Plant Layout and Components

The freshwater branch similarly includes a settling basin and a filtration system. The pump before the filters just provides the head necessary to overcome the pressure drops along the freshwater circuit. 

Provided that the pressure difference between the two sides of the membrane is set to a value lower than the difference in their osmotic pressure, a water flux establishes across the membrane from the low- to the high-pressure side (i.e. from the low to the high salt concentration side). Therefore, the water flow rate on the high-pressure side of the membrane increases. The seawater stream diluted by pure water in the membrane module (brackish stream B1) is then sent to a hydrauhc turbine connected to the same shaft of the main pump. Since the turbine head equals the pump head (apart from the pressure losses along the circuit and the small head provided by the EPl pump) and the turbine flow rate is higher than the pump flow rate because of the water permeated across the membrane, a net mechanical power output is available at the shaft. A generator connected to the shaft finally converts mechanical to electric power. 

The freshwater flow rate at the membrane module inlet is kept significantly higher than the flow permeated across the membrane so as to hmit the increase of salinity concentration (resulting from water permeation) that would consequently reduce the osmotic pressure difference on the two membrane sides. The excess flow rate released from the membrane module (stream F2) is eventually mixed to the brine stream B2 at the turbine outlet and then they are released into a tailrace located downstream the freshwater intake. 

Since the pressure exchanger operates on streams having the same volume, just a fraction of the high-pressure brine stream exiting the membrane module is fed to the device while the remainder (equivalent to the flow rate permeated across the osmosis membrane) is sent to a supplementary hydraulic turbine, whose power output practically represents the plant net effect. 

Mechanical filtration is first performed in dual media components featuring a first layer of anthracite to capture the final oil traces, followed by a bed of quartz sand of proper granulometry to prevent silica entrainment by water. Dual media filters can retain particles larger than few microns and are followed by cartridge filters. They represent the last filtration stage before the membrane and are suited to remove fine particles having size larger than about 1 pm. 

---