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Desalination – From Reverse to Forward

o address the rapid rise in industrial water demand, various membrane and thermal-based technologies

have been developed and implemented which can effectively tap seawater as a source of pure water and can further be used to produce potable/distil led water. These technologies have been a breakthrough in helping secure a continuous supply of pure water for various industries without relying on ground water reserves.

Currently, the most commercially important technologies are based on Multi-Stage Flash Distillation (MSF) and Reverse Osmosis (RO). In the former, distilled water is produced by flashing seawater to steam in multiple stages. The process is used to produce 62 per cent of the world’s desalinated water supply. In the latter, a large external pressure greater than the osmotic pressure is applied on seawater.

Water thus flows opposite to the direction of its natural flow across the semi-permeable membrane, leaving the salts behind. The process is more economical than MSF for lower capacities and is thus

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Hi tches in the cur ren t technolog ies for water treatment have prodded s o m e r e s e a r c h e r s t o explore newer avenues for desalination, one of that is Forward Osmosis (FO). The authors present an overv iew of the FO technology along with its advantages, challenges, on-going research and its opportunities in India.

found in much higher numbers. But while MSF has a very high energy requirement, and RO requires pre-treatment of sea water and has high membrane fouling rates. This makes both the processes capital intensive. To overcome these shortcomings, research on Forward Osmosis Desalination (FO) has caught traction across the world.

ConceptOsmosis involves the flow of solvent from lower concentration to higher concentration across a semipermeable membrane. In RO, seawater is placed across a semi-permeable membrane against pure water. Since the natural flow of water would be from pure water to seawater, a pressure much greater than the osmotic pressure is applied on seawater to facilitate a reverse flow of pure water. But in FO, a highly concentrated solution called draw solution is kept against seawater, described in Figure 1.

Pure water naturally flows from the lower concentration seawater to the draw solution. This draw solution is then treated with a low energy process to give pure water and the added draw solute. The solute is then recycled back into the system to

complete the cycle (Figure 2). The process eliminates any energy intensive steps and greatly increases the life of membranes now operating under much lesser mechanical stress. All this drastically reduces its operational and maintenance cost, making it potentially the most economical desalination technology.

While the concept of FO is already used industrially to produce concentrates and pharmaceuticals, its usage in desalination is not new. Research publications, from as early as the 1970s, show that FO is capable of purifying seawater. Unfortunately, the transition from a lab scale set-up to an industrial scale has had several glitches and to this day, it faces severe technological challenges (Figure 3). Research in FO has primarily focused on the two main issues plaguing the whole FO process such as the draw solute and the membranes.

The Draw SoluteThe key component in the whole mechanism of FO is the use of a draw solution, which essentially ‘draws’ water from the seawater side. If we consider any desalination process, the draw solution has to satisfy

Forward Osmosis Testing System

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several requirements, most of which are health and economic issues. Ideal draw solutions should have:

1. Low cost;2. Low molecular mass;3. Very low (or zero) toxicity; 4. Good chem ica l compa t i b i l i t y w i t h

membranes; 5. High osmotic gradient;6. Should be easily dissolved and then

recovered back.

The last characteristic is particularly important for the economics of the FO process and perhaps the main reason why FO was a lot more expensive than RO when it came to producing potable water. A quick search into the literature shows that researchers have been trying for a long time to find the elusive ideal draw solutes. Volatile gases like sulphur dioxide and precipitating salts like ammonium sulphate have been popular choices in the past.

Even sugars like glucose and fructose have been used owing to their high solubility and ability to generate large osmotic gradient. But the removal of each of these draw solutes has not been efficient enough to meet pure water standards.

Membranes In RO, a good majority of work was initially directed at making membranes from

It is evident that more research is required to optimise the type of material required to build FO membranes.

The other area of research is the development of membrane modules for FO. Most of the commercially available modules for RO are like plate and frame, spiral wound, tubular, etc. The membrane module needs to be changed depending on whether it is a continuous or a batch process. For instance, tubular membranes are used for continuous FO processes. The tubular membranes available in the market have been optimised based on the needs of the RO process. This limits their use in FO.

Available TechnologiesIn 2005, Menachem Elimelech’s Group at Yale university published their seminal work on FO in the journal ‘Desalination’ by using a draw solute of ammonium and carbon dioxide. The primary advantage of this solute was that it forms ammonium carbamine at high concentrations, which is highly soluble in water. Also, we can recover this draw solute by moderate heating (around 60 degrees Celsius) converting it back to its constituent gases. But low water flux and less efficient separation of solute from water stil l escalated the costs of the technology to more than that of RO.

The group has since then, been working on scaling up the whole process. The team of scientists started a spin-off from the university called Osmotic Application Systems (OASYS) Water. Their pilot plant is now able to desalinate water using 90 per cent lesser energy than the traditional RO process and has also achieved a freshwater recovery of 85 per cent compared to just 35-50 per cent in RO. The success of the technology has helped the team raise a whopping USD 10.5 million funding to further develop their technology and bring it to the industry.

A lot of research is also being carried out on developing efficient FO membrane modules

materials, which can withstand the high pressure while having the right pore size. Ideally, FO membranes should have: 1. Very high salt rejection; 2. Good water flux through it; 3. Biological and mechanical stability to last

for a long period; 4. Low concentration polarisation.

In order to meet the above mentioned requirements, the main area of research should focus on developing suitable materials for FO. Most of the membranes used in either FO or RO are polymer membranes, which are porous and have reasonably good mechanical stability but are mostly hydrophobic in nature. As a result, these types of membranes are vulnerable to biological fouling.

