The benefits of using online, automated control of aeration systems using dissolved oxygen (DO) sensors are well known in the industry and have been widely documented (e.g. Olsson et al, 2005). In most wastewater treatment facilities using activated sludge, the electricity provided to the aeration system is one of the most significant operating costs and a good control system can help to significantly reduce these costs whilst ensuring the plant continues to meet its permit requirements. In addition to the obvious financial benefits of online control, a good DO control system provides significant benefits for process optimization particularly in nutrient removal systems.
In Europe, online control using DO sensors has been embraced by most utilities and is considered standard practice for all activated sludge processes. In North America, the experience has been less positive: many utilities have found that the DO sensors have been significantly less reliable than their European counterparts have reported. It can only be postulated why this is the case. Perhaps European utilities place greater importance on instrumentation and control which means the time and effort involved in maintaining DO sensors seems reasonable and the perception of the probes being “unreliable” (i.e. high maintenance) does not surface? European plants typically have fewer operators which means they have to rely on a higher degree of automation and many European countries have higher electricity costs than most regions of North America which means the cost benefits of good control are more notable. It is also possible that the experience in Europe is not so dissimilar to that of North America and that in fact many European plants do have problems with maintaining DO sensor integrity, but these problems are not widely publicized (Watts, 1990).
Whatever the reason, the perception remains in North America that DO sensors are often unreliable and cannot be used effectively for control without a great deal of effort. The 2002 WERF study on Sensing and Control Systems (Hill et al, 2002) says it best: “the dependability and accuracy of the primary sensors is still cited in the literature, in automation workshops, and along practitioners as the single largest impediment to widescale, successful implementation of automation.”
OPTICAL DO SENSORS TO THE RESCUE?
Most DO sensors are electrochemical cells protected by a membrane. In activated sludge, the membrane gradually becomes coated with a biofilm that reduces the ability of oxygen to reach the cell and the signal is diminished. In order to restore the signal strength, the membrane must be periodically cleaned, either by manually lifting the probe out of the aeration basin and wiping it, or using some form of automated cleaning system. The required frequency of manual cleaning is a function of the activity of the activated sludge (measured using OUR) and the presence of fats, greases and other surfactant materials and is often underestimated. The effectiveness of many automated cleaning systems is also open to question and experiences have often been less than positive.
In the past few years, several vendors have launched a whole new breed of DO sensors termed “optical” DO sensors. In these sensors a fluorescent dye is used to detect the amount of oxygen present in the water. Light is shone onto the dye which then re-emits light which can be measured by a detector. The presence of oxygen reduces the fluorescence – a process called “quenching” – which can be used to measure the concentration of oxygen present in the water. Volbeda and Anson (2005) provide a good general description of optical DO probes, their method of measurement and their different features.
Initial experience with these probes has been extremely positive. Several utilities have tested optical DO probes and have been very pleased with their performance. They are generally easy to install and operate and appear to perform well. Testing carried out by Charleston Water System showed that both conventional and optical probe types gave values that compared well with their handheld DO but that the optical DO gave the best performance overall.