Ane Urtiaga

State of the art and review on the treatment technologies of water reverse osmosis concentrates.

The growing demand for fresh water is partially satisfied by desalination plants that increasingly use membrane technologies and among them reverse osmosis to produce purified water. Operating with water recoveries from 35% to 85% RO plants generate huge volumes of concentrates containing all the retained compounds that are commonly discharged to water bodies and constitute a potentially serious threat to marine ecosystems; therefore there is an urgent need for environmentally friendly management options of RO brines. This paper gives an overview on the potential treatments to overcome the environmental problems associated to the direct discharge of RO concentrates. The treatment options have been classified according to the source of RO concentrates and the maturity of the technologies. For the sake of clarity three different sources of RO concentrates are differentiated i) desalination plants, ii) tertiary processes in WWTP, and iii) mining industries. Starting with traditional treatments such as evaporation and crystallization other technologies that have emerged in last years to reduce the volume of the concentrate before disposal and with the objective of achieving zero liquid discharge and recovery of valuable compounds from these effluents are also reviewed. Most of these emerging technologies have been developed at laboratory or pilot plant scale (see Table 1). With regard to RO concentrates from WWTP, the manuscript addresses recent studies that are mainly focused on reducing the organic pollutant load through the application of innovative advanced oxidation technologies. Finally, works that report the treatment of RO concentrates from industrial sources are analyzed as well.

Electro-oxidation of reverse osmosis concentrates generated in tertiary water treatment

This work investigates the application of the electro-oxidation technology provided with boron doped diamond (BDD), an electrode material which has shown outstanding properties in oxidation of organic and inorganic compounds, for the treatment of reverse osmosis (RO) concentrates generated in tertiary wastewater treatment plants (WWTP). Chemical oxygen demand (COD), ammonium and several anions were measured during the electro-oxidation process, and the influence of the applied current density (20-200A/m(2)) was analysed on process kinetics. Analytical assessment showed that several emerging pollutants (pharmaceuticals, personal care products, stimulants, etc.) were presented both in the effluent of the secondary WWTP as well as in the RO concentrate. For this reason, a group of 10 emerging pollutants, those found with higher concentrations, was selected in order to test whether electro-oxidation can be also applied for their mitigation. In the removal of emerging pollutants the electrical current density in the range 20-100A/m(2) did not show influence likely due to the mass transfer resistance developed in the process when the oxidized solutes are present in such low concentrations. Their removal rates were fitted to first order expressions, and the apparent kinetic constants for the anodic oxidation of each compound were calculated. Finally, the formation of trihalomethanes (THMs) has been checked; concluding that after selecting the appropriate operational conditions the attained concentration is lower than the standards for drinking water established in European and EPA regulations.

Boron-doped diamond anodic treatment of landfill leachate: evaluation of operating variables and formation of oxidation by-products.

Landfill leachate with a low BOD/COD ratio was electrochemically oxidized by means of a boron-doped diamond anode. In addition to organic matter removal, this study addressed the issue of formation of both chlorinated organic compounds and nitrate ions as a result of organic matter and ammonia and/or organic nitrogen electro-oxidation in the presence of chloride ions. A factorial design methodology was implemented to evaluate the statistically important operating variables: treatment time (1-4 h), pH (5-8), current intensity (6.3-8.4 A) and addition of chloride (2500-4500 mg L(-1)). The process was evaluated on COD, total nitrogen (TN) and colour removal, as well as on the formation of nitrate, nitrite and chlorinated organics. Of the four variables studied, treatment time and pH had a considerable influence on COD and colour removal. On the contrary, none of the variables had a significant effect on the elimination of TN for which an average removal of 61 mg L(-1) was obtained. The studied variables exhibited different effects on the four groups of organo-chlorinated compounds considered in this study, namely trihalomethanes (THMs), haloacetonitriles (HANs), haloketons (HKs) and 1,2-dichloroethane (DCA). Further analysis at more intense conditions, i.e. current intensity up to 18 A and reaction time up to 8 h revealed that high levels of decolourization (84%) could be achieved followed by low COD (51%) and ammonia (32%) removals. Apart from DCA, the concentration of chlorinated organics increased continuously with treatment time reaching values as high as 1.9 mg L(-1), 753 μg L(-1) and 431 μg L(-1) of THMs, HANs and HKs, respectively.

