Foam separation method for reducing AOX, COD, and color bodies of kraft pulp bleach plant effluents

A method for treating effluent from a kraft pulp bleaching sequence having a chlorine and/or chlorine dioxide stage generating an organic chloride compound-containing filtrate (F.sub.D) and an alkaline extraction stage generating an extraction state filtrate (F.sub.E) wherein the F.sub.D and F.sub.E filtrates contribute to the amount of the chemical oxygen demand (COD), adsorbable organic halides (AOX), color bodies and toxicity in the bleach plant effluent, the method comprising:

• (a) intensely mixing the F.sub.D filtrate with the F.sub.E filtrate in a volume ratio of F.sub.D to F.sub.E within the range of from about 0.5:1 to about 4:1 for a mixing interval sufficient to reduce the amount of AOX in the F.sub.D thereby providing an F.sub.D F.sub.E mixture;
• (b) contacting the F.sub.D F.sub.E mixture with a coagulating compound and a foam concentrate to form a pre-foam mixture;
• (c) treating the pre-foam mixture with a flocculating compound to form a floc in the pre-foam mixture;
• (d) foaming the floc with an amount of air sufficient to form a foam containing the floc and entrained liquid;
• (e) collecting the foam containing floc and entrained liquid;
• (f) breaking the foam in a foam breaker in order to release the entrained liquid wile forming a foam concentrate;
• (g) separating the foam concentrate from the entrained liquid in a foam clarifier;
• and
• (h) recycling a substantial portion of the foam concentrate to the F.sub.D F.sub.E mixture.