Hybrid process for continuous wastewater treatment: coupling of heterogeneous Sono-Photo-Fenton reaction with membrane filtration – SOFENcoMEM

The efficacy of each process was evaluated individually, then their combination in pairs, and finally their full association. Studies were carried out by considering both common operating parameters (temperature, pH, concentration of reagents) and specific process parameters.
The study was completed by an identification of the pollutant degradation pathways and reaction kinetics for a better understanding of the mechanisms involved, including the generation of hydroxyl radicals. Analytical methods have been developed to identify and quantify all intermediates formed during the oxidation of the targeted pollutant (liquid chromatography coupled with UV detection and / or mass spectrometry).
Filtration performance has also been optimized for catalysts immobilization within the reactor. Eventually, a study of membrane ageing in oxidative reaction media was carried out and a strategy for controlling clogging by backwashing was developed.
Comparison of single and coupled processes was conducted in batch reactor on a synthetic effluent. Then, real effluents from local wastewater treatment plants were continuously treated by the best advanced oxidation process.

(i) In homogeneous system: activation of the Fenton reaction by ultrasound and ultraviolet with a real synergistic effect and identification of the pollutant degradation pathways. In heterogeneous system (iron oxide supported on zeolite): additive effect between heterogeneous Fenton oxidation and UV or US irradiation.
(ii) Iron solid phase immobilization avoids metal leaching, the catalyst is stable and the Fenton oxidation reaction is only slightly slowed down.
(iii) The polysulfone membrane module ages during oxidation, without significantly impacting its performance.
(iv) The treatment can be applied without pH adjustment in a synthetic solution, avoiding additional chemical treatment. This it is not the case for the real effluents studied

This project has provided new scientific and technical information in the field of water treatment and chemical reaction engineering (reaction kinetics, activation mechanisms, continuous reactor design), as well as on analytical tools for advanced oxidation processes.
Future prospects could move towards the use of other types of membranes with lower cut-off, to retain the most toxic molecules in the reactor, and improve their abatement rate by concentration effect. This membrane may be catalytic, with iron nanoparticles deposited on its surface.

The work resulted in the writing of a thesis on the activation of the homogeneous and heterogeneous Fenton reaction by ultrasound and ultraviolet (S. Adityosulindro), as well as in three international publications in peer-reviewed journals (on the activation, reaction and filtration aspects, respectively), two papers in French conference proceedings and six communications, including three international ones. This valuation covers various fields such as chemical reaction, activation, membrane separation, water treatment and more generally process engineering.

Environmental and health concerns are driving the wastewater treatment to come up with advanced technologies. Current research on possible treatments for waters polluted by bio-recalcitrant and/or toxic compounds is moving towards a coupling of processes, either traditional or more innovative. Membrane processes are under development but should be coupled with techniques for the destruction of pollutants to provide modern hybrid processes that can be used at the source of pollution rather than as post-treatment. On the other hand, advanced oxidation processes (AOPs) are also investigated, as they generate highly active species, hydroxyl radicals. These radicals are able to attack most organic compounds non-selectively with reaction rate constants as high as 10^9 /M/s. However AOPs proved uneconomical for highly diluted solutions. The coupling of the both is of great interest for science and technology, involving modeling and optimization of both stages.

Among advanced technologies, Fenton oxidation, which is conventionally based on the use of hydrogen peroxide and Fe2+ ions in solution, has been successfully applied for the degradation of various pollutants. However, this process is limited by the narrow pH window (2 to 4) and uneasy iron recovery. To overcome these drawbacks, heterogeneous iron oxide catalysts, either supported on porous materials (e.g. activated carbon, zeolite …) or dispersed in the form of nanoparticles are under development. In the latter case, coupling with membrane filtration (ultrafiltration or microfiltration) will insure the retention of the catalyst during the continuous water treatment.

Fenton reaction (FR) is also classically operated in the presence of ultraviolet/visible (UV/Vis) irradiation (photo-Fenton). This activation prevents the formation of inactive catalyst species and also promotes the reduction of Fe3+ to Fe2+ (limiting step in dark Fenton process) while generating additional radicals.

Similarly, the application of ultrasound (sono-Fenton) should also play several promoting roles in the heterogeneous reaction. In solution, a synergetic effect between FR and ultrasound (US) has been proven, in which US improves Fe2+ regeneration and subsequent formation of radicals. Moreover, acoustic cavitation gives rise to extreme conditions inside and around the collapsing bubble - up to 5000 K and 500 bar - which result in thermal cleavage of volatile compounds, further improving pollutant mineralization. Moreover, low frequency US also induces mechanical effects, including particle aggregate disruption (increase of accessible surface) and possible surface activation or reactivation (structural modification or removal of passivation layer). One last interesting point is that zero-valent iron (ZVI) particles (Fe(0)) can be activated by very short inputs of US. It results in highly effective catalytic species requiring moderate dosages of H2O2. Such a combination will offer the capacity to use inexpensive catalysts (iron shavings) and low cost operating conditions as no long-term external sources of energy are required.

In this context, the following project proposes to study a continuous hybrid process for wastewater treatment, coupling heterogeneous Fenton reaction with activation techniques (ultrasound and/or ultraviolet) and membrane filtration (immersed module). For instance the targeted micropollutants are non-steroidal anti-inflammatory drugs, ibuprofen and diclofenac, whose consumption has exploded during the last decade. Moreover, diclofenac is likely to be part of the list of pollutants targeted by the Water Framework Directive.

The combined hybrid techniques will be examined in terms of process efficiency and global cost, implying the evaluation of catalyst stability, membrane fouling and resistance to radicals, as well as separate and combined effects of activation techniques.