**4.3 Catalyst impregnation**

For the impregnation of the support, the solution prepared with the ZnO catalyst and distilled water was used (**Figure 4a**); later it was dried to remove the liquid trapped in the pores (**Figure 4b**) then it's calcined at 550 °C for 1 h, and the activation was achieved of the catalyst. Calcination increases the surface concentration and generates a catalyst with uniform distribution (**Figure 4c** and **d**).

#### **Figure 3.**

*(a) Catalyst support and placing clay plates on the FFPR, (b) calcination process, observing the porosity and homogeneity on the surface of the plate.*

*Purification of Rainwater Using a Photocatalysis Technique to be Applied to Communities… DOI: http://dx.doi.org/10.5772/intechopen.112579*

#### **Figure 4.**

*ZnO catalyst impregnation process (a) and (b); impregnated support plates and fixation of the support on the FFPR and calcination of a plate in muffle at 550°C (c), completing the impregnation with the catalyst on the FFPR (d).*

#### **4.4 Catalyst doping**

For the doping of the catalyst with Ag<sup>+</sup> nanoparticles, the solution prepared with Ag2SO4 and distilled water (**Figure 5a**) was used to initiate recirculation under exposure to UV light (**Figure 5b**) and thus achieve the photo deposition of the silver (**Figure 5c**). Finally, it was calcined to reach the activation of the bactericidal metal.

#### **4.5 RFPD tests**

The falling film photocatalytic reactor demonstrated its feasibility in the photodegradation of the organic pyridine compound in an initial concentration of 50 ppm using a ZnO – Ag+ -doped catalyst.

**Figure 6** shows the monitoring of the photodegradation of the organic compound utilizing absorbance graphs at a wavelength of 200 to 600 nm. However, more excellent activity of the compound was observed in a range of 200 to 300 nm.

The organic compound presented a characteristic maximum peak of the same at a wavelength of 256 nm and simultaneously showed distinct intermediate peaks of the pyridine [36].

The pyridine photodegradation test was carried out with the same initial concentration for 6 h, taking a sample every 2 h to observe its behavior.

In the degradation of the pyridine, as shown in **Figure 7**, it can be observed how the behavior of the pyridine changes for the reaction time.

It is observed that in the time of zero hours (t = 0 h), the natural behavior of the compound is shown, which ensures that the organic compound is present in the solution, having an initial concentration of 50 ppm and showing absorbance of 2351

#### **Figure 5.**

*Catalyst doping process with Ag+ nanoparticles. (a) Preparation of the solution with silver sulfate, (b) doping process by photo deposition on the support impregnated with the catalyst; (c) support of the FFPR finishing the photo deposition.*

#### **Figure 6.**

*A spectrum of organic compound pyridine with a concentration of 50 ppm.*

**Figure 7.**

*The organic compound pyridine was degraded on the falling film photocatalytic reactor in the presence of the ZnO – Ag<sup>+</sup> -doped catalyst for 6 h.*

arbitrary units (u.a.); then, in the elapsed time of 2 hours (t = 2 h) of the degradation on the photoreactor, it can be observed that the concentration drops to 0.825 u.a., presenting a degradation of 65%; for the 4 hours of reaction (t = 4 h), it shows a decrease in the concentration to 0.731 u.a. resulting in a 69% degradation; later, after 6 hours of reaction (t = 6 h), it presents a slight increase in concentration to 0.845 u.a.; however, after 8 hours of reaction (t = 8 h) in the system, it is possible to drop to 0.695 u.a., reaching a final degradation of 70% [22].

It should be noted that randomly generated peaks appear from the tempo of 2 h to the time of 8 h because the main compound is degrading. Still, alternately, there is the presence of other intermediate compounds being formed.

**Figure 8** shows the spectrophotometer reading carried out in a degradation of the same compound at initial concentrations of 50 ppm. In this image, you can better appreciate the formation of intermediate compounds after the first 2 h of photodegradation; such shapes can be attributed to the presence of 2 hydroxy pyridines, as reported by Montalvo (2009) [36]; however, at 4 h, the pyridine peak disappears.

In **Figure 9**, the trend line shows a reliability of 95.5% concerning the average data represented that oscillate in the initial concentration from 50 ppm to 16.22 ppm, *Purification of Rainwater Using a Photocatalysis Technique to be Applied to Communities… DOI: http://dx.doi.org/10.5772/intechopen.112579*

**Figure 8.** *Degradation of the organic compound pyridine during a period of 4 h.*

**Figure 9.** *Line of tendency and standard deviation of the degradation of the organic compound PYR in a period of 3 h.*

considering a time of 0 to 180 minutes, which was the most representative time in the photodegradation tests.

