Effect of shock and mixed nitrophenolic loadings on the performance of UASB reactors

The effect of nitrophenolic shock loads on the performance of three bench-scale upflow anaerobic sludge blanket (UASB) reactors was studied using synthetic wastewater. Reactors R1, R2 and R3 were fed with 30 mg/L concentration of 2-nitrophenol (2-NP), 4-nitrophenol (4-NP) and 2,4-dinitrophenol (2,4-DNP), respectively, along with methanol (COD=2000 mg/L), sodium nitrate (NO3−-N=200 mg/L), and other nutrients. The reactors were in continuous operation for more than 2 years before the shock loading study was performed. Five nitrophenolic shock loadings of 45, 60, 75, 90 and 120 mg/L d were administrated by increasing the influent nitrophenolic concentration to 45, 60, 75, 90 and 120 mg/L, respectively, while keeping hydraulic retention time as 24 h. The shocks were given continuously for a period of 4 days before switching back to normal nitrophenolic loading (30 mg/L d). The reactors were allowed to recover to normal performance level before administrating the next nitrophenolic shock load. The study showed that the nitrophenolic shock load of as high as 120 mg/L d did not affect the reactors performance irreversibly. After resuming the normal nitrophenolic loading, it took almost 3–18 days for the reactors to recover from the shock effect. The study was further extended to assess the maximum possible mixed nitrophenolic loading (2NP:4NP:2,4:DNP=1:1:1) to which 2,4-DNP acclimated granular sludge containing reactor (R3) can be exposed without hampering the reactor (R3) performance irreversibly. The reactor was able to achieve pseudo-steady-state at a mixed nitrophenolic loading of 180 mg/L d with more than 90% removal of all the three nitrophenols, but failed at a mixed nitrophenolic loading of 225 mg/L d.