Oxygen mass-transfer coefficients for different sample containers used in the headspace biochemical oxygen demand...

To accurately measure the oxygen demand of a wastewater sample in a headspace biochemical oxygen demand (HBOD) or other respirometric test, the rate of oxygen transfer to the aqueous phase must be greater than the oxygen exertion rate by the sample. Oxygen mass-transfer coefficients (Kawa) measured for 28-, 55-, and 160-mL, partially full (18 to 89%) containers placed on their sides on a shaker table and mixed at 200 r/min averaged 8.0 h-1 (range 5.4 to 9.9 h-1). For this mass-transfer coefficient, HBOD values as great as 1340 mg/L.d are possible at the start of an HBOD test, although the maximum daily HBOD declines to 192 mg/L.d at the end of the test because of oxygen depletion in the sample headspace. Mass-transfer coefficients for shaken samples decreased only at low shaking speeds (< 50 r/min). Oxygen mass-transfer coefficients for shaken samples were always larger than those (average of 1.8 h-1) measured for samples in a 250-mL bottle mixed with a stir bar on a stir plate. These mass-transfer coefficients indicate that the oxygen demand of typical full-strength municipal wastewaters can be measured in HBOD tests without oxygen transfer limiting the reaction rate.