be coupled with pre-

As proposed earlier, AOPs by themselves may not be capable enough to reach the desired level of pathogen deactivation as well as to assure quality of treated water because of their lack of disinfectant residual. In the past, different authors have suggested that the application of a strong oxidant (i.e. ozone) followed by a weaker oxidant (i.e. free or combined chlorine) could produce important synergistic effects on the deactivating kinetics of strong pathogens such as *Cryptosporidium parvum* oocysts (Rennecker et al., 2000; Driedger et al., 2000; 2001). The high efficiency demonstrated by sequential disinfection using the ozone-chlorine pair could be related to the generation of different reactive oxygen species (i.e. hydroxyl radicals) by the use of ozone which could synergically enhance the oxidative properties of chlorine, improving the overall inactivation rate. Considering this hypothesis, it could be possible that other methods producing hydroxyl radicals would be

In a recent work, the efficiency of the sequential application of AOPs followed by free chlorine processes was investigated in order to deactivate helminth eggs, another highly resistant waterborne pathogen which is commonly found in surface waters contaminated with untreated domestic wastewater. Detailed information on the experimental matrix and methods used to perform these tests and to evaluate microorganism viability are described elsewhere (Bandala et al., 2011b). Figure 4 depicts the experimental results of sequential deactivation of *Ascaris suum* eggs using solar photocatalysis followed by free chlorine. For comparison purposes, notice that the inactivation results of *A. suum* eggs using free chlorine alone are also shown. It is important to mention that the reaction time, rather than the common *CT* value, is shown on the horizontal axis since the initial concentration of free chlorine (7.0 mg/L) was found to remain constant throughout the duration of the experiments. It is also important to mention that all deactivating tests were conducted in synthetic water at pH 7.0, at which hypochlorous acid (HOCl) was the predominant free

As shown in the experimental results of Figure 4, practically no effect on egg viability was observed when free chlorine alone was used as a single disinfectant. On the other hand, egg deactivation in the range of 25-30% was achieved when free chlorine was applied for 20 minutes after primary treatment with solar photocatalysis was applied. Experimental runs labeled as "Sequential 1" and "Sequential 2" are two different experiments carried out separately under the same reaction conditions ([Fe(II)]= 5mM; [H2O2]= 140 mM after 60 minutes of photo-assisted treatment). As mentioned, the effect of chlorine by itself on egg

**4. Sequential disinfection using Solar-driven Fenton-like processes** 

able to produce similar synergistic effects in similar sequential processes.

treatment and/or other conventional drinking water processes.

duration of the lag phase.

chlorine species.

viability is almost negligible. In a previous work, Bandala et al. (2011b), reported that comparing the effect of chlorine alone with the photo-assisted process alone, the sequential process shows a very close trend to the observed for the AOP. Nevertheless, after the initial 20 minutes of application of sequential chlorine deactivation, the eggs viability was observed remaining unchanged at 10%, whereas in the case of the photo-Fenton process, the helminth eggs deactivation continues until reaching almost 2-log deactivation.

The use of highly-resistant pathogens, such as helminth eggs, as a conservative surrogate for water disinfection is also a very interesting issue because the deactivation of the helminth eggs is a complex task, which was in agreement with previous results from our research group (Bandala et al., 2011b), achieving almost complete helminth eggs deactivation.

Any other less resistant pathogen microorganisms (i.e. bacteria) present in the raw water will be deactivated under the same reaction conditions and after providing the same solar radiation dose. It is reasonable then that the disinfection level reached using AOPs may be, as demonstrated here, improved by the further adding of free chlorine in the sequential process.

Fig. 4. Effect of sequential AOPs-chlorine processes on the deactivation of *Ascaris suum* eggs.
