The estimation of air emissions from wastewater collection and treatment processes is an important and integral piece of the overall air quality permitting and regulatory compliance framework for many industrial facilities, especially those within the petroleum refining and chemical manufacturing sectors. Historically, wastewater air emissions (WWAE) estimates were primarily used in state and federal New Source Review (NSR) permitting, and closely-related reporting programs (i.e., air emissions inventories, Toxic Release Inventories (TRI), etc.). In recent years, the ability to accurately and efficiently estimate a facility’s WWAE has grown in importance due to renewed regulatory focus on reducing Hazardous Air Pollutant (HAP) emissions from certain industrial source categories (e.g., benzene fenceline monitoring [BFM] requirements of the Refinery Sector Rule and recently issued Consent Decrees in the chemical sector).
TRICORD Consulting, LLC conducted a comparative evaluation of three of the most popular tools used for the estimation of Volatile Organic Compound (VOC) emissions generated by wastewater collection and treatment sources. Two of the WWAE estimation tools included in this evaluation were developed and released for public use by the U.S. Environmental Protection Agency (U.S. EPA) – the WATER9 and the Refinery Wastewater Emissions Tool (RWET). The third tool included in the evaluation is TOXCHEM, a commercial modeling software application developed and marketed by Hydromantis Environmental Software Solutions, Inc. (Hydromantis).
The study consisted of a comparative analysis of the qualitative and quantitative performance of the three WWAE estimation tools. The qualitative analysis included an evaluation of each tool with regards to the following criteria:
- Development history and current level of developer support for the product;
functionality and feature set, including any significant design limitations,
with regards to such aspects as:
- Universality of application, i.e., whether the tool is appropriate for use by facilities in a wide range of industries;
- The equipment component types available for emission estimation;
- The comprehensiveness and expandability of the included chemical property database for wastewater species;
- Flexibility/customizability of emissions calculation methodologies.
- User experience, i.e., interface, ease of use, errors/bugs, quality of support documentation, etc., and;
- Calculation methodologies used for a selected list of common wastewater component types.
The quantitative analysis compared the emissions estimates generated by the three tools for six (6) selected common wastewater component types (i.e., drains, junction boxes, sumps/lift stations, API Separators, Dissolved Gas Flotation (DGF) units, and Activated Sludge Plants), using identical input data (i.e., inlet waste characterization and flow rates, equipment component data parameters, etc.). Two separate analysis cases were evaluated – a standalone, component-by-component basis, and a combined case in which the total emissions generated by a simplified wastewater treatment plant were estimated.
The results of the qualitative and quantitative analyses provided a sufficient basis for a number of conclusions to be drawn with regards to the performance of the three WWAE estimation tools, as follows:
- RWET is the simplest and easiest-to-use of the three tools, but has considerable design limitations which may inhibit its usefulness, especially with regards to its lack of many common wastewater component types and design configurations, small chemical property database, and inability to model systems with complex waste and vapor-flow arrangements. Its calculation methodologies are generally very simple, generating emissions estimates which reflect a high level of inherent conservatism, especially for larger components like separators and ASUs.
- WATER9 is an aging software modeling application, which is still available for free download despite no longer being updated or supported by the U.S. EPA. WATER9 has a lack of compatibility with modern computer operating systems, a significant memory handling flaw, incomplete development, and a notably poor user experience. Despite these key challenges, it features comprehensive functionality and highly-detailed input parameter specification. During the quantitative analysis, WATER9 generated lower single-equipment emissions estimates for larger equipment types, on average, than either RWET or TOXCHEM. For the combined case, however, WATER9 appeared to underestimate emissions from the upstream wastewater collection components, and significantly overestimate emissions from the final component in the chain (i.e., ASU). It is likely that WATER9’s significant flaws and compatibility issues will ultimately limit its long-term usefulness for most users.
- TOXCHEM is a commercial software modeling application, still being updated and supported by its manufacturer. TOXCHEM provides full functionality for a wide range of equipment component types, the largest chemical property database, and the highest level of flexibility with regards to the refinement of basic calculation methodologies. It also provides a far superior user experience than the similarly complex WATER9 model, with an intuitive interface and complete, well-written documentation. As such, TOXCHEM generally provides for a higher overall level of confidence in its accuracy and defensibility of estimating wastewater air emissions. Based on the quantitative analysis, TOXCHEM generally estimated higher single-component emission rates than WATER9; however, it also generated the lowest estimated total combined emissions for the wastewater treatment plant case of any of the compared tools.
Contact your TRICORD Client Relationship Manager for the full Wastewater Air Emission Tool Comparison white paper and/or if you need help with estimating your facility’s wastewater emissions.