Where is the water?
As show in the chart (right), the overwhelming majority of water on Earth is seawater. Seawater is characterized by an abundance of salt, which on average makes up 36,000 parts per million (ppm). Seawater is much too salty for human consumption and crops and livestock would not fare much better.
So, where does the water we need come from? Most of it comes from the 1% of fresh water that is found in groundwater or surface water (rivers & lakes).
Groundwater:
Groundwater is the most reliable and consistent source of water and usually requires the least amount of purification before use. In most cases, the quality water from a particular well remains stable over time which allows water treatment specialist to apply the right purification technology with predictable and consistent results. Designing a groundwater treatment system is usually a simple matter of performing a water analysis and applying the correct technologies based on the results.
Surface Water:
Surface water poses several challenges to the application of technology. The greatest challenge is posed by the variability of contaminants that find their way into the lakes and rivers. In addition to being homes to a dizzying array of plant and animal life, surface water is the repository for rain and storm water runoff which carries with it literally everything under the sun. Along with biological and particulate contamination, therefore, surface waters may be loaded with everything from heavy metals to pharmaceuticals. Determining the proper technology for treating surface water streams requires careful analysis, planning and usually a pilot system to prove out the system approach. The pilot system is typically a scaled-down version of the proposed solution with additional controls and monitoring equipment so that the design may be properly evaluated over time and the full range of expected conditions. Once the pilot system has been run for a sufficient period of time, the data gathered is used to modify the design of the operating system to meet water quality and quantity requirements without surprises.
Seawater:
Seawater, or salt water, is nearly as simple to design for as groundwater. It is consistently salty and contaminants are fairly predictable for a given location. Seawater does, however, pose challenges associated with river and lake water in that it is home to myriad life forms from the plant and animal kingdoms. Seawater intakes often get the most attention during the design of a desalination system because the new flow of water into a water treatment system creates an attractive new biome for filter feeders, such as mollusks and gastropods and all of the creatures that feed upon them or their waste products. Pilot systems is large land-based seawater systems are desirable as a means of demonstrating all operational conditions as well as environmental impacts. Ocean-based systems are simpler to design because open ocean system intakes are located high above sea beds and they are typically moving, so that sea life has a hard time setting up shop. Many smaller land-based desalination systems, both residential and commercial, rely on so-called "beach wells" which use the surrounding sand as a giant filter. This greatly reduces the complexity of system design and operation.
Municipal Water:
Not a naturally occurring resource, municipal water is what comes out of your sink's faucet and your showerhead. It may be produced from any of the above sources, but it is usually obtained from ground and surface water. As the population centers have moved to the coasts over the last 30 years, increased demand for water in coastal areas has necessitated the design and implementation of large municipal desalination systems.
Regardless of the original source, municipal water is safe for consumption and most other uses. Regulatory agencies in the United States and most other developed nations, ensure that water quality is routinely monitored and controlled. In industry, however, many processes require water that is purified to a degree that is not available from the tap. In these cases, water purification systems are designed to meet specific flow and quality requirements.