RO Pretreatment Hydrocarbon Engineering - Optimisations of Osmosis

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Courtesy of Kurita

Desalination by reverse osmosis (RO) is a fast growing technology, which is gaining more and more market share in the field of desalination applications. RO plants are able to produce extremely pure water with minimal impurities, even if they are charged with highly contaminated feed water. Based on the fact that the RO membranes themselves are able to separate the smallest ions out of the raw water using sophisticated pores, their performance is heavily influenced by the amount of substances inside the feed stream. Due to that fact, a thorough raw water pretreatment is indispensable for persistent, satisfying membrane productivity.

The situation
One of the first and most important pretreatment steps for the raw water, besides a biocide dosing, is the addition of inorganic coagulants such as ferric chloride or polyaluminum chloride (PAC). By applying one of these additives in the system, mainly charged impurities are forced to coagulate and create flocks due to the contrary charges between the water ingredients and flocculants. This process improves the efficiency of downstream pretreatment systems such as clarifiers, sand and cartridge filters. Nevertheless, almost all RO unit operators are confronted with a certain level and a certain kind of fouling on their membranes, due to an incomplete coagulation of troublesome components upstream. What kind of harasser is creating this issue? In most cases uncharged biopolymers (e.g. polysaccharide) are the root cause. Their missing charge prevents a sufficient removal by charged coagulants. Once they have deposited on the membranes, these polymers negatively affect the membrane filtration due to their higher diffusion resistance. Negative effects to the membranes include increased differential pressure across RO membranes, as well as decreased flux and salt rejection rates.

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