Wastewater management in the pharmaceutical industrytowards the development of new electrochemical reactors for wastewater treatment

Resumen

The pharmaceutical industry is the branch of the chemical sector devoted to the development, fabrication and commercialization of drugs with therapeutic value. Active pharmaceuticals ingredients (APIs) may be fabricated by different processes such as extraction from natural biomasses (fermentation or natural extraction), synthesis via organic reactions or a combination thereof. Subsequently, the API is formulated with different excipients to obtain the commercial drug. Today, most of the APIs are fabricated by chemical synthesis in plants working under batch. Typically, the fabrication of a single API comprises various intermediate stages and plants fabricate more than one API at the same time. With this diversity of processes, comes a similarly diverse set of wastes, including wastewater. The wastewater generated in chemical synthesis pharmaceutical plants is generally toxic and poorly biodegradable, containing a wide range of inorganic and organic (mostly solvents) pollutants. The composition is quite heterogeneous and varies from one day to another, as a function of the production schedule. This complexity hinders the adequate management of those wastes. With this background, the objective of the present PhD is the study of the particularities and the optimization of wastewater management in industries producing in batch. In particular, the issues associated to wastewater management in industries working in batch are tackled in Chapter 5 by using a chemical synthesis pharmaceutical plant located in our region. Samples of every aqueous effluent from all the production processes were collected and analyzed, observing a huge diversity in their characteristics. Conductivity (as a globalizer parameter of inorganic concentration) ranged from 0.15 to more than 200 mS cm-1 and chemical oxygen demand (as a globalizer parameter of the organic pollution) varied from 0 to more than 500,000 mg O2 dm-3). The effluents were classified into five groups as a function of their pollution load. Management strategies were proposed for each one according to the waste management hierarchy: individualized treatment, reutilization/recovery, organic-polluted, inorganic and organic polluted as well as management off-site. In some cases, reutilization or valorization may not be technically possible or economically viable. Under those circumstances, an end-of-pipe technology is required to minimize the environmental impact of wastewater discharge. Currently, the plant is equipped with a Wet Peroxide Oxidation (WPO®) reactor, a technology based on the Fenton reaction. A screening of potentially-applicable technologies both for the elimination of organic compounds (coagulation, Fenton oxidation, conductive diamond electro-oxidation) or concentration of inorganics (electrodialysis and vacuum evaporation) was conducted. Coagulation was discarded as global technology. Fenton oxidation was effective in approximately half of the effluents but cannot achieve high mineralization percentages. Conductive-diamond electro-oxidation (CDEO) was a robust and reliable technology. It could reduce the pollution with an outstanding performance in all the real effluents tested, but further research is needed to confirm the viability of this technology on a real scale. Vacuum distillation effectively separated inorganics from the residual effluent, but the distillated fraction was polluted with organic compounds. On its part, electrodialysis showed a positive performance with the real water but attention is to the paid to the fouling of the membranes. Given the complexity of the situation, an informatic management tool was developed to predict the waste generated and the associated economic and environmental costs. The tool is flexible and could be adapted not only to the ever-changing reality of the plant under study but also to any given chemical plant working in batch. Considering that the most important cost of the WPO® reactor is the acquisition of hydrogen peroxide, in this work the viability of the in situ production via 2 e- electro-chemical reduction of oxygen was studied as an alternative to the acquisition in the market. Because of this, Chapter 6 was devoted to the study of technology to electro-generate hydrogen peroxide. The first aspect to be studied is the cathodic material. For this purpose, a divided cell and a carbon cloth as gas diffusion electrode (GDE) were used. The main conclusion from this part of the work is that the deposition of carbon black and polytetrafluoroethylene (CB/PTFE) boosts the production of hydrogen peroxide. In general, the main limitation of electro-chemical generation of H2O2 is the low solubility of oxygen in water (≅ 8 mg dm-3). Currently, the most effective aeration system is the use of GDEs, but it presents some disadvantages in terms of low oxygen utilization and scalability. Because of this, different alternatives were studied. The first one is the production of H2O2 under pressure to obtain higher concentrations of dissolved oxygen. Those experiments were conducted in the Università degli Studi di Palermo (Italy) under the supervision of Professor Scialdone. Concentrations up to 7,650 mg dm-3 with an instantaneous current efficiency of about 100% were obtained at 30 bar of air pressure and 100 mA cm-2 using CB/PTFE carbon felt cathodes. The value of specific production rate (62.5 mg H2O2 cm2 h-1) represents the highest reported so far. In all the H2O2 electrolyzers studied up to now, a compressor