Electrochemical oxidation of trace organic contaminants in reverse osmosis concentrate using RuO2/IrO2-coated titanium anodes

During membrane treatment of secondary effluent from wastewater treatment plants, a reverse osmosis concentrate (ROC) containing trace organic contaminants is generated. As the latter are of concern, effective and economic treatment methods are required. Here, we investigated electrochemical oxidation of ROC using Ti/Ru_0.7Ir_0.3O₂ electrodes, focussing on the removal of dissolved organic carbon (DOC), specific ultra-violet absorbance at 254 nm (SUVA₂₅₄), and 28 pharmaceuticals and pesticides frequently encountered in secondary treated effluents. The experiments were conducted in a continuously fed reactor at current densities (J) ranging from 1 to 250 A m⁻² anode, and a batch reactor at J = 250 A m⁻². Higher mineralization efficiency was observed during batch oxidation (e.g. 25.1 ± 2.7% DOC removal vs 0% removal in the continuous reactor after applying specific electrical charge, Q = 437.0 A h m⁻³ ROC), indicating that DOC removal is depending on indirect oxidation by electrogenerated oxidants that accumulate in the bulk liquid. An initial increase and subsequent slow decrease in SUVA₂₅₄ during batch mode suggests the introduction of auxochrome substituents (e.g. -Cl, NH₂Cl, -Br, and -OH) into the aromatic compounds. Contrarily, in the continuous reactor ring-cleaving oxidation products were generated, and SUVA₂₅₄ removal correlated with applied charge. Furthermore, 20 of the target pharmaceuticals and pesticides completely disappeared in both the continuous and batch experiments when applying J ≥ 150 A m⁻² (i.e. Q ≥ 461.5 A h m⁻³) and 437.0 A h m⁻³ (J = 250 A m⁻²), respectively. Compounds that were more persistent during continuous oxidation were characterized by the presence of electrophilic groups on the aromatic ring (e.g. triclopyr) or by the absence of stronger nucleophilic substituents (e.g. ibuprofen). These pollutants were oxidized when applying higher specific electrical charge in batch mode (i.e. 1.45 kA h m⁻³ ROC). However, baseline toxicity as determined by Vibrio fischeri bioluminescence inhibition tests (Microtox) was increasing with higher applied charge during batch and continuous oxidation, indicating the formation of toxic oxidation products, possibly chlorinated and brominated organic compounds.     

Electrochemical sulfide oxidation from domestic wastewater using mixed metal-coated titanium electrodes

Hydrogen sulfide generation is a major issue in sewer management. A novel method based on electrochemical sulfide oxidation was recently shown to be highly effective for sulfide removal from synthetic and real sewage. Here, we compare the performance of five different mixed metal oxide (MMO) coated titanium electrode materials for the electrochemical removal of sulfide from domestic wastewater. All electrode materials performed similarly in terms of sulfide removal, removing 78 ± 5%, 77 ± 1%, 85 ± 4%, 84 ± 1%, and 83 ± 2% at a current density of 10 mA/cm² using Ta/Ir, Ru/Ir, Pt/Ir, SnO₂ and PbO₂, respectively. Elevated chloride concentrations, often observed in coastal areas, did not entail any significant difference in performance. Independent of the electrode material used, sulfide oxidation by in situ generated oxygen was the predominant reaction mechanism. Passivation of the electrode surface by deposition of elemental sulfur did not occur. However, scaling was observed in the cathode compartment. This study shows that all the MMO coated titanium electrode materials studied are suitable anodic materials for sulfide removal from wastewater. Ta/Ir and Pt/Ir coated titanium electrodes seem the most suitable electrodes since they possess the lowest overpotential for oxygen evolution, are stable at low chloride concentration and are already used in full scale applications.     

Ozonation of reverse osmosis concentrate: kinetics and efficiency of beta blocker oxidation

Reverse osmosis (RO) concentrate samples were obtained from a RO-membrane system that uses effluents of wastewater treatment plants (WWTP) as feed water for the production of drinking water. A number of different pharmaceuticals (e.g. antibiotics, contrast media, beta blockers) were found in the WWTP effluent as well as in the RO-concentrate. Overall, a concentration factor (feed:concentrate) of approximately 3-4 was measured. Beta blockers (acebutolol, atenolol, bisoprolol, celiprolol, metoprolol, propranolol, timolol) were found in the range of low ng/L to low microg/L. Because metoprolol and propranolol are classified as potentially toxic to aquatic organisms and all beta blocker molecules have moieties, which are reactive towards ozone (amine groups, activated aromatic rings), it was tested whether ozonation can be applied for their mitigation. Rate constants for the reaction of acebutolol, atenolol, metoprolol and propranolol with ozone and OH radicals were determined. At pH 7 acebutolol, atenolol and metoprolol react with ozone with an apparent second-order rate constant ( [Formula: see text] ) of about 2,000 M(-1)s(-1), whereas propranolol reacts with approximately 10(5)M(-1)s(-1). The rate constants for the reaction of the selected compounds with OH radicals were determined to be 0.5-1.0 x 10(10)M(-1)s(-1). Experiments with RO concentrate showed that an ozone dose of only 5mg/L resulted in a quantitative removal of propranolol in 0.8s and 10mg O(3)/L oxidized 70% of metoprolol in only 1.2s. Tests with chlorinated and non-chlorinated WWTP effluent showed an increase of ozone stability but a decrease of hydroxyl radical exposure in the samples after chlorination. This may shift the oxidation processes towards direct ozone reactions and favor the degradation of compounds with high [Formula: see text].     

