Osmotically assisted reverse osmosis (OARO): Five approaches to dewatering saline brines using pressure-driven membrane processes
Introduction
Of the currently available desalination technologies, reverse osmosis (RO) is the most energy efficient choice, but is confined by practical issues to a maximum feedwater salinity of approximately 75 g/L [36]. Being a pressure driven process, irreversible losses associated with a phase transition of the product water, which is required for achieving separation in thermal desalination processes, can be avoided [35]. This allows RO to operate closer to the thermodynamic minimum energy of separation. As described in Elimelech & Phillip [10], the minimum energy of separation increases with process recovery and the resulting increase in the osmotic pressure of the feed solution. However, the maximum operating pressure, required to overcome the feed osmotic pressure, is limited by the membrane burst pressure [3]. This limits the maximum recovery of RO, especially at the higher feed concentrations (>75 g/L).
Unfortunately, lower process recoveries have adverse economic and environmental implications. Larger volumes of feedwater must be extracted and pre-treated for the production of the same amount of freshwater. Furthermore, larger volumes of brine must be disposed of. According to Morillo et al. [23], brine disposal costs constitute between 5% and 33% of the total cost of desalination. A reduction in brine volume can lower the incurred costs associated with its disposal and treatment, which may be required to meet local regulations and to reduce its environmental impact. Thus, more environmentally friendly brine management solutions may become economically feasible with lower brine volumes and higher process recoveries.
Dewatering such highly saline brines (>75 g/L) poses considerable technical challenges. Thermal processes, such as mechanical vapour compression (MVC), are currently still the prevailing techniques applied to achieve this [5]. In Davenport et al. [6], a two-stage MVC process is modelled to concentrate a 70 g/L saline stream to 250 g/L. MVC is chosen due to its effective heat recovery, but it still consumes 24 kWh/m3. In comparison, the minimum energy required by a thermodynamically reversible process per unit volume of product water is approximately 3.5 kWh/m3 [6].
The ability to withdraw water from saline brines (50–350 g/L TDS) in a more energy efficient manner would be beneficial for many industries, such as the oil, gas and energy sectors [3]. For example, it would facilitate the reuse of high salinity wastewaters, and allow other processes, such as inland desalination, to circumvent regulatory, economical and environmental restrictions due to brine volume minimisation [6]. Furthermore, zero-liquid discharge processes could increase profit margins, as valuable by-products, such as minerals, are produced at reduced electricity costs.
Recently, a potentially more energy-efficient solution for dewatering saline brines was proposed by Bartholomew et al. [3] and Chen & Yip [5]. This process is termed osmotically assisted reverse osmosis (OARO) and is also a pressure-driven membrane process.
In this article, the technical and economic viability of five OARO integrated flow processes, of which three are novel, is determined by modelling the processes using the solution-diffusion model. The objective of these processes is to recover 72% and 44% of the freshwater from feed solutions with a respective salinity of 35 g/L and 70 g/L. Both scenarios result in the same final brine concentration of 125 g/L. This brine concentration target is 50 g/L above the generally achievable RO brine concentration, and would result in a 40 % reduction in brine volume. A similar brine concentration target has been set in Chen & Yip [5].
The optimal OARO integrated flow process is chosen, based on the following four considerations:
- Energy consumption:
As suggested in Semiat [30], energy consumption is the main contributor, with a share of 44% of the overall operational expenditures of a typical seawater reverse osmosis (SWRO) desalination plant. The energy consumption is therefore assumed to be representative of the overall process operational costs. In Section 6, the energy consumption of the entire desalination process is modelled. This includes the energy consumed by the seawater intake, pre-treatment, brine discharge and post-treatment.
- Capital cost:
Henthorne & Boysen [12] and Moser et al. [24] both state that the intake, pre-treatment and brine discharge contribute somewhere between 23% and 33% to the total capital expenditures of a SWRO plant. These costs can be excluded from the comparison, as all the OARO integrated flow processes operate with the same feed and recovery and thus have similar intake, pre-treatment and brine discharge costs. However, the cost contribution of the desalination system itself, which is mainly comprised of the cost incurred by the pressure vessels, is approximately 35% of the total capital expenditures of a SWRO plant. As the required number of pressure vessels varies for each process, the capital cost of each OARO integrated flow process will be linked to this number. The required number of pressure vessels is determined for a permeate production of 1000 m3/h at the desired recoveries. Capital costs associated with the high pressure pumps (HPP) and the energy recovery devices (ERD) are also included.
- Permeate salinity:
The maximum allowable permeate salinity is 500 mg/L TDS, as stated in the World Health Organisation (WHO) standards for drinking water [28].
- Operating pressure:
The maximum operating pressure POA is restricted by practical pressure limits of the OARO membrane and module. As explained in Section 2, the burst pressure is assumed to be 48.3 bar. The burst pressure is defined as the maximum allowable transmembrane pressure (TMP) under which the selectivity of the membrane's active layer is not compromised.
