Silt density index

{{short description|Measure for the fouling capacity of water in reverse osmosis system}}

{{Primary sources|date=April 2009}}

The silt density index is a measure for the fouling capacity of water in reverse osmosis systems. The test measures the rate at which a 0.45-micrometre filter is plugged when subjected to a constant water pressure of {{convert|206.8|kPa|psi|abbr=on|sigfig=2}}. The SDI gives the percent drop per minute in the flow rate of the water through the filter, averaged over a period of time such as 15 minutes.{{citation needed|date=October 2015}}[http://www.watertreatmentguide.com/determining_sdi.htm Procedure and calculation]

Typically, spiral-wound reverse osmosis systems will need an SDI less than 5, and hollow fiber reverse osmosis systems will need an SDI less than 3. In these kinds of systems, deep-well waters (with a typical SDI of 3) could be used straight from the source. If fed from surface waters (with a typical SDI greater than 6), the water will need to be filtered before use.{{citation needed|date=October 2015}} Seawater desalination plants utilising reverse osmosis systems also need very efficient filtering due to the typically high but variable SDI of seawater.{{cite web|url=http://www.idswater.com/water/us/WhitePaper_gulf_water/38/paper_information.html|title=Pilot Plant Results Demonstrating Feasibility and Performance of SWRO on Gulf Water (Synopsis)|publisher=IDS Water |archive-url=https://web.archive.org/web/20130320025259/http://www.idswater.com/water/us/WhitePaper_gulf_water/38/paper_information.html|archivedate=March 20, 2013 |url-status=dead|access-date=May 28, 2015}}