# Photocatalytic Inactivation as a Method of Elimination of _E. coli_ from Drinking Water
## 6 Photocatalytic Systems Designed for _E. coli_ Inactivation  
Different photocatalytic set-ups have been used for the inactivation of _E. coli_, and one of them is the liquid-film system. In this set up, light is passed from the top of the set-up through a transparent pyrex window into the suspension of _E. coli_. The suspension is placed on the petri-dish holding a glass coated with the photocatalysts such as titanium oxide. The liquid-film set-up is different from the membrane separated system, where the light is passed from the base of the set-up through a normal glass support to the photocatalyst-coated glass, then to the membrane holding the suspension of _E. coli_. These two systems are shown in Figure 5a,b. Figure 5: (**a**) Liquid-film photocatalytic system and (**b**) membrane-separated system. Reprinted from [59], Copyright (1997), with permission from Elsevier. One other design allows the reduction and oxidation to take place simultaneously, and this makes the investigation of the photocatalytic inactivation to be possible. It is usually employed where a photocatalyst doped with metal is employed for _E. coli_ inactivation. The metal dopant act as the reduction site, which boosts the photocatalytic efficiency via better light-generated charge separation [130]. In this design, the detector that is used is the carbon microelectrode, which is usually positioned close to the surface of the photocatalysts as shown in Figure 6a,b, where palladium doped with titanium oxide was used as the photocatalysts. Materials used to design the carbon microelectrode are copper wire, epoxy resin, mercury, carbon fiber and pyrex glass tube. [PERSON] et al. [107] reported the use of photocatalyst-loaded reactor (Figure 7) made from polyurethane foam and quartz. The polyurethane was used as support that carried the photocatalyst in order to improve its contact area. The photo-reactor is cylindrical in shape, it has a double layer and is of considerable size (35 cm in length and 7 cm in diameter). This reactor has fitted sampler and flow meter to measure the movement of air and aerosol containing _E. coli_ in the system. Figure 6: (**a**) The design of carbon microelectrode, (**b**) set-up for simultaneous monitoring of oxidation and reduction during _E. coli_ inactivation. Reprinted from [130]. Copyright (2000), with permission from Elsevier. Figure 7: Polyurethane foam design for the inactivation of _E. coli_. Reprinted from [107], Copyright (2021), with permission from Elsevier. A portable, easy-to-use photocatalytic design, based on the use of photoelectric has also been reported [131]. It was used to inactivate 5-log of _E. coli_ within 10 s. This reactor has nanotube photoanode made from titanium oxide photocatalyst, which was formed into the shape of teacup, and it is powered by small-sized battery that is rechargeable. The battery is the source of energy for the light emitting diode fitted into the reactor. This novel photocatalytic design is shown in Figure 8.  
A relatively new photoreactor set-up, which also utilizes light emitting diode as a source of illumination was designed with several fixed bed stages (Figure 9). The photocatalyst used for the design was titanium oxide nanoparticles doped with nitrogen, and the nanoparticles were rendered immobile on the beads made from glass. Each fixed bed was made from acrylic sheet with dimensions of 100 mm long, 50 mm wide and 15 mm high. The fixed bed stage was arranged such that there was gravitational flow from one stage to another stage and there was continuous flow of water in and out of the reactor. The suspension of _E. coli_ is usually introduced into the first stage through a peristaltic pump. The incident photon flux in the set-up is usually measured by potassium ferrooxalate actinometer [132]. Figure 8: Photoelectric cup-like photocatalytic design for _E. coli_ inactivation. Reprinted from [131], Copyright (2020), with permission from Elsevier. Figure 9: Multiple stage photoreactors for _E. coli_ inactivation.