Dear Sir. Concerning your issue about the types of foundry sand is most suitable in domestic wastewater treatment. Foundry sand consists primarily of clean, uniformly sized, high-quality silica sand or lake sand that is bonded to form molds for ferrous (iron and steel) and nonferrous (copper, aluminum, brass) metal castings. Although these sands are clean prior to use, after casting they may contain Ferrous (iron and steel) industries account for approximately 95 percent of foundry sand used for castings. The automotive industry and its parts suppliers are the major generators of foundry sand. The most common casting process used in the foundry industry is the sand cast system. Virtually all sand cast molds for ferrous castings are of the green sand type. Green sand consists of high-quality silica sand, about 10 percent bentonite clay (as the binder), 2 to 5 percent water and about 5 percent sea coal (a carbonaceous mold additive to improve casting finish). The type of metal being cast determines which additives and what gradation of sand is used. The green sand used in the process constitutes upwards of 90 percent of the molding materials used. In addition to green sand molds, chemically bonded sand cast systems are also used. These systems involve the use of one or more organic binders (usually proprietary) in conjunction with catalysts and different hardening/setting procedures. Foundry sand makes up about 97 percent of this mixture. Chemically bonded systems are most often used for "cores" (used to produce cavities that are not practical to produce by normal molding operations) and for molds for nonferrous castings. The annual generation of foundry waste (including dust and spent foundry sand) in the United States is believed to range from 9 to 13.6 million metric tons (10 to 15 million tons). Typically, about 1 ton of foundry sand is required for each ton of iron or steel casting produced. I think the following below links and the attached file may help you in your analysis:

https://www.fhwa.dot.gov/publications/research/infrastructure/structures/97148/fs1.cfm

http://foundrygate.com/upload/artigos/Environmental%2C%20Health%2C%20and%20Safety%20Guidelines%20for%20foundries.pdf

Thanks

Sand with specific D10 size and shapes, etc is specifially used for filtration mainly for water supply but suitable for tertiary treatment in small facilities. What is your need?

Sand surface area per filter surface area should not be chosen below 2000 m2/m2 .I suggest you see this Article Slow sand filtration of secondary clarifier effluent for wastewater reuse.