In this research, two natural organic substances; liquorice (Glycyrrhiza glabra) and giant reed (Arundo donax) were investigated as a carbon source in the biological denitrification of drinking water. These materials act as a solid substrate and bio-film carrier. The experiments were carried out in batch, semi-batch and continuous processes. Complete denitrification was achieved with G. glabra. The removal efficiency of A. donax varied between 87 and 100% according to the type of process used. It was found that the nitrate removal rate of G. glabra was 6.96 mg/(L day) N-NO3 and the nitrate removal rate of A. donax was 4.23 mg/(L day) N-NO3. Results showed that these organic substrates could be used as an alternative carbon source for denitrification with complex process control and continuous monitoring. It should be noted that carbon source should be changed periodically for the continuation of the process. (c) 2006 Elsevier Ltd. All rights reserved.