A process for the treatment of liquid effluents by means of clean catalytic oxidation, using hydrogen peroxide and heterogeneous catalysis

Abstract: The present invention concerns a process for the treatment of effluents by means of catalytic oxidation with hydrogen peroxide, using heterogeneous catalysis in the presence of metal pairs with anodic and cathodic characteristics (for instance iron and copper). This process can be applied to the effluents of the food-processing industries (olive oil mills, distilleries, sugar refineries, coffee production, tanneries, etc.). The pre-treated residues can be used as fertilisers or in the production of acetate derivatives. This new process has advantages in economic and environmental terms and allows the self-regeneration of the metals through a process of oxidation/reduction of their ions, which leads to only traces thereof in solution, therefore avoiding the habitual need for a step involving the recovery and/or elimination thereof.

Description: FIELD OF THE INVENTION The process which is the subject of this invention relates to the treatment of effluents, for example from olive mills, by means of catalytic oxidation with hydrogen peroxide, using heterogeneous catalysis, in the presence of metallic pairs with anodic and cathodic characteristics. BACKGROUND OF THE INVENTION The various processes available for the treatment of effluents, for example those generated by olive mills, are demanding in terms of time, space and energy consumption. The application of olive mill effluents to the soil without any previous treatment can damage the structure and the bioactivity of the soil, due in part to the high polyphenol content and the phytotoxicity of these compounds. In addition, aerobic and anaerobic biological treatment processes are often problematical due to the high organic and polyphenol content of olive mill effluent, which leads to long periods of biomass acclimatisation and low success rates. The use of lagoons or other drying systems often produces strong odours due to the direct emission of sulphurous gases during anaerobic processes or organic acids that build up in vapour condensates. As olive mills have greatly varying capacities, a new process should be flexible and easy to use, allowing the safe use of the recovered water or the discharge thereof to a biological treatment station. The black/reddish-brown colour of the effluent should be eliminated in any new approach to the treatment of such effluent. M.F. Rosa e A.M. Vieira, "Perspectivas e limitações no tratamento e utilização das águas residuais de lagares de azeite: situação portuguesa", Boletim de Biotecnologia, n° 52, December 1995, refer to treatment systems which use oxidation or evaporation basins. However, these systems have major inconveniences such as large area requirements, the production of odours and the formation of sludge deposits. A. Lanzani, P. Bondioli, E. Fedeli, A. Ponzetti, G. Pieralisi, "Un processo per lo smaltimento integrale delle acque di vegetazione con contemporanea valorizzazione delle sanse nella lavorazione delle olive", La rivista delle Sostanze Grasse, Vol. LXV, February 1988, describe a process which uses concentration by evaporation. This process too has some disadvantages, such as high energy costs, the caramelisation of sugars and the formation of condensates of organic acids L. Martinez Niet, Garrido Hoyos, F. Camacho Rubio, M.P. Garcia Pareja & A. Ramos Cormenzana, "The biological purification of waste products from olive oil extraction", Biosource Technology 43 , pp. 215-219, 1993, describe a process that uses mushrooms for the biological elimination of polyphenols. The major problem is that the adsorption kinetics is very slow. M. Chakchouk, M. Hamdi, J.N. Foussard, H. Debellefontaine, "Complete treatment of olive mill effluent by a wet air oxidation process coupled with a biological step", Environmental Technology, Vol. 15, pp. 323-332, describe treatment systems by wet oxidation with peroxide. However, these systems require fairly high temperatures and pressures. Treatment systems based on the use of hydrogen peroxide are, in principle, a clean technology. However, the use of catalysts in liquid phase (for instance iron (III) sulphate solution, Fe 2 (SO 4 ) 3 ) leads to the formation of precipitates which are difficult to eliminate (mainly hydroxides), causing severe environmental problems. Furthermore, the use of this type of catalyst requires the control of the addition of the catalyst, which adds additional complications to the process. On the other hand, this process does not achieve colour reductions greater than 70%, under normal pressure and temperature conditions. SUMMARY OF THE INVENTION The present invention allows the elimination of toxicity, colour and odours of effluents of the food-processing industry by catalytic oxidation, with hydrogen peroxide, using heterogeneous catalysis, in the presence of metallic pairs with anodic and cathodic characteristics. The pre-treated residues can be used for irrigation and in the production of acetate or biogas. This process can be applied to effluents of the food-processing industry (olive oil mills, distilleries, sugar refineries, the coffee industry, the tanning industry, etc.). The process eliminates nearly all the toxic polyphenol compounds of liquid effluents, as well as odours and coloration, allowing the discharge thereof to final treatment systems and/or subsequent reutilization. It also involves low energy costs and oxidation occurs at a low temperature. DETAILED DESCRIPTION OF THE INVENTION The catalytic oxidation of effluents, for example