Multiple attached growth reactor system

A method for increasing nitrifying bacteria biomass in an attached growth reactor comprising:

• providing an attached growth reactor having a first end and a second end wherein the first end and the second end are both capable of acting as an inlet or an outlet, said first end having a first access port and said second end having a second access port;
• during warm weather months, transferring a volume of wastewater from a wastewater reservoir into the attached growth reactor in a first direction of flow through the attached growth reactor at the first access port and removing treated effluent from the attached growth reactor at the second access port to an effluent collector for a first period of time;
• then reversing direction of flow through the attached growth reactor by transferring the volume of wastewater from the wastewater reservoir into the attached growth reactor via the second access port and removing treated wastewater from the attached growth reactor at the first access port to the effluent collector for a second period of the warm weather period, thereby forming two populations of nitrifying bacteria, each proximal to one end region of the attached growth reactor.