Dialysis and Reverse Osmosis

Transcription

1 CHEM-E Bioprocess technology II Dialysis and Reverse Osmosis Kwok Liang Hui Elise Jyränoja Thu Ha Dang Instructor: Sandip Bankar Date of Submission: 6 December 2017

2 Abstract Dialysis and Reverse Osmosis are both purification techniques aiming to remove unwanted substances through the use of a semipermeable membrane. Both processes leverage on the difference in particle size of the solid and fluid to attain a separation between the two. This report will give an introduction to dialysis and Reverse Osmosis and explain the theory and mechanism behind the processes. In addition, practical applications used in our daily lives will be explored as case studies. Introduction As an engineer the development of a successful fermentation process is a great achievement. However, a more important step that needs to be tackled is the purification of the product from the fermentation broth. In this report two techniques used in the downstream process are introduced. Both dialysis and reverse osmosis rely on a semipermeable membranes through which molecules diffuse. For us humans water is a necessity essential for the life on earth. Especially in arid regions as the desert fresh water from surface water like rivers and lakes is scarce. In order to overcome this issue desalination of seawater has been considered as a solution. A lot of research and development led to Reverse Osmosis systems/plants. Filtration is a separation process where solid particles in fluids are removed through the use of a semipermeable medium that allows the fluid to pass and retains the unwanted solids. The fluid that has been passed through is known as the filtrate. This separation method is known long ago when our ancestors used it to produce drinking water from muddy water. However, there are always some fine solid particles present contaminating the filtrate and thereby lowering its purity. Thus, there are systems created to improve the filtrate purity such as reverse osmosis and dialysis. A dialysis system consists of a buffer solution called the dialysate and semipermeable membrane. History of dialysis dates back to 1910 where a team led by John Jacob Abel demonstrated the use of dialysis to separate amino acids from blood [1]. He also realised the potential uses of dialysis to cure renal dysfunction in humans. However, the first functioning dialysis machine was only created by Dr. Willem Kolff in the 1940s [2]. Dialysis is also used for non-clinical applications such as in biotechnology where dialysis can be used in fermentation broths to extract important compounds such as ethanol [3].

3 Osmosis is a vital process in our life. In biological systems, osmosis helps to filter out large molecules such as ions and only allows smaller molecules such as oxygen and water to pass. A physicist named Jean-Antoine Nollet observed the process of osmosis through a semipermeable membrane in the 1740s. In the 1950s, the osmosis concept was modified and used to produce pure water from saltwater which would be known as Reverse Osmosis. However, due to inefficient membrane technology at that time, this application was not commercially viable. Only after the development of membrane technology, reverse osmosis was used in desalination plants and by the end of 2001, there were about 15,200 desalination plants worldwide [4]. Mechanism Figure 1. The theory behind osmosis [5]. Osmosis Osmosis is a phenomenon occurring in nature. Examples from our everyday life are the absorption of water from soil through the plants roots and the absorption of water from our blood by the kidneys. It is characterized by the spontaneous passage or diffusion of solvents as water through a semipermeable membrane. [5, 6] The solvents flows from a solution with a low concentration of dissolved solids to a solution with a high concentration of dissolved solids as visualized in figure 1. In this particular case on the left side of the membrane there are more molecules of contaminants. Thus, the water moves in this direction. The flow continues until the equilibrium state meaning the concentration is equal on both sides of the membrane is accomplished. [5]

4 Figure 2. Reverse Osmosis utilizes additional pressure to reverse the water flow and thereby desalinating the water [5]. Reverse Osmosis is the reversal of the osmosis process. A major difference is that energy has to be supplied for reverse osmosis plants. Pressure has to be applied on the more saline solution to move the water through the membrane. Most importantly, the pressure has to be greater than the natural osmotic pressure otherwise the water is not successfully desalinated or demineralized. [5] Figure 3. Simplified overview of the Reverse Osmosis system [5]. After the feed water enters the membrane under pressure, the water molecules pass through the membrane. Foreign particle molecules are retained by the membrane and leave the reverse osmosis system through the reject stream. There are two possible path for the reject stream. It either is discarded or it is recycled and fed back into the feed water supply. A high pressure pump is needed in order to increase the pressure on