Advanced treatment of the reverse osmosis concentrate produced during reclamation of municipal wastewater

The work investigated the treatment of the concentrate produced from the reverse osmosis treatment of an MBR effluent. Two conventional chemical processes, coagulation and activated carbon adsorption, and three advanced oxidation processes (electrochemical treatment, photocatalysis and sonolysis) were applied. Coagulation with alum gave dissolved organic carbon (DOC) removals up to 42%, while FeCl(3) achieved higher removals (52%) at lower molar doses. Adsorption with granular activated carbon showed the highest DOC removals up to 91.3% for 5 g/L. The adsorption isotherm was linear with a non-adsorbable organic fraction of around 1.2 mg/L DOC. The three oxidation methods employed, electrolytic oxidation over a boron-doped diamond electrode, UVA/TiO2 photocatalysis and sonolysis at 80 kHz, showed similar behavior: during the first few minutes of treatment there was a moderate removal of DOC followed by further oxidation at a very slow rate. Electrolytic oxidation was capable of removing up to 36% at 17.8A after 30 min of treatment, sonolysis removed up to 34% at 135W after 60 min, while photocatalysis was capable of removing up to 50% at 60 min.     

Characterisation and removal of recalcitrants in reverse osmosis concentrates from water reclamation plants

Water reclamation plants frequently utilise reverse osmosis (RO), generating a concentrated reject stream as a by-product. The concentrate stream contains salts, and dissolved organic compounds, which are recalcitrant to biological treatment, and may have an environmental impact due to colour and embedded nitrogen. In this study, we characterise organic compounds in RO concentrates (ROC) and treated ROC (by coagulation, adsorption, and advanced oxidation) from two full-scale plants, assessing the diversity and treatability of colour and organic compounds containing nitrogen. One of the plants was from a coastal catchment, while the other was inland. Stirred cell membrane fractionation was applied to fractionate the treated ROC, and untreated ROC along with chemical analysis (DOC, DON, COD), colour, and fluorescence excitation-emission matrix (EEM) scans to characterise changes within each fraction. In both streams, the largest fraction contained <1 kDa molecules which were small humic substances, fulvic acids and soluble microbial products (SMPs), as indicated by EEM. Under optimal treatment conditions, alum preferentially removed >10 kDa molecules, with 17-34% of organic compounds as COD. Iron coagulation affected a wider size range, with better removal of organics (41-49% as COD) at the same molar dosage. As with iron, adsorption reduced organics of a broader size range, including organic nitrogen (26-47%). Advanced oxidation (UV/H₂O₂) was superior for complete decolourisation and provided superior organics removal (50-55% as COD).     

Electrochemical degradation of carbaryl on oxide electrodes

This paper presents the study of a prospective electrochemical treatment system for the pesticide carbaryl. Three different dimensionally stable anodes were employed (Ti/Ru0.3 Ti0.7 O2, Ti/Ru0.3 Sn0.7 O2 and Ti/Ir0.3 Ti0.7 O2) and the effect of current density (10, 20, 40 and 60 mA cm(-2)) and supporting electrolyte (0.1 mol L(-1) NaCl and 0.033 mol L(-1) H2SO4) is discussed. All the electrodes present a low level of carbaryl and total organic carbon removal in H2SO4, even at highly positive potentials, indicating that the application of current is not, in itself, sufficient to promote effective oxidation of the pesticide and its products. However, in the presence of NaCl all the electrodes used present rapid diminishing of the carbaryl and total organic carbon content, thus suggesting enhanced activity. The results demonstrate the participation of partially oxidised Cl- species at the electrode surface, which act as an intermediate in the electron transfer between the pesticide molecule and the electrode. Thus, under such conditions, the feasibility of the electrochemical route for the treatment (total or partial) of waste that contains carbaryl is evident.     

Impact of salinity and pH on the UVC/H2O2 treatment of reverse osmosis concentrate produced from municipal wastewater reclamation

