No, Not solely dependent on dissolved oxygen levels, but rate of oxygen consumption is affected by the DO levels. when the level of dissolved oxygen in the medium falls below a certain point, the specific rate of oxygen uptake is also dependent on the oxygen concentration in the liquid. Oxygen uptake rate and DO are inversely proportional to each other. The dissolved oxygen uptake rate (DOUR) test measures the respiration rate of organisms. Since it measures the rate at which oxygen is used (in mg O2/L/hour), it is a useful tool to evaluate process performance, aeration equipment, and biodegradability of the waste. The oxygen uptake rate is one of the fundamental physiological characteristics of culture growth and has been used for optimizing the fermentation process. oxygen uptake rate (OUR) is the microorganism oxygen consumption per unit time and is one of the few accessible parameters to quantify the metabolism rate of the activated sludge.

thanks for this reply. I also believe that the DO level only affects R (rate of oxygen consumption) when it reaches a critical level, above which R is unaffected. I am not sure about the inverse proportionality between R and DO. I imagine you meant that by stopping the waste flow to the aeration basin being tested, and continuing the sludge recirculation system, the activated sludge will quickly reach endogenous metabolism and will have a uniform oxygen demand rate over the aeration system. Since the oxygen uptake rate R will be relatively low, the DO in the aeration basin will be high. In that sense, the two are inversely proportional to one another. In AWWA 2017, the method used as a standard method to determine the R value of a sample of activated sludge is the BOD bottle method.

This method has a fatal drawback, in that it can severely over-estimate the consumption rate when the sample must be aerated to give a linear straight line of a plot for the DO vs. % saturation.

Looking at Fig. 3 rate of oxygen utilization for the resting cells of Pseudomanas ovalis(Kempe and Bennett 1964), one can see that when the DO conc. is above 17% (say 1.4 mg/L for a saturation C* of 8.8 mg/L) the rate of change of the oxygen electrode reading becomes 100 (i.e., the rate of change of R is zero). R becomes a constant until the sample reaches full saturation. Conversely, when the sample begins with a high DO concentration, say 5 mg/L, R would remain constant until the DO reaches the critical value of 1.4 mg/L beyond which the rate of decline will no longer be linear. This scenario is true provided the initial DO must not be artificially aerated by shaking or injecting DO molecules like in a respirometer. Unfortunately, this is the recommendation by the Standard Methods (AWWA 2017).

Proposed Dilution Method In the article by Doyle and Boyle (1981), an interesting method of testing for alpha was suggested. It appears that it may be possible to use the dilution method as suggested by the article to test out the determination. By first aerating a tank of pure water to an elevated DO, say to 7 mg/L, and then, upon stopping the aeration, gradually and carefully pouring a sample of known volume of the activated sludge mixed liquor into the tank, and then gently mixing them together, it may be possible to measure the slope of the DO decline curve at quiescent conditions, thereby eliminating the first possible explanation for the cause of increased OUR measurement. If the sample has been diluted to 50% by the tank, the resultant slope should then be multiplied by 2 to get the true OUR. This should then be compared with the steady-state column test with an in-situ measurement as recommended by the ASCE Guidelines (ASCE 18-18). Alternatively, mixed liquor can be continuously pumped to the test tank from a position within the existing aeration basin using a displacement pump until a set known volume is withdrawn. This should give the same oxygen depletion curve as the steady-state test, allowing the measurement of the slope of the curve as a measurement of the microbial oxygen uptake rate. To avoid any substrate limiting effect, the test should be done in-situ as quickly as possible just like the off-gas column test.

Proposed Steps for the Dilution Method 1. Aerate a tank of pure water of 4 to 6L volume to an elevated DO, say to 7 mg/L. 2. Collect a sample of mixed liquor to be evaluated into a container of approximately 4 to 6 L. 3. Upon stopping the aeration, gradually and carefully pour the sample of known volume of the activated sludge mixed liquor into the tank. 4. Then gently mix them together by a mechanical stirrer until mutual dilution is complete. 5. Immediately measure the DO by the Winkler Method or by using a calibrated fast-response DO probe with probe time constants less than 0.02 / KLa. 6. Plot the DO versus time on a graph and calculate R using a linear least-squares regression to fit a straight line through the data points. Since the mixed liquor is diluted to 50% its original concentration, the resultant slope of the line is multiplied by 2 to obtain the in-situ R value. The time lapse between sample collection and uptake rate measurement is critical in this ex-situ procedure. The entire process from the collection of samples to starting DO monitoring should take less than 2 min [ASCE 18-18]. The procedure should be replicated at least three times at any given sampling point. A worked example is given below. Suppose the treatment plant in-situ aeration tank has a DO level of 2 mg/L and the bench-scale aeration unit is aerated to 7 mg/L, then the mixture will have a DO concentration of: (a + b)/2 = (2 + 7)/2 = 4.5 mg/L Suppose the DO level dropped to zero or close to the critical level in 7 mins, then the slope of the decline curve would be: 4.5/7 X 60 = 38.57 mg/L/hr The actual oxygen uptake rate in-situ is therefore given by twice this value or 2 X 38.57 = 77 mg/L/hr This experiment should then be compared with the BOD bottle method (AWWA 2017 method 2710A) to confirm whether the disturbance due to shaking was the real cause of overestimation or that it was caused by increased oxygen availability.