DETERMINATION OF OPTIMUM ZLD PROCESS FOR REUSE OF REVERSE OSMOSIS EFFLENENT (CASE STUDY: REVERSE OSMOSIS OF LAR CITY)

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 areas. Reverse osmosis system produces two types of water, one purified water and the other concentrated wastewater. To reuse the concentrated wastewater in reverse osmosis, it is necessary to remove some of the 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 and sodium hydroxide, aluminum sulfate and lime, sodium aluminate and sodium hydroxide and sodium aluminate and lime were 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 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) and hydroxide sodium (500 mg.L-1).