While reverse osmosis (RO) technology is playing an increasingly important role in the reclamation of municipal wastewater, safe disposal of the resulting RO concentrate (ROC), which can have high levels of effluent organic pollutants, remains a challenge to the water industry. The potential of UVC/H₂O₂ treatment for degrading the organic pollutants and increasing their biodegradability has been demonstrated in several studies, and in this work the impact of the water quality variables pH, salinity and initial organic concentration on the UVC/H₂O₂ (3 mM) treatment of a municipal ROC was investigated. The reduction in chemical oxygen demand and dissolved organic carbon was markedly faster and greater under acidic conditions, and the treatment performance was apparently not affected by salinity as increasing the ROC salinity 4-fold had only minimal impact on organics reduction. The biodegradability of the ROC (as indicated by biodegradable dissolved organic carbon (BDOC) level) was at least doubled after 2 h UVC/H₂O₂ treatment under various reaction conditions. However, the production of biodegradable intermediates was limited after 30 min treatment, which was associated with the depletion of the conjugated compounds. Overall, more than 80% of the DOC was removed after 2 h UVC/3 mM H₂O₂ treatment followed by biological treatment (BDOC test) for the ROC at pH 4-8.5 and electrical conductivity up to 11.16 mS/cm. However, shorter UV irradiation time gave markedly higher energy efficiency (e.g., EE/O 50 kWh/m³ at 30 min (63% DOC removal) cf. 112 kWh/m³ at 2 h). No toxicity was detected for the treated ROC using Microtox^® tests. Although the trihalomethane formation potential increased after the UVC/H₂O₂ treatment, it was reduced to below that of the raw ROC after the biological treatment.     

The effect of antiscalant addition on calcium carbonate precipitation for a simplified synthetic brackish water reverse osmosis concentrate

The primary limitations to inland brackish water reverse osmosis (RO) desalination are the cost and technical feasibility of concentrate disposal. To decrease concentrate volume, a side-stream process can be used to precipitate problematic scaling salts and remove the precipitate with a solid/liquid separation step. The treated concentrate can then be purified through a secondary reverse osmosis stage to increase overall recovery and decrease the volume of waste requiring disposal. Antiscalants are used in an RO system to prevent salt precipitation but might affect side-stream concentrate treatment. Precipitation experiments were performed on a synthetic RO concentrate with and without antiscalant; of particular interest was the precipitation of calcium carbonate. Particle size distributions, calcium precipitation, microfiltration flux, and scanning electron microscopy were used to evaluate the effects of antiscalant type, antiscalant concentration, and precipitation pH on calcium carbonate precipitation and filtration. Results show that antiscalants can decrease precipitate particle size and change the shape of the particles; smaller particles can cause an increase in microfiltration flux decline during the solid/liquid separation step. The presence of antiscalant during precipitation can also decrease the mass of precipitated calcium carbonate.     

The fate of Transparent Exopolymer Particles (TEP) in integrated membrane systems: Removal through pre-treatment processes and deposition on reverse osmosis membranes

The abundance of Transparent Exopolymer Particles (TEP) in surface waters has been unnoticed for many years until recently as a potential foulant in reverse osmosis systems. Recent studies indicate that TEP may cause organic and biological fouling and may enhance particulate/colloidal fouling in reverse osmosis membranes. The presence of TEP was measured in the raw water, the pre-treatment processes and reverse osmosis (RO) systems of 6 integrated membrane installations. A spectrophotometric method was used to measure TEP in the particulate size range (>0.40 μm) and was extended to measure TEP in the colloidal size range (0.05–0.40 μm). Ultrafiltration pre-treatment applied in 4 plants, totally removed particulate TEP while microfiltration systems (2 plants) and coagulation/sedimentation/rapid sand filtration systems (3 plants) partially removed this fraction. None of the pre-treatment systems investigated totally removed colloidal TEP. Biopolymer analysis using LC–OCD showed consistency between colloidal TEP and polysaccharide removal by UF pre-treatment and further verified the presence of TEP in the RO feedwater. TEP deposition in the RO system was determined after measuring total TEP concentrations in the RO feed and concentrate. The TEP deposition factors and specific deposition rates indicate that TEP accumulation had occurred in all plants investigated. This observation was verified by an autopsy of RO modules from two RO plants. Further improvement and verification of the (modified) TEP method, in particular the calibration, is necessary so that it can be employed to investigate the role of TEP in the fouling of RO systems.     

Effects of reverse osmosis isolation on reactivity of naturally occurring dissolved organic matter in physicochemical processes

A field reverse osmosis system was used to isolate dissolved organic matter (DOM) from two lacustrine and two riverine surface water sources. The rejection of DOM was on the order of 99% and did not vary significantly with pressure. A simple mass balance model using a single measured value of rejection predicted the concentration within the closed-loop isolation system. The effect of operating pressure and solution flux on mass recovery of DOM was evaluated in laboratory and field trials. Under controlled laboratory conditions, >99% of a lacustrine DOM could be accounted for. A fraction of the isolated DOM was not recoverable using hydrodynamic cleaning; however, this fraction was recovered by using a pH 10 NaOH wash solution. The mass recovered in the NaOH solution increased from <1% to >6% with increasing transmembrane pressures from 414 kPa (60 psi) to1000 kPa (145 psi), respectively. This is consistent with fouling that results from an increase in solution flux, and a decrease in tangential crossflow velocity. Under field conditions, mass balances were generally >95% and mass recovery was >90% in all cases. The effects of temperature on solution flux were consistent with changes in fluid viscosity; effects of temperature on membrane diffusivity or morphological properties were small. RO isolation under low pressure conditions designed to maximize DOM recovery had little effect on DOM reactivity evaluated in terms of nanofiltration membrane fouling, XAD-8 resin adsorption, activated carbon adsorption, competition with trichloroethylene for adsorption sites on activated carbon, and molecular weight distribution measured using size exclusion chromatography.