Abstract

As the population increases, the demand for electricity has also increased. Most of the required electricity is produced by burning fossil fuels at power stations all over the world which has dangerous influences on climate. Therefore, it is important to produce new renewable and maintainable energy sources to decrease the use of fossil fuels. Osmotic power is a renewable source of energy that is generated by using salt and fresh waters. Pressure retarded osmosis (PRO) is a membrane based technology that can produce osmotic power. One of the main challenges for generating osmotic power by using PRO technology is the membrane fouling which reduces the permeate flux and consequently increases the cost and decreases the osmotic power generation. In this research, various pretreatment methods such as different types of ultrafiltration systems and a sand filter were used for removing total organic carbon (TOC), turbidity, and hardness from raw water. Moreover, different feed waters with various qualities were used to investigate the effect of fouling on a commercial membrane in PRO mode in continuous conditions. In addition, two physical and chemical cleaning methods were used to investigate and determine the desirable cleaning methods in PRO processes in order to control and reduce the membrane fouling. Most of the tests were performed at LTE Hydro Québec located at Shawinigan and the rest were performed at Concordia University. Based on the results, highest removal efficiency of turbidity occurred at 3.72 NTU and was 100% and 68.6% for ultrafiltration and the multimedia sand filter, respectively. Maximum TOC removal in ultrafiltration multimedia sand filter was 41% and 1.5% at 6.62 mg/L TOC initial concentration respectively. According to the results, the fouling rate when the draw solution was synthetic salt water followed the order of: untreated river water > multimedia sand filter > microfiltration > ultrafiltration effluents. It was observed that complete blocking, cake filtration and cake enhanced osmotic pressure were the main fouling mechanisms that governed the membrane fouling. The main inorganic foulants using synthetic salt water were iron, aluminum, calcium, sodium, and silica and when sea water was used as the draw solution, iron, aluminum, silica, magnesium, calcium, sodium and potassium were detected as inorganic foulants. The organic foulants were humic substances, polysaccharides and proteins from use of both synthetic salt water and sea water as draw solutions. The successful cleaning method was chemical cleaning using a combination of acidic and basic cleaners. This research indicates that using appropriate pretreatment and membrane cleaning methods can decrease the membrane fouling in PRO processes and consequently can improve the osmotic power generation.