4. Experimental decontamination equipment with metamaterials

For the decontamination of translucent liquids by UV-C radiation, we proposed the equipment in Figure 11, formed from a UV-C transparent core tube, which can be filled with metamaterials. As was estimated in Section 3, the decontamination rate is influenced by the packing and optical properties of metamaterial elements. Two types of metamaterials were used: (a) quartz (SiO2) unordered granules with dimension around 1–5 mm and (b) transmission spectrum in UV region 240–260 nm (Figure 11(a and b)) spheres of glass material with diameter of 2 mm and transmission at 300 nm. The comparative analysis of the decontamination rate for these metamaterials is performed. Optical metamaterials can disperse UV-C light inside the fluid volume and improve the contact zone between radiation and contaminated fluids. In Figure 11(a), the UV-C core tube used for the decontamination of translucent fluids is shown, while Figure 11(b) presents the decontamination equipment for dynamic treatment regime. The decontamination equipment consists of six low-pressure Hg UV-C lamps (30 W) with 90 cm length and about 2.7 cm diameter. These lamps surrounding the decontamination core tube (Figure 10) are placed in the center of a reflecting aluminum cylinder (with a diameter of about 30 cm). The UV-C radiation (with Gaussian distribution) is focused along the axis, i.e., in the decontamination area. The core tube can be filled up with optical metamaterials, while polluted fluids can freely circulate between elements, in interaction with evanescent waves.

The radiation penetration into the core tube offers a significant yield of the contact surface between the flowing fluid and UV radiation in a volume of <sup>0</sup>:<sup>9</sup> <sup>10</sup><sup>4</sup> m3. The fluid circulating through the decontamination zone changes arbitrarily the optical frontiers among metamaterial elements and fluid, in function of pathogen concentration and optical properties. Consequently, the decontamination efficiency depends on the contact surface between the contaminated fluid and periodical optical metamaterial, and it is proportional to the number of elements of metamaterial. The penetration of light radiation into translucent fluids flowing through elements of metamaterials increases due to the optical evanescent field around each element. For the dynamic treatment regime, the core tube is connected to an external reservoir through which the polluted biological fluid flows. The circulation of the fluid is conducted by an electrical pump device. The working principle of the installation can be described as follows. The UV-C irradiation

water for testing was pumped by an electrical device to continuously circulate through the core tube filled up with optical metamaterials (glass spherical bubbles). As follows from experimental results (Table 1), E. coli, Enterococcus, and Coliform bacteria were totally inactivated from a volume of 1 L of contaminated water after 10 min of UV-C irradiation in the presence of glass spheres. This keeps valid after 5 min of treatment, as the bacterial colonies of Coliform and Enterococcus were also annihilated. Nevertheless, an insignificant part of Coliform bacteria survived after 5 min irradiation in dynamic treatment (Figure 12) (see also Table 1). The individual numerical values of the control and treated water samples are collected in Table 1. In Figure 12, images of Petri dishes with bacteria colonies after 48 h of incubation are given. Bacteria in the contaminated water that flows in-between the metamaterial elements in the core tube (glass spheres) are periodically collapsed on the evanescent zone of each element.

Efficient Microbial Decontamination of Translucent Liquids and Gases Using Optical Metamaterials

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As can be observed from Figure 12 and Table 1, the Coliform (including E. coli) and Enterococcus bacteria from a volume of 1 L contaminated water were totally inactivated after 10 min of treatment under UV-C irradiation in dynamic regime, in the presence of glass spheres. The contaminated water is penetrated by UV-C radiation via evanescent field around spheres inside the quartz tube cylinder. The decontamination effect is significant in all cases, leading after 10 min to the eradication of all bacterial strains (B. coliform, E. coli, and Enterococcus).

Next experiments were conducted with beer yeast fermentation. The yeast species transforms by fermentation carbohydrates to carbon dioxide and alcohols [45]. The fermentation was used

About 50 g of fresh yeast were dissolved into 1 L of warm (40C) sweetened water (20%). After 1 h of observation, the fermentation is still active in the untreated solution (Figure 13B), while

Table 1. Characteristic numerical values of untreated (control) and treated water samples in dynamic regime for 5 and 10 min.

]

to estimate the decontamination rate efficiency of the UV-C equipment (Figure 13).

5.2. Yeast inactivation by dynamic treatment regime

Bacterial strain Results [CFU/100 cm<sup>3</sup>

Lake water UV irradiated for 5 min in the core tube filled up with glass spheres

Lake water UV irradiated for 10 min in the core tube filled up with glass spheres

B. coliform 482 E. coli 3 Enterococcus 11

B. coliform 5 E. coli 0 Enterococcus 0

B. coliform 0 E. coli 0 Enterococcus 0

Polluted lake water—control samples

Figure 11. (a) Optical metamaterial used for decontamination and (b) decontamination equipment used for dynamic treatment regime.

of six germicidal lamps is concentrated in the quartz core tube and propagates inside the whole volume through UV transparent metamaterial elements. When using quartz metamaterials, the decontamination volume can be increased by the evanescent zone of UV light radiation forming around each element of the metamaterial.

The microorganism decontamination is achieved in the evanescent area, which can improve the contact zone between radiation and contaminated fluids. The optical force produced by electromagnetic radiation acts as a tweezer, attracting microparticles at the EMF regions with the highest intensity. Microorganisms located in the evanescent zone of metamaterials can be efficiently annihilated. It is worthy to mention that such a UV-C decontamination equipment can be used, as for example, in water distribution system of a city or directly in water pipes of apartments, in order to prevent the biological risk. Moreover, UV-C decontamination reactors can be used for gases (air) decontamination in hazardous situations.
