Strategy for the coupling of photochemical and biological flow reactors useful in mineralization of biorecalcitrant industrial pollutants

This study presents a combined photochemical (Fenton) and biological flow r eactor for the degradation of p-nitrotoluene-ortho-sulfonic acid (p-NTS). T his compound is contained in wastewaters coming from manufactures of dyes, surfactants and brighteners. The non-biodegradability of p-NTS in a fixed b ed reactor (FBR) was proved under theoretically favourable conditions such as the presence of cosubstrates and adapted bacteria. From this ascertainme nt, p-NTS can be considered as a non-biodegradable compound. Afterwards, se veral experiments for sole photo-Fenton treatment were carried out in a lab oratory scale photoreactor. By way of Dissolved Organic Carbon (DOC) and HP LC techniques, it was found that mineralization of p-NTS via photo-Fenton t reatment in continuous or batch mode is not a cost-effective strategy. Howe ver, the chemical and biological characteristics studied for the phototreat ed samples showed that the Fenton system produced within a short time inter mediates with very oxidised functional groups that are biodegradable and no n-toxic. This thus could permit the integration of photochemical and biolog ical processes. During treatments in continuous mode it was found that the main inconvenience of this application is related to the difficulty to cont rol the H2O2 concentration. With this system, it was hard to avoid the inhi bition of bacteria and hence a low biodegradation efficiency. To overcome t he inconveniences of the process mentioned above, the semi-continuous mode was applied. The coupled photochemical-biological reactor was operated at f ive different treatment times (respectively 50, 70, 95, 110 and 125 min). It was found that the most interesting zone for the coupled treatment is at the beginning of the photo-pretreatment when time is short enough to achie ve a cost efficient process and high biological and overall efficiencies. H owever, if the pre-treatment time is too short (i.e., 50 min), the intermed iates present in the solution are still structurally close to the initial b iorecalcitrant compound and the efficiencies of both, the biological-and wh ole coupled process are dramatically diminished. Consequently, the optimal time to stop the photochemical treatment before leading the treated water t o the biological reactor was found to be 70 min. At this moment, appropriat e efficiencies were reached giving the best compromise between time and ene rgy (71 $US per cubic meter) invested in both the biological and the overal l treatment. (C) 1999 Elsevier Science B.V. All rights reserved.