It is customary not to perform hagala on a utensil until twenty-four hours have elapsed (and it is no longer “ben yomo”) since the last time it absorbed a forbidden food. This is because the absorbed taste remains flavorful during these twenty-four hours, and if the boiling water is not sixty times the volume of the utensil, the water will absorb the forbidden taste and transfer it back into the walls of the utensil, rendering the hagala ineffective. But if twenty-four hours have elapsed, the taste in the utensil becomes foul, and the utensil can be koshered even if the water is not sixty times its volume. This is because the utensil releases a foul taste into the water, and such a taste will not render the utensil un-kosher, even if it is reabsorbed. The utensil is only rendered un-kosher if it absorbed positive flavor, in which case it remains un-kosher even once the flavor has become foul. However, if at the time of its absorption the taste was foul to begin with, the utensil is kosher.
Another reason for this twenty-four hour delay is our concern that meat and dairy utensils will undergo hagala in the same water. In such a case, the positive flavors of meat and milk will be released into the water, and if the water is not sixty times the volume of either the meat or the dairy, they will not be batel. Rather, these flavors mingle and then render all of the water forbidden. Consequently, any utensil that undergoes hagala in this water will absorb basar be-ĥalav (the forbidden mixture of milk and meat) and become forbidden. However, once the utensil is not ben yomo, the tastes of milk and of meat in the utensils is foul, and even if the tastes mingle within the water, they do not become forbidden, since noten ta’am li-fgam is permitted (SA 452:2; MB ad loc.). Accordingly, one must ensure that a large pot in which other utensils undergo hagala did not absorb the ben yomo taste of meat, milk, or ĥametz.
The common practice at public hagala stations is to presume that at least some of the utensils brought are not ben yomo. In order to avoid problems, a strong cleanser such as bleach or liquid soap is added to the water, thereby immediately rendering any taste released by the utensils foul. Consequently, there is no concern that meat and dairy flavors will mix or that the tastes released will be reabsorbed by the utensils, because once a taste has become foul, it cannot render utensils forbidden.
Le-khatĥila, no other substance is mixed with the water used for hagala, as explained in Rema 452:5. Be-di’avad, hagala is effective in any liquid. MB ad loc. 26 states in the name of Pri Megadim that if Pesaĥ had not yet begun, one should re-kosher the utensil in boiling water alone. Nevertheless, in communal hagala it is difficult to ensure that everyone’s utensils are not ben yomo, and since we want to avoid any problems that could arise from mixing meat and dairy utensils, the custom is to add soap or bleach to the boiling water. See Sidur Pesaĥ Ke-hilkhato 7:20, which states that the validity of the hagala only comes into question if the water becomes thickened from the added substances. In n. 7 ad loc. it states in the name of Ĥazon Ish that it is preferable to use cleaning fluids to avoid the potential problem of mixing milk and meat and to avoid relying on the opinion that ĥametz before Pesaĥ is considered “isura bala.”