SEWAGE DEPARTMENT TRAINING PROGRAM
How to Build a Free Access Gravity Sandfilter
Install the under drain material.
Add a layer of aggregate 5 inches deep as measured from the bottom of the tank (or 1 inch over the under drain pipe).
The aggregate must meet the Type B requirements found in the Department of Transportation specifications Publication #408, Section 703, Table B. It must have uniform size and grading that meets AASHTO No. 57 requirements as described in Form 408, Section 703.2, Table C. The material must be taken from a PennDOT certified stockpile.
