Mass balance for a novel RO/FO hybrid system in seawater desalination

doi:10.1016/j.memsci.2015.11.047

Journal of Membrane Science

Volume 501, 1 March 2016, Pages 199-208

https://doi.org/10.1016/j.memsci.2015.11.047 Get rights and content

Highlights

•
A novel hybrid reverse osmosis – forward osmosis (RO/FO) system was proposed.
•
Large and small scale seawater reverse osmosis plants have been considered.
•
Increased water recovery and reduced backwash sludge volume were calculated.
•
FO membrane area required and dilution in RO reject was estimated.
•
Small scale plants tend to show better performance with the hybrid system.

Abstract

Seawater Reverse Osmosis (SWRO) desalination is being used by several countries to aid the current demand for fresh water, hence numerous large scale and small scale desalination plants have been built during last decade. Despite major advancements in SWRO technology, the desalination industry is still facing significant practical issues. Two of the major issues are (1) generation of higher volumes of pre-treatment sludge, and (2) overall water recovery. This paper proposes a novel hybrid reverse osmosis (RO) – forward osmosis (FO) system to overcome the above two drawbacks. Mass balance calculations based on laboratory experiments have been used to predict increased water recovery and reduced pre-treatment sludge volume arising from large scale (340,000 m³/day of intake) and small scale (15,000 m³/day of intake) hybrid SWRO desalination plants. The percentage reduction of pre-treatment sludge volume, increase in overall RO water recovery, FO membrane area required and dilution in RO reject have been estimated.

Introduction

It is well-known that the demand for fresh water is increasing and its reserves are depleting. Desalination of seawater has come to aid the demand for fresh water. Desalination processes have evolved from multi-stage flash (MSF) and multi-effect distillation (MED) to reverse osmosis (RO). Approximately 40–50% of the seawater treated by reverse osmosis (SWRO) is converted in to fresh water [1], [2]. SWRO has three major draw backs: (i) high volumes of concentrate due to low water recovery, (ii) significant amounts of pre-treatment sludge that needs treatment and disposal and (iii) high energy consumption due to the use of high pressures to overcome the osmotic pressure of concentrated seawater [3], [4], [5], [6], [7]. Although the last draw back has been addressed well by the introduction of energy recovery devices [8], [9], the first two draw backs still need solutions. Application of forward osmosis (FO) may be able to provide a solution to those two draw backs.

A novel hybrid RO/FO system is proposed that will improve both water recovery and reduce the volume of pre-treatment sludge. In a typical pre-treatment sludge treatment process, clarified backwash sludge gets mechanically treated until the solids content meets the required landfill conditions. However, this process yields high operations and maintenance (O&M) costs [3]. Table 1 shows the O&M cost for a sludge treatment process where daily sludge generation is 275 m³/day. Transportation and disposal of sludge cost AUD 465 and AUD 1978/day, respectively, which is significantly high cost. Fig. 1 shows an existing treatment process (see System E in the Figure) for pre-treatment of sludge in a seawater desalination plant where a centrifuge increases the sludge solids content from 2–4% to 25% [3]). The final sludge solids content is an important factor to be considered when proposing a FO system for sludge dewatering, as solids contents similar to those currently achieved or higher are required. However, existing FO membranes are incapable of producing solids contents of up to 25%, so the FO system considered was to be installed between the clarifier and the centrifuge. The FO system increases the solids content to a designated extent following which the sludge is centrifuged until solids content reaches 25%. This solids content increment may reduce the power requirement [10], as the FO system uses comparatively less energy to function and maintain than a centrifuge. FO system consumes merely 17.3 kW h/day of power to increase sludge content from 3% to 10% (see supplementary material). Further, the volume of filtrate from the centrifuge that is known as centrate will be reduced, which generally needs treatment before discharge.

Three options of RO/FO hybrid system, detailed in Section 2, are considered and mass balance calculations are applied in order to evaluate the feasibility of those systems.

