Erroneous and misleading Oxygen Transfer Rate (OTR)

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WARNING: Beware of the following when choosing an air diffuser—


Erroneous and misleading Oxygen Transfer Rate (OTR)

No OTR measurement standard exists for industrial wastewater treatment in Japan

Because, for example, the fuel efficiency of road vehicles is determined by following a standardized procedure, buyers are able to make fair assessments and comparisons between different makes and models. However, no such standard exists for measuring the OTR of air diffusers used in industrial wastewater treatment in Japan. For sewage treatment, OTR must be measured according to a standard published by the Japan Sewage Works Agency. There is no corresponding standard for industrial wastewater. As a result, some manufacturers of air diffusers advertise massively inflated OTR values that have been obtained through unscientific measurements. Please read on for details.

How to correctly measure OTR

Point 1: Water depth
The deeper the water, the higher the pressure, and therefore also the higher the OTR. Because it is strongly affected by water depth, the OTR must be measured separately at each desired depth (see Fig. 1 below). For example, after obtaining OTR data at a depth of 3.0m, completely drain the water and refill the tank to a new depth of 3.5m, then proceed to measure the OTR at this new depth. Repeat this process for each desired water depth.
After each refill, place one DO meter into the tank and record the change in DO concentration over time. This is the basic procedure for measuring OTR, and is somewhat labor-intensive.

Fig. 1: Correct OTR measurement method

Fig. 2: Incorrect OTR measurement methodHowever, there is one company that attempts to take a shortcut by using a single tank, filling it with water to a depth of 5.0 m, placing multiple DO meters at 1 meter intervals from the surface, and finally measuring the DO at every depth at the same time. These measurements then form the basis for calculating OTR at each depth (see Fig. 2 to the right).
This method is not scientifically valid. Regardless of the depth of the DO meters, it is impossible to calculate OTR data for any depth below 5.0 m using this method.
Based on the impossibly high OTR values that this unscientific method produces, this company has been falsely advertising their air diffuser as "highly-efficient".

Point 2: Position and orientation of the DO meter probe
When measuring the OTR, it is imperative to first remove the entirety of the DO (e.g. by adding a reducing agent such as sodium sulfite). Only then can you proceed to discharge air from the air diffuser and measure the increase in DO with a DO meter.
As explained below, proper use of the DO meter itself is also important.

Discharged bubbles of undissolved oxygen that come into direct contact with the DO meter’s electrode will be measured as if they were dissolved oxygen, which artificially inflates the measured DO value. This is particularly relevant when the electrode is placed directly above the air diffuser, as in Fig. 2 (above).


In order to prevent this, the probe must be positioned and/or oriented in such a way that rising bubbles do not make direct contact with the electrode. The ideal position for the DO meter is up against the inside of the tank, away from the air diffuser and any rising bubbles. The ideal orientation is pointing directly upwards. However, the probe will naturally point downwards when placed in the water. To avoid this, bend the cable back on itself and secure it to the probe with waterproof (e.g. vinyl) tape. This ensures that it will remain pointing upwards as shown in the picture to the right.
When the probe is pointing upwards, bubbles discharged from air diffusers do not make direct contact with the electrode. Only by following this method can the DO be measured accurately.
Unfortunately, the majority of companies obtain DO measurements with the probe facing downwards and therefore obtain invalid data because undissolved oxygen is measured as if it were dissolved.

Point 3: Water temperature
Physical laws tell us that the saturation limit of oxygen varies greatly with the temperature of the surrounding water.
Hot air from air blowers naturally heats up the water during aeration; it is therefore vital to continually record both the temperature of the water and the DO value over the course of the test, and include all of this information in the OTR test report.
Watch out for test reports that do not include water temperature data. These should be regarded with suspicion.

Always thoroughly check the OTR test report

Before purchasing an air diffuser, be absolutely sure to read over the OTR test report and confirm that valid scientific methods were used to obtain the values that are advertised. Pay particular attention when a company advertises an OTR that is much higher than its competitors’. There is also one company that has not conducted any tests at all and uses entirely fictitious OTR values in their brochures.

Our OTR data and methods are open and transparent

OTR data for the OHR AERATOR was obtained using sound scientific methods and independently verified by a third party. We can provide both reports upon request.

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