Power ultrasonic freezing for membrane concentrate treatment

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Application of membrane technologies has increased dramatically in recent years due to increasing demand for drinking water and decreasing unpolluted source water. Membrane filtration, especially, reverse osmosis (RO) and nanofiltration (NF) are the leading technologies for treatment of nontraditional source water such as seawater, brackish groundwater and reclaimed wastewater for potable and nonpotable uses. However, membrane filtration operations generate large volumes of wastewater/liquid waste, including reject-brine, backwash water and membrane cleaning solutions. The product water recovery, for instance, is only 30-40% for some RO/NF membrane based desalination plants. With continued installation of membrane plants, disposal of the concentrate is becoming a major factor affecting the viability of many membrane projects. Currently, most concentrate generated from the membrane filtration operations is discharged into surface water or sanitary sewers without any treatment (except some type of pH adjustment for cleaning wastes). Salts, metals and organic contaminants contained in the feed water become concentrated during membrane filtration processes, along with chemicals and reaction byproducts from pretreatment. Discharging the membrane concentrate of desalination plants into seawater has been considered a safe option but recent studies have shown that the dilution of the concentrate by seawater may be lower than expected. Continue to discharge large volumes of untreated membrane concentrate may result in serious adverse impacts on the aquatic environment. Treatment of membrane concentrate is becoming necessary and volume minimization, even zero-liquid discharge may be soon required for some operations. However, little effort has been made so far to develop practical treatment technologies for environmentally friendly management of membrane concentrate. This research proposal presents an innovative method for membrane treatment and volume reduction. Freeze concentration is a physical process that involves the fractional crystallization of water and subsequent removal of ice. When freeze concentration is used to purify water or liquid waste, impurities are separated from ice crystals and concentrated in the unfrozen liquid. The advantages of freeze concentration treatment include no addition of chemicals, high treatment efficiency, high capacity of waste volume reduction, low corrosion of the treatment facility, and recovery of a pure water stream from the original liquid waste. Power ultrasonic freezing is a promising technology that may significantly improve efficiency of freezing processes. The primary objective of the proposed study is to develop a power ultrasonic freeze concentration (PUFC) process/unit that can be used in new or the existing OR/NF desalination or water/wastewater treatment plants for membrane concentrate treatment. The significance of the proposed research lies in its potential not only to provide an effective technology for membrane concentrate treatment but also to improve the product water recovery rate for the RO/NF membrane filtration plants by generating a pure water stream from the ice formed from membrane concentrate. This research will also advance our knowledge of the effect of ultrasound waves on chemical, physical and/or biological characteristics of freezing materials, ice nucleation and impurity incorporation/rejection in water-ice (solid) interface. The membrane based seawater desalination industry, in-land water and wastewater treatment plants, as well as all freezing related industrial processes such as refrigeration, pharmaceutical, chemical and food processing will greatly benefit from this research.