from olive mills, is an alternative process for the pre-treatment of effluents, allowing them to be used for irrigation in agriculture or to be discharged to municipal sewage systems. In particular, the olive oil industry would be interested in this new, easy to apply and more profitable process (low maintenance costs), which can moreover be adapted to olive mills of any capacity. This catalytic oxidation process can be used in the detoxification of a vast range of agro-industrial effluents, requiring minimal adaptation of the process in order to adjust it to the characteristics of the effluent. The main applications of the process include the treatment of coloured effluents with an odour and a certain level of polyphenols (for example, effluents from olive mills, distilleries, sugar refineries, the coffee industry, tanneries, etc.). The same process can be applied to the production of acetate derivatives, leading to a new industrial use for resources with otherwise little value. This process for the treatment of effluents achieves a high detoxification level, has low demands in terms of energy and time and allows for the possibility of reusing the treated effluent for irrigation. Apart from this, it also allows for subsequent treatment in both aerobic and anaerobic systems. The process is highly effective in decolorisation and it can also be used in the production of organic and inorganic acetate derivatives of industrial value. Another advantage of the present process is that it does not produce sludge, as in the case of homogeneous catalytic oxidation processes (for example, due to iron oxide precipitates). More specifically, the subject of the present invention is a process for the treatment of effluents, by means of catalytic oxidation with hydrogen peroxide, using a heterogeneous catalyst in the presence of a metallic pairs with anodic and cathodic characteristics. The aforementioned metallic pairs should preferably be iron and copper, which are immersed in the effluent to be treated, where the decomposition of the hydrogen peroxide takes place. One of the main advantages of this process over the use of liquid catalysts is the self-regeneration of the metals by means of a process of oxidation/reduction of their ions, which leads to only a trace of the metals in solution, therefore overcoming the usual need for a step involving their recovery and/or elimination from the peroxidised effluent, which means that this invention be classified as a clean technology. Another advantage of the use of heterogeneous catalysts is the fact that it is not necessary to control the addition of the catalyst, contrary to processes that use a liquid catalyst, where the control of the catalyst addition is absolutely necessary. This process allows an organic content reduction of around 50%, a reduction in toxicity of more than 80% and a reduction in colour greater than 75%, with low costs in terms of reagents and energy under normal pressure and temperature conditions. This treatment system has a high degree of automation and is easy to operate. The area required for the installation of the equipment necessary for the treatment depends on the required capacity and on the chosen system - fixed or mobile unit, pond, storage tank or other (for example, for a unit with a treatment capacity of 7 m 3 /h, the area occupied is less than 8 m 2 ). The system can operate continuously without modifications to the unit or the system set-up, for various industrial effluents/applications. EXPERIMENTAL PART The process which is the subject of the present invention consists of the treatment of liquid effluents containing concentration less than 100g/l of Chemical Oxygen Demand (COD) and less than 10 g/l of polyphenolic compounds, by means of catalytic oxidation, with hydrogen peroxide, using as heterogeneous catalyst a pair of two different metal bars; selected according their cathodic or anodic characteristics. Liquid effluents, with a certain COD concentration and polyphenol content, are very diverse, for example effluents generated by olive mills, distilleries, sugar refineries, the coffee industry and tanneries. Olive mill effluents will be considered in detail here. The COD concentration is less than 100 g/L, with average values between 30 and 70 g/L. The initial pH is usually acid with average values of between 3.5 and 5.5. The polyphenol compound content is usually less than 10g/L. The average values are less than 5 g/L and preferably between 1.5 and 4 g/L. The hydrogen peroxide concentration is generally less than 10 % and preferably between 1 and 5 %. Normally the process takes place in a continuously stirred reactor, at approximately atmospheric pressure and temperatures between 20 and 70°C. The process is preferably carried out at atmospheric pressure and at a temperature of between 30 and 60° C. The retention time is usually less than 1 hour, preferably between 15 and 40 minutes. These preferred retention times permit oxidation efficiency greater than 35% for the COD, preferably greater than 45%, and greater than 98% for polyphenolic compounds, preferably greater than 99%. The final pH is lower than the initial pH, preferably between 1.5 and 3.5. Colour removal is usually greater than 75% and preferably greater than 78%. Odour removal is usually greater than 90% and preferably greater than 95%. To improve the efficiency of the process of the present invention, it is advisable to use a retention basin that will contribute to the oxidation efficiency. It is also advisable to neutralise the treated effluent, so that it may be applied as a fertiliser. The following examples are