Using evaporation to dewater Reverse Osmosis reject waste streams
Reverse Osmosis (RO) technology has been used for years in various industries to separate dissolved solids from water by forcing the water through a semi-permeable membrane. RO is also commonly used to purify drinking water and desalinate seawater to yield potable water. The water and other molecules with lower molecular weight (specific weight of molecules allowed to pass through is dependent on the selected membrane) pass through the micropores in the membrane, yielding a purified water stream called the permeate. Larger molecules are retained by the membrane as well as a portion of the water that does not pass through the membrane. This concentrated stream is called the concentrate or RO reject.
In industrial settings, RO is often used to remove total dissolved salts (TDS) from industrial wastewater or treated industrial wastewater and yield permeate with relatively low TDS concentrations.
While RO is an effective method for handling wastewater, what to do with the reject water (typically as much as 20-50% of the RO feed water volume) can be an issue. Specifically, the high salinity in RO reject can make it prohibitive for discharge to a local sewer facility. Hauling the wastewater to a treatment facility can also be prohibitively expensive.
ENCON Mechanical Vapor Compression (MVC) Evaporators and ENCON Thermal Evaporators have proven to be effective technologies for dewatering RO reject waste streams. In brief, evaporation is a time-tested methodology for reducing the water portion of water-based waste. The evaporator converts the water portion of water-based waste to water vapor, while leaving the higher boiling contaminants behind. This greatly minimizes the amount of waste that needs to be hauled off-site.
Evaporation technology has always been more “hands off” than other wastewater treatment methodologies resulting in a dramatically lower labor cost. Evaporation technology can handle a much wider range of waste streams compared to membranes and traditional physical / chemical treatment methodologies. Finally, evaporation does a much better job of concentrating waste streams compared to other methods, thereby yielding a lower disposal volume and cost.
At a typical operating cost $0.01 - $0.02 per gallon of distillate, the ENCON MVC Evaporator is also very energy efficient and yields a high quality distillate. More information on MVC Evaporators can be found here in our Mechanical Vapor Compression section. If lower volumes of RO reject need to be handled, the ENCON Thermal Evaporator heated by natural gas or other available heat sources can evaporate water to atmosphere at a typical cost of $0.10 per gallon of evaporated water, or almost zero is waste oil is available. More information on Thermal Evaporators can be found here in our thermal evaporator system section.
Typical RO reject TDS concentrations are often in the range of 30,000 – 50,000 mg/liter. The salts in solution can range from highly soluble sodium and potassium salts to less soluble or reverse soluble calcium and barium salts. Note: The term reverse soluble implies lower solubility as solution temperature increases. ENCON MVC Evaporators operate at typical temperatures of 225-230°F for high TDS applications like RO reject. Similar or slightly higher (250°F maximum) operating temperatures can be expected with high TDS applications for ENCON thermal evaporators.
Evaporator concentration endpoints tend to correspond closely with saturation concentrations of the various salts at the referenced temperatures. Many sodium and potassium salts such as sodium sulfate, sodium chloride, potassium chloride, and potassium nitrate, have saturation points in the range of 400,000 – 600,000 mg/liter at the referenced elevated temperatures. Based on simple arithmetic, concentrating the RO reject wastewater from 30,000 mg/liter to 400,000 mg/liter corresponds to a volume reduction percentage of approximately 92.5%. Concentrating from 50,000 mg/liter to 600,000 mg/liter would yield a volume reduction percentage of approximately 91.6%.
Less soluble salts such as barium sulfate and magnesium carbonate present more of an issue due to the tendency to precipitate out and scale heat transfer surfaces. To minimize this issue, a type of anti-scale chemistry referred to as Threshold Scaling Inhibitors (TSI) are being used. TSI chemistry is added upstream of the evaporator and modifies the insoluble salt crystals creating a soft slurry that has less of a tendency to adhere to surfaces and cause scale. The resulting soft slurry also has a consistency that minimized the likelihood of erosion on heat exchanger or plumbing surfaces.
When there is an interest or requirement to achieve ZLD, evaporation can be used to concentrate the RO reject, which can then by further dewatered with supplemental equipment. Please refer to the diagram below for a typical layout diagram for a ZLD solution that shows RO reject (or other high TDS wastewater) being fed to the ENCON MVC Evaporator followed by dewatering of the MVC concentrate by a ENCON Thermal Evaporator and a plate and frame filter press.
In the first stage of handling RO reject, an MVC Evaporator is fed the RO reject and it yields distilled water and concentrate. The concentrate goes to a concentrate storage tank, which separates into slurry and supernatant layers. The supernatant layer gets pumped to an ENCON Thermal Evaporator, which further concentrates the liquid while exhausting water vapor to atmosphere. The concentrated slurry from the thermal evaporator is pumped back into the concentrate storage tank. The slurry layer from the tank is pumped into a thickener tank that is then pumped into a filter press. The filter press solids are dumped into a sludge disposal trough with filtrate going back to the concentrate tank. Please refer to the flow chart diagram below.