Nitrogen removal in side stream processes offers a good potential for upgrading wastewater treatment plants (WWTPs) that need to meet stricter effluent standards. Removing nutrients from these internal process flows significantly reduces the N-load to the main treatment plant. These internal flows mainly result from the sludge processing and have a high temperature and a high concentration of ammonia. Therefore, the required reactor volumes as well as the required aerobic SRT are small. Generally, biological treatment processes are more economical and preferred over physical–chemical processes. Recently, several biological treatment processes have been introduced for sludge water treatment. These processes are available now on the activated sludge market (e.g. SHARON®, ANAMMOX® and BABE® processes). The technologies differ in concept and in the limitations guiding the application of these processes for upgrading WWTPs. This paper reviews and compares different biological alternatives for nitrogen removal in side streams. The limitations for selecting a technology from the available ones in the activated sludge market are noted and analysed. It is stressed that the choice for a certain process is based on more aspects than pure process engineering arguments.