Some of the FO membranes in vogue are Cellulose Tri-acetate (CTA), cellulose Acetate (CA), Polybenzimidazole (PBI), etc. These membranes have shown reasonably good stability as well as water flow rate but suffer from a phenomenon called concentration polarisation. Concentration polarisation is the deposition of substance on the membrane surface, which impedes the flow of water.

Some of the FO membranes in vogue are Cellulose Tri-Acetate (CTA), Cellulose Acetate (CA), Polybenzimidazole (PBI), etc. These membranes have shown reasonably good stability as well as water flow rate but suffer from a phenomenon called concentration polarisation.

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Figure 3: FO Desalination Process

Figure 1: FO Desalination Process

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However, a detailed business analysis by the team has shown highly profitable prospects for entrepreneurs in this field.If the results published by Menachem Elimelech’s Group are to be believed, an investment into a FO desalination plant has a payback period 30 per cent lesser than that of an investment in a same capacity RO plant.

Additionally, lower maintenance and continuous costs of the technology will give the entrepreneurs a definite competitive advantage over RO solution providers.

25 per cent. Given the technological superiority of Forward Osmosis (FO), its indigenous development or introduction to the Indian market will be highly beneficial. Educational institutions like the Indian Institute of Technology (IIT) should invest monetary and intellectual expertise in the development of better draw solutions and FO membranes. Entrepreneurs can also look to licensing existing FO desalination technology and meeting the desalination demand in India.

An example of the former is a group of students from the Indian Institute of Technology, Kharagpur. With the guidance from Faculty of the Department of Chemistry, the team has been working on developing an ammonia based FO desalination technology. Their preliminary results showed lower energy consumption than the OASYS technology but further progress has been restrained by lack of availability of FO membranes operating at higher pH.

with higher recovery and life. Hydration Technology Innovations (HTI) is one of the major providers of such modules, while other manufacturers are following suit.

Upcoming Research There has also been significant interest in developing combined RO-FO processes, where the seawater is first pre-treated in a FO chamber. Here, the draw solution is used against seawater to extract pure water. This draw solution is then introduced into a RO chamber, where water is removed. Thus, while the combined process greatly increases the membrane life in the RO chamber, it also increases the set-up cost and the complexity of the system.

Further research on new draw solutes is also underway. In the ammonium bicarbonate process, one of the key aspects is recovering both the gaseous ammonia and carbon dioxide and recycling them. However, it turns out that trapping these two gases is not an easy task and adds to the complexity of the whole process. Recently, novel methods like using magnetic hydrophilic nanoparticles as draw solutes have shown promising results. These nanoparticles give very high pressure gradients and can be easily removed by removal of magnetic field.

Business Opportunities in IndiaDue to the increasing number of industries setting up their manufacturing facilities in India, the desalination industry is expected to grow rapidly at a rate of

Recently, novel methods like using magnetic hydrophilic nanoparticles as draw solutes have shown promising results. These nanoparticles give very high pressure gradients and can be easily removed by removal of magnetic field.

Authors Details D P Ankit

Co-Founder and CEO Think Innoventions

E-mail: dpankit@thinkinnoventions.com

Asmit Bhowmick Head, R&D

Think InnoventionsE-mail: abhowmick@thinkinnoventions.com

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Figure 2: Challenges of FO Desalination :

Figure 3: Challenges of FO Desalination.

The major hurdle for carrying out research or becoming an FO solution provider is that both are capital intensive. In this regard, the Department of Science and Technology (DST) has several schemes under which both institutions and entrepreneurs can avail grants. Moreover, development of a sustainable technology, which shall ensure water security in addition to its immense industrial potential, is seeing great interest from investors across the world. Further interest in FO desalination can help us to find the perfect solution of India’s and the world’s fresh water needs.

References1. Aki l i D Khawaji , Ibrahim K Kutubkhanah,

J o n g - M i h n W i e , A d v a n c e s i ns e a w a t e r d e s a l i n a t i o n t e c h n o l o g i e s ,Desa l ina t ion , Vo lume 221 , I ssues 1 -3 ,1March2008,pp.47-69,ISSN0011-9164,10.1016/j.desal.2007.01.067.

2. T - S C h u n g , e t a l , F o r w a r d o s m o s i sprocesses:Yesterday,todayandtomorrow,D e s a l i n a t i o n ( 2 0 1 0 ) , d o i : 1 0 . 1 0 1 6 /j.desal.2010.12.019.

3. McCutcheon,JR,McGinnis,RL&Elimelech,MA,novelammonia-carbondioxideforward(d i rec t ) osmos is desa l ina t ion p rocess .Desalination174(2005),pp1–11.

4. T Y Cath, A E Childress and M Elimelech,Forwardosmosis:Principles,applications,and recent developments. J Membr. Sci,281(2006),pp70–87.

5. MMLing,KYWangandTSChung,Highlywater soluble magnetic nanoparticles asnoveldrawsolutes in forwardosmosis forwaterreuse.IndEngChemRes,49(2010),pp5869–5876.

6. Assessment of Indian Desalination market“Waterwithoutapinchofsalt”,byFrostandSullivan, July 2009, http://www.frost.com/prod/servlet/cio/176353336.

ProcessProcess

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Figure 1: Principle of FO Desalination :

Figure 2: Principle of FO Desalination.