Different natural organic solid substances (NOSS) were examined as the carbon source for biological denitrification in batch reactors. Among the examined substances, Gracilaria verrucosa, liquorice, and giant reed gave positive response to denitrification and complete nitrate removal was achieved. Throughout the study, the effect of inoculum addition, pH, and carbon amount change on denitrification was also investigated. Results showed that best denitrification performance was achieved with G. verrucosa with the highest protein amount and its large surface area. (c) 2006 Elsevier Ltd. All rights reserved.