Water Treatment Capacity of Forward-Osmosis Systems Utilizing Power-Plant Waste Heat

Abstract

Forward osmosis (FO) has the potential to improve the energy efficiency of membrane-based water treatment by leveraging waste heat from steam electric power generation as the primary driving force for separation. In this study, we develop a comprehensive FO process model, consisting of membrane separation, heat recovery, and draw-solute regeneration (DSR) models. We quantitatively characterize three alternative processes for DSR: distillation, steam stripping, and air stripping. We then construct a mathematical model of the distillation process for DSR that incorporates hydrodynamics, mass-and heattransport resistances, and reaction kinetics, and we integrate this into a model for the full FO process. Finally, we utilize this FO process model to derive a first-order approximation of the water production capacity given the rejected-heat quantity and quality available at U.S. electric power facilities. We find that the upper bound of FO water treatment capacity using low-grade heat sources at electric power facilities exceeds process water treatment demand for boiler water makeup and flue-gas-desulfurization wastewater systems. ■ INTRODUCTIONProposed effluent limitation guidelines at steam electric powergeneration facilities will significantly increase the demand for on-site water treatment. 1 One opportunity to minimize the auxiliary power consumption associated with this treatment capacity is to utilize waste heat available on-site for membranebased water treatment. One potential technology is forward osmosis (FO), where the draw solution is a thermolytic salt (e.g., NH 4 HCO 3 ) ( Figure 1). 2−4 In this two-step process, feedwater is drawn across a semipermeable membrane by a difference in osmotic pressure between the feed solution and the draw solution. The dilute draw solute is then regenerated by thermal decomposition of the thermolytic salt into its constituent gases (i.e., NH 3 and CO 2 ). 5 If waste heat is available, this separation process offers significant electricity savings over reverse osmosis. 6,7 Steam electric power-generation facilities are the largest source of waste heat in the United States, 8 but the feasibility of utilizing this waste heat to drive FO separation processes has yet to be systematically assessed in the peer-reviewed literature. 5,6,9 Past modeling efforts to evaluate the feasibility of waste-heat-driven FO assumed that heat is available at desired quantities and temperatures, 7 whereas experimental demonstrations of FO processes in the peer-reviewed literature utilized electricity or fuel to generate heat. 9−11 This significant gap in the literature exists largely because robust estimates of the quantity, quality, and availability of power-plant waste heat are sparse. 12,13 Our recent work provides estimates of the quantity, quality, and spatial-temporal availability of waste heat for the U.S. power sector over the next 30 years. 14 Demonstrating the feasibility of power-plant waste-heat-driven FO requires the integration of these waste-heat estimates with heat capture, transp...

Nutrient-energy-water recovery from synthetic sidestream centrate using a microbial electrolysis cell - forward osmosis hybrid system

“…It should be noted that an energy consumption rate of 0.73 kWh m -3 would be expected for draw solute regeneration (0.50 ± 0.01 mol L -1 initial diluted draw concentration, low-pressure single distillation column) (McGinnis and Elimelech 2007). Recent research confirmed that this part of energy consumption could be compensated by waste heat, and hence it was not counted in the evaluation in this study (Zhou et al 2015).…”

The synergy between osmotically assisted reverse osmosis (OARO) and the use of thermo-responsive draw solutions for energy efficient, zero-liquid discharge desalination

“…However, the vapour pressure of the thermoresponsive DS considerably exceeds that of water, which is desirable as the draw solute can be easily vaporised in a distillation process operating at relatively low temperatures (McCutcheon et al 2005). Although the draw solute decomposes at low temperatures (60˝C at 1 atm (McGinnis & Elimelech 2007)), the distillation column still has to be operated at significantly higher temperatures to completely remove any ammonia trace (ă 1 ppm) from the brine and product stream (Zhou et al 2015).…”

“…Non-toxic: Toxic solutes in the brine stream or in the final freshwater product can have detrimental effects. Ammonia is toxic above a certain threshold, hence the final water quality needs to present less than 1 ppm of ammonia (including related species such as ammonium and carbamate) in the permeate and brine stream to adhere to the limits specified in the Guidelines for Drinking Water Quality (WHO) (Zhou et al 2015, Chekli et al 2016).…”

“…A, B and S values of the utilised RO and FO/OARO membranes are given in table 1. (Zhou et al 2015) In the two-pass RO system, RO1 and RO2 are operated at 80 bar and 40 bar, respectively. Furthermore, the feed flowrate for each RO pressure vessel is kept constant at 12 m 3 {h (Peters & Hankins 2019).…”

Selection of substrate manufacturing techniques of polyamine‐based thin‐film composite membranes for forward osmosis process

“…The scarcity of drinking water in some locations has motivated the search for new and efficient water desalination solutions. The Forward Osmosis (FO) process has gained considerable interest in recent decades because of its capability of using low‐grade energy [ 1–3 ] from renewable sources [ 4–7 ] to operate. FO is a membrane separation process in which the mass transfer of water across the membrane is driven by the osmotic pressure gradient between a draw solution (DS) and a feed solution (FS).…”