Anthony P. Straub

Pressure-retarded osmosis for power generation from salinity gradients: is it viable?

The enormous potential of harvesting energy from salinity gradients has been discussed for decades, and pressure-retarded osmosis (PRO) is being increasingly investigated as a method to extract this energy. Despite advancements in membranes and system components, questions still remain regarding the overall viability of the PRO process. Here, we review PRO focusing on the net energy extractable and the ultimate feasibility of the most widely explored configurations. We define the maximum energy that can be obtained from the process, quantify losses and energetic costs that will reduce the net extractable energy, and explain how membrane modules can be improved. We then explore the potential of three configurations of PRO: systems designed to control mixing where rivers meet the sea, power plants that utilize the high concentration gradients available from hypersaline solutions, and PRO systems incorporated into reverse osmosis desalination plants to reduce electricity requirements. We conclude by considering the overall outlook of the process and identifying the most pressing challenges for future research.

Thermodynamic limits of extractable energy by pressure retarded osmosis

Salinity gradient energy, which is released upon mixing two solutions of different concentrations, is considered to be a promising source of sustainable power. Of the methods available to harvest the salinity gradient energy, pressure retarded osmosis (PRO) has been one of the most widely investigated processes. In this study, we identify the thermodynamic limits of the PRO process by evaluating the obtainable specific energy, or extractable energy per total volume of the mixed solutions. Three distinct operation modes are analyzed: an ideal case for a reversible process, and constant-pressure operations with either co-current or counter-current flow in a membrane module. For module-scale operation, counter-current flow mode is shown to be more efficient than co-current flow mode. Additionally, two distinct thermodynamically limiting operation regimes are identified in counter-current flow mode—the draw limiting regime and the feed limiting regime. We derive analytical expressions to quantify the maximum specific energy extractable and the corresponding optimal feed flow rate fraction and applied pressure for each operation mode. Using the analytical expressions, we determine that maximum extractable energy in constant-pressure PRO with seawater (0.6 M NaCl) as a draw solution and river water (0.015 M NaCl) as a feed solution is 0.192 kW h per cubic meter of mixed solution (75% of the maximum specific Gibbs free energy of mixing). Considering that this is the theoretical upper bound of extractable energy by the PRO process, we discuss further efficiency losses and energy requirements (e.g., pretreatment and pumping) that may render it difficult to extract a sizable net specific energy from a seawater and river water solution pairing. We analyze alternative source waters that provide a higher salinity difference and hence greater extractable specific energy, such as reverse osmosis brine paired with treated wastewater effluent, which allow for a more immediately viable PRO process.

Role of temperature and Suwannee river natural organic matter on inactivation kinetics of rotavirus and bacteriophage MS2 by solar irradiation

Although the sunlight-mediated inactivation of viruses has been recognized as an important process that controls surface water quality, the mechanisms of virus inactivation by sunlight are not yet clearly understood. We investigated the synergistic role of temperature and Suwannee River natural organic matter (SRNOM), an exogenous sensitizer, for sunlight-mediated inactivation of porcine rotavirus and MS2 bacteriophage. Upon irradiation by a full spectrum of simulated sunlight in the absence of SRNOM and in the temperature range of 14-42 °C, high inactivation rate constants, k(obs), of MS2 (k(obs) ≤ 3.8 h(-1) or 1-log(10) over 0.6 h) and rotavirus (k(obs) ≤ 11.8 h(-1) or ∼1-log(10) over 0.2 h) were measured. A weak temperature (14-42 °C) dependence of k(obs) values was observed for both viruses irradiated by the full sunlight spectrum. Under the same irradiation condition, the presence of SRNOM reduced the inactivation of both viruses due to attenuation of lower wavelengths of the simulated sunlight. For rotavirus and MS2 solutions irradiated by only UVA and visible light in the absence of SRNOM, inactivation kinetics were slow (k(obs) < 0.3 h(-1) or <1-log(10) unit reduction over 7 h) and temperature-independent for the range considered. Conversely, under UVA and visible light irradiation and in the presence of SRNOM, temperature-dependent inactivation of MS2 was observed. For rotavirus, the SRNOM-mediated exogenous inactivation was only important at temperatures >33 °C, with low rotavirus k(obs) values (k(obs) ≈ 0.2 h(-1); 1-log(10) unit reduction over 12 h) for the temperature range of 14-33 °C. These k(obs) values increased to 0.5 h(-1) at 43 °C and 1.5 h(-1) (1-log(10) reduction over 1.6 h) at 50 °C. While SRNOM-mediated exogenous inactivation of MS2 was triggered by singlet oxygen, the presence of hydrogen peroxide was important for rotavirus inactivation in the 40-50 °C range.

