Sampling of Filter Sand

Abstract

We have always had a little curiosity regarding the sampling of filter sand, especially as to whether the samples ordinarily taken represent with fair accuracy the composition of the entire lot of sand. A good opportunity presented itself recently during the construction of some additional filter beds, to check up on this special problem. The sand generally used for filter plants in this part of Texas, is obtained from Goliad, Goliad County, Texas, and is known as Goliad Sand. Numerous tests made at different times have indicated this sand to be about 0.29 to 0.31 mm. effective size, and about 2.15 uniformity coefficient. Inasmuch as this sand apparently was a little too fine, it was decided to place the sand in the filter beds, wash it and then scrape it until the proper size was reached. Accordingly the sand was treated in this manner. The results obtained are shown on table 1. Regarding the methods of sampling, the samples marked grab were taken by simply picking up a small portion here and there, after the sand had been placed in the filter beds, but before washing. The samples marked sample after first washing were taken as follows: After the sand had been washed, the filter waste valve was opened and the excess water allowed to drain out. A piece of 2-inch copper pipe about 4 feet long was then pushed down into the sand for a distance of or 8 inches and then carefully withdrawn. The wet sand packed itself in the tube nicely and was then dislodged easily by striking the tube lightly on the side. The process was repeated until the core was cut through to the gravel layer. The samples marked 6 inches below were taken at a point inches below the surface of the sand bed, and represent the sand at the 6-inch depth, but do not represent the top inches of sand. The same is true for the samples marked 12to 15-inch This depth represents the point half-way between top and bottom of the sand bed.