simply intended to better illustrate the invention and they do not limit the scope of the invention. Examples Example 1 (Comparative) The oxidation of a liquid effluent generated by a continuous olive extraction process was carried out. The initial COD concentration was 45.6 g/L, the pH was 4.15 and the phenol content was 3g/L, with 2% hydrogen peroxide, in a continuously stirred reactor, at a temperature of 45° C and at atmospheric pressure. The efficiency obtained in the process for retention times of 20 and 40 minutes was 21% and 24% respectively for COD removal and 99.9% in both cases for phenolic compound removal. The final pH values were 3.2 and 2.8 respectively. The colour removal was 55% and 62% respectively. Example 2 The oxidation of a liquid effluent generated by a continuous olive extraction process was carried out. The initial COD concentration was 45.6 g/L, the pH was 4.17 and the phenol content was 3 g/L, with 2% hydrogen peroxide, in a continuously stirred reactor, at a temperature of 45° C and at atmospheric pressure. The efficiency obtained in the process for retention times of 20 minutes, in the presence of Fe 3+ and Cu 2+ as metal bars immersed in the reactor was 42% for COD removal and 99.9% for phenolic compound removal. The final pH was 3.1. The colour removal was 80%. Example 3 (Comparative) The oxidation of a liquid effluent generated by a continuous olive extraction process was carried out. The initial COD concentration was 59.8 g/L, the pH was 4.3 and the phenol content was 2.6 g/L, with 2.5% hydrogen peroxide, in a continuously stirred reactor, at a temperature of 38° C and at atmospheric pressure. The efficiency obtained in the process for retention times of 20 minutes was 18% for COD removal and 99.5% for phenolic compound removal, in the presence of 800 ppm of Fe 3+ added in the form of Fe 2 (SO 4 ) 3 . The final pH was 2.4. The colour removal was 66%. Example 4 The oxidation of a liquid effluent generated by a continuous olive extraction process was carried out. The initial COD concentration was 59.8 g/L, the pH was 4.3 and the phenol content was 2.6 g/L, with 2.0% hydrogen peroxide, in a continuously stirred reactor, at a temperature of 54° C and at atmospheric pressure. For a retention time of 20 minutes, the efficiency of the process was 60% for COD removal and 99.9% for phenolic compound removal, in the presence of Fe 3+ and Cu 2+ in the form of solid metal bars. The final pH was 2.15. The colour removal was 78%. Example 5 The oxidation of a liquid effluent generated by a continuous extraction process was carried out. The initial COD concentration was 47.3 g/L, the pH was 4.4 and the phenol content was 2.9 g/L, with 10.0% hydrogen peroxide, in a continuously stirred reactor, at a temperature of 42° C and at atmospheric pressure. For a retention time of 30 minutes, the efficiency of the process was 48% for COD removal and 99.0% for phenolic compound removal, in the presence of Fe 3+ and Cu 2+ in the form of solid metal bars. The final pH was 2.3. The colour removal was 80%. As can be seen from the above table, the present process (Examples 2, 4 and 5) has significant advantages in relation to prior art (Examples 1 and 3). In particular, the increased efficiency obtained for the COD concentration is between 100% and a value greater than 200%, and for the removal of colour between 18% and 45%.

Claims: Process for the treatment of liquid effluents containing concentrations less than 100 g/l of COD and less than 10 g/l of polyphenolic compounds, by means of catalytic oxidation with hydrogen peroxide, characterised in that one pair of two different metal bars selected according their cathodic or anodic characteristics is used as catalyst. Process according to claim 1, characterised in that the metals are iron and copper. Process according to claim 1 or 2, characterised in that the liquid effluents, with high concentrations of COD and containing polyphenolic compounds, are selected from effluents produced in olive oil mills, distilleries, sugar refineries, the coffee industry, tanneries, among others. Process according to any of claims 1 to 3, characterised in that the initial pH value is between 3.5 and 5.5 and by a concentration of hydrogen peroxide of between 1 and 5%. Process according to any of claims 1 to 46, characterised in that the process is carried out at atmospheric pressure and at a temperature of between 30 and 60° C. Process according to any of claims 1 to 5, characterised in that the retention time is normally less than one hour. Process according to claim 6, characterised in that the retention time is between 15 and 40 minutes. Process according to any of claims 1 to 7, characterised by the occurrence of self-regeneration of the metals due to a process of oxidation/reduction of their ions. Process according to any of claims 1 to 8, characterised in that it is not necessary to control the addition of the catalyst. Process according to any of claims 1 to 8, characterised in that the treated effluent is neutralised, in order to allow its application as a fertiliser. Process according to any of claims 1 to 8 characterised in that it can also be applied in the production of acetate derivatives.

Inventors: COSTA GUEDES DA SILVA MANUEL J, MAGGIOLLY NOVAIS JULIO, MARTINS DIAS SUSETE MARIA, COSTA GUEDES DA SILVA, MANUEL JOSE, MAGGIOLLY NOVAIS, JULIO, MARTINS DIAS, SUSETE MARIA, Costa Guedes Da Silva, Manuel José
Applicants: INST SUPERIOR TECNICO, SOLVAY INTEROX PRODUTOS PEROXI, INSTITUTO SUPERIOR TECNICO, SOLVAY INTEROX - PRODUTOS PEROXIDADOS, LDA.
Application year: 1999
Main classification: C02F1
IPC codes: C02F1