Aluminum based chemicals have been used for many years in wastewater treatment applications for suspended solids removal. These include dry and liquid alum, with sodium aluminate used in activated sludge plants for phosphorus removal. The commercial dry alum most often used in wastewater treatment is known as filter alum, and has the approximate chemical formula A12(S04)3 -14H20 and a molecular weight of about 600. Alum is white to cream in color and a 1 percent solution has a pH of about 3.5. The commercially available grades of alum and their corresponding bulk densities and angles of repose are given in Table 1.
Each of these grades has a minimum aluminum content of 27 percent, expressed as A1203, and maximum Fe203 and soluble contents of 0.75 percent and 0.5 percent, respectively.
Since dry alum is only partially hydrated, it has hygroscopic tendencies. However, it is relatively stable when stored under normal temperature and humidity conditions encountered.
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