The Permutit® brand of products, such as the double dish underdrain, became a part of Siemens in 1993. 

Permutit® double dish underdrains have been installed in many applications worldwide and its design is based on the use of a false bottom or second tank head (the double dish) that contains a series of strainers in a calculated and predetermined pattern.  The double dish is of slightly greater radius than the actual tank bottom and minimizes the inactive areas at the bottom of the ion exchange bed.  The strainers retain the ion exchange media and eliminate any requirements for conventional sub-fill, such as gravel, quartz, anthracite, etc.