Removal and fate of total and free cyanide treating real low loaded petrochemical wastewater in a pilot membrane bioreactor (MBR)

This paper deals with about one year experimentation of a pilot MBR operated in parallel with the world's largest industrial MBR and treating continuously real industrial wastewater coming from a petrochemical area. The removal of free and total cyanide was investigated to find an operating solution to face the problems linked to the low organic inloadings, which influenced the composition of the activated sludge and its biosorption and/or biodegradation potentials. In order to gain a deeper understanding on the mechanisms of cyanide removal, several laboratory batch tests were carried out in a range of conditions, using the activated sludge drawn from the pilot and full scale MBR. Results showed good (almost complete) biodegradation of free cyanides, influenced by the F:M ratios, and high stability of iron-complexes. Also, comparing the bench scale, pilot and full scale results, minimal specific removal rate for free cyanides were found to be in the range 0.2÷0.4 μg/gVSS *h, where the limiting substrate concentration was lower than 10 μgCN/L and the biodegradation was the major removal mechanism