Cost is comprised of capital costs, and operation and maintenance costs. The capital costs can be reduced by designing more efficient systems. The operation costs greatly depend on many factors such as the type and concentration of pollutants, level of treatment, and catalyst dose and loading method (fixed and slurry), in addition to the pretreatment (e.g., removal of particles causing high turbidity, which inhibit UV light penetration) and posttreatment (e.g., membrane separation in case of suspension-type TiO2 reactors) costs. Since more efficient systems have been developed, aiming at increase in efficiency and decrease in cost, the estimated costs of such systems have been roughly compared with those of the conventional treatment technologies used. Recent studies have been devoted to scientifically and technically improve TiO2 photocatalysis, particularly solar energy-based detoxification process. The costs for solar photocatalytic process were compared with those of activated carbon and UV/H2O2 systems . The analysis showed that the cost of photocatalysis rapidly decreases while the cost of the other conventional systems more or less remains the same between current and projected period. A field test of a solar photocatalytic process to detoxify BTEX pollutants (benzene, toluene, ethyl benzene, xylene) demonstrated that the treatment cost is competitive with those of conventional treatment technologies . Many similar results supporting the competitiveness and effectiveness of TiO2 photocatalysis were reported with other compounds such as trichloroethylene, pesticides, and polychlorinated biphenyls [103-105]. In most studies so far, the cost for typical TiO2-based AOTs seems higher at this moment than those of conventional technologies mainly due to the UV energy requirement, whose cost fortunately is now decreasing. However, potentially greater cost reductions are expected particularly in case of solar-activated TiO2 systems and AONs.
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