New integrated photocatalytic-biological flow system using supported TiO2 and fixed bacteria for the mineralization of isoproturon

The photodegradation of isoproturon (IP) was performed using TiO2 supported on glass rings in a 1.5 l coaxial rector. After 60 min of phototreatment, IP was completely eliminated and about 80% of dissolved organic carbon (DOC) remained in solution. This efficiency of photodegradation was compared with that using the suspended catalyst in solution and the results indicated that the catalytic activity of TiO2 is not reduced when it is immobilized. The durability of the supported TiO2 was also tested. It was found that after 300 h of photodegradation experiments, its activity was not affected. The chemical nature of the phototreated solution was ass essed following the evolution of the initial compound, the organic carbon, and the formed ions, as well as the toxicity and the biodegradability. These analyses demonstrated that the solution resulting from the phototreatment of IP is biologically compatible and its complete mineralization can be performed by biological means. In this way, for the mineralization of an IP solution, a combined photochemical and biological flow reactor was used operating in semi-continuous mode at laboratory scale. This coupled system employs TiO2 supported on glass rings in the photocatalytic reactor and bacteria supported on biolite in the biological part. In this combined system, 100% of the initial concentration of IP and 95% of DOC were removed. Finally, some field experiments under direct sunlight were carried out at the Plataforma Solar de Almerı́a (PSA), Spain. The photocatalytic oxidation of IP was performed in homogeneous and heterogeneous solutions and two different reactors were compared: a medium concentrating radiation system (Helioman, HM) and a non-concentrating radiation system (compound parabolic collectors, CPC). The degradation rates obtained in the CPC are around 5 times more efficient than the HM collectors. However, in both systems 100% of the initial concentration of IP was removed. The possibility of coupling a CPC photoreactor with a biological system at field pilot scale employing supported TiO2 and fixed bacteria for the treatment of real bio-recalcitrant wastewater is also suggested.

Coupling of photocatalytic and biological processes as a contribution to the detoxification of water

Sandra Parra Cardona

This research contributes to the study and development of a new degradation technique that couples solar and biological processes for the treatment of biorecalcitrant, nonbiodegradable, and/or toxic organic substances present in the aqueous medium. Efficient physicochemical pretreatments are necessary to modify the structure of the pollutants, by transforming them into less toxic and biodegradable intermediates, allowing then, a biological procedure to complete the degradation of the pollutant load in a shorter time and in a less expensive way. The strategy of coupling photochemical and biological processes implicates among others, the study of some fundamental physicochemical properties, the optimization of a coupled reactor at laboratory scale (2 litres), and the study of solar photocatalytic treatment efficacy under direct sunlight using parabolic collectors of 40 to 200 litres. The study of the structure effect on the photoreactivity via TiO2 catalysis is studied using several substituted phenols to cover a wide variety of electronic effects, ranging from strong electron-donating (activating) to strong electron-withdrawing (deactivating) groups and herbicides with very similar molecular structures (metobromuron, isoproturon, chlorbromuron, and chlorotoluron). The photoreactivity of these compounds is affected by the electronic nature of the substituents and their positions in the aromatic ring, being higher when there is a greater electronic density. The Hammett constant, which represents the effect that different substituents have on the electronic character of the aromatic studied compounds, appears to give an adequate descriptor of their photocatalytic degradability. One important consideration in the TiO2-photocatalysed reactions is the adsorption of the organic compound on the surface of semiconductor particles. The dark adsorption isotherms for complete p-halophenols series and four herbicides are measured and correlated with their photoreactivity. The results indicate that no direct correlation exists between the extents of adsorption and the initial photodegradation rates of the studied compounds. Concerning the optimization and utilization of an integrated photocatalytic-biological process at laboratory scale, two kinds of combined systems are developed using immobilized biomass for the biological step and either diluted Fe3+/H2O3 (Fenton reaction) or TiO2 supported on glass rings for the photocatalytic pre-treatment. The advantages of the latter system are that the catalyst can be re-used and that the pH of the solution remains at neutral values. The photo-Fenton reaction instead, renders the phototreated solution acidic making neutralization necessary. The photochemical-biological flow reactors mentioned above, are employed to completely mineralize an isoproturon-herbicide solution. Preliminary experiments concerning the chemical and biological characteristics of the phototreated solution, are carried out to determine the moment at which it becomes biocompatible. Two operation modes (continuous or semi-continuous) of the photo-Fenton-biological coupled reactor are compared by studying the efficiency of the photochemical, biological, and overall treatments of p-nitro-o-toluenesulfonic acid (p-NTS) in solution. The two main parameter affecting the performance of the photo-assisted reactor in continuous mode are related to the very low pollutant concentration that characterise this kind of operation mode and to the high residual H2O3 concentration after the pretreatment. Thus, a semi-continuous mode was applied to try to overcome these inconveniences. In these conditions, 50 to 70 litres of polluted water can be treated per day per litre of photoreactor. The last part of this thesis, addresses the study of the solar photocatalytic treatment efficacy under direct sunlight using parabolic concentrating (Helioman reactors) and nonconcentrating (CPC) collectors. This part, carried out at the "Plataforma Solar de Almería" (PSA) in Spain, indicates that the solar photocatalytic treatment is effective for the purification of water contaminated by herbicides and other substances of industrial origin like the p-NTS. It is demonstrated the utility of both homogeneous (based on Fe3+/H2O3 reaction) and heterogeneous photocatalysis (based on TiO2) as pretreatment methods that can be followed by a biological treatment. Using a coupled system, the treatment of 100 to 300 litres of polluted water per square metre of photoreactor can be envisaged in a sunny day. The low manufacturing, installation, and maintenance costs, and easy operation of the CPC, compared with Helioman collectors, suggest that the former ones are, at present, the best way to apply the solar detoxification technology.

EPFL2001

Photochemical-biological treatment of a real industrial biorecalcitrant wastewater containing 5-amino-6-methyl-2-benzimidazolone

Sandra Parra Cardona, César Pulgarin, Victor Sarria

5-amino-6-methyl-2-benzimidazolone (AMBI), used in the manufacture of dyes, was characterised as a biorecalcitrant compound by means of different biodegradability tests. In order to enhance the biodegradability of this important pollutant, the application of Advanced Oxidation Process (AOPs) as a pre-treatment was explored. Some experiments were addressed to find the most efficient AOP. The systems H2O2/hv, TiO2/H2O2/hv, Fe3+/hv, Fe3+/H2O2 and Fe3+/H2O2/hv were compared. The photo-Fenton system was the most efficient and the optimal conditions (AMBI, Fe3+, H2O2 concentrations) for the degradation of AMBI were found. During the photo-Fenton degradation, experiments were also made to obtain information concerning the evolution of: (a) organic carbon and initial compound concentration; (b) the oxidation state; (c) the toxicity; (d) the biodegradability; and (e) the chemical nature of the intermediates. These analyses show that the solution resulting from the treatment of AMBI is biologically compatible and complete mineralisation can be performed by biological means. A combined photochemical (Fenton) and biological flow reactor for the degradation of AMBI was successfully operated in continuous mode at laboratory scale. 100% of the initial concentration of AMBI and 80.3% of Dissolved Organic Carbon (DOC) were removed in 3.5 hours of total residence time. Finally, some field experiments under direct sunlight carried out at the Plataforma Solar de Almeria, Spain, demonstrated that this solar catalytic system is an effective treatment for this kind of industrial wastewater.

2001

Coupling of photocatalytic and biological processes as a contribution to the detoxification of water

Sandra Parra Cardona

EPFL2001