is required to supply the oxygen. Because of this, the use of a jet aerator, a simple and cheap device based on the Venturi effect, was proposed. It avoids the use of a compressor and the associated acquisition, maintenance and operating costs. In addition, it produces air bubbles and therefore supplies a stream with an oxygen concentration higher than the one of the equilibrium at a given pressure. An efficient production was obtained at 15 mA cm-2 with an instantaneous production of around 9.2 mg H2O2 cm-2 h-1 and a corresponding current efficiency in the order of 90% also using a carbon felt cathode with a CB/PTFE deposition. The next step in the development of the technology was the configuration of the cell. The objective of the innovative layout is the simultaneous optimization of two bottlenecks of electrochemical reactors: minimization of ohmic losses in the electrolyte and maximization of mass transport. The cell consists of placing 3D electrodes very close (in the order of micrometers) and fed the electrolyte through them. This geometry also solves the operative problem of classic microfluidic devices derived from gas evolution and allows the application of high current densities as well as coupling it with a jet aerator. In this case, rigid electrodes are required to avoid short-circuit at such small inter-electrode gaps (between 100 and 400 µm). The two novel aeration systems studied so far (pressure and jet aerator) were synergistically combined in a new and powerful aeration system: the pressurized-jet aerator. The jet increases the aeration capacity of a pressurized circuit and the pressurized system facilitates the aspiration of the jet thanks to the higher density of the gas. The use of pressure required a complete re-design of the system (pipelines, connections, pump, cell, etc.) to withstand higher pressures. A new cathodic material, Duocel® aluminium foam with a deposition of CB/PTFE (Al-CB/PTFE) was used as the cathode in this cell. It is an attractive support thanks to its 3D structure, adequate mechanical resistance and high conductivity. The last characteristic not only reduces the ohmic drop but also improves current distribution, a key for the scale up. All those innovative solutions proved positive considering that the pressurized-jet MF-FT obtained the lowest specific energy consumption reported so far (3.65 kWh kg H2O2-1) at 10 mA cm-3 in 0.05 M Na2SO4 medium. Also, this system is less sensible to electrolyte conductivity, thanks to the low inter-electrode gap, given that a reduction of 15 times in Na2SO4 concentration only results in an increase of less than 2 times in energy consumption (6.62 kWh kg H2O2-1). A preliminary scale up was performed by increasing the electrode thickness (from 5 to 15 mm). Experimental data demonstrate that the system is readily scalable and theoretical calculations suggest that it could be expanded more than one meter under those aeration conditions (160 dm3 h-1 and 6 bar). However, for wastewater treatment the continuous production and activation of hydrogen peroxide (Electro-Fenton) is considered a better alternative than accumulation and further dosage. In addition, this strategy allows the integration of CDEO with Electro-Fenton, two of the appropriate technologies selected in Chapter 5. Because of this, Chapter 7 is devoted to the development of a combined CDEO-EF reactor. In that chapter, the first aspect to be analyzed was the effect of pressure in the Electro-Fenton process. It was concluded that pressure improves the process not only thanks to the faster generation of hydrogen peroxide, but also because of the promotion of a ●OH-induced side-mechanism in which dissolved O2 acts as an oxidant. However, the pressure may also result detrimental for H2O2 catalysis, mainly due to the faster oxidation of Fe2+ to Fe3+ species. Next, the efficiency of mixed metal oxide (MMO) and boron-doped diamond (BDD) anodes for the electro-oxidation of a model organic pollutant was assessed. As expected, the BDD anode was more effective than the MMO anode. A comparison between the new MF-FT equipped with the BDD anode and a commercial cell (Diacell 101®) using conventional diamond flat electrodes, revealed that the novel MF-FT configuration was 4 to 10 times faster and required from 6 to 15 times less energy consumption to completely mineralize 100 mg dm-3 of a model organic pollutant. The last part of the work was devoted to avoid the issues related to the use of a homogeneous catalyst by substituting it with a heterogeneous one. The latter may have several advantages such as low iron leaching (and therefore low secondary pollution), reusability, wide pH operation window or minimum occurrence of refractory Fe by-products. Interestingly, it may not be affected by the increase in oxygen and also do not precipitate on the cathode, two of the most important operational problems observed in previous prototypes. With this aim, goethite was selected as a potential candidate. The main limitation of using a heterogeneous catalyst, i.e. mass transfer, is minimized by fluidizing the particles. Worth mentioning, the adequate dimensioning of the bed of particles can avoid the oxidation of H2O2 (a parasitic reaction) by separating the compartments, not by using a physical separator but thanks to a chemical reaction The final prototype (which integrates a pressurized-jet aerator, a MF-FT cell with state-of-the-art 3D electrodes and a fluidized bed of a heterogeneous Fenton catalyst) stand as a promising design to develop cost-effective electrochemical advanced oxidation processes for wastewater treatment.