**2.4.1 Parabolic through reactor (PTR)**

224 Studies on Water Management Issues

treatment (Chang et al., 2000; Huang et al., 1999; Matthews et al., 1990). The details of these

Titanium dioxide (TiO2) exist as many crystalline forms. The most common forms of crystalline structures are anatase and rutile. Brookite is the most uncommon form due to its instability in terms of the enthalpy of formation. Anatase is the most stable among all the different crystalline forms with 8-12kJmol-1 (Cotton et al., 1999). It can be converted to rutile when it is heated to approximately 700°C (Bickley et al., 1991). Anatase is less dense compared to rutile, has a density of 3900kg/m3 while rutile has a density of 4260kg/m3. In the application of photocatalysis, anatase is a more efficient photocatalyst compared to rutile

It was been used extensively in many studies regarding photocatalytic degradation. Photocatalytic degradation studies utilizing Degussa P25 have been well documented due to its chemical stability, readily availability, reproductive ability, and activity as a catalyst for oxidation process (Bekbolet et al., 1998; Bekbolet & Balcioghu, 1996; Saravanan et al., 2009). Vigorous activities and researches are in process to further develop the existing Degussa P25 or synthesizing new materials, which can initiate photocatalysis using solar energy and hence reduce the cost and shortening the total time needed for the degradation. These developments include increasing the effective surface area, increasing the photoactivity, increasing the active sites, enhancing the absorption of photon energy and reducing the

There are many types of reactors have been developed and can be used in photocatalytic studies. These reactors were developed based on the different needs of applications. The selection of these reactors was according to the experiment conditions and applications. Generally the reactors can be briefly categorized into two groups, a suspension/slurry type and a thin film type. A slurry type reactor uses the catalyst in a suspension form whereas a thin film type reactor uses a thin film catalyst. Both types of reactors can be designed to be an immersion well reactor or flat wall reactor. Immersion well reactors were generally used in laboratory scale works for evaluation purposes. It can be run on either batch or continuous mode. The flow of oxidant and the temperature can be easily controlled and monitored. The source of the light can either be single or multiple with or without any reflectors. A suspension form is preferred because it is normally more efficient compared to the thin film reactor. It is because in a suspension type reactor, the catalyst has a higher effective surface area and hence larger surface area in contact with the substrate. This allows

a larger amount of photon to hit the surface and results in large adsorption capacity.

tubular reactor to concentrate sunlight so that it can enhance the photoreaction.

Other types of reactors are flat wall and tubular photoreactors. These types of reactors are simple and easy to design. Air can be use as an oxidant option for these reactors. Besides, solar energy can be utilized by these types of reactors. Moreover, reflectors are used in a

For the past 20 years, several photocatalytic water treatment reactors have been developed and tested. Different rectors were developed to find the best way of conducting solar

two reactors will be discussed later.

due to its open crystalline structure.

band-gap energy.

**2.4 Photocatalytic reactors** 

**2.3.2 Titanium dioxide (Degussa P25)** 

A parabolic through reactor adopted the principle of parabolic through solar concentrating system' to concentrate the sunlight on the focal point using Dewar tube. The schematic presentation is given in Fig. 1. The PTR concentrates the parallel (direct) rays of the photocatalytically active ultra-violet part of the solar spectrum and can be characterized as a typical plug flow reactor. Borosilicate glass tube which positioned along the focal line was filled with contaminant with titanium dioxide (TiO2) in suspension with a flowrate ranges between 250–3500Lh-1. This type of reactor had been selected as the first solar detoxification loops in Albuquerque and California in USA and Almeria in Spain (Bahnemann, 2004). Several research groups from the European continent have tested the PTR which installed at the Plataforma Solar de Almeria (PSA) in Spain for solar wastewater purification in the early 1990s (Bahnemann, 2004).

Fig. 1. A schematic view of a parabolic through solar concentrator
