Influence of COD/NH₃–N ratio on organic removal and nitrification using a modified RBC

Abstract

 Synthetic wastewaters were prepared with different influent concentrations of ammonia nitrogen (NH₃–N) and COD and the treatment studies were conducted using a rotating biological contactor (RBC). If organic removal and nitrification can be simultaneously effected in one process, it will be an ideal solution to water pollution control. The RBC used in the present study was a four stage laboratory model and the discs were modified by attaching porous netlon sheets to enhance biofilm area. The COD loads (S ₀) used were about 1000 and 1500 mg/l whereas NH₃–N concentrations used were in the range of 20 to 185 mg/l. Hydraulic load (q) of 0.03 m³ . m^-2 . d^-1 and ammonia nitrogen loadings in the range of 0.66 to 5.5 g NH₃–N . m^-2 . d^-1 were used. The RBC was operated at two different rotating speeds of 6 and 12 rpm. The results showed that the nitrification and percentage of COD removal were not affected up to the value of the COD/NH₃–N in the range from 47 to 23 at w=6 rpm and for an average influent COD of 1003 mg/l. Beyond that range only the nitrification rate decreased much whereas the percentage of COD removal was not affected. Similarly, at an influent COD load of 1557 mg/l, the nitrification and percentage COD removal were not affected for the value of the COD/NH₃–N in the range from 44 to 23 but beyond that range only the nitrification rate decreased while the percentage of COD removal was approximately constant and still high. A correlation plot between the NH₃–N removed and NH₃–N applied was presented at a rotating speed of 6 rpm and it was found that the nitrification rate of 3.93 g NH₃–N . m^-2 . d^-1 was achieved at ammonia loading of 5.55 g NH₃–N . m^-2 . d^-1. Also the results at w=12 rpm showed improvement of nitrification rate over those at 6 rpm.