Optical sensing improves dissolved oxygen control

Dissolved oxygen is a key ingredient in the efficient treatment of waste in water processes. A typical wastewater treatment plant uses four main stages of treatment – primary, secondary, tertiary and sludge. The secondary treatment stage is the point at which organic waste is oxidised to form carbon dioxide, water and nitrogen compounds. 

Most modern plants use an activated sludge system, where a culture of bacteria and other organisms feeds on the organic materials in the sewage. Under the right temperature, these bacteria and organisms use dissolved oxygen to break down organic carbons into carbon dioxide, water and energy, clearing the water of harmful substances.

The aeration process efficiency relies on close control of dissolved oxygen levels. Both excessively low and excessively high levels of dissolved oxygen can be equally as harmful to aquatic life, making it essential for water treatment plants to ensure that levels are as close to ideal as possible before water is discharged. Under ideal conditions, dissolved oxygen levels should be maintained at between 1.5ppm to 2ppm. 

Various methods have been used to measure dissolved oxygen, including the Winkler Titration method and portable handheld meters. Although both methods can provide a reliable measurement, the results only offer an indication of dissolved oxygen levels for a particular period and set of conditions, making them unsuitable for applications such as dissolved oxygen blower control, where the supply of oxygen needs to be constantly adjusted. 

Continuously measuring dissolved oxygen levels offers the best way of ensuring the right conditions for maximum aeration efficiency.

When used with modern sensing technology, an online dissolved oxygen measurement system can offer tighter control of dissolved oxygen levels, matching them to actual oxygen demand. When coupled with automatic blower control, significant energy cost savings can also be realised through reduced air consumption.

There are two main types of sensors available for dissolved oxygen monitoring – electrochemical and optical. Electrochemical sensors work on either the polarographic or galvanic cell principles. Both work in a similar way – a polarised anode and cathode with an electrolyte solution surrounded by an oxygen permeable membrane.

The measurement is derived based on the difference in oxygen pressure outside and inside of the membrane. Variations in the oxygen pressure outside of the membrane affect the rate of diffusion of oxygen through the membrane itself. The cathode reduces the oxygen molecules, producing an electrical signal that is relayed first to the anode and then to a transmitter, which converts the signal into a reading. 

The consumption of the oxygen at the cathode requires a constant sample flow in order for a reading to be as accurate as possible. In most cases, this will require the sample to be stirred constantly at the sensor tip to produce the necessary oxygen levels for an accurate reading. One drawback of polarographic sensors occurs during start-up. Unlike galvanic sensors, polarographic sensors require several minutes for their probes to polarise. The requirement for a constant current means that the sensor consumes more power than other sensor types, making it comparatively less cost-effective. 

Although electrochemical sensors have been proven to offer similar levels of accuracy to optical devices, their requirement for a constant flow and their susceptibility to fouling or clogging, make them comparatively less reliable under non-ideal monitoring conditions. Where this occurs, the risk of inaccurate measurement and inefficient blower control is greatly increased. 

Continued sensor drift, coupled with fouling of the sensor membrane, also means that frequent maintenance, including calibration, is needed, ranging from once a month to once a day in extreme circumstances. 

Optical sensors can overcome many of the limitations associated with their electrochemical counterparts. The most advanced dissolved oxygen sensors work on the ‘dynamic luminescence quenching’ principle, a light-based measurement technique. These sensors are comprised of lumiphore molecules embedded in a sensing element, plus blue and red LEDs and a photodiode. 

When a reading is taken, the lumiphore molecules are excited by blue light from the blue LED. When excited, these molecules emit a red light, which is detected by the photodiode. Any oxygen molecules present will quench the excited lumiphore molecules, reducing the amount of red light being emitted. The shift in the amount of red light is then measured by the red reference LED. 

As dissolved oxygen concentration and the amount of red light being returned are proportional, a measurement can be taken and converted into a reading based on mg/l. A key benefit of optical measurement technology is its stability and accuracy. The luminescence lifetime technique is used to measure the phase shift between the returned red light and the red reference light. 

Using optical sensors for dissolved oxygen measurement can help overcome many of the problems associated with electrochemical devices. With no requirement for sample flow or stirring to artificially raise dissolved oxygen levels, they can provide high accuracy even in low oxygen (hypoxic) conditions. 

They also have a low maintenance requirement compared to membrane-based sensors. ABB’s ADS430 optical DO sensor, for example, is robust enough to withstand the problems that can affect conventional membrane-based sensors, such as abrasion, fouling or poisoning, while the sensor lumiphore is not affected by photobleaching or stray light. 

The sensor is also immune to the effects of sulfides, sulfates, hydrogen sulfide, carbon dioxide, ammonia, pH, chloride and other interferences which enables it to provide consistent, accurate readings over long periods of time without suffering from sensor drift.

The sensor also features a smart sensing cap, which comes pre-loaded with factory calibration coefficients, serial number, lifetime indication, and manufacture date which are automatically uploaded to the sensor, eliminating the time normally required for set-up. By automatically prompting the user when replacement is due, the cap removes the risk of unexpected sensor failure. When cleaning is necessary, the cap can also be cleaned and redeployed without calibration.

With aeration accounting for well over half of a plant’s total energy costs, accurate control of dissolved oxygen levels presents a key step in minimising operational costs. Coupled with the inherent maintenance benefits of optical sensors this makes the technology an attractive solution for use in aeration processes.