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The upgrading of municipal wastewater treatment plants (WTPs) often triggers the need to identify controls necessary to ensure that nuisance-based, off-site ambient air standards are not contravened. This can be particularly challenging as it is necessary to consider not only the spatial and temporal emissions variability inherent in routine facility operations, but also the air quality “credit” to be realized by improvements to the wastewater process resulting from the upgrade itself. While these odors obviously need to be controlled, it is imperative to avoid overcontrol as the resultant capital and recurring costs can be extreme.

The 26th Ward WTP is located in Brooklyn, New York and operated by the New York City Department of Environmental Protection. It is a 170 million-gallon-per-day (mgd) activated sludge treatment plant with partial combined sewage overflow (CSO) treatment. A major upgrade is currently underway to correct a variety of plant deficiencies, as well as to improve instrumentation and process-control capabilities. One of the upgrade components is the inclusion of odor controls such that strict compliance with hourly, off-site hydrogen sulfide (H2S) standards is ensured.

This paper presents the results of a facility-wide H2S emissions characterization, as well as a discussion of predicted off-site impacts and resultant control requirements based on field measurements made between July and September, 2001. For each process source, H2S emissions were measured during times when the facility was operating normally (conservatively reflective of the “build scenario”) as well as when it was shown to be in upset mode. Upset conditions resulting in increased H2S formation and subject to remedy under the upgrade were considered.

The emission rate representative of the build scenario was conservatively defined as the highest measured rate which could not be explained by the occurrence of a documented upset condition.

The source-attribution technique, essentially a mass-balance approach, was used to characterize H2S emissions. For each open process source, this involved calculation of a single, downwind, path-averaged H2S concentration for each 15-minute “monitoring event.” Each event consisted of 34 near-ground (1m height) point measurements using two comparably performing Jerome meters. Emission rates were assessed using Gaussian dispersion relationships and on-site measurements of wind speed, wind direction, and atmospheric stability (as a proxy for vertical dispersion). Higher-emitting areas (such as weirs and other turbulent areas) were addressed by periodically measuring H2S at representative locations, directly above the source surface, from which relative “hot-spot” source strengths were derived and assigned.

A refined characterization was performed for the preliminary settling tanks, as this was, by far, the highest-emitting source. The accuracy of the emission estimates was significantly improved by the site-specific treatment of vertical dispersion (sigma-z) during each monitoring event. An open-path Fourier-transform infrared (FTIR) spectrometer was configured along the H2S measurement path to monitor two tracer gases released in a controlled manner from different upwind locations, thereby facilitating the direct determination of sigma-z coefficients across the downwind source dimension. A second-order sigma-z curve, unique to each of 77 monitoring events for this source, was developed and substituted directly into the dispersion model for H2S emissions calculation. This eliminated the need to rely on somewhat crude relationships between atmospheric stability class and sigma-z values.
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Document Type: Research Article

Publication date: 2004-01-01

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