Continuous Monitoring of VOCs in Industrial Wastewater
Many priority pollutants in industrial wastewater, such as chlorinated hydrocarbons and aromatics, are characterized by low solubility and high vapor pressures.
by W.M. Doyle
Many priority pollutants in industrial wastewater, such as chlorinated hydrocarbons and aromatics, are characterized by low solubility and high vapor pressures. For these substances, the system discussed here offers rapid, fully-automatic monitoring. It can provide simultaneous measurements of multiple species with detection limits in the low ppb range and updates every few minutes.
Discharge of volatile organic chemicals (VOCs) into wastewater is an endemic problem in chemical processing. As a result, most plants employ remediation facilities to purify wastewater before discharging it into the environment. These facilities, however, can be easily overloaded by a large spill. Conventional detection methods involve periodic collection of samples and laboratory analysis by relatively slow techniques such as gas chromatography and mass spectroscopy. This approach suffers from limitations:
- Inability to detect a major spill before it can reach - and possibly damage - the remediation facility.
- Failure to detect many short term spills due to low sampling frequency.
- Inaccuracies such as underreporting of concentrations due to VOC evaporatin between sample collection and measurement.
- Frequent inability to determine the source of a given spill.
A fully automated system (see Figure 1) based on a sparging infrared (IR) technique developed by Axiom Analytical offers continuous monitoring of VOCs. This technique uses a stream of air bubbles to transfer pollutants from a fixed volume of water to an air stream, where concentrations can be measured quickly and easily by using infrared spectroscopy.
Figure 2. Major elements of the Sparging -IR wastewater analysis system.
The speed of the sparging IR technique enables it to overcome limitations of previous systems outlined above. In addition, since it’s fully automated, it greatly reduces manpower requirements, resulting in substantial cost savings. Finally, by providing a real-time chemical signature of each spill, it makes it possible to trace a spill to its exact location so action can be taken to prevent a recurrence.
Key hardware elements of this wastewater analysis system are illustrated in Figure 2. These include a sparging subsystem, a low volume IR gas cell (Axiom Analytical LFT-210), an FT-IR spectrometer (Bruker IR Cube), a data system, and sources of pressurized air and clean water. The sparging subsystem includes a sparging vessel, air flow controller, water pump, and various valves and sensors. All elements of the system are controlled by the Symbion-DX process analytical software suite (Symbion Systems Inc.). This controls and sequences operation of the spectrometer, pumps and valves - and provides for analysis, display, alarms and archiving of data. The system is designed for continuous, unattended 24/7 operation with reporting and alarms via a plant-wide data network.
Figure 1. A fully automatic Sparging-IR system for 24/7 on-line operation.
The system typically makes a discrete measurement every two to three minutes. The measurement cycle involves first pumping a 200-mL sample of wastewater from the plant’s effluent stream into the sparging vessel. Air is then blown into the bottom of the vessel at a constant flow rate via a sparging nozzle. The solute containing air emerging from the vessel is directed to the gas cell where it’s analyzed by the spectrometer operating in conjunction with the Symbion-DX software and a multivariate analysis program.
Principles of Operation
Operation of this system is based on the fact each organic chemical species has a unique infrared spectrum, or fingerprint (see Figure 3). Despite uniqueness, direct measurement of low concentrations of VOCs in liquid water is made very difficult by presence of very strong and broad water absorption in the fingerprint spectral region. As mentioned, the system solves this problem by transferring organics from the water to a vapor stream. Low solubility of many VOCs, combined with the fact their vapor pressures are typically much greater than water’s, leads to a dramatic enhancement of relative concentrations in the vapor stream. For chlorinated hydrocarbons and aromatics of greatest environmental concern, relative concentrations can be many orders of magnitude greater than in liquid water. At the same time, spectral absorptions of water vapor are far weaker and narrower than those of liquid water. As a result, these absorptions generally don’t overlap those of the species of interest and hence don’t limit the system’s sensitivity.
The wastewater analysis system will usually be calibrated to report on those VOCs that may be present in a given plant environment. Typically, this will be a small number of components, but as many as 15 different species have been measured simultaneously during lab simulation (see Figure 3). In routine operation, measured concentrations are reported via the plant’s network. For analytical purposes, time dependent depletion curves can be monitored on the system’s video display.
Figure 3. Vapor phase fingerprint region spectra of chloroform (red).
Sensitivity of the system for measuring individual species will depend on solubilities and vapor pressures as well as various factors related to operating requirements and nature of the installation. An example of measurement limits for a particular set of components is given in Table 1.
Figure 4. Depletion plots of four organic species in water obtained during laboratory simulation, with y-axis units in ppm. Components are chloroform (black), dichloromethane (blue), touene (green), and tetralin (red).
The first fully automated sparging IR wastewater system of this type has been in almost continuous operation for nearly two years at a major chemical manufacturing complex. It has successfully detected significant spills on a number of occasions in time for appropriate action to be taken, thereby minimizing consequences and costs often associated with such spills.
Table 1. Selected detection limits.
Although it operates nearly continuously, the system does require maintenance. Items for periodic consideration include the water pump and software database. It’s also sometimes necessary to clean the sparging vessel and blow out water lines by means of compressed air. False positives haven’t been a problem since changes in system performance are generally in the form of loss of optical signal. Such effects are easily distinguished from changes in sample stream composition.
Several additional benefits have resulted in addition to elimination of costs associated with major spills. For example, the system has made it possible to eliminate the very significant costs associated with the previous approach of discrete daily sampling and off-line analysis.
Use of continuous on-line monitoring also revealed inaccuracies inherent in the previous system. These resulted from the evaporation of the volatile pollutants between the time a sample was gathered and its eventual analysis.
Finally, by providing a chemical signature of each spill, the system has made it possible to trace the spill to a particular plant location. As a result of these benefits, this particular facility has now changed over to complete dependence on the new system.
About the Author: W.M. Doyle is president and chief executive officer of Axiom Analytical Inc. Contact: 949-757-9300, email@example.com or www.goaxiom.com