How to evaluate photochemical methods for water treatment

Abstract

Photochemical advanced oxidation processes (AOPs) generally imply generation of hydroxyl radicals which are initiating the oxidative degradation by well defined reactions (hydrogen abstraction, addition and electron 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 evaluation of photochemical AOPs is based on the absorption spectrum of the oxidant to be added and on the spectral distribution of the emission of commercially available light sources. Dominating light absorption, in particular in the UV-C spectral domain, by the solutes of the aqueous system to be treated may lead to exclude some of the degradation processes, as excitation of the oxidant and, consequently, production of the initiator become inefficient with increasing inner filter effects. The evaluation of photochemical AOPs in terms of volume independent rates is convenient and highly advocated, but such comparisons should only be made for processes applied to a restricted number of model substrates which are to be degraded in optimized equipment. Taking into account the volume independent rates determined in the range of realistic pollutant concentrations, the number of m3 of contaminated water of known pollutant nature and concentration to be treated per unit of time, the list of a commercially available light sources and their geometry, a final selection of the degradation process or of a combination of processes may be made, and the total electrical energy required and the number of photochemical reactors to be built may be calculated.