Avoiding Sewer Fires with Vapor Space Organics Monitoring

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ABSTRACT
Regulation of flammable gases released from wastewater discharges is facilitated by the use of the Vapor Space Organics test method. Such regulation can reduce the risk of a flash sewer fire, resulting damage and potential injury. The updated 2003 VSO process developed by Severn Trent Laboratories produced data that was equivalent to the 1990 VSO process developed and used by the Cincinnati Metropolitan Sewer District. The similarity of the VSO results was apparent when the data were compared statistically with 99% and 95% confidence intervals and when compared graphically. Overall, the average difference was 6.8%. The precision, as estimated by the average RSD, is about 5% for VSO 2003 and 11% for VSO 1990. This represents a significant and expected improvement in precision when comparing the automated 2003 process to the manual 1990 process.

BACKGROUND
Vapor Space Organics (VSO) analysis was developed by the Cincinnati Metropolitan Sewer District (MSD) in response to a sewer fire in nearby Louisville, Kentucky and potential hazards in the Cincinnati system in the early 1980s. Extensive property damage occurred in Louisville. Testing for flammables in the sewer discharges was started as one tool to prevent a recurrence. The VSO test method was developed to facilitate regulation flammable gases released from wastewater discharges.

The original VSO test was equipment and process specific, resulting in a method defined parameter designed to monitor volatile organic compounds present in the room temperature headspace of the wastewater sample. The vapor space organics method includes hydrocarbons from propane (C3) to nonane (C9) and other solvents that also contain carbon.

Since the VSO method was originally developed in 1984, GC technology and Quality Assurance practices have advanced. Severn Trent Laboratories has worked closely with the Cincinnati MSD to update the method in order to facilitate the use of state of the art GC technology such as capillary columns and headspace autosamplers. In addition, more comprehensive quality assurance procedures have been added in order to improve data quality, method ruggedness and productivity.

METHOD SUMMARY
This updated VSO (2003) method uses the same fundamental principles as the original VSO method (1984-1990) published by the Cincinnati MSD. Headspace sample preparation is coupled with gas chromatography with flame ionization detection (FID). The water sample is collected using standard volatile organic compound (VOC) collection techniques in 40mL VOC vials. Room temperature headspace allows a portion of the VOC content in the water to partition into the gas phase above the water sample. These VOCs are subsampled and injected into a gas chromatograph. VOC peaks detected with the FID and eluting in the VSO retention time window are summed to produce the VSO result. The VSO retention time window includes C3 through C8 hydrocarbons and some C9 hydrocarbons as well as halogenated organics, alcohols, ketones and esters. The final VSO result corresponds to the concentration of hexane in parts per million by volume (ppmv) in the prepared sample headspace (HS) that would produce the same total peak area as that recorded in the sample. Thus, the result is “VSO as hexane” with units of “ppmv HS”. Key updates include the addition of retention time marker compounds, the use of a headspace autosampler, aqueous calibration standards, capillary gas chromatographic column, five point initial calibration, laboratory control samples and matrix spike samples.

VALIDATION DATA
The updated 2003 VSO process developed by Severn Trent Laboratories produced data that was equivalent to the 1990 VSO process developed and used by the Cincinnati Metropolitan Sewer District. The similarity of the VSO results was apparent when compared statistically with 99% and 95% confidence intervals and when compared graphically (Figure 1). Overall the average difference was 6.8%. The precision, as estimated by the average RSD, is about 5% for VSO 2003 and 11% for VSO 1990. This represents a significant and expected improvement in precision when comparing the automated 2003 process to the manual 1990 process.

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