Oxygen Uptake Rate (OUR) and Specific Oxygen Uptake Rate (SOUR)

Oxygen uptake rate (OUR) represents the volumetric oxygen consumption rate. It can be directly correlated with food to volume (F/V) ratio since the amount of substrate to be decomposed is proportional to the amount of oxygen required at steady state.

By definition, volumetric oxygen dissolution rate must exactly match with OUR unless dissolved oxygen (DO) level changes. Or, if there is no additional oxygen dissolution at all, declining DO is a direct consequence of OUR. The non-steady state desorption method is relying on this principle to estimate oxygen transfer efficiency (OTE).

The specific activity of microorganisms is measured by specific oxygen uptake rate (SOUR) and can be obtained by dividing OUR by MLVSS or MLSS. As OUR is related to F/V, SOUR is directly related to F/M. The low SOUR in MBR is a natural consequence of the low F/M ratio employed in MBR design. Typical design and operational parameters are summarized in Table 1.

Although microorganisms are capable of very high SOUR such as >50 mg O2/g MLVSS/hr, oxygen dissolution cannot keep up with such high demand due to the lowering OTE at high aeration rate. As air flow rate increases, more air bubbles coalesce and surface area does not increase proportionally to the air flow rate. At a certain point, additional air flow no longer create more surface area, where specific air dissolution reaches its plateau. Perhaps economically feasible maximum OUR for CAS and MBR, which can be supported by diffuser system, are less than 150 mg/L/hr and 100mg/L/hr, respectively.

Table. 1. Comparison of preferred ranges of operating parameters in MBR and CAS

Design Parameter Unit MBR CAS
 

F/M

g BOD/g MLSS/day 0.04-0.12 0.16-0.24
g COD/g MLSS/day 0.08-0.24 0.32-0.48
g BOD/g MLVSS/day 0.05-0.151), 2) 0.2-0.3
g COD/g MLVSS/day 0.1-0.303) 0.4-0.6
F/V g BOD/L/day 0.5-1.5 0.6-0.9
g COD/L/day 1.0-3.0 1.2-1.8
MLSS g/L 8-121) 2-4
MLVSS g/L 6-10 1.7-3.4
SRT days 10-30 5-10
SOUR mg O2/g MVLSS/hr 2-54) 6-12
OUR mg O2/L/hr 15-50 20-40
DO mg/L 1-2 1-2

1) Judd, 2006
2) Brepols, 2010
3) Converted from the BOD based F/M assuming COD/BOD=2.0 for municipal wastewater
4) Ng, 2006

It must be emphasized that OUR and SOUR do not necessarily indicate how healthy microorganisms are because they are a consequence of organic loading. For instance, SOUR decreases as F/M ratio decreases just because microorganisms’ activity decreases. Specific ATP concentration, which equals to total ATP concentration in mixed liquor divided by MLVSS, sharply decreases as SRT decreases as shown in Fig. 1. Therefore, OUR/SOUR alone cannot be used as a performance indicator of microorganisms in biological system.

OUR.ht2Fig. 1. Total and specific ATP content in mixed liquor in lab scale MBR treating synthetic feed (Maeng et al., 2013).

 

© Seong Hoon Yoon