실증 파일럿 플랜트를 이용한 역삼투-압력지연삼투 공정의 최적화

Abstract

Reverse osmosis (RO) is the most common process for seawater desalination. A common problem in the RO and thermal processes is the high energy requirements for seawater desalination. Many researches have been conducted to use renewable energy sources for power supply to the seawater reverse osmosis (SWRO) plant in order to reduce the cost of desalination process. One method to generate osmotic power is through pressure retarded osmosis (PRO) process. Osmotic power is to produce electric power by using the chemical potential of two flows with the difference of salinity. PRO process produces electric energy by working the turbine or hydraulic pressure by energy recovery device (ERD). In the PRO process, water permeates through a semipermeable membrane from a low concentration feed solution to a high concentration draw solution due to osmotic pressure. Experimental studies have demonstrated the technique potential for power generation using reverse osmosis membrane of high water permeability and salt rejection rates. Recently, the majority of PRO researches are focused on mixing of seawater and river water, from which up to 2.6 TW osmotic energy is projected to be produced globally. However, the seawater-river water PRO system has a low energy density due to its low osmotic pressure difference. In addition, Flat-sheet and hollow fiber PRO membranes have been used for the PRO process, but most of those have limitation structures, of the membranes, such as a limited hydraulic pressure applied and membrane deformation during operation. Also, despite many studies were carried out to investigate the PRO system, there have been only few studies which have evaluate the performance of the spiral wound membrane module and large scale PRO plant using SWRO brine for a draw solution and wastewater secondary effluent for a feed solution. In a PRO process, river water and wastewater are commonly used as low salinity feed solution, whereas seawater and brine from the SWRO plant are employed as draw solution. During PRO process using wastewater effluent as feed solution, PRO membrane fouling is usually caused by the convective or diffusive transport of suspended or colloidal matter or by biological growth. So pretreatment process of PRO is the most critical step of PRO membrane in order to prevent membrane fouling. However PRO membrane fouling have little been studied. The main objectives of this study is to assess the power production from PRO performance test using flat-sheet membrane and 8 in spiral wound membrane modules and the reduce energy consumption from a 240 m3/d scale SWRO-PRO hybrid system. In addition, the economic efficiency of SWRO-PRO hybrid system was also evaluated. Other objectives of this study is to assess the PRO water flux change from pretreatment to remove organic matter by coagulation-UF membrane process and PAC adsorption-UF membrane process. The experimental results obtained from flat-sheet membrane test showed that increasing the draw solution salt concentration, flowrate, and temperature resulted in increasing the PRO water flux and power density due to high effective osmotic pressure. Comparison of flat-sheet membrane and 8 in spiral wound module for the PRO performance showed that the power density of flat-sheet membrane was approximately 60% higher than 8 in spiral wound module. Also, comparison of single-stage PRO and two-stage PRO for the PRO performance of module showed that the power density of the single-stage PRO was 28∼30% higher than two-stage PRO, and water recovery of the two-stage PRO was 70∼80% higher than single-stage PRO. In addition, comparison of CDCF flow configuration and CDDF flow configuration for the PRO performance showed that the water recovery of CDDF flow configuration was approximately 5% higher than CDCF flow configuration. Comparison of co-current flow mode and counter-current flow mode for the performance showed that the energy production of counter-current flow mode was higher than co-current flow mode under the experimental conditions of 5.0 L/min, and 20 bar. Results of PRO membrane backwashing efficiency test in flowrate 10.0 L/min, operation pressure 20 bar, and backwashing time 10 min condition showed that rate of the water flux recovery was approximately 100%. Based on the results of PRO performance test, continuous operation SWRO-PRO hybrid pilot plant resulted in the energy reduction of approximately 10∼30% higher than SWRO process.

In addition, the results of pretreatment tests for PRO were as follows: ∘ The optimum ferric chloride dosage for removal of organic matter applied for the coagulation process was 50 mg/L. ∘ The most effective pH condition for the removal of organic matter was found to be pH 5.5. ∘ The optimum PAC dosage for removal of organic matter applied for the PAC adsorption process was 200 mg/L. ∘ Both PAC-UF pretreatment process (200 mg/L as PAC) and coagulation-UF pretreatment process (50 mg/L as FeCl3) were higher removal efficiency of organic matter, as also resulting in the substantial improvement of water flux of PRO membrane.