Methods of OTR-Measurements
There are three methods of otr-measurements: the absorption method, the desorption method and the offgas method. All methods are described in the German guideline DWA-M 209. We are able to perform each of these measurements according to the state of the art. All methods allow the determination of the commonly interesting values (α)SOTR (oxygen transfer rate), (α)SSOTR (specific oxygen transfer rate) and (α)SAE (aeration efficiency). Advantages, disadvantages and application of the various methods are briefly explained below.
Absorption Method
Absorption measurements can be applied in pure water as well as in activated sludge. For measurements in pure water the oxygen is depleted by sodium sulfite. The addition of cobalt sulfate is required as a catalyst. Although the concentrations of the necessary chemicals are generally harmless the disposal of the water should be coordinated with the responsible environmental authority.
To avoid disposal issues, absorption measurements in pure water would require pure nitrogen for oxygen depletion by stripping. Unfortunately, this method is usually ruled out due to the comparatively high amount of nitrogen required. Another clean but at the same time better affordable method of otr measurements in pure water are desorption measurements.
Absorption measurements in activated sludge are a cost-effective alternative to pure water measurements. Oxygen depletion is easily achieved by respiration (microorganisms) and thus neither chemicals nor pure gases are required. However, this type of measurement requires a complete interruption of the influent to the aeration tank, as the method requires a constant oxygen uptake rate.
Desorption-Method
Desorption measurements are the most elegant method for the proof of guaranteed performance values. Although possible in activated sludge, too, they are almost exclusively used in pure water. In this type of measurement, the oxygen concentration in the aeration tank is raised to around 30 mg/L by super saturation (with pure oxygen). Subsequently aeration with ambient air causes the oxygen concentration to drop to the saturation value valid under atmospheric conditions. From a physical point of view desorption and absorption measurements are the same. Oxygen transfer rates can be equally well determined by both methods.
Offgas-Method
The offgas method is preferably used to determine the oxygen transfer rate under operating conditions and when tank geometry or diffuser configuration are not suited for absorption measurements. It is always performed in activated sludge. For this method the air leaving the activated sludge tank is collected by a floating hood and subsequently analyzed. Thanks to our newly developed mobile offgas hood, the complicated hose routing out of the tank (the common method in the past) is no longer necessary. Gas quantity and composition are analysed directly on the hood. The measurement of oxygen content in the offgas allows direct determination of the specific oxygen transfer rate in the corresponding tank section.
Verification of guaranteed performance values
Checking guaranteed performance values of newly installed aeration systems is a frequently requested service. It ensures plant operators of the expected operating costs. At an expense of usually significantly less than 1% of the investment, the benefits clearly outweigh the costs. Because one thing is clear: only an independent verification reliably detects deficiencies affecting aeration performance (e.g., design flaws, damage, or low quality of the materials used).
The verification of guaranteed performance values usually takes place in pure water. We prefer desorption measurements using pure oxygen for initial super saturation. Of course, other methods can be applied as well, be it in pure water or in activated sludge.
We know that newly equipped aeration tanks are often scheduled to go into operation immediately after verification of the guaranteed performance, we therefore continuously strive to further reduce the delay between measurement and final report.
General Performance
We support operators of wastewater treatment plants in monitoring the lifecycle of their aeration system through our general performance measurements. This allows for the estimation of the economically most feasible moment for reinvestment. We recommend such measurements, which are carried out in activated sludge, at intervals of two to three years.
We prefer the offgas method for this kind of measurement, which can be conducted under full operation of the plant. The only operational requirement is a constant air flow. Parallel on-site determination of the alpha factor can further improve the comparability of the measurements over the years.
Measurements in pure water
The oxygen transfer rate of diffuser elements is defined for pure water (drinking water quality). Surfactants reduce the oxygen transfer (alpha factor), while an dissolved solids (salt) cause an increase (beta value).
Therefore, measurements in pure water which can be performed as absorption or desorption, constitute the most accurate method to verify manufacturers' guaranteed performance values.
If pure water cannot be provided, e.g. for reasons of security of supply or cost, effluent or river water is sometimes (and increasingly) used. Even though alpha factor tend to stay above 0.90 for such waters significantly lower values (down to 0.80) have also been measured. Therefore we recommend an additional on-site determination of the alpha factor in our mobile test unit.
Measurements in Activated Sludge
OTR-Measurements in activated sludge are a cost-efficient alternative compared to measurements in pure water. These measurements do not require any dosing of auxiliary substances. Above all, however, there is no need for costly and often time consuming filling of the tank with pure water.
Measurements in activated sludge can be conducted as absorption measurements or according to the offgas method. While the absorption method requires an interruption of the influent to the activated sludge tank, the offgas method can be carried out under normal operation (at a constant air flow rate).
Both methods are used for the verification of guaranteed performance values as well as for general performance assessment (monitoring of aeration efficiency). Limits of applicability do exist: for absorption measurements these are too low air volumes, meaning an achievable (apparent) oxygen saturation concentration of below 5 mg/L. Measurements by the offgas method, however, fail when the air volume is too high, as the difference between the oxygen concentration in the offgas and the ambient air becomes too small.
For better comparability between OTR-Measurements in activated sludge and measurements in pure water, an additional on-site determination of the alpha factor is recommended.
On-site Determination of the alpha factor (α)
Oxygen exchange between water and air (bubbles) takes place on the boundary between the two phases. This interface can be blocked by surfactants, leading to a reduced exchange rate, which is described by the factor α. By definition its value is 1 for pure water. Lower value mean a reduced oxygen transfer. In activated sludge the α factor is influenced by numerous parameters. Typical values for submerged diffusion lie in the range from 0.6 and 0.75 but can also be significantly lower and somewhat higher, depending on wastewater composition and operating conditions. Surface aeration systems, on the other hand, generally produce alpha factor of 0.95 and higher.
An on-site determination of the factor α is especially recommended if the plant's effluent is to be used to run a pure water measurement for the verification of guaranteed performance values. In this case a reference measurement with pure water is conducted in our mobile test unit (approx. 500 L), followed by a measurement in the chosen medium. Our mobile test unit allows the performance of all methods of OTR measurements (absorption, desorption, offgas) In order to reproduce the real conditions as closely as possible water height, air flow rate as well as diffuser area and material can be adjusted.
When measurements for general performance assessment are performed regularly, year-to-year comparability of the results can be improved by additional alpha factor determination.
