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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2004.11.010 Byline: Gerd Sagawe (a), Rodolfo J. Brandi (b), Detlef Bahnemann (a), Alberto E. Cassano (b) Keywords: Photocatalysis; Water pollution; Solar irradiation; Recycle reactors; Photonic efficiency; Chemical oxygen demand Abstract: The concentration of dissolved oxygen in water, in equilibrium with atmospheric air (ca. 8ppm at 20[degrees]C), defines the limits of all practical oxidizing processes for removing pollutants in photocatalytic reactors. To solve this limitation, an alternative approach to that of a continuously aerated reactor is the use of a recirculating system with aeration performed after every cycle at the reactor entering stream. As defined by the nature of a single recirculating step (the need of a reactor operation at a rather low concentration range), this procedure results in a very low photonic efficiency (thus requiring a large photon collecting area and consequently increasing the capital cost). The design engineer will have to resort to a series of several reactors with recirculation. This solution may then lead to a very high Photonic Efficiency for the entire process (i.e., a reduced light harvesting area) at the price of an increase in the required capital cost (due to the larger number of reactors). This paper provides a very simple analysis and analytical expressions that can be used to estimate, for a desired degree of degradation, a trade-off solution between a high number of reactors and a very large surface area to collect the solar photons. Author Affiliation: (a) Institut fuer Technische Chemie. Universitaet Hannover Callinstrasse 3, D-30167 Hannover, Germany (b) INTEC Universidad Nacional del Litoral and CONICET, Guemes 3450, 3000 Santa Fe, Argentina Article History: Received 22 June 2004; Revised 19 November 2004; Accepted 22 November 2004 Article Note: (miscellaneous) Communicated by: Associate Editor Gion Calzaferri

A solar photoreactor operated in the batch, recirculating mode is analyzed in terms of very simple observable variables such as the impinging photon flux, the incident area, the initial concentration, the flow rate, the reactor volume and a property defined as the Observed Photonic Efficiency. The proposed equipment is made of a tubular reactor, a tank, a pump and the connecting pipes. The analysis is formulated in terms of the photon input corresponding to an equivalent batch system that is derived as a new reaction coordinate for photoreactions. Employing several plausible approximations, the pollutant concentration evolution in the tank is cast in terms of very simple analytical solutions. Process photonic efficiencies are defined for the system operation and calculated with respect to the maximum achievable yield corresponding to the differential operation of the solar recirculating reactor. Keywords: Photocatalysis; Water pollution; Solar irradiation; Recirculating reactors; Photonic efficiency; Photon input