A gas chromatographic-based headspace biochemical oxygen demand test

The 5-day biochemical oxygen demand (BOD₅) test is an established tool for measuring the concentration of biodegradable organic matter in wastewater effluents. Unfortunately, the BOD test is time consuming, labor intensive, and, due to the dilution of the wastewater, takes a relatively long time to complete. A few years ago a headspace BOD (HBOD) test was developed that avoided the need to dilute wastewater samples. A disadvantage of the original HBOD test was that the sample had to be transferred from the HBOD tube to another vessel for a dissolved oxygen measurement. This study demonstrates that it is possible to conduct HBOD tests using a gas chromatograph to measure oxygen utilization in the sealed tube and that a 3-day HBOD provides a reliable estimate of the BOD₅. The HBOD₃ values measured for primary and secondary clarifier effluents from an activated sludge plant were 114 and 23 mg/L; BOD₅ measurements were 116 and 24 mg/L. Changes in headspace volumes did not significantly change the HBOD₅. Secondary settled wastewater had a HBOD₅ of 83 ± 3 mg/L (mean ± SD) for headspace volumes of 10-20 mL (36- 71%) in a 28-mL tube. The HBOD₅ was also constant (84 ± 6 mg/L) when samples were diluted 20-60% using BOD dilution water at a fixed headspace volume of 8 mL in HBOD tubes. Nitrification increased the HBOD of nondiluted wastewaters after 4 days, although nitrification appeared to be sufficiently inhibited using a standard nitrification inhibitor. A calibration test for the HBOD was developed using a 300-mg/L glucose and glutamic acid (GGA, 50% each) solution and secondary clarifier wastewater that provided HBOD₃ values similar to those obtained in the analogous BOD₅ test. It is argued that the simpler procedures, added precision of GC-based protocols, and more rapid exertion of oxygen demand make the HBOD test superior to the conventional BOD test.