How exactly, You can drink seawater
Summary: Desalination systems have a lot of work to do, removing about 98-99% of the salt in seawater to make it drinkable. There are two main systems for accomplishing this, one of which involves pushing the water through a rubber membrane that the salt can't fit through (reverse osmosis). The other of these is distillation, which involves boiling water and then letting it re-condense without the salt, similar to the way alcohol is purified.
Types:
Seawater contains on average about 130 grams of salt per gallon, but the maximum safe consumption level for humans is about
2 grams per gallon. There are a variety of means of attaining this safe consumption level.
This website also has schematics of systems
Reverse Osmosis
- Thought of in the 4th century BC by Aristotle, who imagined using successive filters to remove the salt from seawater. Since the 1950s, researchers have been developing membranes to filter out salt, which requires about 1/4 of the energy and 1/2 the cost of distilling water, depending on the location and specific designs. The process works by pumping feedwater through permeable membranes at high pressure to separate salts from the water.
- Even with current membranes, 14kWh of energy are required to produce 1,000 gallons of desalinated water. If half of the current water used in the US were to come from reverse osmosis desalination, it would require approximately 100 GW of additional energy.
- Solar reverse osmosis is being considered, which is similar to other reverse osmosis procedures, but that it is driven by solar power.
Distillation:
The heating and evaporating of water to separate dissolved materials. The difference between most of these systems is how the feedwater is evaporated, and systems may be hybrids of those listed below.
Insert basic distillation description - Thermal Desalination: The first actual practice of desalination, which involved boiling water on ships as early as 200 AD. The energy required for this process makes it prohibitively expensive on a large scale in most locations, but has occurred in the Middle East.
- Solar Thermal Desalination: Evaporation ponds used to evaporate salts out of seawater represents one of the largest commercial applications for solar energy in use today.
- Solar Humidification-Dehumidification Process (HDH): Mimics the natural water cycle on a shorter time frame by evaporating and condensing water to separate it from other substances. Solar thermal energy is used to produce water vapor, which is then later condensed in a separate chamber. The waste heat from the condenser is then used to preheat incoming water. This is currently an effective system from small- to mid-scale desalination systems
- Multistage Flash (MSF): In this system, the feedwater is heated and pressure lowered so the water flashes into steam. The pressure is then lowered at different stages to remove different materials from the water.
- Multiple Effect Distillation (MED): The feedwater in this system passes through a number of evaporators in series. Vapor from one series is used to evaporate the water in the next series.
- Vapor Compression (VC): This process involves evaporating the feedwater, compressing the vapor, and then using the heated compressed vapor to heat incoming feedwater.
Comparison of Distillation and Reverse Osmosis Technologies:
- Advantages for Distillation:
- Greater potential for economies of scale
- Do not shut down for maintenance as often
- Less pretreatment because of the need of RO plants to remove large particles
- Do not generate waste from backwash of pretreatment filters
Advantages for Reverse Osmosis: - Do not generally require heating, eliminating issue with thermal impacts of the discharged water
- Fewer corrosion problems
- Lower energy requirements
- Tend to have higher recovery rates, approximately 45% in general
- Can remove unwanted contaminants
- Take up less surface area for the same amount of water production
Pretreatment:
Pretreatment is needed to remove substances that would interfere with the desalting process. A biocide (usually less than 1 mg/L of chlorine) is required in both types of systems to prevent algae and bacteria growth. Some RO systems then need to be dechlorinated because the membranes can't always tolerate chlorine. Ozone or ultraviolet light may be used to remove marine organisms, which may then require more pretreatment. In RO plants, suspended solids and other particles need to be removed to reduce damage to the membranes.
Wastes:
Desalination plants produce liquid wastes that contain some or all of the following: high salt concentrations, chemicals used during pretreatment, and toxic metals. These are then discharged into the ocean, sometimes after being washed by being mixed with other inlet water sources, or they may be dried out and disposed of in a landfill.