The Whole Plant Approach to Air Emission Modeling for NESHAP Compliance
Long overlooked, the air emissions associated with biological treatment of process wastewaters continue to be further scrutinized and regulated. The common approach is to utilize EPA’s Water9 as a model to present findings and support permitting activities. It is essential when using a model that not only is the facility modeled appropriately, but also that the specific compound properties utilized by the model are appropriate. This paper presents a methodology utilized to modify industrial wastewater treatment facilities while using Water9 to not only accurately model emissions, but also to guide the design to achieve emissions goals.
Various needs or opportunities, such as improving operating consistency, improving mechanical reliability, enhancing processing efficiency, or reducing wastes and/or emissions can motivate a site to evaluate their waste treatment plant. At a SOCMI facility, a similar approach spurred a program to evaluate upgrading the waste treatment plant. The facility is regulated by the National Emission Standards for Organic Hazardous Air Pollutants from the Synthetic Organic Chemical Manufacturing Industry (HON) and therefore must control the air emissions of hazardous air pollutants (HAPS) not only from production related activities, but also wastewater treatment. All upgrades to the aerobic biological process would need to consider the resulting air emission profile in relation to the HON. Using Water9 as a predictive tool, various potential aerobic modifications could be evaluated to determine the alignments and technologies which would minimize air emissions from the wastewater treatment facility.
The steps followed to develop a Water9 model and consider different unit operations and the associated air emissions were to:
- Characterize the process wastewaters and identify the compounds of concern (hazardous air pollutants, HAPs, and volatile organic material, VOM)
- Review the Water9 default properties for compounds of concern and verify appropriateness
- Determine site specific biodegradation rates
- Develop Water9 model to mimic physical system
- Run model, evaluate output, and modify system to minimize air emissions
As with most industrial wastewater treatment facilities there was a significant amount of monitoring data, both of conventional parameters (flow, temperature, total organic carbon (TOC), total suspended solids (TSS), etc.) and regulated compounds such as the HAPs and VOM concentrations. This existing data was then scaled using future production estimates to develop a future design wasteload. Any appropriate aerobic biological treatment alternative would be required to successfully process the design wasteload while complying with the current NPDES permit limitations.
In addition to simple organic removal through traditional biological pathways, it was necessary to consider the loss of HAPs and VOM to the air. Therefore, it was essential that this design wasteload reflect not only the conventional parameters but also the regulated compounds. Furthermore, based on production activities, the conventional parameters (TOC, etc) may not vary significantly but the chemical constituents which make up this TOC may. This made it important to consider several compound mixes given a singular TOC load. Table 1 presents this concept.