Likewise, the standard deviation representing the variations of the average obtained in the data of the different photodegradation tests carried out to get these results is evidenced.

**Figure 9** describes the degradation of the organic compound pyridine; it starts from a concentration of 50 ppm in its initial time. After 60 min of the photodegradation test, it degrades to 30.2599 ppm; after 120 min of degradation, the concentration of the compound drops to 26.3962; for a time of 180 min, the concentration is lowered to 16.2276 ppm.

In **Figure 10**, you can satisfactorily see the increase in the photodegradation of the organic compound; at time zero, there is no result since the presence of the contaminant is intact in the solution; for a time lapse of 60 min of photodegradation, there is a reduction from 50 ppm to 30.2599 ppm, achieving a percentage of photodegradation of 39% concerning the initial concentration of the original compound; it can also be observed that at 120 min, the photodegradation increases by 20% and after 180 minutes, a photodegradation of 68% [27].

Such results obtained are adequate due to the conditions managed, referring to the volume worked under direct exposure to sunlight.

**Figure 10.**

*Average percentage of degradation of the organic compound pyridine during 3 h.*

#### **4.6 Obtaining kinetic parameters**

Various studies show that the first step in the photocatalytic degradation of organic compounds follows first-order or zero-order kinetics. The results clearly show that the reaction rate depends fundamentally on the concentration of the reaction (**Figure 9**), so it can be said that it follows first-order kinetics or is like first-order kinetics:

$$r\_a = \frac{dC}{dt} = K \tag{1}$$

By integrating the velocity equation, the following function is obtained:

$$\text{In}\frac{\text{C}}{\text{Co}} = -K\_{\text{apparent}}\tag{2}$$

Which is equivalent to *Co K*− apparent.

**Figure 11** plots the logarithms of the normalized concentrations log(C/Co) vs. reaction time. The values of the apparent reaction constant were obtained by linear regression.

The data are necessary to obtain the Kapparent kinetic constants from the line equation. **Figure 11** shows the photocatalytic degradation of pyridine, and this follows a pseudo-first-order reaction since the reaction rate is the same as the concentration increases. **Table 5** shows the values of this constant obtained as the same from **Figure 11**.

#### **4.7 Sample collection**

A collection system was built to capture rainwater directly from the source, in the open sky, and thus collect 25 samples that were preserved, stored, and refrigerated until their characterization and the final analysis (**Figure 12**). These samples were collected during the rainy season from June to September in the municipality of Carmen city, Campeche.

*Purification of Rainwater Using a Photocatalysis Technique to be Applied to Communities… DOI: http://dx.doi.org/10.5772/intechopen.112579*

#### **Figure 11.**

*Kinetics of the pyridine reaction using ZnO-Ag+ as a catalyst.*


#### **Table 5.**

*The apparent velocity (min−1) for the degradation tests utilizing a ZnO-Ag+ reactor.*

#### **Figure 12.**

*Sample collection. a) Construction of the rainwater collection system, b) water collected during seasonal rain, and c) samples stored and refrigerated.*

#### **4.8 Characterization and analysis of rainwater samples**

The Official Mexican Standard NOM-127-SSA1–1994 [37], referring to Environmental Health regarding water for human use and consumption, establishes the permissible limits of quality and treatments to which water must be subjected for purification. For this reason, it is considered to carry out the pertinent characterization tests on the water samples, following the criteria it establishes.

The average results obtained from the tests carried out during the characterization and physical and chemical analyses obtained are shown in **Tables 6** and **7**, respectively, as well as their maximum permissible limit according to regulations.

#### *Water Purification – Present and Future*


#### **Table 6.**

*Average results obtained from physical tests carried out on rainwater according to the corresponding regulations.*


#### **Table 7.**

*Average results obtained from chemical tests carried out on rainwater according to the corresponding regulations.*

The analyzed parameters show average results below the maximum permissible limit since, as previously mentioned, the rainwater was collected directly from the source in the open sky without having contact with any contaminated surface and was later stored in refrigeration, to preserve its properties until reaching the corresponding characterization analysis.

#### **4.9 The treatment of collected rainwater**

When carrying out the photodegradation treatment of rainwater collected in a cistern, the 20-L sample was recirculated in the RFPD to maintain contact of liquid with the surface of the reactor with the supported and impregnated catalyst and under the effect of sunlight for eight hours, a 1 L sample was taken every 4 h to carry out the tests on the main parameters and verify the effectiveness of the system when degrading the organic components present. The data shown in **Table 8** were obtained from the tests carried.

