TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In method (I): the circulating scrubber water is maintained at a pH of less than 8, preferably less than 7.5. The hot syngas stream passes to a separator for the removal of most of the particulates before passing to the scrubber; the acidic solution comprises acetic acid, and the acidic solution is added to the circulating scrubber water to increase the scrubber efficiency by reducing the pH in the scrubber to capture the ammonia gas; the portion of the scrubber waste water effluent stream enters a biological treatment zone for the removal of carbonyl sulfide and ammonia; the hot syngas stream contains hydrogen cyanide and at least a portion of the treated syngas stream effluent from the scrubber is sent to a hydrogen cyanide scrubber and at least a portion of a hydrogen cyanide scrubbing water effluent is sent to the anoxic biological treatment step for the removal of hydrogen cyanide; the portion of the scrubber waste water effluent stream undergoes an anoxic biological treatment step to produce an anoxic effluent stream, where at least a portion of the anoxic effluent stream undergoes an aerobic biological treatment step, a biologically treated waste water stream is recovered from the aerobic biological treatment step, and a portion of the biologically treated waste water stream is recycled back to the anoxic biological treatment step; and the process that produces at least a portion of the purge water comprises a fermentation process, where at least a portion of the treated syngas stream effluent from the scrubber enters a fermentation zone of the fermentation process that converts the treated syngas to soluble chemical products by contact with anaerobic microorganisms in an aqueous fermentation broth, and the acidic solution comprises a portion of the aqueous fermentation broth. In method (II): the acidic solution comprises purge water from a process for converting the treated hot syngas stream to chemical products and the addition of purge water is controlled to maintain the circulating scrubber water in a pH of less than 8, preferably less than 7.5; the at least a portion of the scrubber waste water effluent stream passes to an anoxic biological treatment step in the biological treatment zone for the partial conversion of nitrate formed from ammonia, and ammonium into nitrogen gas and for the at least partial conversion of carbonyl sulfide to primarily carbon dioxide and hydrogen sulfide to produce an anoxic effluent stream, where at least a portion of the anoxic effluent stream passes to an aerobic biological treatment step in the biological treatment zone for the nitrification of ammonium to nitrates, the biologically treated waste water stream is recovered from the aerobic biological treatment step, and a portion of the biologically treated waste water stream is recycled back to the anoxic biological treatment step; the purge water provides sufficient carbon for cellular growth of microorganisms and for the reduction of nitrate to nitrogen in the anoxic biological treatment step; the hot syngas stream comprises hydrogen cyanide, where at least a portion of the treated syngas stream effluent passes to a hydrogen cyanide scrubber for contact with a hydrogen cyanide scrubbing water stream and at least a portion of a hydrogen cyanide scrubbing water effluent passes to the anoxic biological treatment step in the biological treatment zone that converts the hydrogen cyanide to compounds that are benign to microorganisms in the aerobic biological treatment step; the portion of the particulates in the scrubber waste water effluent is removed before passing the scrubber waste water effluent to the biological treatment zone; and the portion of the biologically treated waste water effluent stream passes to the hydrogen cyanide scrubber for contact with the hydrogen cyanide scrubbing water stream. In method (II): the pH of the scrubber water is less than 7.5; the portion of the particulates in the scrubber waste water effluent is removed before passing the scrubber waste water effluent to a biological treatment zone; the portion of the hydrogen cyanide scrubbing water effluent passes to a hydrogen cyanide removal system, where at least a portion of the hydrogen cyanide is removed to a produce a hydrogen cyanide removal system effluent and at least a portion of the hydrogen cyanide removal system effluent passes back into the hydrogen cyanide scrubber; and the portion of a hydrogen cyanide treated syngas stream effluent passes to a fermentation system for the conversion of syngas to chemical products e.g. ethanol and other liquid products by contact with anaerobic microorganisms in an aqueous fermentation broth and the acidic scrubbing solution comprises at least a portion of the aqueous fermentation broth. The process for converting the treated hot syngas stream to chemical products is a fermentation process and the purge water comprises acetic acid and water. The amount of biologically treated waste water recycled from the aerobic biological treatment step to the anoxic biological treatment step is adjusted to maintain a predetermined effluent nitrate concentration and the anoxic biological treatment step is maintained at a pH of 6-9, preferably 6.5-8.5. EXAMPLE - No suitable example given.