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- Comparison of Energy Efficiency and Power Density in Pressure Retarded Osmosis and Reverse Electrodialysis. [JOURNAL ARTICLE]
- Environ Sci Technol 2014 Aug 26.
Pressure retarded osmosis (PRO) and reveres electrodialysis (RED) are emerging membrane-based technologies that can convert chemical energy in salinity gradients to useful work. The two processes have intrinsically different working principles: controlled mixing in PRO is achieved by water permeation across salt-rejecting membranes, whereas RED is driven by ion flux across charged membranes. This study compares the energy efficiency and power density performance of PRO and RED with simulated technologically-available membranes for natural, anthropogenic, and engineered salinity gradients (seawater-river water, desalination brine-wastewater, and synthetic hypersaline solutions, respectively). The analysis shows that PRO can achieve both greater efficiencies (54-56%) and higher power densities (2.4-38 W/m2) than RED (18-38% and 0.77-1.2 W/m2). The superior efficiency is attributed to the ability of PRO membranes to more effectively utilize the salinity difference to drive water permeation and better suppress the detrimental leakage of salts. On the other hand, the low conductivity of currently available ion exchange membranes impedes RED ion flux and, thus, constrains the power density. Both technologies exhibit a tradeoff between efficiency and power density: employing more permeable but less selective membranes can enhance the power density, but undesired entropy production due to uncontrolled mixing increases and some efficiency is sacrificed. When the concentration difference is increased (i.e., natural→anthropogenic→engineered salinity gradients), PRO osmotic pressure difference rises proportionally but not so for RED Nernst potential, which has logarithmic dependence on the solution concentration. Because of this inherently different characteristic, RED is unable to take advantage of larger salinity gradients, whereas PRO power density is considerably enhanced. Additionally, high solution concentrations suppress the Donnan exclusion effect of the charged RED membranes, severely reducing the permselectivity and diminishing the energy conversion efficiency. This study indicates that PRO is more suitable to extract energy from a range of salinity gradients, while significant advancements in ion exchange membranes are likely necessary for RED to be competitive with PRO.
- Integrating reverse electrodialysis with constant current operating capacitive deionization. [JOURNAL ARTICLE]
- J Environ Manage 2014 Aug 19.
The presence of a salinity gradient between saline water streams may result in the production of electricity via either reverse electrodialysis (RED) or forward osmosis. While the former system generates electricity because of the ionic current, the latter process produces electricity due to the osmotic pressure. In this study, RED is coupled with capacitive deionization (CDI) so that highly pure water, fresh water and electricity could be generated simultaneously. A CDI cell is operated at constant current, and it generated ultrapure water and two streams (a lower salinity stream of approximately 17.4 mol NaCl per m(3) and a high salinity stream of approximately 512.8 mol NaCl per m(3)) to be fed to the RED stack from a 15,000 ppm CDI feed concentration. The performed simulation reveals that, the total power generated from the RED using infinitely divided electrodes is 0.57 W/m(2) electrode area. The use of RED in a CDI plant introduces a new approach to minimize CDI brine concentration, which would otherwise have a negative impact on the environment if it were disposed directly without prior treatment.
- Water permeability of nanoporous graphene at realistic pressures for reverse osmosis desalination. [Journal Article]
- J Chem Phys 2014 Aug 21; 141(7):074704.
Nanoporous graphene (NPG) shows tremendous promise as an ultra-permeable membrane for water desalination thanks to its atomic thickness and precise sieving properties. However, a significant gap exists in the literature between the ideal conditions assumed for NPG desalination and the physical environment inherent to reverse osmosis (RO) systems. In particular, the water permeability of NPG has been calculated previously based on very high pressures (1000-2000 bars). Does NPG maintain its ultrahigh water permeability under real-world RO pressures (<100 bars)? Here, we answer this question by drawing results from molecular dynamics simulations. Our results indicate that NPG maintains its ultrahigh permeability even at low pressures, allowing a permeate water flux of 6.0 l/h-bar per pore, or equivalently 1041 ± 20 l/m(2)-h-bar assuming a nanopore density of 1.7 × 10(13) cm(-2).
