Effect of COD/NO3 -N ratio on the performance of a hybrid UASB reactor treating phenolic wastewater

– The effect of COD/O3-N ratio on the biodegradation of complex phenolic mixture was studied in bench scale hybrid upflow anaerobic sludge blanket (HUASB) reactors. HUASB reactor is a combination of a UASB unit at the lower part and an anaerobic fixed film at the upper end. The aim of this study was to evaluate the biodegradability of phenolic mixture (from synthetic coal wastewater) as the only carbon and energy source in continuous experiments using nitrate as the final electron acceptor. Synthetic coal wastewater contained phenol (490 mg/L); m-,o-,p-cresols (123.0 mg/L, 58.6 mg/L, 42 mg/L); 2,4-, 2,5-, 3,4- and 3,5 dimethyl phenols (6.3 mg/L, 6.3 mg/L, 4.4 mg/L and 21.3 mg/L) as major phenolic compounds representing the complex phenolic mixture. Nitrate nitrogen loading was – increased from 0.11 g/m3/d to 0.5 g/m3/d in order to keep COD/NO3 -N ratio as 20.1, 14.85, 9.9, 6.36 and 4.45. An input phenolics concentration of 752 mg/L and hydraulic retention time (HRT) of 24 h was maintained through out – the study. Removal of phenolic mixture was found to increase with the lowering of COD/NO3 -N ratio. Maximum – phenolics removal of 98% was achieved at a COD/NO3 -N ratio of 6.36. However, phenolics removal got adversely – affected when COD/NO3 -N ratio was reduced below 6.36. A nitrogen production efficiency of 78% was obtained according to nitrate consumption. Simultaneous denitrification and methanogenesis was observed in all the reactors throughout the study, demonstrating that denitrification is a feasible alternative for the treatment of coal wastewater. Granules degrading complex phenolic mixture were of diameter 1.6–2.25 mm.

The present study revealed that the removal of complex phenolic mixture from coal wastewa– ter improved with the lowering of COD/NO3 -N ratio. Specific conclusions are as follows: – • The optimum COD/NO3-N ratio for maximum COD and phenolics removal was about 6.36 in the hybrid UASB reactor. • COD removal increased from 89.57% to – 95.75% with the reduction of COD/NO3 -N ratio from 20.1 to 6.36, which drastically re– duced to 11.57% upon descent of COD/NO3 N ratio to 4.45. • Phenolics removal increased from 90% to 99% – with the reduction of COD/NO3-N ratio from 20.1 to 6.36, which drastically reduced to 48% – upon descent of COD/NO3-N ratio to 4.45. • Simultaneous removal of phenolic compounds could be observed at all COD/NO3–-N ratios.