Photocatalytic membrane reactors for degradation of organic pollutants in water

Different flowsheets (batch and continuous) of photocatalytic membrane reactors, to be used for degradation of organic pollutants present in water, together to some experimental results, are reported. 4-Nitrophenol (4NP) was used as a probe polluting agent and titanium dioxide in suspension was the catalyst. The photodegradation tests in the batch system were carried out without membrane changing the characteristic variables of the process (light intensity, TiO2 concentration, 4NP concentration, O-2 concentration, pH) to find their optimum values. The batch system consisted of a water jacket thermostatted and stirred beaker irradiated from above with a UV-Vis lamp (light intensity on the surface of the liquid 3.4 mW/cm(2)), An empirical predictive equation was obtained describing the reaction rate as a function of the reported variables. Photodegradation tests in the membrane reactors (total volume from 400 to 700 mi) were carried out coupling the batch to a re-circulation cell containing various types of flat sheet membranes which were able to retain the suspended catalyst and partially selective to the pollutant. The membranes were: NTR7410 and NTR7450 (Nitto Denko); N30F and NF-PES-010 (Hoechst); MPCB0000R98 (SEPAREM). The measured permeate flux was in the range 5-30 l/h m(2) at 4 bar and all membranes showed both a rejection and a capacity to adsorb the pollutant with a transitory phase varying from 80 to 400 min at 4 bar. This behaviour could be a benefit for the process because oscillations in the pollutant concentration are not transmitted in the permeate. Three factors: rejection, photocatalytic degradation and adsorption were able to maintain the 4NP concentration in the permeate at very low values. For the continuous system, the lowest 4NP concentration in the permeate was 6-7% (w/w) of the initial 4NP concentration (40 mg/l) after a transient period of 300 min. Further improvements of this process are under investigation. (C) 2001 Elsevier Science B.V. All rights reserved.