​Lokesh- I ​concur with ...

​Lokesh- I ​concur with ​George.  ​First - check ​your source ​water and your ​pre-treatment ​processes. ​ Then - ​the filter ​material itself.​ One annoying ​problem we ​faced was the ​angular grain ​shape of the ​filter sand, ​ which had ​orders of ​magnitude lower ​k, than would ​have been ​expected based ​only on grain ​size.  ​Look at your ​filter material ​under a ​microscope - if ​it is ​moderately sub-​rounded to well-​rounded, then ​that is not the ​issue.  ​However, if it ​is sub-angular ​to angular, ​that may be an ​important part ​of the issue. ​