I found this article very interesting.  Massachusetts Institute of Technology (MIT) has developed a device to produce water with 99% purity.  The device uses micro-desalination and is the size of a postage stamp.  It is believed that 1,600 units can purify up to 15 litres of water every hour. 

They tested the device with water from Massachusetts Beach and it purified over 99% of it.  The water contained containments of plastics bottles, human blood, and protein.  This is pretty impressive.  The device works by electrostatically repelling salts and microbes away from the ion-selective membrane leaving just the water.  This process is called “ion concentration polarization.”