ANAEROBIC TREATMENT OF CYANIDE-RICH CASSAVA STARCH WASTEWATER
I Nyoman Widyatmika, Moustafa S. Moussa, Peter van der Steen, Huub Gijzen
UNESCO-IHE Institute for Water Education, The Netherlands
(free)
A study was conducted to evaluate the response of Upflow Anaerobic Sludge Blanket (UASB) reactor to cyanide-rich wastewater. Three laboratory scale UASB reactors (named R1, R2 and R3) with 3 L effective volume each were used to treat cassava starch wastewater with a cyanide concentration of up to 25 mgCN/L.
The wastewater was prepared from commercially grinded cassava and only the settled supernatant was used as reactor’s influent. The supernatant already containing cyanide concentration of 1.5 – 5 mgCN/L. KCN was used as an additional source of cyanide, while NaOH and carbonate-phosphate buffer were added for pH regulation. All three reactors were seeded with granular sludge from an industrial UASB reactor of a potato chip industry. Hydraulic Retention Time (HRT) in the three reactors was maintained at 12 hours through the whole period of experiment. Temperature in the reactors was maintained at 350 C using doublejacketed reactor. The organic load applied to the reactors ranged from 3.6 to 11.1 gCOD/L.day.
Adaptation period of 98 days was obtained by stepwise increase in organic load without any addition of external cyanide during this period. By the end of the adaptation period, all the three reactors achieved COD removal efficiency higher than 90%. The cyanide concentration to R3 was increased in two steps, 5 mgCN/L for each (on days 99 and 160). R2 received additional shock load of 20mgCN/L once on day 160. R1 was kept as a control reactor without any addition of external cyanide.
During the addition of the first 5 mgCN/L to R3 (on day 99), temporary deterioration of the reactor performance was observed in terms of reduction in biogas production and VFA accumulation. Cyanide in the effluent also increased. It took the reactor 5 days to achieve full recovery and to reach the steady state performance. Unexpectedly, the shock load of 20 mgCN/L to R2 (adapted to 2 mgCN/L for 150 days) hardly affected the performance of the reactor. The three reactors had similar performances, although they received different cyanide loads. All reactors had satisfactory COD removal efficiency above 90% and percentage of methane in the biogas was above 76%. Cyanide in the effluent significantly decreased to 0.85, 3.7, and 2.5 mgCN/L for R1, R2 and R3 respectively at the end of the experiment. In-situ methanogenic activity tests also gave similar results for the three reactors. In conclusion, the results demonstrate the potential of anaerobic treatment to treat cassava starch wastewater and to withstand higher concentration of cyanide.
KEYWORDS Cassava starch wastewater, UASB, anaerobic process, cyanide inhibition
