TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method, an acid solution is added to the scrubber to control the pH, where the acidic solution comprises purge water from a process for converting the treated hot syngas stream to chemical products, where the addition of purge water is controlled to maintain the circulating scrubber water at a pH of less than 8, preferably less than 7.5. The process for converting the treated hot syngas stream to chemical products is a fermentation process, where the purge water comprises acetic acid and water. In the method, particulates in the scrubber water are passed through a separator to remove the particulates from the stream prior to biological treatment. In the method: 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 partial conversion of HCN and glycol nitriles into nitrogen and carbon dioxide to produce an anoxic effluent stream; 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, and for the partial conversion of glycol nitrile into primarily and carbon dioxide; 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 method further comprises: measuring at least one indicator of an amount of cyanide or aldehyde remaining in the scrubber gas and adjusting a rate of aldehyde addition to the scrubber circulating water in response to maintaining the level of HCN in the syngas below the desired concentration required; or measuring at least one indicator of an amount of aldehyde, glycol nitrile or cyanide contributed by the scrubber waste water effluent stream passed to the anoxic biological treatment step, and adjusting a rate of aldehyde addition to the scrubber circulating water in response to the indicated contribution of aldehyde to the anoxic biological treatment step from the aldehyde containing scrubber waste water effluent. 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, where the anoxic biological treatment step is maintained at a pH of 6-9, preferably 6.5-8.5. The purge water comprises acetic acid that provides sufficient carbon for cellular growth of microorganisms and for the reduction of nitrate to nitrogen in the anoxic biological treatment step. In the method, at least a portion of the treated syngas stream effluent passes to an additional aldehyde scrubbing stage or additional scrubbing tower to remove aldehyde from the syngas, where at least a portion of the biologically treated waste water effluent stream passes to the additional aldehyde/HCN scrubbing stage(s) or scrubber for contact with the cleaned syngas. Preferred Components: The aldehyde is formaldehyde, acetaldehyde and/or acrolein. The glycol nitrile is 2-hydroxyacetonitrile.