Clear misrepresentation of Oxygen Transfer Rate efficiency

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

Clear misrepresentation of Oxygen Transfer Rate efficiency

There is no official standard of measuring OTR for industrial wastewater treatment in Japan

For example, because the fuel consumption performance of a car is measured based on strict standard criteria, the user can judge the superiority or inferiority of performance fairly. However, there is no official Oxygen Transfer Rate (OTR) measurement standard for air diffusers in industrial wastewater treatment in Japan. (There is a standard determined by the Japan Sewage Works Agency for sewage treatment which air diffusers for sewage treatment must comply with. However, for industrial wastewater there is no official standard to measure OTR.)
As a result some air diffuser manufacturers advertise a high OTR based on non-scientific principles when in fact, if based on scientific principles, their OTR would be very low. For details, see the followings:

How to correctly measure OTR:

1) Water depth
The deeper the water, the stronger the water pressure, so the OTR increases. The difference in water is an important factor to increase/decrease OTR. Therefore when measuring OTR it is necessary to obtain data at each water depth (i.e. 3.0m / 3.5m / 4.0m / 4.5m / 5.0m / 5.5m …). For example, after obtaining the OTR data at the 3.0m water depth, drain the water and fill with new water to 3.5m water depth and take the data at 3.5m water depth, then repeat the process for each water depth. Set one DO (dissolved oxygen) meter in the tank each time and record the temporal change in DO concentration. See fig.1 below. This is the basic method to obtain OTR data which requires time and effort.

fig-1: Correct OTR measurement method

fig-2: Incorrect OTR measurement method However, there is a company that takes shortcuts and sets DO meters at 1.0m, 2.0m, 3.0m and 4.0m from the surface in one tank filled with water up to 5.0m depth, and obtains DO data for each water level at the same time. Then based on this data, OTRs for each depth are calculated. See the fig.2 on the right.
This testing method is scientifically incorrect. Regardless of the drop point of the DO meter, it is impossible to accurately obtain OTR at lower than 5.0m water depth. Due to the impossibly high OTR obtained by this inaccurate method, the company have been advertising their air diffuser as a ”highly efficient air diffuser”.

2) Position and direction of DO meter’s electrode tip
When measuring the OTR, it is necessary to remove all DO from water (e.g. tap water) in the tank to zero by adding sodium sulfite firstly, and then discharge air from the air diffuser and measure the increase in DO value using a DO meter. How to use the DO meter is important. See the following points:

(1) When the bubbles discharged from an air diffuser directly touch the DO meter’s electrode tip, the correct DO value can not be measured accurately because it measures undissolved oxygen (oxygen gas in the bubbles) as if it were DO. This is especially relevant in the fig.2 case above where the electrode tip is set above the air diffuser as shown. To prevent this, it is necessary to set the electrode tip in a position where the rising bubbles do not hit it directly.

(2) When you set the DO meter in water normally, the electrode tip will face downwards. To avoid this, bend the cable and fix the electrode with the cable using vinyl tape, then the electrode tip will face upwards as shown in the picture on the right.
This improvised method prevents air bubbles discharged from air diffusers directly hitting the electrode tip and measuring undissolved oxygen (oxygen gas in the bubbles) as if it were DO. When the electrode is facing upwards, discharged bubbles from the air diffusers do not attach directly to the electrode tip. Only following this improvised method can the DO be accurately measured.
This method is very important, but in most cases it is measured with the electrode tip facing downwards. This means there are many cases where DO is not measured correctly and undissolved oxygen is measured as if it were DO.

3) Water temperature
Due to the law of nature, the saturation value of DO depends greatly on the water temperature. When aerated with an air diffuser, the water temperature increases due to the hot air discharged by the air blower, so both the water temperature and the DO value at the time of the OTR test must be recorded and that data should be included in the test report. An OTR test report with no water temperature data is questionable, so be careful of this.

You need to check the OTR test report

It is imperative that you check the OTR test report of the air diffuser before purchase to ensure the advertised OTR is obtained based on correct scientific methods. (Particular attention is required for air diffusers advertised with a much higher OTR than other air diffusers.) There is a manufacturer that advertises false OTR in their brochure even though they did not obtain OTR data at all.

OHR will clearly show the OTR test report

OHR obtained OTR data from the OHR AERATOR through a scientifically proven method. In addition, there is an OHR AERATOR OTR evaluation report conducted by another company. Please request these reports from us.