In Sweden many advanced sewage treatment plants for BOD and phosphorus removal have to be extended with nitrogen removal. Due to existing plant configuration and wastewater composition, denitrification with supply of an external carbon source can be a cost-effective solution in many cases. At the Klagshamn wastewater treatment plant in Malmo investigations for extensive nitrogen removal have been made in a single-sludge system with pre-precipitation and post-denitrification where methanol was added for denitrification. Results from the tests showed that a high level of nitrogen removal can be reached, and that the process was stable and easy to operate. The process application gave less supplementary cost for an extended nitrogen removal than for upgrading the plant with larger basin volumes. In order to examine the purification performance caused by the addition of methanol, the starting period was followed extensively with online nitrate sensors and daily composite samples. The development of the denitrif ication capacity of the sludge with methanol and acetate as carbon sources was followed and microbiological changes were examined microscopically. Complete denitrification was obtained after approximately one month at 10°C. The denitrification capacity of the sludge with methanol reached that of acetate after about the same time. The microscopic examination revealed a growing population of budding and/or appendaged bacteria, presumably Hyphomicrobium spp, reaching a stable maximum at the time when optimal nitrate removal occurred.