The cyanide removals of about 70% and 90% are observed in C-AS and E-AS sets respectively. The ability of the control system to remove cyanides at high influent concentration of 86 mg l−1 is attributed to the presence of acclimatized microorganisms that are able to remove cyanide in the wastewater. Additionally, other abiotic removal processes might be involved. Previous researchers have reported almost 60% cyanide removal in activated sludge process by stripping i.e. the volatilization of HCN from aqueous phase to air phase. This is highly likely to occur in the current study since the system pH was pH 7–8.5 (Fig. 6) and cyanide in this pH range is normally in the form of HCN which is highly volatile. Adding A. vinelandii resulted in increased COD and cyanide removals. It seems that A. vinelandii is able to co-cultivate with the mixed microbial microorganisms within the AS wastewater treatment and aid in an enhanced removal of cyanide. This is the first study in which A. vinelandii is illustrated to possess the ability to remove cyanide under wastewater conditions. The improved COD removal under A. vinelandii supplemented conditions is in agreement with Kargi and Ozmuci [17] although their comparisons were made for the biological treatment of nitrogen deficient synthetic wastewater.
An interesting observation in the present study is the apparent link between the decrease in cyanide concentrations and the reduction in the levels of COD in the wastewater. This is illustrated when comparing the cyanide and COD reductions with and without A. vinelandii. Analogously, tetracyanonickelate degradation by A. vinelandii is found to be dependent on the availability of a carbon source [36].
4. Conclusions
The ability of A. vinelandii, a N2-fixing bacterium, to grow and remove cyanide in cassava wastewater was evaluated. This study demonstrates that A. vinelandii are able to survive and thrive within the typical range of CN concentrations found in the cassava mill industry. The A. vinelandii supplemented activated sludge cultures showed that A. vinelandii possess the ability to co-cultivate with other MMS and aid in an enhanced removal of cyanide. Cyanide removal increased from 70% to 90% in the presence of A. vinelandii. This is the first study in which A. vinelandii was shown to be able to remove cyanide under wastewater conditions.