3. Results and Discussions
The initial emission data quantified from the UV radiation source yielded a value of 2.38 mW/cm
2, which represents the nominal irradiation within the annular reactor configuration. This information is crucial for understanding the intensity of the UV radiation delivered to the water within the reactor [
1,
4,
18].
Additionally, the total volume of water employed in the experiment was measured to be 800 mL. Meanwhile, the internal capacity of the reactor itself was determined to be 147 mL. These outcomes are significant as they provide key parameters for the delineation of the flow profile within the reactor system.
By knowing the volume of water used and the internal capacity of the reactor, parameters such as residence time and flow rate can be calculated. These parameters are essential for understanding how the water flows through the reactor, how long it remains within the reactor, and how effectively it is exposed to UV radiation for microbial inactivation.
To assess the capacity of the system to deactivate microorganisms effectively, it is essential to establish the requisite nominal UV dose for distinct types of microorganisms. The UV dose requirement varies depending on the type of microorganism being targeted. The nominal UV dose requirements for different microorganism types are described according to
Table 2.
The capacity to illustrate the variance in nominal dosage administered to diverse microorganisms is indeed feasible. This involves understanding the different UV dose requirements as outlined in
Table 2. By comparing these requirements, it is possible to visualize the range of UV doses needed to effectively deactivate various microorganisms. Furthermore, an essential requirement involves approximating the direct delivery of irradiation within the flow and evaluating the reactor’s ability to render microorganisms inactive [
1,
4,
18]. This evaluation is crucial for determining the effectiveness of the UV disinfection process.
Figure 3 depicts the distribution of UV irradiation within the reactor and how it interacts with the flowing water.
Given that the flow rate employed in this study is approximately 62.5 mL/s, and each data point receives a quantified energy of 0.194 mJ, the comprehensive fluence (total energy delivered) is derived through the incorporation of residence time.
Residence time represents the interval during which the fluid undergoes irradiation across the experimental arrangement, essentially the throughput within the reactor. To accomplish this, it is essential to delineate the control volume, characterized as an infinitesimal unit, and apply Equation (2).
Equation (2) represents an equation relating the fluence (total energy delivered) to the residence time and other pertinent parameters. By applying this equation, the comprehensive fluence received by the fluid as it flows through the reactor can be calculated. This comprehensive fluence accounts for the total energy delivered to the fluid and is crucial for assessing the effectiveness of the UV disinfection process in terms of deactivating microorganisms.
As a result, leveraging these data allowed for the estimation of the complete volume residing within the reactor at 147 mL, while the overall quantity of water employed amounted to 800 mL. The duration necessary for the entirety of the volume to traverse the system at least once is 70 mL. These latter figures establish the correlative framework between dose and temporal variation, a dynamic that found application in the experimental depiction presented in
Figure 4.
In the vegetal broccoli, microbiological inactivation occurred, and when the Tukey test was applied, it indicated that there were two groups with a statistically significant difference, surpassing the value of α = 0.05. Specifically, at time 0–20 min (Group A), the values were 1, 0.56, and 0.56, respectively. At this point, it is considered that there is no difference between the values within Group A. Conversely, at times 30 and 40 min (Group B), there is no difference observed within the group itself; however, there is a significant difference when this group is compared to Group A. The table below shows the complete data (
Table 3).
Having elucidated the dose dynamics and garnered endorsement for these mathematical outcomes, the research transitioned into the experimental phase, with a concentrated emphasis on microbiological considerations. The initial stride in this segment encompassed substantiating the viability of leaching vegetable constituents within this configured apparatus, operating under the stipulated physical property parameters.
To achieve this objective, the controlled introduction of clean water was administered into the system, coupled with the incorporation of broccoli, known to harbor E. coli within its foliage. The initial concentration of the microorganism was set at 10
6 CFU/mL. The ensuing outcomes are graphically depicted in
Figure 5.
The contaminated water was recirculated and irradiated to achieve the above shape. Guided by theoretical insights, it was ascertained that a duration of 35 to 40 min of fluid recirculation was imperative for the deactivation of microorganisms within the solution. Following this procedural phase, the assessed outcomes about both the vegetable and the water within the system were graphically represented. This graphical representation juxtaposes the initial concentration of microorganisms against the temporal dimension (
Figure 5).