Extraction of Cr(VI) with aliquat 336 in hollow fiber contactors: mass transfer analysis and modeling

In this work the mass transfer analysis and modeling of the hollow fiber non-dispersive liquid—liquid extraction of Cr(VI) with Aliquat 336 is reported. Experimental results corresponding to different values of the initial concentration of Cr(VI) in the aqueous phase in the range 50 g/m3 ⩽ C0 ⩽ 500 g/m3 showed three different diffusional regimes depending on both the initial concentration of Cr(VI) and the linear velocities of the aqueous phase: (i) kinetic control of the mass transport in the aqueous phase, (ii) kinetic control of the mass transport through the membrane fiber wall and (iii) an intermediate region where the control is shared between the aqueous and the membrane phases. The integration of the mass conservation equation with a nonlinear equilibrium condition at the fiber wall agrees satisfactorily with the results of experiments performed at different initial concentrations of Cr(VI) and different values of the linear velocity of the aqueous phase in the range C0 ⩾ 50 g/m3 and 2.95 × 10−3 m/s ⩽ v ⩽ 1.18 × 10−2 m/s. An optimization of the parameters D, solute diffusivity in the aqueous phase, and Keq, equilibrium constant of the extraction chemical reaction, with all the experimental results, using as criterion the minimum weighted standard deviation, gave as a result the value of the parameter D = 2.3 × 10−9 m2/s and values of Keq dependent upon the initial concentration of Cr(VI) in the feed solution. The mass transfer model and parameters reported in this work are useful for the design and optimization of the nondispersive extraction of Cr(VI) in a hollow fiber module.

Kinetics of electro-oxidation of ammonia-N, nitrites and COD from a recirculating aquaculture saline water system using BDD anodes

The viability of the electro-oxidation technology provided with boron doped diamond (BDD) electrodes for the treatment and reuse of the seawater used in a Recirculating Aquaculture System (RAS) was evaluated in this work. The influence of the applied current density (5-50 A m(-2)) in the removal of Total Ammonia Nitrogen (TAN), nitrite and chemical oxygen demand (COD) was analyzed observing that complete TAN removal together with important reductions of the other considered contaminants could be achieved, thus meeting the requirements for reuse of seawater in RAS systems. TAN removal, mainly due to an indirect oxidation mechanism was described by a second order kinetics while COD and nitrite removal followed zero-th order kinetics. The values of the kinetic constants for the anodic oxidation of each compound were obtained as a function of the applied current density (k(TAN) = 7.86 × 10(-5) · exp(6.30 × 10(-2) J); kNO2 = 3.43 × 10(-2) J; k(COD) = 1.35 × 10(-2) J). The formation of free chlorine and oxidation by-products, i.e., trihalomethanes (THMs) was followed along the electro-oxidation process. Although a maximum concentration of 1.7 mg l(-1) of total trihalomethanes was detected an integrated process combining electrochemical oxidation in order to eliminate TAN, nitrite and COD and adsorption onto activated carbon to remove the residual chlorine and THMs is proposed, as an efficient alternative to treat and reuse the seawater in fish culture systems. Finally, the energy consumption of the treatment has been evaluated.

Assessment of the formation of inorganic oxidation by-products during the electrocatalytic treatment of ammonium from landfill leachates

This work investigates the formation of oxidation by-products during the electrochemical removal of ammonium using BDD electrodes from wastewaters containing chlorides. The influence of the initial chloride concentration has been experimentally analyzed first, working with model solutions with variable ammonium concentration and second, with municipal landfill leachates. Two different levels of chloride concentration were studied, i) low chloride concentrations ranging between 0 and 2000 mg/L and, ii) high chloride concentrations ranging between 5000 and 20,000 mg/L. Ammonium removal took place mainly via indirect oxidation leading to the formation of nitrogen gas and nitrate as the main oxidation products; at high chloride concentration the formation of nitrogen gas and the rate of ammonium removal were both favored. However, chloride was also oxidized during the electrochemical treatment leading to the formation of free chlorine responsible of the ammonium oxidation, together with undesirable products such as chloramines, chlorate and perchlorate. Chloramines appeared during the treatment but they reached a maximum and then started decreasing, being totally removed when high chloride concentrations were used. With regard to the formation of chlorate and perchlorate once again the concentration of chloride exerted a strong influence on the formation kinetics of the oxidation by-products and whereas at low chloride concentrations, chlorate appeared like an intermediate compound leading to the formation of perchlorate, at high chloride concentrations chlorate formation was delayed significantly and perchlorate was not detected during the experimental time. Thus this work contributes first to the knowledge of the potential hazards of applying the electro-oxidation technology as an environmental technology to deal with ammonium oxidation under the presence of chloride and second it reports efficient conditions that minimize or even avoid the formation of undesirable by-products.