Section snippets

Option 1

In addition to an existing 2 stage RO desalination process, a FO system is proposed to reduce the volume of pre-treatment sludge. This Option is suggested for the RO processes, where 2nd pass RO concentrate (significantly low in salt concentration since 2nd pass RO treats the permeate from the 1st pass) is used to backwash media filter. Fig. 2a shows the process flow diagram. An optimised proportion of 1st pass RO concentrate is used to draw water through FO as it has high concentration (hence

Mass balance

Several factors need to be considered while conducting the mass balance for a hybrid RO/FO system. It is essential to establish the water recovery of RO at various osmotic pressures of the feed, as the feed will be a mixture of pre-treated sea water and the diluted draw solution from the FO. Information on the amount of back wash water required for the pre-treatment process (generally sand filters) will help to decide how much of this volume could be reduced through the FO process. The above

Materials and method for FO experiments

Flat sheet CTA membranes were purchased from HTI, USA and Fe(OH)₃ sludge (feed solution) was received from the Perth Seawater Desalination Plant (PSDP), Australia. Draw solution (RO reject) was prepared at laboratory scale following the SWRO process in Fig. 1. Spiral wound RO membranes with effective area of 0.32 m² were used. Operating pressure and the frequency of the RO unit were 30 bar and 20 Hz, respectively. The concentrate was recirculated back to the feed until the electrical conductivity

FO experiments

Pre-treatment sludge solids content was 3.4% total solids (TS), as received. When diluted by 1:4, the solids content reduced to 0.6% TS. Fig. 3 shows the effect of solids content on the water flux, along with the solids content values. Significantly higher water flux was observed when EC of the feed solution was unadjusted, compared to constant EC feeds. Water flux of EC adjusted and constant EC samples were 6.1 and 8.0 LMH, respectively (at 1:4 dilution). Added EC controller increases the salt

Conclusions

Experiments and mathematical modelling suggest that proposed FO/RO hybrid systems are capable of reducing the volume of pre-treatment sludge. A draw to feed flow ratio of less than 4 for both scales would be the optimum ratio, with a membrane area of 900 m² and 500 m² for large and small scale plants, respectively. Table 4 shows the final volume reduction, increase in water recovery and final solids content of both large and small desalination plants considered. By increasing FO membrane area up

References (17)

S. Jamaly et al.
A short review on reverse osmosis pretreatment technologies
Desalination
(2014)
J.E. Blank et al.
The real cost of desalted water and how to reduce it further
Desalination
(2007)
M. Latorre
Environmental impact of brine disposal on Posidonia seagrasses
Desalination
(2005)
M. Ahmed et al.
Brine disposal from reverse osmosis desalination plants in Oman and the United Arab Emirates
Desalination
(2001)
C. Fritzmann et al.
State-of-the-art of reverse osmosis desalination
Desalination
(2007)
C.P. Chu et al.
Energy demand in sludge dewatering
Water Res.
(2005)
R. Valladares Linares et al.
Water Res.
(2016)
J.E. Miller, Review of water resources and desalination technologies, Sandia National Laboratories Report,...

There are more references available in the full text version of this article.

Cited by (21)

