Described herein are attached growth reactor systems which increase nitrifying bacteria biomass through a variety of means during warm weather. As a consequence, the attached growth reactor system contains sufficient nitrifying bacteria biomass to remove ammonia from wastewater in cold to moderate climates. In one example, there are two attached growth reactors into which wastewater is distributed discontinuously. Specifically, wastewater is transferred to the first attached growth reactor for a first period of time and then is transferred to the second attached growth reactor for a second period of time during warm weather which effectively doubles the nitrifying bacteria biomass in the system. During cold weather, approximately half of the wastewater is applied to each reactor simultaneously.