- Direct phosphorus recovery from municipal wastewater via osmotic membrane bioreactor (OMBR) for wastewater treatment. [JOURNAL ARTICLE]
- Bioresour Technol 2014 Aug 4.:221-229.
This work reports, for the first time, a new approach to direct phosphorus recovery from municipal wastewater via an osmotic membrane bioreactor (OMBR). In the OMBR, organic matter and NH4(+) were removed by biological activities. PO4(3)(-), Ca(2+), Mg(2+) and unconverted NH4(+) were rejected by the forward osmosis (FO) membrane and enriched within the bioreactor. The resultant phosphorus-rich supernatant was then used for phosphorus recovery. By adjusting the pH to 8.0-9.5, PO4(3)(-) was recovered via precipitation with Ca(2+), Mg(2+) and NH4(+). The OMBR showed up to 98% overall removal of TOC and NH4(+)-N. At pH 9.0, more than 95% PO4(3)(-)-P was recovered without addition of magnesium and calcium. The precipitates were predominantly amorphous calcium phosphate (ACP) with phosphorus content >11.0%. In principal, this process can recover almost all the phosphorus, apart from the portion assimilated by bacteria. The global phosphorus recovery efficiency was shown to be 50% over 84days.
- Reverse osmosis and nanofiltration of biologically treated leachate. [Journal Article, Research Support, Non-U.S. Gov't]
- Environ Technol 2014 Sep-Oct; 35(17-20):2416-26.
Experiments of nano-filtration (NF) and reverse osmosis (RO) were conducted to remove most pollutants from the biological treated leachate. For example, the purified permeate after reverse osmosis treatment with spiral membranes reached effluent water quality as follows: COD of 57 mg O2/l, BOD7 of 35 mg O2/l, and suspended solid of 1 mg/l which satisfies the discharge standards in Estonia. For both RO and NF, conductivity can be reduced by 91% from 6.06 to 0.371 mS/cm by RO and 99% from 200 to 1 mS/cm by NF. To test the service life of the RO spiral membranes, the process was able to reduce chemical oxygen demand (COD) and biological oxygen demand (BOD) of biologically treated leachate by 97.9% and 93.2% even after 328 and 586 hours, respectively. However, only 39.0% and 21.7% reductions of Ptot and Ntot were achieved. As a result, neither RO (spiral membranes process) nor NF was able to reduce the total nitrogen (TN) to the required discharge limit of 15 mg/l.
- Evaluation method of membrane performance in membrane distillation process for seawater desalination. [Journal Article]
- Environ Technol 2014 Sep-Oct; 35(17-20):2147-52.
Membrane distillation (MD) is an emerging desalination technology as an energy-saving alternative to conventional distillation and reverse osmosis method. The selection of appropriate membrane is a prerequisite for the design of an optimized MD process. We proposed a simple approximation method to evaluate the performance of membranes for MD process. Three hollow fibre-type commercial membranes with different thicknesses and pore sizes were tested. Experimental results showed that one membrane was advantageous due to the highest flux, whereas another membrane was due to the lowest feed temperature drop. Regression analyses and multi-stage calculations were used to account for the trade-offeffects of flux and feed temperature drop. The most desirable membrane was selected from tested membranes in terms of the mean flux in a multi-stage process. This method would be useful for the selection of the membranes without complicated simulation techniques.
- Does hindered transport theory apply to desalination membranes? [JOURNAL ARTICLE]
- Environ Sci Technol 2014 Aug 19.
