Desalination

Last chapter, we explore the two important source water’s features and which one will satisfy more water demands. In our study, seawater has the great potential, yet, the high salinity property make it unfavorable to factories. We hope our project will be a part of power which pushes this water more to our daily life.

Desalination method has been used for hundreds of years to produce fresh water, but large-scale municipal drinking water distillation plants began to operate during the 1950s (Gleick, 2006). Over years, this industry become more and more important due to the increasing water demand of human activities. Until 2013, global desalination water production capacity has increased to 85.9 mililion m3/d. (Shumo, 2015)

Various desalination method

As table 7.3.1 shows, the methods varies. For any ones concerns, we gives explanation to every methods listed in this table.

We will look close up to one of the method, RO method and its mechanism in next section.

Definition of Predominant desalination methods (NEP, 2008): Electrodialysis (ED)—A process of desalination whereby an electrical current is used to separate out salt and impurities in the intake water through the use of semipermeable, ion-selective membranes operating in a DC electric field. The major method in the Middle East is thermal desalination because of it rich oil deposits.

Ion exchange - A chemical process involving the reversible exchange of ions between a liquid and a solid.

Multiple effect distillation (MED) - A thin film evaporation process where the vapor formed in a chamber, or effect, condenses in the next, providing a heat source for further evaporation.

Multi-stage flash distillation (MSF) - A desalination process where a stream of brine flows through the bottom of chambers, or stages, each operating at a successively lower pressure, and a proportion of it flashes into steam and is then condensed.

Reverse osmosis (RO) - A method of separating water from dissolved salts by passing feedwater through a semipermeable membrane at a pressure greater than the osmotic pressure caused by the dissolved salts. RO operates via a solution/diffusion mechanism whereby the water dissolves into and diffuses through the nonporous membrane, leaving the majority of the salts behind in the concentrate. RO membranes are also capable of removing some larger organic contaminants. Small uncharged species can pass through the membrane. Water that passes through the membrane leaves the unit as permeate or product water; most of the dissolved impurities remain behind and are discharged in a brine or waste stream.

Vapor compression evaporation (VC/VCE)—Thermal desalination processes that utilize heat from compression of water vapor for subsequent evaporation of feedwater, either with a mechanical compressor (mechanical vapor compression, MVC) or a steam ejector (thermal vapor compression, TVC). Vapor compression processes are particularly useful for small to medium installation.

Which desalination method will be the key to this industry? In 2008, membrane and distillation processes equally share current desalination production capacity worldwide (Greenlee, 2009). We can’t find the current information, yet, RO technology can produce fresh water (from seawater) at one-half to one-third of the cost of distillation in 2008 already (Miller, 2003).

The Middle East holds approximately 50%of the world’s production capacity (and 2.9% of the world’s population) and has forged ahead as the leader in large-scale seawater desalination (Greenlee, 2009). In 2005, Israel opened the world’s largest seawater RO desalination plant, with a production capacity of 330,000m3/day (Sauvet-Goichon, 2007).

These might not prove that RO method will be the most popular method, but it will certain prove that it will be one of the most popular methods. With this in mind, we thought it may be great to visit a RO membrane research center.

Visit to Hangzhou Water Treatment Technology Development Center

On Aug. 8, 2015, we visit the Hangzhou Water Treatment Technology Development Center. It is the supporting unit of National Engineering and Technological Research Center for Liquid Separation Membrane and a base for the national membrane sector. Their vice director, Hongqiang Li, and several people from the research team received us.

During our visit, the center gave us a detailed presentation of RO method including its current situation, mechanism, whole process, technological and other difficulties. When we explained our project, they showed great interest on this new approaching way. The question they asked most is how efficient this method can be. Unfortunately, that time we were half-way of transformation, so we can’t answer very clearly. Yet, they still inspired us on combing two methods instead of replacing one.

After this visit, we follow the big picture that center gave us and filled in with details below.

The mechanism of reserves osmosis membrane