Kattrina, First check the ...

Kattrina,

First check the colour and density of the foam. If it is brown and dense it is probably caused by filamentous bacteria. If it is light and white then it is likely to be an organic residue from incomplete treatment of the wastewater, often created under anaerobic conditions.

In either case you need to get the conditions right to fix the problem. Don't waste money on buying bacteria or chemicals to "fix" the problem as this will only be a short-term measure.