Serially connected forward osmosis membrane elements of pressure-assisted forward osmosis-reverse osmosis hybrid system: Process performance and economic analysis

Abstract

Pressure-assisted forward osmosis (PAFO) has been considered an alternative of forward osmosis (FO) process to reduce capital expenditure (CAPEX) of FO. This is mainly attributed to the enhanced water flux through the FO membranes which leads to the improved dilution of draw streams. In this regard, it has been expected that employing PAFO in the FO-reverse osmosis (RO) hybrid system for seawater desalination can reduce the overall economics of the hybrid process. However, replacing FO with PAFO causes an additional energy cost in the seawater dilution step which inevitably leads to a question that PAFO-RO hybrid is truly an economically beneficial option over FO-RO and stand-alone RO desalination. More importantly, though serial connection of FO elements (SE) improves the dilution of initial draw water which can induce enhanced energy cost saving in the following RO step, this economic benefit is also compensated with the additional membrane cost in the PAFO unit process. To rationalize its overall performance and economic benefit, the thorough performance and economic evaluation were conducted based on actual pilot-scale PAFO operations for serial connection of three 8040 FO elements(SE1, SE2 and SE3). A scenario was assumed that PAFO is implemented as an additional pretreatment step of existing RO process. The results showed that the FO-RO hybrid process is not an economically feasible option unless a significant unit FO element cost cut-down is guaranteed. On the other hand, PAFO-RO hybrid showed some benefits with regards to a higher range of target RO recovery and unit FO element cost, particularly when two FO elements are serially connected (SE2). However, consideration on overall plant construction and operating cost components led to a conclusion that the skepticism on the hybrid process has to be maintained. This skepticism can be alleviated when a new PAFO-RO hybrid desalination plant is commissioned primarily due to significant potential CAPEX and OPEX savings in the RO process originated from the significant reduction of Highlights  Significant impact of initial draw flowrate and hydraulic pressure on the economics of PAFO in hybridization with RO  Hydraulic pressure dependence of FO membrane element performance  Drastic pressure-drop in the draw channel of the last element in serial configuration  Target RO recovery and FO element cost in PAFO-RO hybrid are important for economics

A Review of CFD Modelling and Performance Metrics for Osmotic Membrane Processes

“…While the above-mentioned parameters are useful to describe module performance, they do not explicitly address the economic impacts of energy consumption, especially for a large-scale system. Despite previous studies having evaluated the economic impacts [ 144 , 145 , 146 ] or total cost [ 147 ] of large-scale desalination systems, only a handful of studies have evaluated the impact of spacer design on total cost [ 148 ]. A recent techno-economic study by Toh et al [ 149 ] found that improving spacer design is more crucial than increasing permeance at same recovery, especially for SWRO systems.…”

Unlocking the application potential of forward osmosis through integrated/hybrid process

“…The energy penalty posed by the pressureassisted operational mode could potentially be compensated by enhanced permeate throughput and reduced membrane area [20]. The economic potential of integrated PAFO-RO has been proven higher than integrated FO-RO process, though the plausibility of implementing PAFO to existing RO plants still remain uncertain due to the additional capital expenses associated with PAFO [21].…”

Exploring the Operation Factors that Influence Performance of a Spiral-Wound Forward Osmosis Membrane Process for Scale-up Design

“…A diamond-type mesh spacer was adopted for the feed channel (1.15 mm), and a combination of tricot-type and diamond-type spacers was adopted for the draw channel (1.98 mm). The detailed configuration of the spiral-wound FO element can be found elsewhere [41].…”