Measurement of dissolved oxygen (DO) concentration is important in a wastewater treatment process. In the secondary treatment, biodegradable organic matter (BOD) and suspended solids (TSS) are removed through the processes of aeration and filtration. After leaving the primary treatment stage, sewage is pumped into aeration tanks. The sludge is loaded with microorganisms and mixed with air or pure oxygen. Through the diffuser or aeration tube, dissolved oxygen (DO) is added to the aeration basin to enhance the oxidation process. The process provides oxygen to aerobic microorganisms so they can successfully turn organic wastes into inorganic byproducts. A shortage of dissolved oxygen (DO) will kill the microorganisms, therefore affecting the effluent quality.
Here are some tips and information to ensure successful installation of the dissolved oxygen (DO) sensor and analyzer:
- Select the installation location that allows easy access for maintenance and calibration
- The point of measurement should be away from the diffusers or aeration tube, to prevent the sensor being hit by air bubbles
- Accumulation of air bubble around the sensors will cause inaccuracy or inconsistency in readings
- Select the location which produces a typical oxygen concentration
- Do not leave the sensor cable dangling into the aeration tank, pond or basin
- Sensor must be submerged completely into the activated sludge
- Sensor must be mounted with an immersion holder
- To prevent leakage, wrap the sensor threads with pipe tape before inserting into its mounting (Do not over tighten the sensor)
- Run the sensor cables through a conduit to prevent moisture intrusion/signal distortion
- Moisture caused by improper protection or broken cable may reduce the insulation resistance of the cable connections causing a partial short circuit
- Long cable distances may degrade the signal. For best performance, install cable in a noise-free environment (Refer to datasheet for the maximum length and manufacturer’s recommended distance from the meter)
- Sensor connection cable consist of a layer of special layer of graphite for screening. Cutting of the surplus may decrease the isolation resistance between the core and the screen of the cable
- Calibration is recommended at regular intervals to ensure accurate measurements
- If the meter and sensor is exposed to harsh environments ie high-temperature or exposure to chemicals, frequent calibration may be necessary
- Zero calibration: Immerse the dissolved oxygen (DO) sensor into the zero oxygen solution. Allow the meter to stabilize and adjust according to the solution’s temperature (Refer to manufacturer’s manual)
- Air calibration: Hold sensor in air and wait for reading to equilibrate
Dissolved oxygen concentrations in water are affected by:
- Temperature: The most significant variable is temperature, so it is essential to measure it in conjunction with dissolved oxygen. The solubility of oxygen in water is inversely related to temperature – as temperature increases, DO decreases. Therefore, as a water body cools overnight, more oxygen can be dissolved. The same applies for cooler seasons – a water body in winter will have a higher DO concentration than in summer, assuming other variables are held constant. However, it is important to keep in mind the impact photosynthesis and respiration have on DO concentrations during the day and night.
- Salinity: The solubility of oxygen in water is inversely related to salinity – as salinity increases, DO decreases. Seawater can hold about 20% less oxygen under the same temperature and atmospheric pressure as freshwater. Therefore, it is critical to also measure salinity (using a conductivity sensor) when collecting DO data in estuaries, wetlands, coastal areas, aquaculture, or any other application where salinity can vary.
- Barometric Pressure: There is a direct relationship between barometric pressure and DO levels in water – as pressure decreases, DO decreases. At lower elevations, the barometric pressure is high, so there is more pressure to push gaseous oxygen from the atmosphere into water. But at higher elevations, the barometric pressure is much lower.
