The study examined how the COD/N ratio (2.0, 2.5, 3.0, 3.5, 4.0) and cycle length (24 h and 12 h), influence the effectiveness and the rate of nitrite reduction, and the final gaseous products of denitritation, when glycerine was a carbon source. In addition, daily sludge production (Delta X) was both estimated by calculating heterotrophic sludge yield (Y-HD) (Delta X-YHD), and determined by making daily measurements of biomass concentration (Delta X-exp). The results indicated that there were discrepancies between Delta X-exp and Delta X-YHD, which shows that estimating daily biomass production by calculating heterotrophic sludge yield gives estimates which are not particularly accurate. Therefore, to find the optimal COD/N ratio, Delta X-exp was taken into consideration. The study showed that at a COD/N ratio of 2.0, complete nitrite elimination was not observed (nitrite effectiveness removal 68%); at COD/N ratios of 2.5-4.0, denitritation effectiveness was around 100%. However, at a COD/N ratio of 2.5, N2O concentration was 2 times higher than at the higher COD/N ratios (3.0-4.0). With a 12 h cycle length, the denitritation rate was highest (18.85 mg N-NO2/g VSS h) at a COD/N ratio of 3.5, whereas the rate was 1.61-times lower (9.0 mg N-NO2/g VSS h) at a ratio of 3.0. Although the denitritation rates differed at these COD/N ratios, nitrites were completely reduced in the first 2 h of the cycle. However, Delta X-exp was 0.66 g VSS/d at a COD/N ratio of 3.5, which was 30% higher than at a ratio of 3.0. This means that, taking into consideration the effectiveness and the rate of denitritation, its final gaseous products and sludge production, the most optimal COD/N ratio was 3.0 when glycerine was a carbon source. (C) 2015 Elsevier B.V. All rights reserved.