The measurement of DO in water

The oxygen concentration in activated sludge is a central parameter of biological wastewater treatment. Aerobic, anoxic or anaerobic zones are required for carbon breakdown, nitrification, denitrification and bio-logical phosphorus removal. Knowledge of the DO (dissolved oxygen) in activated sludge is imperative for this task. Thus, from the process point of view, the question is not whether, but how the oxygen concentration can be measured continuously and accurately.

Between 60 per cent and 70 per cent of the energy used in sewage treatment is for the aeration of activated sludge. Control and regulation strategies for reducing the energy required for biological wastewater treatment mostly concentrates on the optimisation of the oxygenation in the aeration tank. Therefore, the continuous, accurate measurement of DO in activated sludge is essential from an economic point of view.

Electrochemical measuring technique

Electrochemical techniques for DO measurement have been in use for more than 40 years, both galvanic and polarographic. In principle, the measuring cells comprise an anode and cathode of different metals immersed in a common electrolyte. On some types of cells, the electrolyte chamber is separated from the sample by a gas permeable membrane. Oxygen from the sample diffuses through the membrane until partial pressures of oxygen are the same on both sides. On cells without a membrane, the sample itself is the electrolyte.

In galvanic cells, a potential difference is established between the anode and cathode. Oxygen reduces at the cathode with corresponding oxidation at the anode. The potential difference is proportional to the oxygen concentration in the sample. Galvanic measuring cells are self-polarising, therefore ready immediately after switch-on.

Polarographic cells require an external polarisation voltage to be applied to the anode and cathode, as the potential between the electrodes is not sufficient to reduce oxygen. The current, proportional to the oxygen concentration, is then evaluated. A stable polarisation voltage is not established spontaneously between the anode and cathode, so a polarisation period is required. This can be up to 2 hours, so the sensor is only ready for use at the end of this polarisation time.

Optical measuring technique

The new optical technique for the determination of DO eliminates the disadvantages of traditional electrochemical methods. The Luminescence Dissolved Oxygen principle, as used by the Hach LDO sensor, is based on physical luminescence. This is defined as the property of a luminophore to emit light that is produced due to excitation, utilising light energy for this excitation. The intensity and the decay of the luminescence radiation over time are dependent on the oxygen concentration that surrounds the material.

The sensor, for example, comprises two components: a sensor cap with luminophore applied to a transparent carrier material with a protective, gas-permeable coating, and a sensor body with blue and red LEDs, a photodiode and an electronic analysis unit. The sensor cap is screwed to the sensor body and immersed in the water. Oxygen molecules from the sample to be analysed are thus in direct contact with the luminophore.

For the measurement phase, the blue LED emits a pulse of light that transfers part of its radiation energy to the luminophore. Electrons within the luminophore are exited to a higher energy level, with the resulting difference in energy being re-emitted as radiation in the form of longer wavelength red light.

As oxygen molecules contact the luminophore, they absorb energy from the electrons in the exited state, allowing them to return to their initial state with a corresponding reduction of emitted radiation. With increasing oxygen concentration and the resulting reduction of the intensity, the luminophore returns to a normal energy state faster. The lifetime of the radiation emitted is therefore shortened.

Intensity (Imax) and the decay time (T) for the red radiation are dependent on the surrounding oxygen concentration. To determine the oxygen concentration, the decay time, T, of the red radiation is analysed. In this way the oxygen measurement is reduced to a purely physical measurement of the time.

Additionally, the sensor response is continuously checked with the aid of the red reference LED fitted in the probe. Prior to each measurement, this LED emits light of a known characteristic that is reflected at the luminophore and passes through the entire optical system. Changes in the measuring system are thus detected without delay.

Advantages in use

The established electrochemical techniques for the measurement of DO require users to perform regular maintenance. Membrane and electrolyte replacement, cleaning, calibration and anode polishing are today considered necessary and unavoidable to keep the inherent disadvantages of these sensors within acceptable limits.

With the luminescence technique, an alternative is now available. In comparison to electrochemical techniques, there are significant advantages for the user relating to the quality of results and the maintenance required by the use of the optical method.

• No membrane or electrolyte replacement

  With the luminescence process, the oxygen sensitive luminophore layer replaces the electrolyte, electrodes and membrane. In the example given above, the sensor cap is simply replaced once a year by the user.

• Reduced calibration requirements

  Combined with the continuous monitoring of sensor response with the red reference LED, generally, a one-off electronic calibration is all that is required. Any wear during operation or fading of the luminescent material affects the intensity, but not the decay of the radiation emitted. This reduces the measurement of DO to a drift-free measurement of time.

• No damage to the sensor by H2S

  If gaseous H2S passes through the membrane of an electrochemical cell with a silver anode, a silver sulfide layer is formed that is very difficult to remove. This process results in irreparable damage to the electrochemical cell. The luminescence technique is not affected by H2S (or numerous other chemicals), therefore it is possible to use this method in many difficult applications.

• No flow requirement

  Electrochemical techniques evaluate the current or voltage produced by the reduction of oxygen at the cathode. To balance this 'oxygen consumption' within the cell, a continuous diffusion of oxygen into the electrolyte is required. Maintaining a continuous flow is the only way of preventing a reduction in the sample oxygen concentration around the sensor, otherwise known as 'oxygen starvation'.

  With the luminescence technique, no oxygen is consumed. The oxygen only needs to contact the luminophore layer and does not need to be in a flow.

• Fast response times

  With the optical technique, oxygen molecules only need to contact the luminophore, producing response times in the region of seconds. If a slower response time is required, signal damping can be set at the controller.

• High sensitivity to low oxygen concentrations

  By applying the analysis of decay time, the sensitivity of the measuring effect (DT/DCO2) increases as the oxygen concentration drops. The measuring principle therefore has particularly good resolution in the lower measuring range.

• Mechanically robust sensor

  This sensor cap is extremely robust when compared to measuring cells covered with membranes. Membrane fracture during operation or cleaning by the user is not possible.

Summary

The development of units, such as the Hach LDO sensor, is not only an improvement on existing electrochemical methods, but represents a new paradigm in the all-important determination of dissolved oxygen concentrations.

The result is an almost 'ideal' oxygen sensor that features exact measurements with very low maintenance and that is simple to use. The work to be performed by the user is limited to the annual replacement of the sensor cap and occasional cleaning of the sensor.