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Saudi Journal of Kidney Diseases and Transplantation
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LETTER TO THE EDITOR Table of Contents   
Year : 2010  |  Volume : 21  |  Issue : 4  |  Page : 748-749
Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems

Internal Medicine Department, Zagazig University, Zagazig, Egypt

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Date of Web Publication26-Jun-2010

How to cite this article:
Talaat KM. Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems. Saudi J Kidney Dis Transpl 2010;21:748-9

How to cite this URL:
Talaat KM. Dialysis fluid regeneration by forward osmosis: A feasible option for ambulatory dialysis systems. Saudi J Kidney Dis Transpl [serial online] 2010 [cited 2022 Aug 16];21:748-9. Available from: https://www.sjkdt.org/text.asp?2010/21/4/748/64669
To the Editor,

In forward osmosis (FO), water is drawn from the feed solution to dilute a more concentrated draw solution across a water permeable, but solute impermeable membrane. Recently, there has been a great interest in the possible appli­cations of FO including osmosis pumps for drug delivery, water desalination and the hy­dration bags made by Hydration Technologies, Inc., Oregon. [1],[2],[3]

A similar concept may be adopted for dia­lysis fluid regeneration, whereby a concentra­ted draw solution consisting mainly of sodium chloride is used to draw water from the spent dialysis fluid across a FO membrane. After FO treatment, the fluid obtained on the draw so­lution side of the membrane increases in vo­lume and its composition becomes more si­milar to the fresh dialysis fluid while the spent dialysis fluid looses a substantial portion of its water content and the uremic toxins become more concentrated in the remaining fluid, which is then discarded. Theoretically, if a highly concentrated draw solution containing physiologically balanced electrolytes is used, up to 50% of the spent dialysis fluid water may be retrieved by a FO treatment.

An efficient method for dialysis fluid regene­ration is a pre-requisite for developing a fea­sible ambulatory dialysis system. The small bulk of the necessary equipment, mainly a FO mem­brane and a low pressure pump as well as the very low external energy needed, make FO a very attractive option for dialysis fluid regene­ration. The CTA forward osmosis membrane made by Hydration Technologies, Oregon out­performed other membranes in FO applica­tions. The United States Patent Application 20060226067 disclosed an asymmetric FO membrane having a salt rejection ratio of more than 99%, and comprising cellulose fibers. A disadvantage of the cellulose membrane is its suboptimal urea rejection ratio. [2] However, the clinical toxicity of urea is minimal, [4] and a decreased urea clearance may not adversely affect the clinical usefulness of the process. In another approach to more efficient FO mem­branes, Kumar et al developed a highly per­meable polymeric membrane into which the bacterial water channel protein, aquaporin Z was incorporated. This membrane showed ex­cellent water permeability and solute rejection while its urea rejection ratio was almost com­plete. [5]

The source of the energy that drives FO is the thermodynamic energy generated by the con­centration difference between the draw solu­tion and the feed solution. [1] Consequently, in­creasing the concentration of the draw solution increases the intrinsic potential energy of the system and the amount of water that can be retrieved from the spent dialysis fluid. A more concentrated draw solution will decrease the bulk of the fluid carried by the patient and increase the efficiency of the FO system.

In a system using FO for dialysis fluid rege­neration, sodium chloride must be the main solute in the draw solution. Other physiologi­cally important electrolytes such as calcium, potassium, magnesium and a bicarbonate ge­nerating base must also be added in physio­logically balanced proportions to obtain a re­usable regenerated solution on the draw solu­tion side of the FO membrane. Given that the amount of salt in the draw solution is equal to the amount of salt in the spent dialysis fluid, the water retrieval ratio will always be less than 50% of the water content of the spent dia­lysis fluid and the reclaimed fluid will always be hyperosmolar in comparison to the spent dialysis fluid. These obvious drawbacks must be corrected by a complimentary form of treat­ment to make FO a viable option for dialysis fluid regeneration. Adding free water from a refillable water reservoir attached to the sys­tem is a very feasible option. However, the water volume needed will be 50% of the vo­lume of reclaimed dialysis fluid. Adding nut­rients like glucose to the draw solution will increase the water fraction that can be re­trieved by approximately 50 %, while the final glucose concentration in the regenerated dia­lysis fluid will be approximately 5%. Conse­quently, in a ten hour/day dialysis prescription, the total energy gain from the dialysis fluid will be unacceptably high (2000 calories/day).

Because of the limitations and peculiarities of the proposed concept, modified dialysis the­rapy prescription is necessary when FO is used for dialysis fluid regeneration. In essence, a prolonged daily therapy will be acceptable to the patients because they remain ambulatory during the therapy. An extended time dialysis prescription may also result in a better removal of phosphate and protein-bound uremic toxins, if a more permeable hemodialysis membrane is used. Also, slow extended time ultrafiltra­tion may offer better volume control and car­diovascular stability during the dialysis treat­ment. It may be possible to combine a short daily conventional home hemodialysis therapy with a prolonged ambulatory dialysis therapy using FO for dialysis fluid regeneration. The proposed dual modality therapy may allow the use of two hemodialyzer membranes, each optimized for a particular prescription. Urea and small solutes are satisfactorily removed during a short treatment time if high dialysate flow rate and a dialyzer with high urea clea­rance are used. Phosphate, middle weight ure­mic molecules and protein-bound uremic subs­tances are better removed during a prolonged dialysis therapy using a highly permeable mem­brane even with a low dialysate flow rate. Small priming volume, low thrombogenicity, high permeability for the larger uremic toxins and better biocompatibility are the major pre­requisites of the membrane used for the pro­longed dialysis prescription in which the dia­lysis fluid is regenerated by FO.

To test the proposed concept, a multilayer FO device e.g. a 6-8 layer parallel plate and frame module should be specifically built for the purpose of dialysis fluid regeneration. The water retrieval rate of the proposed module and its rejection ratio of the large number of the well recognized uremic toxins should be determined in vitro. If satisfactory in vitro re­sults are obtained, animal studies should be conducted.

   References Top

1.Cath TY, Childress AE, Elimelech M. Forward os­mosis. Principles, applications, and recent deve­lopments. J Membrane Sci 2006;281(1-2): 70-87.  Back to cited text no. 1      
2.Cath TY, Gormly S, Beaudry EG, Flynn MT, Adams VD, Childress AE. Membrane contactor processes for waste water reclamation in space: Part I: Direct osmosis concentration as pretreatment for reverse osmosis. J Membrane Sci 2005; 257(1-­2):85-98.  Back to cited text no. 2      
3.McCutcheon JR, McGinnis RL, Elimelech M. Desalination by ammonia-carbon dioxide for-ward osmosis: influence of draw and feed solution concentration on process performance. J Membrane Sci 2006;278(1-2):114-23.  Back to cited text no. 3      
4.Johnson WJ, Hagge WW, Wagoner RD, Dinapoli RP, Rosevear JW. Effects of urea loading in patients with far-advanced renal failure. Mayo Clin Proc 1972;47(1):21-9.  Back to cited text no. 4      
5.Kumar M, Grzelakowski M, Zilles J, Clark M, Meier W. Highly permeable polymeric membranes based on the incorporation of the functional water channel protein Aquaporin Z. Proc Natl Acad Sci USA 2007;104:20719-24.  Back to cited text no. 5  [PUBMED]  [FULLTEXT]  

Correspondence Address:
Khaled M Talaat
Internal Medicine Department, Zagazig University, Zagazig
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Source of Support: None, Conflict of Interest: None

PMID: 20587888

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