“…The largest reduction in COD was attained at (V b /V R ) = 0.55. An increase in COD removal with an increase in the (V b /V R ) from 0.50 to 0.55 can be attributed to the fact that for increasing (V b /V R ), more biomass grown on the particles participated in degradation of the constituents of the wastewater [5,13]. On the other hand, a decrease in COD removal observed with an increase in (V b /V R ) from 0.55 to 0.60 was due to the fact that in this case, a significant volume of the reactor was occupied by the support media, and consequently the aeration characteristics of the bed has worsened [5,8].…”