A system and method for the treatment of process streams is provided. A catalyst mediates a wet oxidation process at elevated temperatures and pressures for treating at least one undesirable constituent in an aqueous mixture. The aqueous mixture may be contacted with a catalyst and an oxidizing agent at an elevated temperature and a superatmospheric pressure in a reactor (206). At least a portion of the catalyst may be precipitated by a pH adjustment by the addition of acid or alkali compound from a source of pH adjuster (234) and recycled back to contact the aqueous mixture via line (238) after clarification in clarifier (230). The pH of the recycled catalyst containing effluent may be adjusted by the addition of an acid or alkali compound from source of pH adjuster (226).