The data shown in **Table 8** show that the rainwater that was not promptly used for domestic use began to be collected through the roof, conducted by pipes to be stored in a typical cistern, where, by not having an immediate disinfection treatment, it became rich in microbiological contaminants (bacteria, fungi, algae, etc.) by carrying garbage, organic matter from the environment, bird droppings or other animals that circulate on the roof, and therefore, stored water becomes a cause of concern for health care.

Each microorganism grows individually. However, ambient temperature and an average pH of 6.69 are critical factors that increase the growth rate of *Purification of Rainwater Using a Photocatalysis Technique to be Applied to Communities… DOI: http://dx.doi.org/10.5772/intechopen.112579*


#### **Table 8.**

*Results obtained from the analysis of rainwater collected according to the corresponding regulations.*

microorganisms, mainly mesophilic bacteria, total coliforms, fecal [thermotolerant] bacteria, and *Escherichia coli*.

Regarding the dissolved salts, it can be noted that there was a decrease from 452 to 196.5 mg/L, which favors the water quality, an indicator of the effectiveness of the photocatalytic process of the water disinfection treatment.

Chloride ions are one of the most widespread ions in natural waters. It is not usually an ion that poses portability problems to drinking water, although it is an indicator of water contamination due to human action. The maximum permissible chloride concentration for human consumption is 250 mg/L. Therefore, an average of 25.22 mg/L is an acceptable average value for using water stored in a cistern.

Magnesium and calcium salt depend fundamentally on the geological formations that the water traverses before its collection. In this case, the hardness increases from a value of 68.79 to 220.33 mg/L; it is considered hard water, which means that it contains more calcium and magnesium minerals; as the hardness of water increases, more calcium and magnesium are dissolved. Magnesium and calcium are positively charged ions. Due to their presence, other positively charged ions will dissolve less easily in hard water than in water that does not contain calcium and magnesium.

Considering the total alkalinity parameter with an average value of 122.15 mg/L, which is an acquired value of the materials added in domestic uses due to the cistern water storage system, showing the presence of hydroxides, carbonates, and bicarbonates of elements such as calcium, magnesium, sodium, potassium, or ammonia, considered as biological nutrients.

Total coliforms and fecal coliforms are the most significant indicators of microbiological contamination; the presence and degree of fecal contamination are essential factors in evaluating water quality [38, 39]. The initial tests (t = 0 h) present a significant NMP of total and fecal coliforms, 92,000 and 6400 NMP/100 mL, respectively. In both cases, the effectiveness of the photocatalytic reaction is verified in the tests carried out at t = 4 h, eliminating thoroughly any coliform present in the medium.

### **5. Conclusions**

Given the nature of the construction of the falling film photocatalytic reactor, it is concluded that it satisfactorily fulfilled the function for which it was designed when testing its functionality in the previous tests and the same degradation of rainwater collected in the cistern. It is an alternative for the development of future research with different contaminants and at larger scales since it did not show any complication in the mineralization of organic compounds and microorganisms using zinc oxide catalyst doped with Ag+ nanoparticles, interacting under the incidence of sunlight.

The clay plates, due to their composition and their manufacturing, had deformations due to the preparation on the surface face and the cuts made to adapt the reactor to the size, creating pathways through which the water had greater ease in moving, and at the same time, crests were created where the passage of the flow of the treated liquid over the reactor occurred to a lesser extent.

However, for use as support, the clay was an excellent receptor of the catalyst since a uniform impregnation of ZnO on the clay was achieved, increasing the contact area of the catalyst from 1 m2 of the surface area to 852.5 m<sup>2</sup> .

The doping of the ZnO catalyst with the photo deposition of Ag<sup>+</sup> nanoparticles is a proven good alternative for the photocatalytic degradation in the mineralization of contaminating microorganisms, showing the breakdown of their cell wall in a period of 4 hours. At the same time, the degradation of the organic compound "pyridine" is verified, showing degradation efficiency of up to 68% in a time of 3 h.

The operating parameters of the falling film system were tested at an angle of 30°, at a flow rate of 8 L/min, the results obtained in the degradation of the organic compound pyridine being sufficient and optimal, using the low ZnO – Ag+ doped catalyst—the incidence of sunlight.

The rainwater collected in a cistern was disinfected with the advanced oxidation treatment applied with the RFPD, complying with the NOM-127-SSA1–1994 regulations, considering the principal physical, chemical, and microbiological parameters for water use. It guarantees that this water can be used as drinking water, since the quantitative determination of each parameter was below the maximum permissible limits referenced in the Official Mexican Standard.

### **Conflict of interest**

The authors declare no conflict of interest.

*Purification of Rainwater Using a Photocatalysis Technique to be Applied to Communities… DOI: http://dx.doi.org/10.5772/intechopen.112579*