Renewable energy integration in water desalination: State-of-the-art review and comparative analysis
2023, Applied Energy
This paper offers a comprehensive overview of advanced water desalination systems powered by renewable energy sources, specifically focusing on developments in the past decade. Solar energy is particularly emphasized as a pivotal resource for powering diverse desalination methods, including membrane distillation, reverse osmosis, humidification-dehumidification, forward osmosis, and hybrid systems. The primary objectives of this study encompass evaluating current advancements in renewable energy systems, especially solar energy, for driving water desalination processes. Furthermore, the study aims to identify gaps in research, challenges, and recommendations within these processes, and to assess the performance and refinement potential of hybrid desalination systems. Through a comprehensive analysis of various investigations, integrated effectiveness and cost-efficiency are examined, focusing on practical energy utilization. Notably, the analysis reveals the potential of hybrid desalination systems to reduce energy consumption. For instance, the hybridization of forward osmosis with membrane distillation showcases a significant energy ratio decrease from 0.89 to 0.64. Moreover, the combination of humidification-dehumidification with reverse osmosis is shown to reduce energy usage while enhancing freshwater output by up to 38% compared to standalone reverse osmosis. However, challenges persist, such as the need for improved integration of hybrid systems with concentrated solar collectors like photovoltaic thermal and concentrated photovoltaic thermal technologies.
Performance of a solar hybrid desalination plant integrated with photovoltaic-thermal collectors for remote regions: Experimental and modeling investigation
2023, Solar Energy
Energy and freshwater are the most prioritized needs for any nation nowadays, especially for arid areas. Giving attention to renewable energy sources like solar energy is a must towards a cleaner environment and a world with no need of temporary sources like fossil fuels. Also, a solution to lower the power requirements and costs of desalination hybrid systems should be investigated to overcome the water problem. The present study aims to design and build a novel hybrid desalination plant working with the highest productivity and lowest power consumption. This was done by incorporating a reverse osmosis desalination unit with the conventional solar distiller. The photovoltaic/thermal collectors were utilized to produce electricity as well as a heating unit in addition to the evacuated tube solar collectors, to preheat the hybrid desalination plant feed water. The results presented that the use of photovoltaic/thermal panels and evacuated tube solar collectors as preheating units improved water recovery by 46% and reduced power consumption by 38.34% for the small reverse osmosis unit but the salt rejection decreased by 4%. Cooling the photovoltaic panel increased its electricity production by 24.1% on average. Moreover, the combination of the solar distiller with the reverse osmosis unit on the feed line represents the best configuration which improves the solar distiller’s productivity by 98.32% as compared to separate solar distillers. The cost of freshwater produced is 0.719 $/ $m^{3}$ which is cost competitive compared to previous solar desalination systems.
Economic evaluation of wastewater and water treatment technologies
2020, Current Trends and Future Developments on (Bio-) Membranes: Recent Achievements in Wastewater and Water Treatments
Because of recent advances, separation processes based on membrane technology are being increasingly considered as good alternatives to conventional methods used for treatment of water and wastewater. Researchers are finding that membrane technology offers reduced environmental impact and high efficiency. Because of economic problems and high cost of membrane based separations which have caused to their application limitation and most of the costs belong to operating and capital costs which membranes associated. This chapter discusses membrane technology applications in wastewater and water treatment from an economic viewpoint. The consequences of extensive experimental models got to decide the expense of treatment utilizing of membranes which are used for ultrafiltration and microfiltration processes. The major variables affecting the expense were observed to be the cost of the membrane and its replacement, power, and membrane replacement recurrence. The significance of choosing the most reasonable membrane and optimal operational requirements are illustrated. A comparative study between conventional treatment methods and expenses of membrane frameworks has been made. Some economically important topics have been highlighted in membrane usage.
Reverse osmosis and forward osmosis in integrated systems
2019, Current Trends and Future Developments on (Bio-) Membranes: Reverse and Forward Osmosis: Principles, Applications, Advances
