Seawater Pretreatment Process
Desalination or treatment of seawater is a process in which fresh water that is suitable for consumption is extracted from the saline water (Pearce, 2007). Many methods are used in the desalination process, but the most applied and efficient one are reverse osmosis. The sea water that is fed to the reverse osmosis plants, many times has large particles which end up spoiling the membranes of the plants due to the high level of large particles in the seawater (Van Hoof, Hashim, & Kordes,1999). Therefore, to reduce operation cost and treatment time, there is a need for a pretreatment process. The most dominant process of seawater pretreatment process is the use of ultrafiltration (Bonnelye, Sanz, Durand, Plasse, Gueguen, & Mazounie, 2004).
Ultrafiltration is the process of removing suspended solids, fine suspensions and colloidal materials from water. It is capable of even removing oil, bacteria and virus. This is a membrane filtration process, which involves the use of low hydrostatic pressure to force a liquid against a semipermeable membrane (APEC, 2016). Water is pumped through a membrane module, where it is filtered and the filtrate stored in a separate storage vessel, awaiting the next process of reverse osmosis to remove the salinity (Kennedy, Kamanyi, Rodríguez, Lee, Schippers, & Amy, 2008).
Figure 1: An Ultrafiltration Process Sketch
In the mechanism above, the water is pumped back and forth through the membrane module, which contains hollow tubes placed back to back (Fig. 2). These tubes can filter in both directions; that is when water is pumped into them in the filtrate mode and also when it is pumped out in the backwash (Fig. 3). The membrane is then able to retain some amount of solids; depending on how fine-tuned it is through the size exclusion mechanism (Post, Huiting, Cornelissen, & Hamelers, 2011). Filtrates, such as water and also other low molecular weight solvents can permeate through the membranes of the tubes (Shahabadi & Reyhani, 2014).
Figure 2: Hollow Fiber Membranes in the Membrane Figure 3: Flow of Water in and out of the Tubes
This process is similar to the reverse osmosis; the main difference is in the size of the pores contained in the filtering membranes (Wilf & Klinko, 1998). The ultrafiltration process of sea water is a simple process; whose setting up is very cost effective. Therefore, it is necessary so as to save costs and reduce the complexity of the reverse osmosis plant and also increases the lifespan of the treatment plants (Chua, Hawlader, & Malek, 2003). It is also important to note that in the ultrafiltration, no chemicals are used and that the quality of water received as output is good since the membranes can even remove turbidity (Schaep, Van der Bruggen, Uytterhoeven, Croux, Vandecasteele, Wilms, & Vanlerberghe, 1998).
References
APEC. (2016, March 31). Water Quality. Retrieved from Free Drinking Water: http://www.freedrinkingwater.com/water_quality/quality1/43-08-ultrafiltration-membrane-filter-tech.htm
Bonnelye, V., Sanz, M. A., Durand, J. P., Plasse, L., Gueguen, F., & Mazounie, P. (2004). Reverse osmosis on open intake seawater: pre-treatment strategy. Desalination, 167, 191-200.
Chua, K. T., Hawlader, M. N. A., & Malek, A. (2003). Pretreatment of seawater: results of pilot trials in Singapore. Desalination, 159(3), 225-243.
Kennedy, M. D., Kamanyi, J., Rodríguez, S. G., Lee, N. H., Schippers, J. C., & Amy, G. (2008). Water treatment by microfiltration and ultrafiltration. Advanced Membrane Technology and Applications, 131-170.
Pearce, G. K. (2007). The case for UF/MF pretreatment to RO in seawater applications. Desalination, 203(1), 286-295.
Post, J. W., Huiting, H., Cornelissen, E. R., & Hamelers, H. V. M. (2011). Pre-desalination with electro-membranes for SWRO. Desalination and Water Treatment, 31(1-3), 296-304.
Schaep, J., Van der Bruggen, B., Uytterhoeven, S., Croux, R., Vandecasteele, C., Wilms, D., & Vanlerberghe, F. (1998). Removal of hardness from groundwater by nanofiltration. Desalination, 119(1), 295-301.
Shahabadi, S. M. S., & Reyhani, A. (2014). Optimization of operating conditions in ultrafiltration process for produced water treatment via the full factorial design methodology. Separation and Purification Technology, 132, 50-61.
Van Hoof, S. C. J. M., Hashim, A., & Kordes, A. J. (1999). The effect of ultrafiltration as pretreatment to reverse osmosis in wastewater reuse and seawater desalination applications. Desalination, 124(1), 231-242.
Wilf, M., & Klinko, K. (1998). Effective new pretreatment for seawater reverse osmosis systems. Desalination, 117(1), 323-331.
