Pesticide removal by combined ozonation and granular activated carbon filtration

E. Orlandini

Research output: Thesisinternal PhD, WU

Abstract

<p>Since the seventies, new water treatment processes have been introduced in the production of drinking water from surface water. Their major aim was to adequately cope with the disinfection of this water, and/or with the removal of pesticides and other organic micropollutants from it. This research focused on Biological Activated Carbon (BAC) filtration, which is a combination of ozonation and GAC filtration. Its general goal was identification and understanding of the mechanisms that underlie the expected beneficial effect of ozonation on the removal of organic micropollutants by GAC filtration. Detailed investigations for all organic micropollutants present in surface waters were not possible, thus, one model compound - pesticide atrazine - was chosen.</p><p>The initial pilot plant experiment with Rhine River water (pretreated by coagulation, sedimentation and rapid sand filtration) confirmed the expectation that ozonation significantly improves the removal of atrazine by GAC filtration. This improvement is not only due to the well-known effect of ozone-induced oxidation of atrazine, but also due to the effect of ozone-induced oxidation of a part of Background Organic Matter (BOM) present in water. BOM refers to the organic matter in the influent of GAC filters other than the target compounds that need to be removed. BOM is mostly of natural origin, <em>e.g.</em> compounds such as humic substances, but it also includes - especially in Rhine River water - many compounds of anthropogenic origin.</p><p>Subsequent (pilot-, bench- and lab-scale) experiments aimed to verify which of the anticipated processes and relationships underlie the improved atrazine removal observed in filters receiving ozonated influent. Namely, an important part of BOM compounds is partially oxidized due to ozonation. This partial oxidation increases the biodegradability, and reduces the adsorbability and molecular mass of BOM compounds. Consequently, enhanced biodegradation of BOM and its reduced adsorption are expected in filters receiving ozonated influent. Both biodegradation and adsorption of pesticides are expected to be improved as a result.</p><p>These experiments led to the following conclusions. Improved removal of BOM observed in filters that received ozonated influent can be attributed to the enhanced biodegradation of BOM in these filters. This can be concluded because ozonated, rather than non-ozonated BOM, was also better removed in filters filled with non-activated carbon, in which the removal of BOM is via biodegradation only. It could not be demonstrated that biodegradation of atrazine accounts for its improved removal in GAC filters that received ozonated rather than non-ozonated influent. Namely, no indication of atrazine biodegradation in these GAC filters was found in either of the experiments conducted: (i) no metabolites of atrazine were detected in the effluent of atrazine-spiked GAC filters, (ii) atrazine was not removed in filters filled with non-activated carbon, (iii) atrazine was not removed in the liquid media inoculated with the bacteria taken from atrazine-spiked GAC filters, and (iv) after three years of pilot plant operation, more atrazine was desorbed from GAC taken from the filter that received ozonated rather than non-ozonated influent. The enhanced biodegradation of BOM in filters receiving ozonated influent improves adsorption of atrazine in GAC filters. This can be concluded because atrazine was better adsorbed onto GAC preloaded with ozonated water that passed through filters filled with non-activated carbon (in which the removal of BOM is via biodegradation) than onto GAC preloaded directly with ozonated water.</p><p>The results also revealed that the improved adsorption of atrazine in filters receiving ozonated influent is the effect of both the higher adsorption capacity of GAC for atrazine, and the faster external and internal mass transfer rates of atrazine in these filters compared with filters receiving non-ozonated influent. Higher adsorption capacity and faster mass transfer were explained as due to reduced competitive adsorption and reduced preloading of ozonated BOM. Competitive adsorption of BOM occurs when BOM adsorbs simultaneously with atrazine, and competes with it for the adsorption sites available on GAC. BOM preloading is adsorption of BOM onto GAC before the adsorption of atrazine. Reduced competitive adsorption and reduced preloading of ozonated BOM are the consequence of increased biodegradability of a part of BOM compounds that are partially oxidized by ozonation. Namely, this increases the amount of BOM that is biodegraded rather than adsorbed in GAC filters. Besides increased biodegradability, decreased adsorbability of oxidized BOM also contributes to the improved adsorption of atrazine.</p><p>Finally, the two commonly applied models, <em>i.e.</em> the simple Adams-Bohart model and the more complex Plug Flow Homogenous Surface Diffusion model, were applied for the prediction of atrazine breakthrough in GAC filters with and without ozone-induced bioactivity. Neither model resulted in an accurate prediction. This can be expected considering that, due to the complexity of the processes that simultaneously take place during GAC filtration, the prediction of its performance involves many inevitable assumptions and simplifications.</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Schippers, J.C., Promotor, External person
Award date22 Nov 1999
Place of PublicationRotterdam
Publisher
Print ISBNs9789054104148
Publication statusPublished - 1999

Keywords

  • water treatment
  • water quality
  • pesticides
  • ozone
  • waste water treatment
  • filtration
  • drinking water
  • carbon

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