Abstract: Described herein is a Submerged Attached Growth Reactor (SAGR) which provides nitrification (ammonia removal) from wastewater in cold to moderate climates. The system described herein is novel in that the SAGR reactor includes more than one influent distribution point. Specifically, in addition to the first influent distribution point at the front end of the reactor, there is provided at least one additional distribution point(s) downstream of this first distribution point for introduction of influent into the reactor. As a result of this arrangement, when a second low carbonaceous biochemical oxygen demand (CBOD), high nitrogen influent is distributed into the reactor at a location downstream of the initial influent entry point, a second (or more) population of bacteria (mainly nitrifying bacteria) is established and/or maintained in a physically discrete part of the overall treatment reactor.