Question

Describe in detail the chemicals used in the pre-treatment, post-increment and membrane cleaning (CIP) stages used in desalination plants where ro membranes are used, and their intended use.

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Answer 1

Desalination plants are those which works on the principle of reverse osmosis. Water is being pushed at higher pressure in the membrane and sufficient amount of energy is required for the process.

Pre-treatment is a process in which suspended solids and colloidal particles are being removed by the disinfection method and membranes that can withstand bacteria degradation. It helps in reducing the foul smell coming from the plant. The chemicals being used in the process are ferric chloride and some organic polyelectrolytes. Some additional inorganic coagulants like aluminum sulphate and aluminum chloride are also used and all these chemicals helps in coagulation and flocculation and reduce suspended solid quantity.

After pre-treatment the process that is to be looked in desalination plants is post treatment of the waste water. The water from the MSF and RO desalination process would require post treatment so that the water is adequate for drinking purpose. What we have to do is to reduce the pH, alkalinity and hardness of the water and the chemical used here are hydrated lime and sodium hypochlorite. Usually there is also some amount of boron present which is to be removed strictly as it may form some toxic substances in water.

As the desalination plants follow the reverse osmosis process, the membrane used here are RO membranes and their cleaning is also important to provide quality water. The process of autopsying and cleaning membrane is used before the initiating of plant work. High strength iron cleaners are being used across to slow down the flow of permeate across the period and the chemicas used here are hydroacetic acid and phosphoric acid as the membrane is of iron and these are high strength iron cleaners and sodium hydroxide can also be used which is dilute in nature.