The effect of HRT on TPH removal
TPH removal efficiency versus different concentrations of TPH at two HRTs (18 & 24 hr) for each reactor is shown in Figure 3.
Figure 3  TPH removal efficiency at two HRTs of 18 and 24 hours versus ratios of TPH/COD molasses in (a) HMBR (b) RBC.
As shown in Figure 3 increasing HRT has led to increasing the TPH removal efficiency, because pollutants contacted microorganisms for a long hydraulic retention time.
Increasing the ratio of TPH/COD molasses to 0.6 has led to increasing the TPH removal efficiency in both reactors but when the ratios of TPH/COD molasses was greater than 0.6, the efficiency of both systems in removing the pollutant was reduced. This is due to the fact that the increase in the concentration of hydrocarbons on biofilm distorts the cellular metabolism of microorganisms and prevents them from using carbon molasses for their metabolism and reproduction. This will, in turn, reduce MLSS in system and the potential for removing the pollutant will be significantly reduced. Thus, in treating the oily wastewater in such reactors, it is recommended not to choose the ratio of TPH/COD molasses more than 0.6.