I am working on this for my paper. and I tried to use a probe for the measurement of the OUR. you will need also a DO controller to stop aeration at 75% Cs* at the process condition.

But 75% saturation is hard to achieve. In normal WWTP, DO concentration is around 20%. The probe that measures the OUR at 75% is not what actually happens at the normal DO level? Of course, one can aerate the basin water to 75% DO, but then it comes back to the question, would the increase in oxygen availability be a factor in determining the OUR?

As with the substrate, if you assume that the OUR is independent of DO then elevating the DO is simply a means to allow the measurement time to be prolonged to get a better measurement.

An alternate method is to use the Kalman filter-based methods to estimate the OUR - they also estimate the KLa at the same time. The approach has been published many times, and is subject to the assumption that OUR is at a point where it is not affected by DO. It can be time varying, reflecting an effect from other substrates.

I have used the first, but with no means of knowing how representative the result was.

I have also used the second, where all I could say was that the results were believable, in the same area as KLa and OUR estimated from an on/off aeration test, and that when tested against a simple model with known KLa and OUR, and then adding noise, the results picked up against the true (model) parameters.

But I have seen your reference to a paper that the re-aeration step test may not give the best results - added to the pile for when I have some spare time, or work comes back into this area.

With 'typical' DO half-saturation coefficients at 0.1 - 0.2 mg/l, then starting at 2 mg/l is 'good enough' for the assumption of independence of OUR from DO - elevating DO for the test is then also 'good enough.'

Dear Jeremy:

Has anyone tried the steady state column test? As per ASCE 18-96, to estimate oxygen uptake rate, mixed liquor is pumped from a position within the existing aeration tank. Mixed liquor is continuously pumped to the test column from a position within the existing aeration tank using a submersible pump. The liquid detention time in the column is typically maintained between 10 and 15 min. The mixed liquor should be aerated using a fine pore (fine bubble) diffuser. The oxygen transfer efficiency of the diffuser used in the column using process mixed liquor is measured using the off-gas techniques. The airflow rate to the test diffuser is adjusted so that the DO concentration in the steady state column is maintained in the range of those found in the test section of the aeration basin. The off-gas measurements provide oxygen transfer efficiency data at the test conditions.

Oxygen uptake rate is determined by a mass balance of oxygen around the column system as:

oxygen uptake rate = (oxygen transfer rate - net change in DO)/column volume,

or R = (OTRf - (DOo - DOi)Qi)/V

For an example, an ex situ column test is performed at a test section of the aeration basin. The following data are collected:

DOi (at transfer pump) = 0.55 mg/L

DOo (in test column) = 0.80 mg/L

qi (airflow rate to column) = 1.07 NL/s

OTEf (measured in column) = 0.130 mg O2 transferred / mg O2 supplied

Qi (mixed liquor pump rate) = 2.16 L/s

V (column volume) = 1,460 L

OTRf = 1.07 NL air/s × 299.3 mg O2/ NL air × 0.13 mg O2 transferred / mg O2 supplied = 41.6 mg O2/s

R = [41.6-(0.80-0.55) × 2.16]/1,460 = 0.0278 mg/L•s × 3600 s/hr = 101 mg/L/hr

Unfortunately, there is no comparable data using the other methods that you mentioned.

Regards,

Johnny Lee

I haven't. I hadn't heard of this method!

The beauty of this method is that it does not require artificially aerating to a higher DO level, and KLa doesn’t come into the picture as well. I suggest we do an experiment in one treatment plant with an acrylic column 3 m or so high; and compare the result with the traditional BOD bottle method and observe the difference, esp. for low oxygen-limiting conditions? The key element of success is the off-gas analyzer that must measure the offgas accurately, since the OTE is highly sensitive to changes in the gas composition. We can try this in Stevenage?

Will be difficult, since the research funding dried up years ago. But difficult is not impossible.

