Design and experiment on RO brine desalination system by solar vacuum membrane distillation in cold region

Abstract: In order to solve the problems of zero discharge of reverse osmosis (RO) brine, the coupling matching of solar collector and membrane distillation, the cooling water reduction and phase change heat recovery in the steam condensation process of membrane distillation system, the RO brine desalination system by solar vacuum membrane distillation in cold area was designed and verified in the test. The system consists of RO concentrated tank, membrane distillation module, solar collector, condensing unit, circulation pump, vacuum pump, fresh water tank, and so on. The condensing devices are self-made stainless steel heat exchange coil placed in the RO freshwater pool and geothermal coil placed in the underground of the greenhouse. The components of the system are designed in detail, and the self-made condensing device is the focus in the design. The system is placed in a greenhouse in Xinrong Village, Helan County, Yinchuan City, Ningxia. The front end of the system is an RO system for greenhouse irrigation. Since RO concentrated water cannot be discharged, it is further diluted by vacuum membrane distillation. In order to reduce energy consumption, the RO concentrated water treated by membrane distillation is heated by a solar collector. The fresh water vapor produced by membrane distillation is cooled by the stainless steel heat exchange coil in the RO fresh water tank and the greenhouse geothermal coil, which solves the problem of cooling water reduction in the steam condensation process of the membrane distillation system, and realizes phase change heat recovery and heats the greenhouse. Through measuring the relevant parameters of each component of the system, the solar collector effect, the RO brine desalination effect of the vacuum membrane distillation, the condensation effect of the condensing device and the heating effect of the greenhouse crop were tested and analyzed. The tests show that under the test conditions, the selected collector area can basically meet the heat requirement of membrane distillation for the whole year, the length of the stainless steel heat-dissipating coil designed in the greenhouse fresh-water tank can meet the steam condensation requirement, and the geothermal coils laid in 30-40 cm depth below the surface can meet condensation requirements and greenhouse heating requirements. The amount of freshwater produced by membrane distillation increases with the increase of the inlet temperature of the RO brine on feeding side and the vacuum pressure on the permeate side. When the inlet temperature of the RO brine on feeding side is 80 °C and the vacuum pressure of permeate side is -0.080 MPa, the water production is 37.62 L/h. When the operating time of the vacuum membrane distillation system reaches 240 h, the membrane fouling increases and the water production decreases. When the hollow fiber membrane is cleaned, the water production after washing is increased by nearly 1.5 times compared with that before washing. It has been verified by experiments that the RO concentrated water desalination system by solar vacuum membrane distillation in cold area can basically meet the requirements for use. This study has important significance for reducing the energy consumption of membrane distillation system, realizing the zero discharge of RO brine, and reducing the concentration of brine in the environment.