Water Quality of the Product, Desalinated Water
Desalination used for municipal water does not remove all dissolved solids, and thus, one of the issues to address in desalination is the water quality of the desalted water--the product water . The fraction and character of the constituents left in the product water depends on the process, the feed water quality, and recovery rate of the desalination process.
Reverse Osmosis membranes separate dissolved substances from water based on size and charge of the ion or molecule. The membranes used for desalination do not have true pores; ions diffuse through the polymer matrix at a much slower rate than water does. Charged ions are caught up in the surface charge of the polymer and diffuse very slowly – to the extent that only a fraction of a percentage of ions is able to pass through the membrane. Uncharged molecules are able to pass through at approximately the same rate as water molecules. If they are larger than the open spaces in the membrane, they might be retained with the concentrate, but long narrow molecules can still manage to slip through.
Electrodialysis moves charged ions through the membrane rather than water. Therefore, only charged ions are removed from the product stream. All uncharged molecules, particles, and microbes need to be removed through another processes.
Thermal processes such as distillation, evaporate water molecules which are then condensed to form “pure” water. No dissolved minerals are transported with the vapor, but volatile organic molecules are vaporized with the water. Distillate must be cleansed of organic contaminants through a separate process.
Examples of potential contaminants of desalted water:
Arsenic in groundwater is in the form of arsenate (As5+) and arsenite (As3+). In a reducing environment with alkalinity (often found in groundwater) the As3+ form is more soluble and uncharged in the form of Arsenious acid (H2AsO3). Raising the pH above 8.5 will drive the reaction back toward charged molecular species and improve rejection in RO systems. The Safe Drinking Water Maximum Contaminant level limit for arsenic in is 0.010 mg/L.
Boron concentrations in seawater range from 4 – 5 milligrams per liter in the forms B(OH)3 or B(OH)4-. The uncharged form is not highly rejected by reverse osmosis membranes. Boron is not regulated in drinking water by the EPA, but California Department of Public Health has a notification level of 1 mg/L and the World Health Organization guideline is for 0.5 mg/L. Boron can be removed to a lower concentration by treating seawater RO permeate with a second RO pass or by using ion exchange on a portion of the permeate.
Bromine concentration in seawater is around 65 mg/L in the form of Bromide, (Br-). Bromine is just below Chlorine on the periodic table, meaning that it has a similar chemical nature. A typical seawater RO membrane should reject most of the bromide for a final concentration below 1 mg/L. However, when seawater is chlorinated before the RO system for disinfection purposes, the bromide reacts with organic molecules in the seawater and the chlorine to form brominated trihalomethanes. Some of these are uncharged, small organic molecules that can pass through the membrane easily. The EPA has not yet set a Maximum Contaminant Limit Goal for Brominated Trihalomethanes, but they have issued a Drinking Water Health Advisory and are gathering data on health effects.
Organic contaminants can be transported through a membrane process in a manner similar to the above examples of inorganic molecules. If they are uncharged and small enough, or oriented in just the right manner, they will be easily carried through the membrane with water.