Can batch or semi-batch processes save energy in reverse-osmosis desalination?

Abstract

Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the minimum practical energy of desalination substantially exceeds the thermodynamic minimum. Batch processes can theoretically approach the thermodynamic minimum, suggesting the possibility for further energy savings. In this study, we aim to quantify what energy reductions may be possible for batch-like processes when process inefficiencies such as frictional losses and concentration polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive analytical approximations and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this analysis, we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and twostage RO would save 13% and 15% energy, respectively, over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.

Predicting the Specific Energy Consumption of Reverse Osmosis Desalination

“…These desalination factors, summarized in Table 2, represented the explanatory variables in our multiple linear regression model for small-scale desalination facilities, referred to here as the small-scale model. Recovery was included as "inverse recovery" in the models as 1 1−R since SEC is proportional to this value [40,47]. The use of energy recovery systems was included as a binary variable with 0 indicating no energy recovery technology and 1 indicating the use of at least one energy recovery device, since the amount of energy savings is not often reported and different energy recovery technologies save similar percentages of operational energy [42].…”

“…Approaches to reduce SEC include use of high permeability membranes [43], use of energy recovery devices, intermediate chemical demineralization, use of renewable energy, and optimal process configuration [44]. With advanced materials, increased water-solute selectivity has become more important than additional increases in water permeability, since increasing permeability negligibly decreases SEC [45][46][47]. Since reported energy consumption typically represents 19% to 44% of the cost of desalination [16,23,25,[48][49][50], understanding which factors most significantly affect the SEC of desalination processes becomes important for the future environmental and social sustainability of desalination as an alternative water source.…”

Addressing Desalination’s Carbon Footprint: The Israeli Experience

“…This is unfortunate as they can probably do better: an RO plant located in southern California has shown even lower energy consumptions of only 3.1 kWh per m 3 of water [58] with many experts arguing that seawater desalination with energy consumption levels as low as 2 kWh/m 3 are eminently feasible [59]. Recent research shows that two-stage, batch-like processes and processes with increased staging could offer energy savings as high as 15% over present one-stage reverse osmosis seawater operations [60]. Indeed, there are many suggestions for innovative approaches to desalination that could reduce energy requirements dramatically [61,62].…”

Split-feed counterflow reverse osmosis for brine concentration

“…Operating at high pressures, although offering a high theoretical energy efficiency [14], also requires more expensive pumps, pressure vessels, and pipes, increasing system capital costs. Some of these challenges are being addressed by new variations on standard RO technology, such as multi-stage RO, closed circuit RO, and batch RO [15,16,17,18,19], but some of these challenges, such as a recovery ratio limited by membrane properties and feed conditions, remain.…”