Measuring the Biological Oxygen Uptake Rate (also known as the Oxygen Uptake Rate, or OUR) accurately is essential for understanding microbial activity in various environments, such as wastewater treatment plants, bioreactors, or natural ecosystems. Here’s a step-by-step guide to measuring it correctly:

1. Determine the Type of System:

The method for measuring the oxygen uptake rate depends on the system you are studying. For instance:

• Wastewater Treatment Plants: The oxygen uptake rate in activated sludge can provide insights into microbial activity and process efficiency.
• Bioreactors: In biotechnological processes, OUR is used to monitor cell metabolism and optimize conditions for microbial growth.
• Aquatic Systems: In lakes or rivers, oxygen uptake can give clues about the health of the ecosystem.

2. Equipment Needed:

• Respirometer: A device used to measure the oxygen consumption rate of biological systems. It can be closed (measures oxygen depletion over time) or open (measures the flow of oxygen into the system).
• Dissolved Oxygen (DO) Sensor/Probe: This measures the oxygen concentration in the system. Modern DO probes are often membrane-based or optical.
• Stirred Reaction Vessel (for wastewater/bioreactor measurements): Ensures proper mixing and uniform oxygen distribution in the sample.
• Temperature Control: Oxygen solubility is temperature-dependent, so controlling the temperature is crucial for accurate results.

3. Procedure for Measuring OUR:

Step 1: Calibrate the Oxygen Sensor

• Before measurement, calibrate the DO probe according to the manufacturer’s instructions. Calibration typically involves setting the probe in a saturated air environment (100% oxygen saturation) and a zero-oxygen environment (such as sodium sulfite solution).

Step 2: Prepare the Sample

• For Wastewater: Collect a sample of activated sludge or other biological material. Ensure that the sample is fresh and representative of the system.
• For Bioreactors: Use a well-mixed and representative portion of the microbial culture.
• Ensure the sample is at the same temperature and pH as the system from which it was taken. Maintain optimal conditions for microbial activity to avoid introducing errors.

Step 3: Saturate the Sample with Oxygen

• In a closed vessel, aerate the sample by stirring or sparging with air to raise the dissolved oxygen level near saturation. This ensures a sufficient initial oxygen concentration to measure the uptake.

Step 4: Measure Oxygen Uptake Rate

There are two main methods to measure OUR:

A. Static (Closed) Respirometry:

• Record Initial DO: After saturating the sample with oxygen, turn off the aeration or stirrer to stop oxygen exchange with the environment. Start recording the dissolved oxygen concentration immediately.
• Monitor Oxygen Decline: Measure the dissolved oxygen concentration at regular intervals (e.g., every 1-5 minutes). Continue the measurement until the DO drops by a significant amount (e.g., 1-2 mg/L or more, but not to zero).
• Calculate OUR: Plot the DO concentration over time, and calculate the slope (rate of decline in oxygen concentration). The OUR is the rate of oxygen consumption per unit of time, normalized by the volume of the sample.

According to ASCE/EWRI 18-18 Standard Guidelines for in-process oxygen transfer testing, additional discrepancies between in-situ OUR and ex-situ estimates happen when the ex-situ OUR device increases the DO of the withdrawn sample compared to the mixed liquor DO at the sample location (and vice versa). This is especially true for nitrifying aeration systems, where nitrification rates increase substantially between a DO of 0.5 and 2.0 mg/L. Using the previously developed biological model [not stated], withdrawing a nitrifying mixed liquor from an aeration tank with a DO of 0.5 mg/L and raising its DO to 2 mg/L in the ex-situ device resulted in a 50% increase in nitrogenous OUR in the ex-situ device. How do we address this problem?

For any wastewater treatment aeration tank, the paper presents an interesting technique to deal with oxygen uptake rate (OUR) measurements. Aerobic metabolism rates are frequently reflected in oxygen consumption rates, although inorganic carbon remineralization and abiotic reoxidation of reduced species can also contribute to oxygen consumption rates. This paper focuses on the former type of oxygen consumption; hence the technique is primarily related to the biological uptake rate only. It would be fascinating to try the proposed dilution vs. the “shake it up” aeration approach to avoid shearing the floc which may increase the OUR artificially, jeopardizing the true measurement in the aeration tank. In any bioreactor, when the level of dissolved oxygen in the medium falls below a certain point, the specific rate of oxygen uptake is also dependent on the oxygen concentration in the liquid. Oxygen uptake rate and dissolved oxygen (DO) are inversely proportional to each other. The dissolved oxygen uptake rate (DOUR) test measures the respiration rate of organisms. Since it measures the rate at which oxygen is used (in mg O2/L/hour), it is a useful tool to evaluate process performance, aeration equipment, and biodegradability of the waste. The oxygen uptake rate is one of the fundamental physiological characteristics of culture growth and has been used for optimizing the fermentation process, and so it needs to be measured accurately. Oxygen uptake rate (OUR) is the microorganism oxygen consumption per unit time and is one of the few accessible parameters to quantify the metabolism rate of the activated sludge in a wastewater treatment plant. The manuscript observes that if a sample of mixed liquor is withdrawn from an aeration tank operating at low DO, the OUR measured after dilution with oxygen (or other means of aeration) will be higher than the true actual in-tank OUR which is limited by the aeration supply. Once a high DO is provided, then oxygen is no longer a limiting factor and the OUR will increase. With sufficient DO the OUR will become a function of the substrate concentration and the biomass concentration in the collected sample for measurement. Also, it is possible that the point at which the DO becomes limiting may itself be a function of the OUR. In other words, if a graph of OUR vs DO is made as shown in this document, one may find a different point at which the OUR starts to drop off (as the DO goes from close to saturation to lower levels). To alleviate the many problems of measurement, the proposed method using dilution with saturated DO may give a more accurate measurement than the current standard method using a sample shaking technique as described in APHA 2017. With a more accurate measurement of the OUR, it may lend credence to justification for the modification of the fundamental equation for oxygen transfer in a respiring system, as applied to an example provided by ASCE/EWRI 18-18 recently published.