The City of New York is in the process of upgrading most of its 14 municipal wastewater treatment plants (WTPs) which collectively serve some 7.25 million people throughout the 5 boroughs (Manhattan, Bronx, Brooklyn, Queens, and Staten Island). Typically, these upgrades involve capacity expansions and the replacement of antiquated processes and infrastructures, and will continue for the next 10 years or longer. The capital cost of these upgrades is estimated at more than $2 billion. Because they are being funded with public monies, all facility upgrades must be performed in strict accordance with applicable CEQR (City Environmental Quality Review) requirements. Much of MSI's current work involves performance of the air component of the attendant CEQR analyses for two of these facility upgrades.

The CEQR process for air can be thought of in terms of the following four steps. The first step is the development of a facility-wide emissions data base to support subsequent analyses for both the "future build" and the "future no-build" scenarios. The second step is a regulatory applicability analysis based on a tons-per-year assessment of emission rate potential and the net emissions change resulting from the project. The third step, based largely on results of air quality dispersion modeling, is to assess whether applicable air quality thresholds (standards and guideline concentration levels) are met for each of these scenarios. Finally, based on results of the CEQR analysis, the fourth step involves modification of the facility upgrade to incorporate those controls (process or emissions) shown to be necessary for achieving compliance with the above thresholds. It is only after compliance has been demonstrated that the lead City agency in the CEQR determination can issue a "negative declaration" and the upgrade can proceed.

Hydrogen sulfide (H2S) is generally the most problematic compound for the WTP upgrades, as strict City and State off-site, odor-related standards exist, and uncontrolled emissions from the higher-emitting process sources can often result in contravention of these standards by several factors. Because the New York City Department of Environmental Protection (NYCDEP) wastewater treatment plants are so large, the cost associated with installation of emissions controls can run in the tens of millions of dollars, exclusive of recurring (operating) costs. Therefore, from a financial perspective, it is imperative that the CEQR analysis be performed in such a manner as to minimize the conservatism in the results -- thus avoiding expensive over-engineering of emissions controls -- while at the same time ensuring the scientific integrity of the work.

As a subcontractor to one of the NYCDEP's premiere engineering design contractors, Hazen and Sawyer, P.C., MSI has been tasked with performing upgrade-related CEQR air quality analyses for the Bowery Bay and 26th Ward facilities. For each, this involves estimating pollutant emission rates from all process and building sources for subsequent use in dispersion modeling to assess compliance with air quality standards at off-site receptors. Compounds of concern are: criteria pollutants (sulfur dioxide, nitrogen dioxide, carbon monoxide, and particulate matter) and total non-methane volatile organic compounds (VOCs); hazardous air pollutants (HAP), including speciated VOCs and non-criteria pollutants regulated by the State; and H2S.

Sources of criteria pollutants and non-methane VOCs are generally limited to the boilers which provide energy to the facilities. Emissions for these sources are generally based on information provided by the equipment manufacturers.

The process sources, including the sludge thickeners, tanks, and uncovered channels, are the principal sources of HAP and H2S. HAP emissions are estimated based on the NYCDEP's own aqueous monitoring data which was used as input to the TOXCHEM+ Model. For both facilities, criteria pollutant and HAP compliance was demonstrated under the upgrades as conceived (i.e., without the need to consider controls). For H2S, however, significant exceedances were shown, thus requiring emission controls to be included in the future build scenario.

For any emitting source, only two means of estimating off-site air quality impact exist: ambient air monitoring (in which the impact is measured directly) and dispersion modeling (in which the impact is predicted, or estimated, based on the source emission rate and on-site meteorology). Considering cost and resources, dispersion modeling represents the only practical means of assessing the air quality impact downwind of a source as large and complex as a municipal wastewater treatment plant.

From a technical perspective, the key to any dispersion modeling effort of this type lies in the parameterization of the source term(s) to be used as input to the model. Our ability to conceive and execute field measurement programs to generate scientifically defensible and legally admissible emission-rate estimates from complex area sources, such as municipal wastewater treatment plants, is what sets us apart from our competitors. For example, for each of the WTP facilities with which we are currently involved, our emissions-measurement field work has allowed us to represent the entire plant as a collection of nearly 300 individual source components, each with unique emission factors. These emission factors, in turn, are then used as input to a dispersion model to assess off-site compliance with applicable H2S standards. With this level of precision, through an iterative process, the combination of controls minimally necessary for each source sub-area to achieve compliance can be identified precisely. In this manner, huge savings are realized by strictly avoiding the costly over-engineering of emissions controls.

Others have attempted to use flux chambers placed over the process area sources to characterize H2S emissions for input to the dispersion model. We consider this method to be generally infeasible, first because of the heterogeneous nature of the emissions and the resultant inability to properly address data-representative considerations, and second because of a variety of complex analytical considerations.

It is our experience that the "area-source technique" (as modified for use with point monitors) is the best method to accurately estimate H2S emissions from WTP process-area sources. This technique is applicable to all area-type sources, i.e., homogeneous sources (uniformly emitting) and non-homogeneous sources (having "hot spots"). It involves identification of a source "attribution" based on a series of near-ground (1m height) upwind and downwind measurements, and the subsequent back-calculation of emission rates based on Gaussian dispersion relationships inherent in most USEPA Guideline models (e.g., ISCST). In addition to the source-attribution information, coincident on-site measurements of wind speed, wind direction, and atmospheric stability are required.

The area-source technique was originally developed for use with some type of optical remote sensing (ORS) technology, such as open-path Fourier-transform infrared (FTIR) or ultraviolet (UV) spectroscopy. This is because open-path spectroscopy directly generates the source-attribution information in the required path-integrated form directly. Because H2S is a poor IR and UV absorber, however, a point-monitoring approach had to be employed to allow simulation of the path-integrated concentration representation. The point-monitoring method of choice was the Jerome meter, manufactured by Arizona Instruments.

Click onto modified area-source technique for a detailed discussion of the area-source emissions-estimation technique (modified for use with Jerome meter point monitors for H2S measurements), as applicable to municipal wastewater treatment plants and other large area sources. Also included on that page is a more detailed discussion of the on-site meteorological data requirements.

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