OTE is a key indicator of air diffuser (aerator) performance, and ASCE/EWRI 2-22 sets out standardized methods for its evaluation in an objective and reproducible manner. Devised by the Environmental & Water Resources Institute (EWRI) of the American Society of Civil Engineers (ASCE), this standard is one of the world’s most reliable.

While for many years we had been using our own OTE figures, we recently engaged a third party in Europe in order to obtain new data with worldwide currency, following this standard. The results closely matched our in-house measurements, corroborating the reliability of our data, and we have now incorporated these values into our calculations.

Incidentally, in one case of a Japanese diffuser that is professed to be “high efficiency”, OTE was 60–70% lower than claimed.
In Japan, which lacks a unified standard in the vein of ASCE/EWRI 2-22, the industry is founded on the uncynical assumption that ”surely every company obtains their data via scientifically-rigorous methods”. But whether intentionally or not, some companies advertise unreasonably high OTE. Caution is advised.
For more information on common mistakes made during measurement, please see our FAQ page, or contact us directly.
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As will be explained more fully in the following section, the ASCE/EWRI 2-22 standard specifies each of its measurement procedures in meticulous detail; and by adhering to them, near-identical results can be obtained regardless of where in the world or by whom they are carried out. Note, however, that this standard has not been adopted in Japan — it is not even recognized here.
We had tests conducted to this standard as we sell extensively to overseas markets, but the absence of any domestic organization that undertakes compliant measurements presents a significant hurdle, with mounting expenses.

In Europe and the US, where this standard has been widely adopted, the simple fact that a performance test report complies with ASCE/EWRI 2-22 guarantees a certain level of reliability. Japan, by contrast, finds itself in a situation where measurement techniques and conditions differ from company to company because no unified standard for assessment has been established — a situation that has led to companies collecting data using faulty methods. It is therefore essential not to simply ‘swallow’ official OTE figures at face value, but to obtain the test reports and scrutinize the conditions and methods used.

The fact that ASCE/EWRI 2-22 systematically sets out measurement procedures based on the phenomenon of oxygen transfer makes it possible to obtain highly-reliable results even without strict compliance with every detail of every stipulation, provided that the underlying principles have been properly grasped.
This page aims to provide an understanding of these scientifically-sound measurement procedures by explaining the content of the ASCE/EWRI 2-22 standard, and in so doing enable you to ascertain whether advertised OTE data was obtained by valid means — for any company.

The ASCE/EWRI 2-22 standard prescribes the following test procedure. (Note that this information is not intended to be a substitute for the original document and should not be used as such. Please purchase a copy from ASCE, such as via the ASCE Library online bookstore, if you require full details.)

Step1: Fill the tank with clean water

Fill the tank with clean water to the required level for measurement.

While diffusers are normally used in the treatment of wastewater, there is enormous variation in the nature and quantity of wastewater pollutants (organic matter, suspended solids, microorganisms) depending on the source. Measurements are thus taken in clean water as a provisional unified standard. This is true of all oxygen transfer tests, not only ASCE/EWRI 2-22.
For information on the extent to which clean-water OTE drops in wastewater, please see this page: ”What is the ‘alpha factor’?”

Step2: Lower the DO

Before commencing testing, the dissolved oxygen (DO) level in the test water must be lowered sufficiently.
The following two methods are prescribed:

● Sulfite deoxygenation method:

This method involves adding sodium sulfite as a reducing agent, which reacts with the oxygen to form sodium sulfate. Cobalt chloride is added as a catalyst. The DO level must be depressed below 0.5 mg/L at all measurement points in the tank.

● Nitrogen deoxygenation method:

Introduced in the 2022 (latest) revision, this method involves adding nitrogen gas to replace the dissolved oxygen. It does not use any chemical agents, so does not create any byproducts, enabling a cleaner measurement. Unlike the sulfite method, aeration begins once the DO concentration falls below 1.0 mg/L.

Step3：Commence aeration and measure change in DO over time

Aeration is begun, and continues until the DO concentration has reached at least 98% of its saturation level, during which period the airflow rate, change in DO, water temperature and other parameters are recorded over time.

DO measurement may be performed either continuously using DO meters, or by using the Winkler method (iodometric titration).
Here is an outline of the key provisions in the case of DO meters:

• ❶ Use a minimum of four DO determination (measurement) points
• ❷ Distribute the points vertically and horizontally so as to be as representative as possible of the contents of the tank
• ❸ The points must include one at a shallow depth, one at mid-depth, and one at a deep location
• ❹ Locate the points sufficiently far (at least 0.6 m) from the walls, internal structures, floor and surface.
  Further, the points must be no closer to the surface than 10% of the minimum tank dimension.
• ❺ Secure the DO probes to minimize movement by water currents

Step4：Determine KLa

KLa (the volumetric mass transfer coefficient) is determined from the DO-versus-time data, then corrected to the standard temperature of 20°C. Please see the glossary for an explanation of the meaning of KLa.

Step5：Perform at least three replicate tests under identical conditions

At least three replicate tests are conducted under identical conditions. The temperature-corrected KLa values in each replicate must not vary by more than ±15% from the mean value. Only test results that fulfil this condition are considered valid.

Step6：Convert to SOTR (Standard Oxygen Transfer Rate)

For each replicate test, the results are corrected to standard conditions and the SOTR is calculated.
The average of these SOTR values is then taken.

Standard conditions:

• ● 20°C water temperature
• ● Atmospheric pressure
• ● Zero DO concentration

A further correction to a reference TDS (total dissolved solids) concentration of 1,000 mg/L is made to account for its effect on KLa.

Step7：Calculate SOTE (Standard Oxygen Transfer Efficiency, %)

Finally, the SOTE is calculated from the SOTR.

This is the procedure as prescribed in the standard.

Actual testing involves collecting data at multiple different water depths, as the depth greatly affects the OTE.
This means that Steps 1–7 must be repeated for each water depth under measurement; testing thus takes a considerable amount of time, effort and expense.

ASCE/EWRI 2-22 specifies methods for testing and calculating the following four quantities:

1. Overall volumetric mass transfer coefficient (KLa)

Meaning: an indicator of how readily oxygen moves into a liquid

KL denotes the oxygen transfer coefficient (ease of movement) in a liquid film; a denotes the gas–liquid interfacial area (bubble surface area)

It is a key indicator of the performance of air diffusers and agitators

Unit: h^-1

2. Oxygen transfer rate (OTR)

Meaning: the amount of oxygen actually transferred to the liquid per unit time

KLa × (saturation DO – actual DO)

Unit: kg/h

3. Standard oxygen transfer rate (SOTR)

Meaning: OTR measured under standard conditions (clean water, 20°C, atmospheric pressure)

Unit: kg/h

4. Oxygen transfer efficiency (OTE)

Meaning: the percentage of the supplied oxygen that is actually dissolved into the liquid

Unit: %