EMISSIONS OF VOCS FROM A MUNICIPAL SEWER

In response to increasing regulatory scrutiny related to sources of hazardous air pollutants and reactive organic gases, several recent studies have addressed the characterization and control of volatile organic compound (VOC) emissions from municipal and industrial wastewater treatment plants. VOC emissions from wastewater collection systems have received much less attention. A naturally well-ventilated municipal sewer interceptor that receives significant quantities of VOC-laden industrial wastewater was studied to quantify VOC emissions. Headspace outgassing rates, when summed across four manhole covers, were as high as 2300 m(3)/h, far exceeding headspace turnover rates previously estimated and published in the technical literature. Individual concentrations of five target VOCs (benzene, toluene, ethylbenzene, total xylenes, and tetrachloroethene) in the sewer headspace ranged from 0 to 46 ppm during three daytime monitoring events and one 24-hour event. Emissions were greatest for toluene, approaching 100 g/h from a single manhole cover during midday of the 24-hour event. Significant diurnal and weekday/weekend trends were observed for aromatic VOC emissions but not for emissions of tetrachloroethene. Combined emissions of toluene, xylenes, and ethylbenzene from only four manhole covers rivaled or exceeded those summed over aerated grit chambers and aeration basins at each of four large municipal wastewater treatment facilities in Southern Ontario. Furthermore, average and peak emissions of total nonmethane hydrocarbons (TNMHC) from a single manhole cover were 265 g/h (2300 kg/yr) and 630 g/h (5500 kg/yr), respectively, during a 24-hour sampling event. The primary source of VOC stripping was observed to be a series of two large drop structures, which lead to 29-44% removal of individual VOCs from wastewater. An important conclusion of this study is that large fractions of VOCs may be removed from wastewater and emitted to the ambient atmosphere prior to entering a downstream treatment facility.