تعیین فرآیند بهینه ZLD برای استفاده دوباره از پساب سیستم‌های اسمز معکوس (مطالعه موردی:سیستم اسمز معکوس شهرستان لار)

Iran is one of the arid and semi-arid countries which face with many problems in water supply. Membrane process such as reverse osmosis (RO) is one of the advanced methods of water treatment which are widely used in arid and semi-arid regions. Reverse osmosis system produces two types of waters, one purified water and the other concentrated wastewater. To reuse the concentrated wastewater in reverse osmosis, it is necessary to remove some critical elements that cause problems for the membrane. In this research ZLD processes were used for removing the critical elements of reverse osmosis wastewater of Lar city in Fars Province, Iran. The purpose of this study was to evaluate ZLD processes and determine the optimal process for recycling of reverse osmosis wastewater.
In this research different scenarios were selected for reverse osmosis wastewater treatment including the processes of absorption, chemical precipitation and combination of these processes. Fluidized bed crystallization (calcium carbonate particles) was used for absorption process and sodium hydroxide, lime, sodium aluminate and aluminum sulfate were used for chemical precipitation processes. These chemicals were combined with wastewater in concentrations of 100, 200, 300, 400, 500 and 600 mg/L and some parameters including calcium, magnesium, chloride, sodium, potassium, pH, salinity, silica and turbidity were measured and optimum chemical concentration was determined. Optimal concentration was chosen based on maximum removal efficiency of harmful elements for reverse osmosis membrane.
Optimum removal efficiency of sodium hydroxide, lime, sodium aluminate and aluminum sulfate, were in concentration of 500, 400, 200 and 100 mg.L^-1, respectively. To improve removal efficiency, the chemicals were combined together based on the optimal concentration of each chemical which was determined in the previous step. Therefore, aluminum sulfate with sodium hydroxide, aluminum sulfate with lime, sodium aluminate with sodium hydroxide and sodium aluminate with lime was mixed together. At this step the best removal efficiency was obtained in combination of optimal concentration of sodium aluminate and sodium hydroxide. Absorption process in fluidized bed crystallization was investigated for difference fluxes and the results showed that the optimum flux was 1.94 m³.h^{- 1}.m^-2. The maximum removal efficiency of harmful elements for reverse osmosis membrane was observed in the combination processes of chemical deposition and absorption which occurred in fluidized bed crystallization with flux of 1.94 m³.h^-1.m^-2 with adding optimized values of sodium aluminate (200 mg.L^-1) with hydroxide sodium (500 mg.L^-1).
In combination of chemical precipitation with solute adsorption process using only one chemical, the fluidized bed crystallization process with the addition of sodium hydroxide (500 mg.L^-1) can be used as a desirable option because the removal efficiency of calcium and silica were 82.2 and 97.2%, respectively. Of course, magnesium removal was 31.1%, which is not appropriate. So this option is only suitable for removing calcium and silica. The removal of magnesium was 31.1% and not suitable, respectively. So this option is only suitable for removing calcium and silica. In the process of chemical precipitation in the form of combination of chemicals optimum concentrations, the combination of optimum concentrations of sodium hydroxide (500 mg.L^-1) and sodium aluminate (200 mg.L^-1) was suitable process for silica removal because calcium, magnesium and silica reduced 68.8, 54.2 and 93.3%, respectively. In the combination of Sodium hydroxide (500 mg.L^-1) and aluminum sulfate (100 mg.L^-1), the removal efficiency for calcium and magnesium was 88.2% and 100%, respectively. Therefore, to remove silica, combination of sodium hydroxide and sodium aluminate and to remove calcium and magnesium, combination of sodium hydroxide and aluminum sulfate were appropriate. But finally, by combining the process of solute adsorption and chemical precipitation, the mode of combining the optimum concentrations of sodium hydroxide (500 mg.L^-1) and sodium aluminate (200 mg.L^-1) and adding them to the fluidized bed crystallization, the highest removal efficiency for calcium, magnesium and silica (95.8, 97.9 and 94.4%, respectively) and maximum flux (1.94 m³.h^-1.m^-2) were obtained. Therefore, this is optimum ZLD process.