Why would iron salts be used as defoamers in waste-water treatment?
I have never heard of iron salts being used as a defoamer in waste water treatment. It is very common to use FeCl3 as a flocculant in such applications. This is the primary use of iron chloride in water treatment. The iron chloride forms a voluminous floc which coagulates suspended matter and precipitates this for removal. The iron chloride can also be used in combimation with organic flocculants such polyacrylamides, for increased efficiency. There may be a concurrent reduction of foaming substances due to them being coagulated by the flocculant. But the primary function of the FeCl3 is not defoaming.