Since the osmotic pressure as driving force in forward osmosis is created by the osmotic gradient between feed solution and draw solution, the posttreatment of draw solution is generally required and conducted by reverse osmosis to recover product water and reuse draw solutes. Therefore, forward osmosis and reverse osmosis are often combined to be an integrated membrane system. The integrated forward osmosis-reverse osmosis system has been studied for saline water desalination, wastewater treatment, and renewable energy generation. The harsh fouling of conventional reverse osmosis process can be converted to reversible fouling of forward osmosis in the integrated system. It is possible to perform the desalination, water treatment, and energy generation simultaneously by employing different water (such as quality impaired water, saline water, and concentrated brine) in the integrated system. The integrated forward osmosis-reverse osmosis membrane system can play a promising role for the comprehensive management of water resource and promote the sustainability in water-energy nexus.
Intermediate concentrate demineralization techniques for enhanced brackish water reverse osmosis water recovery – A review
2019, Desalination
Citation Excerpt :
The sludge, mainly CaCO3 and impurities, was dewatered and collected by vacuum filtration. Recycled CaCO3 was then calcined for quicklime recovery [121]. In general, desalination water cost reduction is favored by the industry growth rate, plant production capacity, competition between various processes and technological improvements.
Brackish water (BW) desalination represents an untapped potential for augmenting water resources by the reliable technology of reverse osmosis (RO). There is a significant economic incentive to operate the process at the highest possible water recovery. A high recovery has the important advantage of reducing the amount of concentrate waste stream whose safe disposal is of significant environmental concerns. The degree of RO water recovery possible with a given raw water is dictated by the scaling problem. To overcome this hindrance, a variety of processes have been developed for conditioning the concentrate emanating from the primary RO unit for additional permeate production by a secondary RO unit. This paper provides a critical review of the treatment technologies that enable intensification of the water recovery in BWRO desalination by the intermediate concentrate demineralization techniques. The principles of each technique as well as its strength, weaknesses, technology readiness level, and economics are discussed. Extensive application of ICD technologies requires overcoming challenges such as lowering energy demand and total cost and reducing membrane scaling and use of chemicals.
Reverse osmosis desalination: A state-of-the-art review
2019, Desalination
Citation Excerpt :
A similar setup with focus on reducing the pre-treatment sludge in SWRO, directed the sludge from media filter backwash to an FO membrane, where concentrate from the SWRO was used as a draw solution. The results of the study showed decrease in sludge volumes, increase in water recovery, and increase in the solid contents of the sludge [504]. Applications for FO/RO hybrid were also investigated for use in space missions, where FO, together with osmotic distillation (OD) is used as pretreatment for RO.
Water scarcity is a grand challenge that has always stimulated research interests in finding effective means for pure water production. In this context, reverse osmosis (RO) is considered the leading and the most optimized membrane-based desalination process that is currently dominating the desalination market. In this review, various aspects of RO desalination are reviewed. Theories and models related to concentration polarization and membrane transport, as well as merits and drawbacks of these models in predicting polarization effects, are discussed. An updated review of studies related to membrane modules (plate and frame, tubular, spiral wound, and hollow fiber) and membrane characterization are provided. The review also discusses membrane cleaning and different pre-treatment technologies in place for RO desalination, such as feed-water pre-treatment and biocides. RO pre-treatment technologies, which include conventional (e.g., coagulation-flocculation, media filtration, disinfection, scale inhibition) and non-conventional (e.g., MF, UF, and NF) are reviewed and their relative attributes are compared. As per the available literature, UF, MF and coagulation-flocculation are considered the most widely used pre-treatment technologies. In addition, this review discusses membrane fouling, which represents a serious challenge in RO processes due to its significant contribution to energy requirements and process economy (e.g., flux decline, permeate quality, membrane lifespan, increased feed pressure, increased pre-treatment and membrane maintenance cost). Different membrane fouling types, such as colloidal, organic, inorganic, and biological fouling, are addressed in this review. Principles of RO process design and the embedded economic and energy considerations are discussed. In general, cost of water desalination has dropped to values that made it a viable option, comparable even to conventional water treatment methods. Finally, an overview of hybrid RO desalination processes and the current challenges faced by RO desalination processes are presented and discussed.

View all citing articles on Scopus

View full text

Mass balance for a novel RO/FO hybrid system in seawater desalination

Highlights

Abstract

Introduction

Section snippets

Option 1

Mass balance

Materials and method for FO experiments

FO experiments

Conclusions

Desalination

Desalination

Desalination

Desalination

Desalination

Water Res.

Water Res.