Elsevier

Desalination

Volume 458, 15 May 2019, Pages 1-13
Desalination

Osmotically assisted reverse osmosis (OARO): Five approaches to dewatering saline brines using pressure-driven membrane processes

https://doi.org/10.1016/j.desal.2019.01.025Get rights and content

Highlights

  • ARO integrated flow processes can dewater concentrated brines (>75 g/L), while operating within practical limits.

  • OA-5 is a novel flow process, which consumes 4 kWh/m3 to concentrate a 35 g/L saline stream to 125 g/L.

  • OA-3 is a novel flow process, which consumes 6.37 kWh/m3 to concentrate a 70 g/L saline stream to 125 g/L

Abstract

The ability of osmotically assisted reverse osmosis to draw water from concentrated brine (>75 g/L) can be attributed to the combined effect of reducing the transmembrane osmotic pressure difference via a draw solution, and operating at high hydraulic feed pressures. This approach has been incorporated into a standard ultrafiltration and reverse osmosis desalination plant to increase the process recovery. In total, five different OARO integrated flow processes are modelled numerically to determine their technical and economical feasibility in maximising the process recovery. Three of the five presented OARO integrated flow processes are novel, and offer technical and economical advantages over the previously proposed OARO processes.

At lower feed salinities, OA-5, which is a new process, is the optimal OARO integrated flow process. Recoveries of up to 72% from a 35 g/L saline feed are possible when operating at the membrane burst pressure of 48.3 bar. Furthermore, the energy consumption of OA-5 is approximately 4.00 kWh/m3, which is significantly lower than that in currently employed high recovery thermal processes, such as mechanical vapour compression.

OA-3 is another original OARO integrated flow process and is the most attractive at a higher feed salinity of 70 g/L. The maximum recovery of 44% is achieved at an average energy consumption of 6.37 kWh/m3. The results presented in this article demonstrate that pressure-based membrane processes can competitively concentrate brine streams to concentrations of up to 125 g/L.

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.

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