Gravel is used and a base in water softeners carbon filters and other backwashing devices. Water softener usually only use gravel when they are in a tank with a diameter greater than 10". Most other device use gravel to allow the device to correctly backwash and to assure larger particulates do not migrate into the purified water.

select DISTINCTROW top 10 p.CatalogId,p.ccode from products p, prodcategories cc, categories c where cc.intCatalogId=p.CatalogId and cc.intcategoryid=c.categoryid and cc.intcategoryid = 1099 and p.hide = 0 and (p.customermatch is null or p.customermatch = '') and (p.loggedin = false or p.loggedin is null) and (c.customermatch is null or c.customermatch = '') and (c.loggedin = false or c.loggedin is null) AND (higherCatalogId is null) order by p.ccode