The purest water in the world is not, as you might think, rain but the ultrapure water used in the manufacture of silicon semiconductor devices.
The reason is quite simple: the spacing of circuit components on an integrated circuit chip is measured in nanometres so any impurity in the water used for rinsing them during fabrication could bridge between them resulting in a failure of the chip. Impurity concentrations in ultrapure semiconductor rinse water are measured in nanogrammes per litre (ng/L) or parts per trillion – that’s equivalent to an average sized needle in a large haystack.
When one of Europe’s largest silicon wafer manufacturers needed a new water treatment plant, one of their main concerns was ensuring that the water was totally sterile and part of this process is regular sanitisation of the recirculating distribution ring main system.
Water purity Due to the need to maintain water purity, traditional sanitising chemicals cannot be used. A high dose (20mg/l) of ozone effects sanitisation within a few hours, reducing colony forming unit count (CFU), and Total Organic Compounds (TOC). Because it ultimately converts into oxygen and water, ozone does not introduce any breakdown products into the water as other sanitising agents do.
It also means that the ultrapure water can be used more than once, providing both economic and environmental advantages. However, to restore ultrapure water quality after sanitisation, the residual ozone must be removed.
The manufacturer contacted leading Scandinavian manufacturer Ozonetech to provide the sanitisation system. Ozonetech is a Swedish multi-awarded cleantech company that develops and markets premium systems and solutions, based on proprietary ozone generation technology, to purify, disinfect and deodorize air and water. Their headquarters is located in Stockholm with offices in Finland, Estonia and Germany and installations worldwide.
Ozone is easily and quickly destroyed by ultraviolet radiation so Ozonetech worked with atg UV Technology to develop an ozone destruction system to meet the 3m3/h flow rate during sanitisation.
Ozone destruction The result was model UVM-25-4 with a 2.5kW medium pressure lamp delivering a UV dose of 120mJ/cm². The standard reactor had to be modified to suit the restricted installation space in the ring main but this was carried out during manufacture so that there were no delays to installation on site.