(545ar) Impact of Seasonal Salinity Variations in Estuarine Systems: Thermodynamic Feasibility Analysis of Pressure Retarded Osmosis (PRO) and Reverse Osmosis (RO) Combinations

In a recent effort by the authors, the potential to extract energy from the salinity gradients present during the mixing of river and sea water was investigated for locations where river water properties vary significantly with the seasons. It was found that estuarine systems coupled with heavy rainfall pose considerable challenges if the technology of pressure retarded osmosis (PRO) is to be used to generate energy that can be fed back to the grid. The feasibility of such a process is heavily dependent on the salinity of the river water, which as a result of precipitation, changes drastically over a 12-month period for which the analysis was carried out. The highest mixing energies were 0.77 kWh/m³, which is close to the thermodynamic limit, while the lowest obtained was 0.02 kWh/m³. The benefit of energy generation through PRO- that the only by product is brackish water- is outweighed by the fact that it is not feasible in such locations. In order to make such a technology feasible, the membrane configurations must change with the change in salinity. Although energetically feasible, such configurations are not commercially viable yet, as membranes with different power densities will be required during different seasons so as to maintain a constant rated output throughout the year. This is a significant finding, since it demonstrates that for a difference of 10 bars in osmotic pressure differential, the membrane permeability has to be increased almost 3 folds. In estuarine systems, seawater intrusion is also a reason why salinities vary. Here, salinities vary with the distance from the point of confluence with the sea. A representative analysis has graphically shown that the pumping energy requirements exceed the energy generated through the PRO process. Temperature effects were also accounted for by using literature data from 3 decades ago, comparing with the latest data and analyzing the difference. The overall variation in temperature for the 12 months is approximately 4-5.C, for both the periods. However, an interesting observation is that the average temperatures throughout both the 12-month periods under study, varies by 1.2.C, with the average temperature being higher in 2014-15 period, than in 1981.The results are indicative of change in hydrological regimes of rivers, due to temperature change, which leads to increased mixing energies. Thus, having carried out the above studies, we conclude that using PRO process to generate energy is not a viable option in locations with seasonally varying river water properties. Alternate strategies must be adopted so as to make it energetically feasible, and economically viable. In attempts to address this problem, and eventually arrive at a solution, a hybrid system consisting of PRO and reverse osmosis (RO) system has been discussed.

The result of the above study was that greater the salinity gradient, greater the mixing energies emanated. However, for locations where the salinity and other properties change seasonally, the PRO unit alone was proving to be infeasible for an all year round operation as a result of varying salinities. To address the problems raised, the authors envisioned a configuration consisting of a RO desalination unit that pretreats the sea water entering the PRO, making it more saline for the draw solution, while the treated low salinity water from the RO unit can be used as the feed solution for the PRO unit. Energetically, this proved to be infeasible- using the thermodynamic limits, set by Gibbs energy formulations. However, such a system leads to membrane fouling that further prove its commercial infeasibility and ineffectiveness at tackling a major issue that the authors had sought to solve. It was thus found that if the PRO unit is thought of as a supplement to desalination RO unit, rather than the focus of the plant, then there could be multiple benefits, as opposed to having a hybrid PRO-RO system- which is not energetically feasible.

To address the problems raised in the previous study, the authors have worked on a different configuration of PRO process- in conjunction with an RO unit, such that the primary focus shall be on the RO unit that desalinates brackish water so as to provide potable water for use by households. A novelty of the configuration being studied is that OPEX of RO desalination process can be reduced by supplementing the energy from the PRO unit. Here, the PRO unit is being studied as an accessory to the desalination plant, rather than an energy generating unit by itself. Another major advantage of such a supplement to the normal RO desalination plant is that high salinity brine discharge to the water bodies can be avoided. Previously, high saline discharge would lead to degradation of the marine ecosystem in the region. Thus, energy requirements can be reduced and balance in the marine ecosystem is maintained as a result of this configuration change in the standard RO desalination unit. Reduced energy requirements benefit the consumer of the potable water as now the cost of producing same volume of potable water requires lesser energy than before.

The authors have attempted to tackle issues of energy and water- the two top challenges for the recent millennium and discover a solution which address both. They firmly believe that this is impactful, and can be further investigated before commercialization.