Are your effluent BOD5 results significantly higher than your suspended solids results? Are you having trouble meeting your BOD5 permit limits? Would you meet your permit limits if you could report CBOD5? Do you experience a significant discrepancy between effluent BOD5 and CBOD5 results? If the answers to these questions are “yes,” you may be experiencing the negative impact of nitrification in your 5-day biochemical oxygen demand (BOD5) test. You are not alone. John Hall and Robert Foxen reported in 1983 that as many as 60 percent of the BOD5 violations nationally may have been caused by nitrification in the BOD5 test rather than improper design or operations.1