SEWAGE DEPARTMENT TRAINING PROGRAM
How to Build a Free Access Gravity Sandfilter
Over the under drain material, add another layer of aggregate 4 inches deep.
The aggregate used in this layer must meet the Type B requirements posted in the Department of Transportation specifications Publication #408, section 703, Table B. The size and grading shall meet AASHTO No. 8 requirements as described in Form 408, Section 703.2, Table C. The aggregate must be taken from a PennDOT certified stockpile.
