Cyanides & thiocyantes in biological wastewater treatment

Cyanides are often present in industrial waste water and can be toxic to microbes (especially chemoautotrophic nitrifiers (AOB & NOB) - in anaerobic digesters, CN is highly inhibitory to methanogens. In non-acclimated biomass, we see microbial respiration inhibited by cyanide of 0.3 mg/L. Cyanide is often found in petrochemical, steel/coking, and pharmaceutical waste waters as a by-product of industrial processes. It is not uncommon to see CN concentrations well above the 0.3 mg/L threshold.

While cyanide is toxic to many organisms, a large number of microbes have the ability to detoxify cyanide and actually use the carbon/nitrogen for growth. While multiple biological pathways exist, the most efficient is an aerobic pathway. Below are the chemical transformations for CN and SCN.

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In practice, I have worked with systems experiencing both inhibition by influent cyanide and shock from sudden increases in cyanide concentrations. Cyanide inhibition is readily addressed by bioaugmentation with cultures having strong CN detoxification pathways. We have identified multiple strains of Pseudomonas, Bacillus, and Rhodococcus with excellent cyanide detoxification and growth on multiple resistant organic compounds including phenol which is often found in high CN wastewater.

These cultures can be concentrated in shelf-stable form and added to systems with CN related shock or problems with stable CN removal. Once added the CN detoxification starts and the MLSS returns to a healthy state within days as respiration inhibition decreases. Once detoxified, both AOB & NOB return to normal ammonia and nitrite oxidiation rates.

Biological cyanide removal can produce CN concentrations in the ppb range at a lower cost than chemical oxidation or metal complexing options. While it cannot treat extremely high CN concentrations seen in some plating and mining wastes, most industrial high CN wastewater can be treated efficiently with biological treatment.