Modeling Air Emissions from Wastewater Treatment Operations

Wastewater treatment facilities, both industrial and municipal, are often subject to state air permitting and annual emission reporting requirements. Additionally, Federal and state regulatory agencies scrutinize emissions to the air from wastewater treatment facilities. In each of these conditions, air permitting, emission reporting, or responding to regulatory agency requests for information, quantification of pollutant emissions to the air is required. Emission sources of concern at wastewater treatment facilities typically include gas combustion (e.g. digester gas flares or engines), material handling, and fugitive emissions from wastewater treatment operations. Quantification of emissions from combustion and material handling processes is relatively straightforward; emission calculations are typically performed using published emission factors, equipment manufacturer's guarantees, or facility specific stack testing results. Quantification of emissions from wastewater treatment units is more complex, with variability in influent composition and flow rate along with a wide variety of different treatment units each with its own emission characteristics. Typically, characterization of influent water composition and treatment unit parameters must be used with an emision model to quantify emissions from wastewater treatment units. There are several choices for modeling emissions from wastewater treatment units. Some of the more common choices include the WATER9 model, the TOXCHEM model, equations found in AP-42, or Pooled Emission Estimation Program (PEEP) emission factors. This paper provides a description of these models, a discussion of how they can be used, and considerations for model selection.