The final phase entailed the determination of the kinetics law and the establishment of the mathematical order and dynamics governing microorganism inactivation. Initially, it was inferred that the process adheres to first-order kinetics. Subsequently, a comparison was conducted between the experimental concentration rate over time and the mathematical kinetics law, with the outcomes succinctly presented in
Figure 6.
The behavior of the inactivation rate of the microorganism by dose is described in the kinetics laws of the first order, thus the experimental results will have a shape close to the curve, for it plots the estimated curve versus the experimental to increase the dose irradiation level (
Figure 7) [
1,
4,
18].
The results above include a comparison of the experimental results obtained and those estimated by the kinetics law. It can be observed that the shapes do not overlap at the initial stage of irradiation.
This observation stems from the protective mechanisms present in microorganisms. When exposed to irradiation doses below the threshold necessary to break the bonds and/or cause DNA/RNA protein deconfiguration, microorganisms generate photopolymers.
In the kinetics law, this aspect of the equation formulation was not considered, resulting in a disparity between the theoretical and experimental results.
The imperative requirement for advancing novel technologies to bolster productivity, curtail expenditures, and concurrently alleviate environmental harm is progressively escalating.
The impact of ultraviolet radiation on DNA has been extensively documented in the scientific literature over a prolonged period. Depending on the wavelength of incident UV radiation, two distinct categories of damage manifest within the DNA structure. In the UVA range (320–400 nm), the predominant influence is that of indirect effects, owing to the lack of light absorption by nucleic acids within this domain. Nevertheless, this radiation can potentially engender the production of reactive oxygen species (ROS) [
19,
20].
ROS emerge from the oxidative processes involving molecular oxygen within mitochondria and other molecules. This oxidative activity leads to the generation of superoxide (O
2−), which subsequently has the potential to evolve into hydrogen peroxide (H
2O
2) and molecular singlet oxygen (1 O
2). These ROS can oxidize nitrogenous bases, incite strand fractures, and even precipitate DNA protein cross-linking [
21,
22].
Within the scientific literature, the documented minimal nominal dosage needed for the deactivation of E. coli ATCC 8739 is approximately 8.1 mJ/cm2, whereas for E. coli 0157:H7 CCGU 29193, it rests around 3.5 mJ/cm2. In the scope of the present study, the circulation system, operating at a flow rate of 62.5 mL/s, demonstrates the potential to administer a dosage of up to 12 mJ. This highlights that the system can provide fourfold the requisite energy for microorganism inactivation.
These findings signify a 99% and 99.999% diminishment of microorganisms, respectively, compared to their initial concentrations at time zero. The ensuing inquiries pertain to the correlation between time and microorganism inactivation, alongside the reactor’s efficacy in curtailing biological activity.
The employed technique reduced 90% of microorganisms within 20 min, 99.9% within 30 min, and 99.999% within 40 min. This condition underscores the efficacy of microorganism inactivation and facilitates the formulation of a theoretical estimation, approximating that around 35 min are requisite for specific outcomes—a projection remarkably close to actuality.
Nevertheless, in the context of irradiation of the vegetable, the outcome deviates from that observed in the case of water. This divergence can be attributed to the presence of a vortex induced during the leaching procedure of the broccoli within the tank. It becomes imperative to enhance mixing efficiency in this scenario. This objective could be accomplished by augmenting the separation between low-pressure and high-pressure regions, attainable through the elevation of the flow rate or the introduction of a barrier during the re-circulation phase [
22].
Microbiological inactivation by UVC radiation occurs initially through the absorption of radiation by DNA. In this process, two main excited states, 1ππ* or 1 nπ*, are formed due to the absorption of photons by pyrimidine molecules present in the nitrogenous bases [
22].
Consequently, the process triggers the formation of pyrimidine dimers, resulting in the generation of three distinct photoproducts: 6-4 photoproducts (6-4 PPs), alongside Dewar dimers. Within this spectrum, it is the cyclobutene pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4 PPs) that inflict the most substantial harm to DNA, accounting for 70–80% and 20–30%, respectively.
The dosage administered to the microorganism facilitates the generation of these photoproducts, subsequently culminating in the inactivation of the bacteria. Consequently, a reduced dosage administered to the microorganism possesses the inherent potential for regeneration, thereby facilitating the recovery of the microorganism.