5 the salt side of the Reverse Osmosis. The amount of pressure required depends on the salt concentration of the feed water meaning if the feed water is more concentrated, a higher pressure has to be applied to overcome the osmotic pressure. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it. The water stream that carries the concentrated contaminants hindered by the membrane from passing is called the reject (or concentrate) stream. [5] An important feature of an RO system is the use of cross filtration which leads to two outlets: the filtered water goes one way and the contaminated water goes another way. This ensures less build up of contaminants, as the cross flow filtration allows water to sweep away contaminant build up and also allows enough turbulence to keep the membrane surface clean. [5] Performance of Reverse Osmosis Reverse Osmosis is able to retain up to 99% of dissolved particles, ions but also bacteria. The rejection of a certain contaminant is based on the size (if the molecular weight is greater than 200) and charge. The higher the ionic charge is, the more probable the Reverse Osmosis membrane will retain it. It is a highly effective purification technique for brackish, surface and ground water. Reverse Osmosis plants are used in different fields of industries as pharmaceutical, food and beverage and most importantly for the production of potable water. However, a proper pretreatment is necessary to prevent fouling of the Reverse Osmosis membrane and ensure its functionality. Particulate, organics and microorganisms are causing fouling. [5] Dialysis Dialysis is a way of separating small, unwanted compound from macromolecules in solution by using a semipermeable membrane [ 7,8]. The flow through the membrane is caused by selective and passive diffusion. The membrane separates molecules of the sample that are larger than the membrane pores while small compounds and buffer solutions can pass freely through the membrane. This is shown in the figure 1. Because, the sample and the buffer solution are placed on the opposite sides of the membrane, the flow of the smaller compounds from sample to buffer solution, reduces the concentration of the small compounds in the sample. [7] The reduction of the

6 concentration in the sample continues until reaching the equilibrium with the buffer solution. [9] By this reduction of the concentration, the level of unwanted and contaminated compound in the solution is reduced to a acceptable level. To increase the purity of the sample, the buffer solution should be changed so that it contains less small compounds from the sample. This enables more small compounds to flow through the membrane. [7] Figure 4. shows the principle of dialysis where the molecules that are larger than the pores are not able to pass through. The buffer solution and the sample are placed on the opposite sides of the semipermeable membrane. [7] The efficiency of dialysis is described as how much the concentration and composition of the sample can be changed. The efficiency depends on the concentration difference between the volumes of sample and the buffer solution. The bigger the buffer solution is the more efficient the dialysis is. The figure 5 shows a schematic description of the dialysis process in laboratory. In this figure the sample is marked as dialysed solution (V 1 ) and the buffer solution as dialysing solution (V 2 ). To increase the efficiency of the process and the purity of the sample, the buffer solution should be changed after the equilibrium has been reached. This enables more small compounds to flow through the membrane. [7,9]

7 Figure 5. Shows the dialysis in the biochemical laboratory practice [9]. The time needed for dialysis to happen is influenced by many factors. These factors determine the rate of dialysis. For example, increasing temperature or stirring the buffer solution rises the rate of dialysis because it speeds the diffusion. In addition to this, the concentration of molecules and molecule s molecular weight also affects the rate of dialysis. As the concentration of a molecule increases the probability of the molecules to diffuse through the membrane increases and thereby the rate of dialysis rises. However, in the case of increasing molecular weight the diffusion through the membrane is more difficult and the rate of dialysis decreases. The pore sizes defines how big molecules can diffuse through the membrane. The molecular weight cut-off (MWCO) describes the pore size as the smallest average molecular mass of the molecule that cannot pass through the membrane. The surface area and the thickness of the sample also affects the rate. As the surface area of the membrane increases and the thickness of the membrane decreases the rate of dialysis rises. [7]

