A facility normally maintains a 1.0 mg/L total chlorine residual (TRC) at the end of the contact basin. Over the last 2 days, the chlorine dose had to be increased from 6 mg/L to 12 mg/L to keep any detectable residual, yet the basin effluent TRC is still only 0.1 mg/L. The lab reports effluent from the aeration basin is NH3-N = 0.2 mg/L and NO2-N = 2.0 mg/L (NO3-N is low). Operators also note lower-than-normal DO in the nitrification zone yesterday. Assume 1.0 mg/L NO2-N consumes about 5.0 mg/L chlorine (as Cl2). Flow is 1.8 MGD and contact time is 40 minutes. Which finding MOST directly explains the sudden chlorine demand increase and points to the best operational direction?