As reverse osmosis (RO) and nanofiltration (NF) polyamide membranes become increasingly used for water purification, prediction of pollutant transport is required for membrane development and process engineering. Many popular models use hindered transport theory (HTT), which consider a spherical solute moving through an array of fluid-filled rigid cylindrical pores. Experiments and molecular dynamic simulations however reveal that polyamide membranes have a distinctly different structure of a "molecular sponge", a network of randomly connected voids widely distributed in size. In the view of this disagreement, this study critically examined the validity of HTT by directly measuring diffusivities of several alcohols within polyamide film of commercial RO membrane using attenuated total reflection-FTIR. It is found that measured diffusivities deviate from HTT predictions by as much as 2 - 3 orders of magnitude. This result indicates that HTT does not adequately describe solute transport in desalination membranes. As a more adequate alternative, the concept of a random resistor networks is suggested, with resistances described by models of activated transport in "soft" polymers without a sharp size cut-off and with a proper address of solute partitioning.
- dc step response of induced-charge electro-osmosis between parallel electrodes at large voltages. [Journal Article]
- Phys Rev E Stat Nonlin Soft Matter Phys 2014 Jul; 90(1-1):013007.
Induced-charge electro-osmosis (ICEO) is important since it can be used for realizing high performance microfluidic devices. Here, we analyze the simplest problem of ion relaxation around a circular polarizable cylinder between parallel blocking electrodes in a closed cell by using a multiphysics coupled simulation technique. This technique is based on a combination of the finite-element method and finite-volume method for the Poisson-Nernst-Planck (PNP) equations having a flow term and the Stokes equation having an electric stress term. Through this analysis, we successfully demonstrate that on application of dc voltages, quadorapolar ICEO vortex flows grow during the charging time of the cylinder for both unbounded and bounded problems and decay during the charging time of the parallel electrodes only for the bounded problem using blocking electrodes. Further, by proposing a simple model that considers the two-dimensional (2D) PNP equations analytically, we successfully explain the step response time of the ICEO flow for the both unbounded and bounded problems. Furthermore, at low applied voltages, we find analytical formulations on steady diffused-ion problems and steady ICEO-flow problems and examine that our numerical results agree well with the analytical results. Moreover, by considering an ion-conserving condition with 2D Poisson-Boltzmann equations, we explain significant decrease of the maximum slip velocity at large applied voltages fairly well. We believe that our analysis will contribute greatly to the realistic designs of prospective high-performance microfluidic devices.
- Role of water quality assessments in hospital infection control: Experience from a new oncology center in eastern India. [Journal Article]
- Indian J Pathol Microbiol 2014 Jul-Sep; 57(3):435-8.
Water quality assessment and timely intervention are essential for health. Microbiology, total dissolved solids (TDS) and free residual chlorine were measured for water quality maintenance in an oncology center in India. Impact of these interventions over a period of 22 months has been demonstrated with four cardinal events. Pseudomonas in hospital water was controlled by adequate chlorination, whereas high TDS in the central sterile supply department water was corrected by the installation of electro-deionization plant. Contaminated bottled water was replaced using quality controlled hospital supply. Timely detection and correction of water-related issues, including reverse osmosis plant was possible through multi-faceted approach to water quality.
- A health risk assessment of reclaimed municipal wastewater for industrial and miscellaneous use. [Journal Article]
- Water Sci Technol 2014; 70(4):750-6.
The study evaluated the safety of reclaimed water using health risk assessment and biotoxicity tests. The reclaimed water was produced from reverse osmosis and used in industrial and miscellaneous purposes. The health risk assessment was conducted based on the concentrations of detectable pollutants in reclaimed water in a hypothetical scenario. The estimated carcinogenic and non-carcinogenic risks are lower than the generally accepted level. Biotoxicity evaluation included three genotoxicity tests, a chronic toxicity test using medaka fishes, and a subchronic toxicity test using mice. The reclaimed water is not genetically toxic, and does not cause significant chronic effects on these model organisms. These results confirm the safety of using reclaimed water from municipal wastewater treatment plants.