Electro-oxidation of reverse osmosis concentrates generated in tertiary water treatment
Introduction
The increasing scarcity and pollution of freshwater resources has created a monumental new challenge for researchers, water professionals and policy makers: satisfy increasing water demand from an ever-growing population, while protecting the environment and public health and avoiding local, regional and international conflicts over water. In this context, reclaimed water appears as a potential water source of great interest to several sectors such as, agriculture, industry and municipal uses, especially in countries with lack of natural sources.
Advanced membrane technology has become increasingly attractive for reclamation of municipal wastewater because compared with conventional technologies, membrane processes are highly efficient, economical, easily to operate and automat, do not require intensive use of chemicals as pre-treatment and occupy less space (Ravazzini et al., 2005, Qin et al., 2004, Alonso et al., 2001, Reith and Birkenhead, 1998). Moreover, membrane treatments combine process stability with an excellent effluent quality. RO treatment gives water with a very high quality for reuse (Oron et al., 2008, Oron et al., 2006, Qin et al., 2006, Qin et al., 2005, Qin et al., 2004, Abdel-Jawad et al., 2002). However, the cost of disposal or treatment of the resultant RO concentrate represents one of the main disadvantages (Van Hege et al., 2004, Van Hege et al., 2002, Van der Bruggen et al., 2003). These membrane concentrates include an increased amount of salts, organics and biological constituents. Among those organic compounds appears the so-called “emerging pollutants” which include pharmaceuticals and personal care products. Around 3000 different pharmaceutical ingredients are commonly used today, including painkillers, beta blockers, contraceptives, lipid regulators, antidepressants, tranquilizers and impotence drugs. These pharmaceuticals are discharged after consumption from private households or from hospitals and reach municipal wastewater treatment plants within the wastewater. In contrast to pharmaceuticals, personal care products do not have to pass through the human body. They enter the wastewater via their regular use, for instance during showering or bathing. Since the major part of these compounds is not totally removed during conventional wastewater treatment, they are usually present in wastewater effluents (Ternes and Joss, 2006) and they appear in the RO concentrates because they are highly rejected by the reverse osmosis membranes (Radjenović et al., 2008, Watkinson et al., 2007, Drewes et al., 2005). Although these compounds are not covered by any directive, they are being deeply investigated because their presence was pointed out as a possible cause of damage of the quality of natural water (Gómez et al., 2007). Thus, treating these concentrates would minimize environmental impacts associated to their discharge or management and even treated water could be further reused.
Electrochemical technologies have been widely used for treating water and wastewater (Westerhoff et al., 2009, Dialynas et al., 2008, Van Hege et al., 2004, Van Hege et al., 2002). Specifically, with regard to RO concentrates electrochemical treatment present several advantages, such as moderate to high salinity which ensures an excellent electric conductivity that could reduce the energy consumption, high chloride concentration that improves the indirect oxidation, through the electro-generation of strong oxidants like hypochlorite and the capacity for oxidising recalcitrant organics. Several materials have been proposed for the anodes, such as Pt, TiO2, SnO2, IrO2, RuO2, etc, but some of them have shown loss of activity due to surface fouling or limited service life. Among them, BDD electrodes appear like an ideal anode owing to its promising characteristics: hardness, stability up to high anodic potentials, high reactivity towards organics oxidation and efficient use of electrical energy (Polcaro et al., 2009, Comninellis et al., 2008, Cabeza et al., 2007a, Cabeza et al., 2007b, Van Hege et al., 2004, Rychen et al., 2003). Its large electrochemical window, the widest known, allows generating very strong oxidising agents like hydroxyl radicals and a wide range of electrochemical reactions (Anglada et al., 2009, Cabeza et al., 2007a, Cabeza et al., 2007b). They may oxidize the organic compounds only in a thin reaction layer adjacent to the anodic surface, so the organic removal is often a mass transfer controlled process. Moreover, ·OH radicals may be the responsible for the reactive oxygen species (ROS) formation, O3 and H2O2 which may react with the organic matter in the solution bulk. Furthermore, if sulphate or phosphate ions are contained in the water, other oxidants like peroxodisulphate and peroxodiphosphate may be formed and can contribute to the oxidation reactions in the bulk solution (Comninellis and Guohua, 2010, Polcaro et al., 2009, Polcaro et al., 2003, Rychen et al., 2003). The presence of chloride inevitably conducts to the formation of hypochlorite which represents the most efficient way to oxidize ammonium (Cabeza et al., 2007b, Vanlangendonck et al., 2005, Comninellis and Nerini, 1995).
This work aims at the study of the application of the electro-oxidation technology with BDD electrodes to treat the concentrate stream of a reverse osmosis unit employed in the tertiary treatment of the secondary effluent of a WWTP. The viability of the technology was studied in terms of removal of chemical oxygen demand (COD), ammonia and anions. Preliminary results of the reduction of selected emerging pollutants contained in the WWTP effluent have been also obtained and analysed.
Section snippets
Water source
The RO concentrate from a UF/RO pilot plant which treats the secondary effluent of a WWTP was used in the experiments. The ultrafiltration unit is equipped with two parallel hollow fiber membrane modules, AquaFlex (NORIT), of 40 m2 each one and operates in a dead-end filtration continuous mode with a maximum flowrate of 3 m3/h. With a nominal pore size of 0.02 μm, UF attained total removal of turbidity and E. Coli. The RO pilot plant was composed of two spiral wound composite polyamide membrane
Electro-oxidation kinetics
A first set of experiments was performed with no addition of further chemicals in order to analyse the influence of current density on the electro-oxidation results. Three different current density values were assessed, 50, 100 and 200 A/m2, for treating the reverse osmosis concentrates of sample C2, that corresponds to the operation of the RO unit with a productivity of 70%. Experiments were replicated so data shown correspond to average concentration values with an experimental error of 4.1%.
Conclusions
From the preliminary results presented in this study, it is established that electrochemical oxidation with BDD electrodes appears like a promising technology to treat reverse osmosis concentrates obtained in the tertiary treatment of municipal wastewaters. BDD electro-oxidation does not require the addition of further chemicals. The pollutants present in reverse osmosis concentrates (ammonium, COD and emerging pollutants) can be successfully eliminated during electro-oxidation treatment.
Acknowkedgements
Financial support from projects Consolider CSD 2006-44, CTQ 2008-00690/PPQ, CTM2006-00317 (Spanish Ministry of Science and Innovation) and 062/SGTB/2007/3.1 (Spanish Ministry of Environment) is gratefully acknowledged. The collaboration of MARE S.A. is also acknowledged.
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