Security concerns are driving current research to develop means of detecting toxic chemicals in wastewater influents as few online instruments are currently available for this purpose. Online respirometers such as the STIPTox are the obvious choice using existing, established technology. One of these units was recently used successfully in the WERF project 03-CTS-7S “Feasibility Testing of Support Systems to Prevent Upsets” to detect 5 out of 6 known toxins.
In the same project a decision support system (DSS) was developed that used three simple sensors – pH, ORP and conductivity – to detect known toxins. The main difficulty in getting the DSS to work was in dealing with the noisy data caused by filter blockages and sensor fouling. With very good sensor maintenance the DSS based on simple sensors could work well, as proven by testing the DSS with “cleaned up” data which had much of the noise removed.
Future advances in sensor technology will add more sensors to the suite that can be used in a DSS.
Through the Water Environment Research Foundation (WERF), the US EPA is funding several projects investigating different aspects of security at wastewater treatment facilities. One area where significant research effort is being focused is the detection of chemical, biological and radiological (CBR) contaminants in the wastewater stream, preferably before they are able to cause a detrimental impact on the wastewater facility. The most important element in any detection system is the primary sensor i.e. the instrument or instruments used to generate the signal that provides some form of measurement that indicates a problem. WERF and the EPA recently brought together instrumentation experts from the water and wastewater sectors to discuss research needs for sensor development at a workshop in Cincinnati in 2005. Sensor development and sensor selection are high priorities in developing systems to protect wastewater treatment plants from upsets caused by either accidental or intentional introduction of CBRs into the wastewater.
Aside from the obvious benefit of developing sensors and protocols to protect plants from terrorist activities, the increased focus on measuring toxic substances in wastewater will also benefit facilities that routinely experience process upsets caused by toxic chemicals from other, less malicious, sources.
INSTRUMENTATION FOR PLANT PROTECTION
In order to protect a wastewater treatment plant from a toxic chemical, a sensor or monitor should be used detect the chemical somewhere upstream of the biological treatment process in the plant as this is the process susceptible to significant upsets from toxins. In an ideal system, the detection device would be located at the plant inlet or even somewhere further upstream in the collection system, but these locations are harsh environments for automated sampling systems and sensors and so it may be more realistic to locate the system downstream of preliminary or even primary treatment. The closer the monitoring point is to the biological treatment, the more difficult it will be to detect the toxin and provide an expeditious response.
There are many online instruments available for monitoring process streams and an increasing number of them are being investigated for water security. Most of these instruments were developed for monitoring clean water and not specifically for the harsh environment of wastewater. Care must be taken in applying these kinds of instruments to wastewater monitoring and in many cases they are completely inappropriate. A number of monitors have been developed for wastewater applications and among them are a handful of systems explicitly designed for detecting toxicity. These toxicity monitors are described below, followed by a brief mention of some more advanced sensors in various stages of development and finally a discussion of the use of other conventional “off-theshelf” sensors for toxicity detection.