# Packed Bed Photoreactor for the Removal of Water Pollutants Using Visible Light Emitting Diodes
## 1 Introduction  
The main causes of water contamination are industrial discharges, excess use of pesticides and fertilizers, the presence of pharmaceutical residues, and landfilling of domestic wastes [1]. Traditional water treatment processes are mainly based on mechanical (sedimentation and filtration), biological, and physical-chemical processes (flocculation, sterilization, or chemical oxidation) [2; 3]. Advanced treatment technologies are based on the pollutants oxidation by potassium permanganate, ozone, hydrogen peroxide, high-energy ultraviolet light, photo-Fenton, electro-Fenton, and photo-electro-Fenton processes [4; 5; 6; 7; 8]. Typically, these technologies are not cost-effective [9]. On the other hand, the conventional wastewater treatments have different disadvantages, such as use of chemicals and production of sludge, as well as low efficiency in removal of bio-recalcitrant compounds [10]. Heterogeneous photocatalysis is one of the advanced oxidation processes that couples low-energy light with semiconductors acting as photocatalysts and, therefore, it could be considered as sustainable process for the treatment and purification of water and wastewater [11; 12]. Although in recent years, a large number of publications have shown the potential of photocatalytic treatment for the removal of different categories of toxic compounds in water [13; 14; 15] using also innovative photoreactors configurations [16; 17; 18], the research is still limited to the development of photocatalytic laboratory reactors or to photoreactors very difficult to scale up. It must be considered that the design of a photocatalytic reactor is not an easy step; in particular, the development of a photocatalytic reactor is complicated because it is necessary to have an adequate irradiation of the overall reaction volume for the activation of the entire mass of photocatalyst and to favor an easy catalyst separation after treatment [19]. Typically, these reactors are designed to study the photocatalytic degradationof pollutants through the application of conventional lamps as light sources [20]. However, since conventional lamps are expensive in terms of manufacturing, as well as electricity consumption, many studies have been devoted to photocatalytic reactors that exploit the solar spectrum [21]. But it is necessary to take into account the many limitations related to solar photoreceptors. For example, in a solar compound parabolic collector (CPC), despite the fact that the reactor is formed by small tubes, the area equipped by the parabolic collector for light harvesting is huge. So, the installation of these systems also requires a large area [22] to avoid that the single modules projecting shadows upon each other [23]. Another limitation this kind of system is connected to the use of only direct sunlight [24; 25]. Thus, the solar photoreceptors do not work efficiently during cloudy days and night time. The recent rapid advances in light-emitting diodes (LEDs) technology have made possible a new frontier for a wide range of lighting applications, including photocatalysis and, consequently, photoreceptor design [26; 27]. In fact, the energy conversion efficiency of LEDs has increased exponentially in recent years, making them a suitable light source for the design of compact photocatalytic reactors for water and wastewater treatment [28; 29], as well as for the removal of hazardous compounds from gaseous streams [30], overcoming the limitations related to the developing of photocatalytic reactors into pilot scale level. With the use of LEDs, the design of photoreceptors could be significantly facilitated and, by using a flexible LEDs strip as the external light source, it is possible to use simple cylindrical geometry for photoreceptor, thereby enhancing the contact between the photocatalyst and the water to be treated.