There is certainly a need to get to the bottom of this. Eckenfelder (1952) published the article "Aeration Efficiency and Design" in which he described two methods of testing for the microbial respiration rate. Both the steady-state method and the non-steady state method have been used universally ever since. The example presented in the article appears to have "validated" these two methods beyond a shadow of doubt, that these two methods are compatible with each other. The results were given in a table. In this table was shown the test results for 6 runs, for an aeration tank of 33 inches tall, and aerated at different flow rates from 33 cu ft/hr to 92 cu ft/hr. In terms of SI units, the air flow rate (AFR) and the height-averaged air flow rate would essentially be the same given the small height. Eckenfelder used the log-deficit method to calculate the mass transfer coefficients, corresponding to each AFR. I reproduced and converted his data and then used the non-linear regression analysis (NLLS) as recommended in the standard (ASCE 2-06) to re-calculate the Kla's. Next, I used the steady-state method to again calculate Kla, after using the measurements of the individual respiration rates from the BOD bottle method, and equating those with the oxygen transfer rate as Kla(C*-c), and the results are similar to those of the non-steady state method, proving the validity of both test methods. Unfortunately, clean water tests were not performed, and so there is no way to estimate alpha. However, the respiration tests were done at 2 p.p.m. as reported in the article, and so these samples must have been re-aerated by vigorous shaking to at least twice the value of the in-situ dissolved oxygen concentration. If my hypothesis is correct, i.e., that the microbial oxygen uptake rate is linearly proportional to the oxygen availability, then the measured R values must have been at least twice the actual values in the aeration basin where the samples were withdrawn.The resultant Kla values would then be half the actual values measured by the non-steady state method. This experiment then in fact did not prove the validity of either one or the other, but proved that these two methods give results that are off by 50%.

In my thesis, I postulated that the equation for the steady-state method should come out to be Klaf = 2R/(C* -cr) instead of a single R as conventionally used, because of the gas depletion effect in the air bubbles. With this modification, this would then given the exact results of the Klaf as before using the ASCE method, if it is reckoned that the BOD method of measuring the OUR is incorrect (over-estimated) because of the additional aeration. It is therefore vital, to prove one way or another, that an in-situ oxygen uptake rate test be performed similar to that described in the Guideline ASCE 18-96 (or more recently ASCE 18-18) for the steady-state column test using the off-gas measurement techniques. This may confirm, once and for all, whether oxygen availability has an effect on the oxygen uptake rate in a sample upon re-aeration.

Ref: Eckenfelder, W. (1952). Aeration Efficiency and Design: I. Measurement of Oxygen Transfer Efficiency. Sewage and Industrial Wastes, 24(10), 1221-1228.

In this paper "Evaluation of Activated Sludge Oxygen Uptake Rate test procedures", a series of bench‐ and pilot‐scale experiments was conducted to evaluate the ability of biochemical oxygen demand (BOD) bottle‐based oxygen uptake rate (OUR) analyses to represent accurately in-situ OUR in complete mix‐activated sludge systems. Aeration basin off‐gas analyses indicated that, depending on system operating conditions, BOD bottle‐based analyses could either underestimate in-situ OUR rates by as much as 58% or overestimate in-situ rates by up to 285%. A continuous flow respirometer system was used to verify the off‐gas analysis observations and assessed better the rate of change in OUR after mixed liquor samples were suddenly isolated from their normally continuous source of feed. OUR rates for sludge samples maintained in the completely mixed bench‐scale respirometer decreased by as much as 42% in less than two minutes after feeding was stopped. Based on these results, BOD bottle‐based OUR results should not be used in any complete mix‐activated sludge process operational control strategy, process mass balance, or system evaluation procedure requiring absolute accuracy of OUR values. This echos my previous suspicion about Eckenfelder's experiment for calculating the oxygen transfer efficiency based on sample testing of the respiration rate which required artificial boosting of the DO concentration in a BOD-bottle.

re: Chiesa S.A. et al. (1990) Evaluation of Activated Sludge Oxygen Uptake Rate test procedures. J. Environmental Eng. May 1990 | Volume 116, Issue 3

In the BOD bottle test, the boosting of the DO concentration may make the sample measurement artificially high, and so the true uptake rate in the aeration tank is not measured correctly.

I reckon there are only three possible explanations. Firstly, when the sample is agitated by vigorous shaking, the activity level of the microbes might increase. Like athletes under hard work, they respire more. Secondly, the oxygen level in the sample may be limiting (although at 2 ppm, it shouldn't be); thirdly, Garcia-Ochoa suggested a 'cell economy principle' by which the microbes voluntarily reduce the respiration level at low DO and changes the respiration rate at elevated level due to increased oxygen availability. There have been many literature on this but none seems to have given a definite answer. How do we correct this error?

An article by Doyle (1981) suggested an interesting method of testing for alpha. It seems to me that it may be possible to use such a dilution method to test out the determination of OUR. By first aerating a tank of pure water of same temperature to an elevated DO, say 7 ppm, and then pour the activated sludge mixed liquor into the tank, and then gently mix them together, it may be possible to measure the slope of the DO depreciation curve at quiescent conditions, thereby eliminating the first possible explanation for the cause of any increased OUR measurement. If the sample has been diluted to 50%, 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 insitu measurement.

Would this work? Does anybody have any data with the steady-state column test as described in the recent ASCE/EWRI Guidelines (ASCE 18-18)?