Section snippets
Osmotically assisted reverse osmosis (OARO)
Osmotically assisted reverse osmosis incorporates both the forward osmosis (FO) and reverse osmosis working principles. However, compared to FO, the draw solution is less concentrated than the feed solution and is only utilised to lower the osmotic pressure difference across the membrane [19]. Thus, OARO can be operated at lower applied hydraulic pressures than conventional RO, which permits dewatering of more saline feed streams without exceeding the maximum allowable hydraulic pressure of the
Analysis of the stand-alone OARO process
Before the OARO integrated flow processes are analysed in Section 5, it is important to understand the effect of varying the operating parameters of the stand-alone OARO process, which is depicted in Fig. 1. Here, ‘stand-alone’ refers to a process not yet integrated with the RO unit. All comparisons of the stand-alone OARO process are performed at a constant seawater (draw) and brine (pressurised saline feed) inlet salinity of 35 g/L and 70 g/L, respectively. Further assumptions are that NaCl
The standard UF-RO process
In the following section, the OARO process is incorporated into the standard UF-RO process, depicted in Fig. 7, to achieve the specified recoveries. Although a single UF and RO process are depicted, each process consists of a number of membrane modules in parallel (NUF and NRO) to process the overall feed rate. The operating conditions of this UF-RO process will remain constant for all simulations. A single stage RO process is chosen, where each pressure vessel contains 7 8-inch spiral wound
The OARO integrated flow processes
Five different OARO incorporated processes are displayed in Figs. 8 and 9. For clarity, the feed side (FS) and draw side (DS) of each OARO process are indicated in the figures. As mentioned in Section 1, the objective is to achieve a final system water recovery of 72% and 44% for a seawater inlet salinity of 35 g/L and 70 g/L, respectively. These recoveries are obtained for each OARO integrated flow process, with various feed pressures POA and a total number T of OARO stages. In this work, the
Simulation results and discussion
In this section, all OARO integrated flow processes are evaluated for both recovery scenarios. Fig. 11 shows the simulation results obtained for scenario 1 (CIN = 35 g/L and YS = 72%) and scenario 2 (CIN = 70 g/L and YS = 44%) on the left-hand and right-hand columns, respectively. Although the final brine concentration is equivalent for both processes (125 g/L), major differences in POA, specific energy consumption, required number of RO and OARO pressure vessels, and permeate salinity can be
Conclusions
In this article, five OARO integrated flow processes are presented. These are compared according to their technical and economic feasibility, at 72% and 44% freshwater recoveries from water sources with respective salinities of 35 g/L and 70 g/L. Prior to modelling the entire process, the stand-alone OARO process was investigated and the optimal OARO process parameters were determined. The influence of each OARO parameter can be summarised as follows:
- Membrane properties:
Higher recoveries at
Abbreviations
- AL-DS
active layer-draw side
- AL-FS
active layer-feed side
- BWRO
brackish water reverse osmosis
- CTA
cellulose triacetate
- DS
draw side
- ECP
external concentration polarisation
- ERD
energy recovery device
- FO
forward osmosis
- FS
feed side
- HPP
high pressure pump
- ICP
Internal concentration polarisation
- MSRO
multi-stage reverse osmosis
- MVC
mechanical vapour compression
- OARO
osmotically assisted reverse osmosis
- PAFO
pressure assisted FO
- PRO
pressure retarded osmosis
- ROSA
RO system analysis software
- SEC
specific energy consumption
- SDI
silt
Acknowledgments
The authors would like to thank The University of Bahrain (in the Kingdom of Bahrain) for supporting and funding this research work.
References (44)
Computational model for estimating reverse osmosis system design and performance: part-one binary feed solution
Desalination
(2012)- et al.
Osmotically assisted reverse osmosis for high salinity brine treatment
Desalination
(2017) - et al.
Pressure assisted osmosis using nanofiltration membranes (PAO-NF): towards higher efficiency osmotic processes
J. Membr. Sci.
(2017) - et al.
Solution-diffusion with defects model for pressure-assisted forward osmosis
J. Membr. Sci.
(2014) A grand challenge for membrane desalination: more water, less carbon
Desalination
(2018)- et al.
State-of-the-art of reverse osmosis desalination pretreatment
Desalination
(2015) - et al.
Assessing the current state of commercially available membranes and spacers for energy production with pressure retarded osmosis
Desalination
(2016) - et al.
A short review on reverse osmosis pretreatment technologies
Desalination
(2014) - et al.
A simple modeling approach for a forward osmosis system with a spiral wound module
Desalination
(2018) - et al.
Osmotic's potential: an overview of draw solutes for forward osmosis
Desalination
(2018)
Analysis of enhancing water flux and reducing reverse solute flux in pressure assisted forward osmosis process
Desalination
Practical considerations for operability of an 8′ spiral wound forward osmosis module: hydrodynamics, fouling behaviour and cleaning strategy
Desalination
Analysis of an osmotically-enhanced dewatering process for the treatment of highly saline (waste)waters
J. Membr. Sci.
Osmotically enhanced dewatering-reverse osmosis (OED-RO) hybrid system: implications for shale gas produced water treatment
J. Membr. Sci.
The pretreatment with enhanced coagulation and a UF membrane for seawater desalination with reverse osmosis
Desalination
Pressure retarded osmosis from hypersaline solutions: investigating commercial FO membranes at high pressures
Desalination
Comparative study of brine management technologies for desalination plants
Desalination
Effect of hydraulic pressure and membrane orientation on water flux and reverse solute flux in pressure assisted osmosis
J. Membr. Sci.
UF/MF pre-treatment to RO in seawater and wastewater reuse applications: a comparison of energy costs
Desalination
Investigation of different operational strategies for the variable operation of a simple reverse osmosis unit
Desalination
Ashkelon desalination plant: a successful challenge
Desalination
Forward osmosis: where are we now?
Desalination
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