Module-Scale Analysis of Pressure Retarded Osmosis: Performance Limitations and Implications for Full-Scale Operation

We investigate the performance of pressure retarded osmosis (PRO) at the module scale, accounting for the detrimental effects of reverse salt flux, internal concentration polarization, and external concentration polarization. Our analysis offers insights on optimization of three critical operation and design parameters--applied hydraulic pressure, initial feed flow rate fraction, and membrane area--to maximize the specific energy and power density extractable in the system. For co- and counter-current flow modules, we determine that appropriate selection of the membrane area is critical to obtain a high specific energy. Furthermore, we find that the optimal operating conditions in a realistic module can be reasonably approximated using established optima for an ideal system (i.e., an applied hydraulic pressure equal to approximately half the osmotic pressure difference and an initial feed flow rate fraction that provides equal amounts of feed and draw solutions). For a system in counter-current operation with a river water (0.015 M NaCl) and seawater (0.6 M NaCl) solution pairing, the maximum specific energy obtainable using performance properties of commercially available membranes was determined to be 0.147 kWh per m(3) of total mixed solution, which is 57% of the Gibbs free energy of mixing. Operating to obtain a high specific energy, however, results in very low power densities (less than 2 W/m(2)), indicating that the trade-off between power density and specific energy is an inherent challenge to full-scale PRO systems. Finally, we quantify additional losses and energetic costs in the PRO system, which further reduce the net specific energy and indicate serious challenges in extracting net energy in PRO with river water and seawater solution pairings.

Iron oxide amended biosand filters for virus removal.

Laboratory studies were performed to determine if the addition of iron oxides throughout biosand filter (BSF) media would increase virus removal due to adsorption. The proposed mechanism is electrostatic adsorption of negatively charged virion particles to positively charged iron oxides formed during the corrosion of zerovalent iron. Initial tests conducted using continuous flow, small-scale glass columns showed high MS2 bacteriophage removal in an iron-amended sand column (5log10) compared to a sand-only column (0.5log10) over 20 pore volumes. Additionally, two experiments with a column containing iron particles revealed 4log10 and 5log10 removal of rotavirus in the presence of 20 mg/L total organic carbon. Full-scale BSFs with iron particles removed>4log10 MS2 for the duration of the experiment (287 days), while BSF with steel wool removed>4log10 MS2 for the first 160 days. Plug flow for the BSF was shown to depend on uniformity between the iron oxide material and sand media grains. The results suggest that the duration of effective virus removal by iron-amended biosand filtration depends on source water conditions and the quantity and composition of iron material added. Overall, this study provides evidence that iron-amended BSFs may advance the field of point-of-use technologies and bring relief to millions of people suffering from waterborne diseases.

Impaired Performance of Pressure-Retarded Osmosis due to Irreversible Biofouling.

Next-generation pressure-retarded osmosis (PRO) approaches aim to harness the energy potential of streams with high salinity differences, such as wastewater effluent and seawater desalination plant brine. In this study, we evaluated biofouling propensity in PRO. Bench-scale experiments were carried out for 24 h using a model wastewater effluent feed solution and simulated seawater desalination brine pressurized to 24 bar. For biofouling tests, wastewater effluent was inoculated with Pseudomonas aeruginosa and artificial seawater desalination plant brine draw solution was seeded with Pseudoalteromonas atlantica. Our results indicate that biological growth in the feed wastewater stream channel severely fouled both the membrane support layer and feed spacer, resulting in ∼50% water flux decline. We also observed an increase in the pumping pressure required to force water through the spacer-filled feed channel, with pressure drop increasing from 6.4±0.8 bar m(-1) to 15.1±2.6 bar m(-1) due to spacer blockage from the developing biofilm. Neither the water flux decline nor the increased pressure drop in the feed channel could be reversed using a pressure-aided osmotic backwash. In contrast, biofouling in the seawater brine draw channel was negligible. Overall, the reduced performance due to water flux decline and increased pumping energy requirements from spacer blockage highlight the serious challenges of using high fouling potential feed sources in PRO, such as secondary wastewater effluent. We conclude that PRO power generation using wastewater effluent and seawater desalination plant brine may become possible only with rigorous pretreatment or new spacer and membrane designs.

Selectivity and Mass Transfer Limitations in Pressure-Retarded Osmosis at High Concentrations and Increased Operating Pressures.

Pressure-retarded osmosis (PRO) is a promising source of renewable energy when hypersaline brines and other high concentration solutions are used. However, membrane performance under conditions suitable for these solutions is poorly understood. In this work, we use a new method to characterize membranes under a variety of pressures and concentrations, including hydraulic pressures up to 48.3 bar and concentrations of up to 3 M NaCl. We find membrane selectivity decreases as the draw solution concentration is increased, with the salt permeability coefficient increasing by a factor of 2 when the draw concentration is changed from 0.6 to 3 M NaCl, even when the applied hydraulic pressure is maintained constant. Additionally, we find that significant pumping energy is required to overcome frictional pressure losses in the spacer-filled feed channel and achieve suitable mass transfer on the feed side of the membrane, especially at high operating pressures. For a meter-long module operating at 41 bar, we estimate feedwater will have to be pumped in at a pressure of at least 3 bar. Both the reduced selectivity and increased pumping energy requirements we observe in PRO will significantly diminish the obtainable net energy, highlighting important new challenges for development of systems utilizing hypersaline draw solutions.

Energy Efficiency and Performance Limiting Effects in Thermo-Osmotic Energy Conversion from Low-Grade Heat

Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m2) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.

Social work and engineering: Lessons from a water filtration project in Guatemala:

International service learning (ISL) programs seek to facilitate community inclusion, but such participation can prove elusive. For technical projects, such ventures can undermine local leadership, generate mistrust in communities, and even create an aversion to technological solutions. In this article, we document how social work and engineering students collaborated to bring clean water to rural Guatemala, and demonstrate how we employed social work principles to address the myriad issues encountered in the project. We contend that the inclusion of a social work perspective, with its emphasis on relationships, can help mitigate some of the challenges ISL projects tend to encounter.