The benefits of hybridising electrodialysis with reverse osmosis
Graphical abstract
Introduction
Based on a comparison of the cost of water, we establish guidelines for choosing between stand-alone electrodialysis (ED) and hybrid electrodialysis-reverse osmosis (ED-RO) systems. By modelling the energy and equipment costs of electrodialysis as a function of product salinity we demonstrate the opportunity to reduce costs by shifting salt removal to higher salinity. Hybridisation of electrodialysis with reverse osmosis allows such a shift. Therefore, we model hybrid electrodialysis-reverse osmosis systems to establish when the benefits of hybridisation outweigh the costs of the reverse osmosis unit. We frame our models such that the decision between hybrid and stand-alone systems is based upon a cost ratio between reverse osmosis and electrodialysis systems, and consider this as a variable in our analysis.
Our interest in hybrid ED-RO systems is to further minimise the environmental impact and economic cost of brackish desalination, of which the latter has grown at an estimated annualised rate of 12% over the past 10 years [1] (see Appendix A). Brackish desalination involves the treatment of waters of slight (1,000–3,000 ppm total dissolved solids, TDS) to moderate salinity (3,000–10,000 ppm TDS) [2] present in naturally saline inland aquifers or coastal aquifers that have become subject to the intrusion of seawater [3] (see Fig. 1).
From environmental and cost perspectives, the ratio of water recovered to that withdrawn, known as the recovery ratio, RR, is an important consideration. A higher recovery ratio allows the following benefits: a reduction in the size of the desalination plant intake; a reduction in the volume of brine produced, which requires disposal to the sea, surface waters or confined aquifers below the aquifer from which water is withdrawn [5]; and a reduction in the rate of aquifer recharge required, which might be done continuously with treated waste water [4] or periodically with water sourced from another location during periods of low demand [6]. Conversely, a higher recovery ratio results in the production of higher salinity brine, which, depending upon the degree of dispersion and/or dilution employed at the point of disposal, can have adverse effects on plant and animal life [7]. We focus on scenarios where the benefits of reduced volumes outweigh those of increased salinity and consider technologies offering high recovery ratios.
Electrodialysis is well suited to applications requiring high recovery ratios for at least three reasons. Firstly, electrodialysis is a salt removal rather than a water removal technology, and so the majority of the feed water is easily recovered as a product. This is in contrast to reverse osmosis, where high recovery ratios require multiple stages in a continuous process (Fig. 2a) or longer process times in a semi-batch (or batch) process [8]. Secondly, electrodialysis is capable of reaching brine concentrations above 10% total dissolved solids (TDS), which is beyond the osmotic pressures reachable by current reverse osmosis systems [9], [10]. Thirdly, seeded precipitation of scalants in the ED process can in some cases circumvent the barrier on water recovery imposed by the solubility of feedwater solutes; this has been demonstrated by recirculating the electrodialysis concentrate loop through a crystalliser [9], [11], [12] or a combination of a crystalliser and an ultrafiltration unit [10].
Although ED enjoys the advantage of high water recovery, costs increase with the amount of salt removal required (Fig. 2b). This is particularly true at low salinity where salt removal rates, which scale with the electrical current, are limited by the rate of diffusion of ions to the membrane surface. This phenomenon, known as the limiting current density, as well as the high electrical resistance of solutions at low concentrations, increases the costs of electrodialysis at low salinity. Thus, it is the synergy of ED providing high recovery and RO providing final high product purity that gave rise to analyses of hybrid ED–RO systems. The technical feasibility of these systems has already been demonstrated [10], [12], [13], [14], [15], [16], but there are a limited number of studies benchmarking hybrid ED–RO systems against other technologies. To date, one study has compared hybrid ED–RO to a reverse-osmosis-mechanical-vapour-compression system and concluded that the hybrid system has lower upfront capital costs and lower operational costs [13].
In summary, electrodialysis can offer the benefit of higher recovery relative to reverse osmosis systems. Although the cost of water from a reverse osmosis system operating at lower recovery may be smaller, when brine disposal costs are taken into account, electrodialysis can be more cost effective [17]. In this paper we focus on scenarios where, overall, ED is more cost effective than RO and analyse the question of when it is preferable to hybridise electrodialysis with reverse osmosis rather than operate with electrodialysis alone. We also compare simple hybrid and recirculated hybrid system configurations.
Section snippets
The rationale for hybridising electrodialysis with reverse osmosis
The rationale for hybridising electrodialysis with reverse osmosis is to relax the product purity requirements on the electrodialysis unit. Later, we will demonstrate how these requirements can be relaxed by comparing simple hybrid and recirculated hybrid designs to a stand-alone ED system, Fig. 3. First, to understand why the relaxation of product purity requirements can reduce ED costs, we focus on the stand-alone ED system and consider the dependence of the specific cost of water on product
Reasons to prefer a simple ED–RO hybrid configuration
One way to shift salt removal to higher salinity is via the simple hybrid ED–RO configuration [12] illustrated in Fig. 3. This configuration has two benefits over a stand-alone electrodialysis system: the total membrane area (or number of stacks required) is reduced as higher rates of salt removal (current densities) are possible at higher salinities; and electrodialysis product requirements are relaxed since the final product consists of a blend of high purity RO permeate and the
Reasons to prefer a recirculated hybrid ED–RO system
While the simple hybrid configuration of Fig. 3 can shift salt removal to higher salinities, the hybrid configuration [10], [13], [14], [15], [16] that incorporates recirculation can furthermore facilitate salt removal within a narrower band of higher salinity (closer to what is illustrated in Fig. 5). The effect of hybridising with recirculation is thus to cut down more drastically on ED costs than in the simple hybrid configuration, but at the expense of greater reverse osmosis costs, since a
Sensitivity analysis
Table 1 provides the sensitivity of the critical cost ratio, CR⁎, to key input parameters for a simple hybrid system operating with a feed of 3,000 ppm, a product stream of 500 ppm and an applied voltage per cell pair corresponding to 70% of the limiting current density at 350 ppm. The sensitivity to each input parameter, X, is calculated according to
CR⁎ is more sensitive to percentage changes in the feed salinity SF than to percentage changes in the product salinity SP. This is
Conclusion
Hybrid ED–RO systems will be preferred over stand-alone ED systems where a high purity product is required and provided the cost of water from RO is low relative to ED. The break-even point between a hybrid ED–RO and a stand-alone ED system occurs when the cost of water from a single stage RO system, operating at 50% recovery, is between about 60–70% of the cost of water from a stand-alone ED system. At break-even, the savings in ED costs, brought about by the elimination of low salinity stages
Acknowledgments
The authors are grateful for the support of the Hugh Hampton Young Memorial Fellowship and of the King Fahd University of Petroleum and Minerals through the Center for Clean Water and Clean Energy at MIT and KFUPM under project number R15-CW-11.
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