Mutual Industries silt sack has a foam backing that helps in collecting debris and measures 4 x 3 x 2 . Mutual Industries silt sack measuring 4 x 3 x 2 helps in decreasing the amount of salt and debris that enters the storm water inlet during construction. Sack is placed inside the storm drain. • Silt sack. • Used for reducing the amount of silt and debris that may enter a storm water inlet during construction. • These sacks are placed inside the storm drain to collect debris as it enters the opening. • Sold as 1 Each.