8 Importance to industry The largest and most important application of reverse osmosis is the separation of pure water from seawater and brackish waters. Since the early 1970s, it has also been used to purify fresh water for medical, industrial, and domestic applications. [4] It has been shown that contaminants posing potential health hazards as pharmaceuticals, hormones and food preservatives may be removed by reverse osmosis plants. This indicates the potential that this technology holds. However, RO requires electricity to pump the water through the semipermeable membranes. Another important fact is that contaminants can easily clog the pores of the membranes and thereby raising maintenance costs. [10] Dialysis is an important filtration technique in the modern industry. It has applications in medical fields such as clinical dialysis and in biotechnology fields such as extraction of important salts or proteins from fermentation processes. However, despite the popularity of using dialysis in industries, there are some limitations that will be further explained. One of the limiting factors is that the dialysis process is really slow. Dialysis would take a few hours to days to completely remove the contaminants from the desired product. This explains why a typical clinical dialysis session would lasts around four hours to be completed [11]. Another limitation is that dialysis requires the inefficient act of manually replacing the dialysate solution. Such an act would be inefficient and costly for any biotechnology plant as stopping the reactor to change the dialysate would hinder the extraction of the desired product [12]. With such limitations in mind, some biotechnology companies have adopted an alternative extraction method like gel filtration technique. Case studies Hermodialysis is a treatment for patients who have kidney dysfunction. It is a process of purifying the blood by removing waste and metal ions. As the name suggests, hermodialysis uses dialysis to filter out unwanted substances from the blood to the dialysate through the use of a membrane called dialyzer [13]. The dialyzer is a semipermeable membrane which only allows water and waste to pass through. Thus clean blood is produced and it will flow back to the patient. Dialysate is a solution of pure water, electrolytes and salt [14]. A dialysate solution s main purpose is to remove toxins from the water through dialysis. Salt from the dialysate solution would be introduced into the bloodstream through hermodialysis.

9 However, if the dialysate solution does not contain a high purity water, ions that are introduced into the bloodstream may build up to a hazardous level which can be fatal to the patient. Thus dialysis centers typically obtain high purity water for their dialysate solution through the use of reverse osmosis [15]. Reverse osmosis is an important technique of producing drinking water for many countries e.g. Singapore. Singapore is a small country with no natural resources such as water. Thus, Singapore has been relying on neighbouring countries such as Malaysia to obtain clean water. In order to resolve the dependency issue, the government prioritised the research in water recycling and they produced NEWater. NEWater is a reclaimed water which is suitable for drinking that has been purified using reverse osmosis and microfiltration. Through the process of reverse osmosis and ultraviolet technologies, Singapore is able to produce high purity water that is suitable for drinking and thus reduce the problem of water shortage. Conclusion In conclusion, we have extensively discussed dialysis and Reverse Osmosis which are important purification techniques having been developed to improve our everyday s life. Through the use of dialysate and a semipermeable membrane, dialysis allows essential compounds to be extracted and contaminants to be removed from any given sample. Hence, dialysis is applied to different areas of our industries today as shown in the case study above. Reverse Osmosis has been developed with the goal of obtaining highly purified water from salt or waste water. It is an important technique developed from the general osmosis technique which can solve water shortage problems in many countries such as Singapore. Although it seems to be promising due to its removal of a variety of contaminants, issues concerning membrane clogging and fouling still are relevant.

10 References [1] - [2] [3] Adapted from: Continuous production of ethanol by yeast immobilised in a membrane fermentor (Kyu H. Kyung, Phlipp Gerhardt) March 1984 [4] [5] [6] [7] Dialysis methods for protein research, thermofisher.com. Thermo Fisher Scientific Inc Collected from nter/protein-biology-resource-library/pierce-protein-methods/dialysis-methods-protein-research.h tm l [8] Luo, J., Wu, C., Xu, T., Wu, Y. Diffusion dialysis-concept, principle and applications, Journal of Membrane Science 366 (2010) [9] Hegyi, G., Kardos, J., Kovacs, M. et al., Units, solutions, dialysis in Introduction to practical biochemistry, Eötvös Loránd University, Hungary Web. [10] [11] [12] [13] [14] [15] s-water-system-991