# Photocatalytic Inactivation as a Method of Elimination of _E. coli_ from Drinking Water
## 1 Introduction
Consequently, _E. coli_ is also used to evaluate the efficiency of water disinfection methods. If the water sample is free from _E. coli_ after a disinfection process, it is concluded that the method is efficient and the water is free from faecal contamination--thus making it unnecessary to analyze the water sample for other pathogens [8,10,11,12]. Several conventional water disinfection methods such as chlorination, ozonation, radiation, advanced filtration, and the use of ultraviolet (UV) light have been used over the years [3,13]. Water decontamination by chlorination, for example, uses chlorine as a strong oxidant, and it is one of the important, most common, relatively simple, effective and inexpensive conventional ways to provide biologically potable water [3,4,10,13,14]. Unfortunately, conventional water purification technologies have several drawbacks, some of which include:  
* Formation of Disinfection Byproducts (DBPs) and their consequent carcinogenicity: Chemical disinfection methods such as chlorination and ozonation could also lead to the formation of toxic and corrosive disinfection byproducts such as trihalomethanes (THM), chloroform, chlorite, and haloacetic acids. This is due to high amount of content of free residual chemicals after the disinfection process, and these compounds have been proven to be extremely carcinogenic [3,4,8,9,13,15,16]. * High operation and maintenance cost: Expensive chemicals or equipment are required for the operation and maintenance of many of these methods. For instance, water decontamination with ultraviolet light applies shortwave radiation (<280 nm) and is associated with increased energy utilization. This requires expensive equipment for the production [13,15,16]. * Antibiotic drug resistance: Microorganisms soon become resistant to some of these conventional methods with prolonged use, thereby making the method ineffective for water disinfection over time. Some biohazards are even naturally resistant to chlorination and UV treatments [9; 16; 17; 18]. * Low disinfection efficiency: For example, chlorination is not able to eliminate all pathogens. This is due to its slow biodegradation kinetics [8; 13]. Moreover, the conventional disinfection technologies are often met with implementation challenges. A high concentration of residual chemicals could cause an unpleasant taste and smell in the drinking water [13; 16]. Ozonation, chlorination and other techniques have been used for removing these bacteria, but they usually produce toxic by-products and some of these byproducts may be mutagenic or carcinogenic [19; 20]. In fact, disinfection with the help of UV radiation alone, which is considered as a good alternative due to the fact that the introduction of chemicals is not required, has now been discovered to be disadvantageous. This is because some bacteria are resistant to UV radiation since they regenerate after a few hours of irradiation [19; 20]. Hence, there is a great need for alternative water disinfection technologies that could make up for the inherent limitations of the conventional water decontamination methods [3]. Recent studies have been focused on the development of an advanced antibacterial technology against microbial contaminants in water. Specifically, a more efficient method of inactivating bacteria such as _E. coli_ is desirable. Photocatalytic disinfection is considered as one of the innovative and promising options with high disinfection potential for water purification [13; 15]. It is regarded as one of the most prominent advanced oxidation technologies (AOT) and has so many applications in water and air purification, viral and bacterial inactivation, and deodorization [16; 21; 22]. It has been an area of great interest in recent years, since it is applicable in many fields including environmental and energy related research areas [13]. Photocatalysis has so many advantages over the conventional water disinfection methods. Compared to the other conventional methods of inactivating _E.