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 IMPLICATIONS National Emission Standards for Hazardous Air Pollutants (NESHAPs) are currently being developed in the United States in response to the Clean Air Act (CAA) Amendments of 1990. These standards will address emissions of hazardous air pollutants (HAPs) from many municipal and industrial wastewater sources, including on-site industrial sewers and possibly municipal sewers. Many HAPs are also highly reactive organic gases that contribute to ozone formation, a concern that may lead to the implementation of reasonably available control technology (RACT) control requirements for wastewater systems located in ozone nonattainment areas. This paper includes a description of a methodology for determining volatile organic HAP and total nonmethane hydrocarbon (TNMHC) emissions from sewers. Results of six field sampling events indicate that, under certain sewer operating conditions, a significant fraction of volatile organic HAPs can be removed from wastewater and emitted to the ambient atmosphere prior to entering a wastewater treatment plant. Thus, relative to wastewater treatment facilities, gravity-flow sewers should not be neglected as sources of volatile organic HAP emissions.

Municipal and industrial sewers have come under increased regulatory scrutiny as sources of volatile organic compound (VOC) emissions to the ambient atmosphere. A well-ventilated municipal sewer interceptor that receives significant quantities of VOC-laden industrial wastewater was studied to quantify VOC emissions. Headspace outgassing rates across four manhole covers were as high as 2300 m3/h. Emissions were greatest for toluene, approaching 100 g/h from a single manhole cover at the mid-point of the 24-hour event. Significant diurnal and weekday/weekend trends were observed. Emissions from a single manhole cover rivaled or exceeded those summed over aerated grit chambers and aeration basins at four large municipal wastewater treatment facilities in Southern Ontario. The primary source of VOC stripping was observed to be a series of large drop structures, with aromatic VOC stripping efficiencies ranging from 25 to 38% across two drops. Finally, an existing model that predicts VOC emissions from sewers was observed to reasonably predict measured stripping efficiencies. 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.

Odour emissions and corrosion concerns can be a constant focus for many wastewater treatment and collection system owners, usually from the first day that their collection systems begin operation. Many sewer systems are reaching the end of their useful life, or have experienced either odour or corrosion issues. This paper shows a link between odour generation and corrosion potential, and how modelling can be used to assist in odour/corrosion assessments of existing wastewater collection systems. A model has been developed that is capable of predicting liquid-phase sulphide generation and subsequent release to overlying sewer headspace as hydrogen sulphide (H2S), where it can be the source of either odour or corrosion problems. This paper presents an overview of the model, and uses a case study involving both odour and corrosion issues to demonstrate the utility in modelling. The model was used to identify potential locations within the system where odour/corrosion may be problematic, as well as assisting in the evaluation of potential odour control alternatives.

New federal and state regulations have placed increased scrutiny on sources of hazardous air pollutants (HAPs), including many volatile organic compounds (VOCs). The U.S. Environmental Protection Agency will continue to develop National Emission Standards for HAPs (NESHAPs) and subsequent Maximum Achievable Control Technology (MACT) implementation requirements for industrial source categories over the next several years. To a great degree, industryspecific NESHAPs will be developed based on estimates of VOC emissions from specific process streams. These estimates may be in the form of (1) extrapolations from VOC emissions measurements at a limited number of similar processes, and/or (2) empirical emission models. This paper focuses on VOC emissions from junction boxes and drop structures. It is based on the results of 17 pilot experiments designed to illustrate the effects of entry pipe location, liquid flow rate, headspace ventilation rate, and drop height on VOC stripping efficiency. Experimental results are compared to predictions based on standard estimation procedures as outlined in AP-42. Experimental stripping efficiencies were observed to be highly sensitive to VOC physicochemical properties, drop height and liquid flow rate. Except at very low liquid flow rates, experimental stripping efficiencies were lower than those estimated using AP-42 estimation methods. Finally, it was observed that passive modifications to existing junction boxes or drop structures can lead to significant reductions in VOC stripping efficiency. IMPLICATIONSSignificant uncertainties in VOC emissions estimates for on-site industrial sewers exist. This paper involves a comparison of AP-42 emissions estimates and experimental data associated with a pilot junction box and drop structure. Limitations of AP-42 emissions estimates are identified, as are the relative effects of several system operating conditions and chemical properties on VOC stripping efficiencies. Results should be of value to those interested in improving VOC emissions estimates or employing passive control measures to suppress VOC emissions.

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