Cyanide toxicity and cyanide degradation in anaerobic wastewater treatment

The effect of cyanide on the anaerobic treatment of synthetic wastewater, containing starch and volatile fatty acids, was evaluated. A laboratory-scale UASB reactor, operated at hydraulic retention time of 12 h, was successfully acclimatised to CN influent levels as high as 125 mg l−1. Evaluation of cyanide levels in the effluent demonstrated removal efficiencies of this compound of between 91 and 93% at volumetric CN loading rates of about 250 mg l d−1. First addition of CN at 5 mg l−1 and subsequent sudden increases in influent CN levels during the acclimatisation process resulted in temporary deterioration of reactor performance in terms of methane production and COD conversion, while CN levels in the effluent were temporarily increased. Recovery from CN inhibition was observed within 3–4 weeks, when effluent CN levels decreased again below about 10 mg l−1. Sludge activity measurements demonstrated an increased tolerance against CN, once sludge had been acclimatised to this toxic compound. The effect of CN inhibition on methanogenic activity was more pronounced for acetoclastic than for hydrogenotrophic methanogens. The findings of this study demonstrate the potential of anaerobic treatment for COD removal in CN contaminated waste waters. The results also suggest a potential application specifically for CN removal from waste streams.