Integrated treatment of landfill leachates including electrooxidation at pilot plant scale.

This paper reports the integration of advanced and conventional technologies to deal with the treatment of landfill leachates. The raw leachate, with average values of COD=4430 mg/L and N-NH(4)(+)=1225 mg/L, was first treated on site by an activated sludge large-scale process reducing the former parameters to 1750 mg/L (av.) of COD and 750 mg/L (av.) of N-NH(4)(+). Next, 50 L/h of the effluent were pumped to a pilot plant that included Fenton oxidation followed by an electrooxidation unit, provided with boron doped diamond anodes (anode area=1.05 m(2)); almost complete removal of the organic matter and ammonium nitrogen was achieved. Comparison of the results with those obtained in the laboratory (70 cm(2) of anode area) was performed observing a similar performance in the kinetics of COD removal, while differences were found in the ammonium removal rates. The specific energy consumption necessary to electro-oxidize the organic load below the disposal limit (COD<160 mg/L) at pilot plant scale was 35 kWh/m(3).

Parallelism and differences of pervaporation and vacuum membrane distillation in the removal of VOCs from aqueous streams

In this work two gas–liquid separation processes, pervaporation (PV) and vacuum membrane distillation (VMD), have been compared in their application to the separation of chloroform–water mixtures. After selection of the adequate separation membrane the comparison of the PV and VMD should be based on the kinetics and selectivity towards the desired compound. The kinetic models and parameters previously reported by the authors (A.M. Urtiaga, E.D. Gorri, J.K. Beasley, I. Ortiz, J Membr Sci 156 (1999) 275–291 and A.M. Urtiaga, G. Ruiz, I. Ortiz, J Membr Sci 165 (2000) 99–110) for the separation of chloroform from aqueous solutions in the range of concentrations 200<C<2000 mg/l, temperature 5<T<44°C and flowrate 140<Re<1400 have been used to compare the mass transfer flux and selectivity in both separation processes. The use of hollow fiber membranes of polydimethylsiloxane in the PV system and microporous polypropylene in the VMD process resulted in the same rate of removal in both systems under the simulated conditions. Particularly, the PV system offered the uppermost selectivity, providing a high concentration of chloroform in the permeate.

Electrochemical oxidation of landfill leachates at pilot scale: evaluation of energy needs.

Two of the main drawbacks to be overcome before full scale implementation of boron-doped diamond electro-oxidation were addressed in this work; its energy consumption and hazard of formation of chlorinated organics. This was framed within a study in which the technical and economic feasibility of BDD electro-oxidation of landfill leachate was evaluated at pilot scale. On one hand, its technical feasibility was assessed based on COD and NH(4)(+) removal, allowing the selection of the operation conditions that provide optimal energy efficiency, and special attention was paid to the risk of formation of undesired by-products such as nitrite, nitrate ions and trihalomethanes. On the other hand, treatment costs were estimated based on energy consumption required to reach the disposal limits to natural watercourses established by legislation. The results were compared with those of other commonly used AOPs. Under the operating conditions studied, the concentration of COD decreased below the disposal limit (160 mg L(-1)) and ammonia concentration reached values as low as 30 mg L(-1). The energy consumption needed was estimated at 54 kWh/m(3) and the formation of chlorination by-products appeared to be low as the maximum total trihalomethane concentration detected was 432 microg/L.

Electrochemical disinfection of secondary wastewater treatment plant (WWTP) effluent.

In this work the electrochemical disinfection of the effluent of a secondary wastewater treatment plant is investigated. In the experimental work, performed on-site with real effluents of the WWTP located in Vuelta Ostrera (Cantabria, Spain), boron-doped diamond (BDD) electrodes were employed. The initial concentration of E. coli in the effluent of the WWTP varied in the range 1.3 x 10⁴-5.2 x 10⁵ cfu/mL. The influence of two operation variables on the kinetics of E. coli deactivation was investigated: i) The applied current density was varied in the range J=40-120 mA/cm², showing first order kinetics, and linear dependency of the apparent kinetic constant with the applied current density; and ii) the chloride concentration was varied in the range 60-1,050 mg/L, showing that increasing chloride content also enhanced the kinetics of the E. coli deactivation. The latter parameter is particularly important in coastal areas, as in the case of the present study. The formation of disinfection by-products (DBPs) was followed by measuring the content of trihalomethanes (THMs) that nevertheless was maintained below 100 μg/L, so it can be concluded that the formation of DBPs is not a disadvantage of electrochemical disinfection of secondary effluents of WWTP.