HOW TO EVALUATE PHOTOCHEMICAL METHODS FOR WATER-TREATMENT

Photochemical advanced oxidation processes (AOPs) generally imply gene ration of hydroxyl radicals which are initiating the oxidative degrada tion by well defined reactions (hydrogen abstraction, addition and ele ctron transfer) with available organic substrates. This limitation of the scope of applications may be avoided in implementing combinations of different photochemical and/or thermal processes. A simple evaluati on of photochemical AOPs is based on the absorption spectrum of the ox idant to be added and on the spectral distribution of the emission of commercially available light sources. Dominating light absorption, in particular in the W-C spectral domain, by the solutes of the aqueous s ystem to be treated may lead to exclude some of the degradation proces ses, as excitation of the oxidant and, consequently, production of the initiator become inefficient with increasing inner filter effects. Th e evaluation of photochemical AOPs in terms of volume independent rate s is convenient and highly advocated, but such comparisons should only be made for processes applied to a restricted number of model substra tes which are to be degraded in optimized equipment. Taking into accou nt the volume independent rates determined in the range of realistic p ollutant concentrations, the number of m(3) of contaminated water of k nown pollutant nature and concentration to be treated per unit of time , the list of a commercially available light sources and their geometr y, a final selection of the degradation process or of a combination of processes may be made, and the total electrical energy required and t he number of photochemical reactors to be built may be calculated. (C) 1997 IAWQ. Published by Elsevier Science Ltd.