PREINDUSTRIAL EXPERIENCE IN ADVANCED OXIDATION AND INTEGRAL PHOTODEGRADATION OF ORGANICS IN POTABLE WATERS AND WASTE-WATERS BY PHOTOPERM(TM) MEMBRANES IMMOBILIZING TITANIUM-DIOXIDE AND PROMOTING PHOTOCATALYSTS

A membrane module, utilizing photocatalytic membranes, has been employed in a pilot plant both to investigate integral photodegradation of trichloroethene and atrazine, as model molecules of chloroaliphatics and chloro-triazine pesticides, and to transform typical industrial non-biodegradable wastes into fully biodegradable ones. In the first kind of experiments, by Langmuir-Hinshelwood plots relative to the pilot plant itself, with dioxygen as oxidising agent, the kinetic constants for degradation at 'infinite' concentration, k, and the apparent adsorption equilibrium constants, K, have been evaluated. Kinetic constants k were (3.68+/-0.25)X10(-5) and (6.67+/-0.65)X10(-4) mol/min for trichloroethene and atrazine respectively, and corresponding values of K (11.2+/-0.6)X10(3) and 150+/-45 M(-1) respectively. Apparent reaction order was substantially zero for trichloroethene over the entire range of initial concentrations examined (9.1X10(-5) to 1.0X10(-2) M); for atrazine, in contrast, it was close to unity at concentrations lower than about 10(-3) M, but it decreased progressively, approaching 0.2 at higher concentrations. Variation of apparent reaction order with concentration is discussed on the basis of a kinetic character of parameter K. Advantages brought about by immobilization of titanium dioxide semiconductor, with respect to its use in aqueous suspensions (rate factor 2.5), and by co-immobilization of suitable promoting photocatalysts together with titanium dioxide (rate factor 28) are shown. In the second kind of experiments, times necessary to reach a BOD/COD ratio of 2/3 have been measured for some characteristic wastes containing: i) non-biodegradable surfactants; ii) phenolic components; iii) organic solvents and paint components from wood and metal finishing; iv) lubricating fluids and refinery effluents. By employing ozone, instead of dioxygen, as oxidizing species during the photocatalytic transformation, reactivity was higher by a factor ranging from 1.1 to 1.7, but, even with dioxygen, times for complete induction of biodegradability were satisfactorily low (from 8.5 to 26 h per m(3) of effluent containing 10,000 ppm COD, in a pilot plant fitted with 1.2 m(2) of photocatalytic membrane, and a nominal irradiating power of 80 W).