0957–5820/03/$23.50+0.00 # Institution of Chemical Engineers www.ingentaselect.com =titles=09575820.htm Trans IChemE, Vol 81, Part B, March 2003 THE DESIGN OF AN INDUSTRIAL WASTE-WATER TREATMENT PROCESS USING ADSORBED OZONE ON SILICA GEL C. TIZAOUI and M. J. SLATER School of Engineering, Design and Technology, University of Bradford, Bradford, UK A new technique using ozone loaded on adsorbents for industrial waste-water treatment has been investigated. This is a three-step process: ozone adsorption on a xed bed column; water treatment on the bed loaded with ozone; and nally drying= regeneration of the wet bed. Silica gel of mean particle size of 1.5 mm has been tested for ozone loading capacities followed by water treatment, both at ambient temperature. The mechanisms of the mass transfer process during ozone loading and water treatment have been established. The drying of the adsorbent using vacuum and dry air is feasible, but it is a time- consuming operation. A design procedure is described for the whole system. Keywords: water treatment; environment; adsorbents; reactive dye; ozone; silica gel. INTRODUCTION During the past few years there have been numerous statements by regulatory bodies worldwide calling for the gradual banning of the use of chlorine and its derivatives in water treatment and replacement by ozone. In many cases it is found that oxidants such as chlorine, hypochlorite and chlorine dioxide, which are widely used in water treatment, generate compounds of higher toxicity than the original ones (LaGrega et al., 1994). Ozone is increasingly used in drinking water sterilization, waste-water treatment, food and agricultural industries, the textile industry for colour removal, and pulp bleaching. Most processes apply the gas as it is produced diluted in a mixture of oxygen or air by simply bubbling it into water. The direct contact of liquid and ozone at low concentrations suffers from various problems: ozone solubility in water is low and total destruc- tion of the pollutant and total utilization of ozone may not be achieved. In many cases reactions involving ozone are not selective and=or have slow kinetics. Usually pollutants are found at low levels dissolved in water and in most cases the kinetics of pollutant oxidation include both ozone and pollutant concentrations with positive reaction orders (vonGunten and Laplanche, 2000). Therefore increasing the concentration of ozone in water will enhance the kinetics. A new concept consists of loading ozone on a bed of an adsorbent such as silica gel then passing polluted water through the bed. Sites of high ozone concentration are created and enhancement in the mass transfer and in the chemical reaction kinetics with adsorbed or unadsorbed pollutants is thought to be possible due to localized higher ozone concentration. Some control over the contact time of ozone with pollutants may also prove benecial for pollu- tants that react only slowly with ozone; this can be achieved by using a packed bed and water ow rates to match reaction rates. With direct gas–liquid contact, long contact times require a stirred or loop reactor in which gravity does not control gas bubble relative velocities and in that case back- mixing is deleterious compared with a plug ow reactor or counter-current contact system. The counter-current ow of adsorbent is possible but not usually an attractive proposi- tion, so a xed-bed system operating cyclically is investi- gated. The last step in this cyclic packed bed process is to dry the bed (vacuum followed by dry air) and then ozone ow is recommenced. Any pollutant residues on the dry adsorbent may be oxidized during ozone loading (Tizaoui and Slater, 2001a). ADSORBENTS The adsorbent should have high ozone adsorption capacity at ambient temperature, and be able to adsorb the initial pollutants and=or by-products if possible. It is known that silica gel can adsorb ozone, but previous studies have concentrated on sub-zero Centigrade temperatures to obtain substantial ozone loading (Cook et al. , 1959). The adsorbent should be strong enough to withstand repeated wetting and drying and also be suitable for use in a packed bed (pellet form not powder). Various adsorbents were batch or semi- batch tested for ozone adsorption (metal oxides mainly). It was determined that materials containing either strong Lewis acids such as alumina or strong Lewis bases destroy ozone (Bulanin et al., 1995; Bulanin et al. , 1997; Li and Oyama, 1998; Thomas et al. , 1997; Tizaoui, 2001). In contrast, silica gel adsorbs ozone physically without promoting its decom- position at higher levels (Ouederni et al., 1996; Tizaoui, 2001). Therefore silica gel obtained from Gjay Chemicals Company was chosen to carry out this investigation. The equilibrium adsorption and desorption of ozone in oxygen was studied at ambient temperature and a moisture content of the adsorbent of about 2% H 2 O by loading a xed 107