Navigating Wastewater Compliance Under HON, MON, BWON, and Permitting/Emission Limitations

As environmental regulations evolve and enforcement intensifies, facilities in the chemical and refining industries must keep pace with increasing expectations for wastewater compliance. Among the most technically demanding requirements are those tied to the National Emission Standards for Hazardous Air Pollutants (NESHAPs), particularly the Hazardous Organic NESHAP (HON), the Miscellaneous Organic NESHAP (MON), and the Benzene Waste Operations NESHAP (BWON).

A cornerstone of compliance under these rules is accurately determining the Point of Determination (POD) and Point of Generation (POG); locations where regulatory obligations for wastewater streams begin. Misidentifying these points can have far-reaching consequences, potentially exposing facilities to enforcement action, flawed permitting, or ineffective emission controls. Proper identification and documentation of these points is not only critical for compliance but forms the basis for nearly all subsequent regulatory actions involving air emissions and wastewater.

POD and POG Determinations: Laying the Foundation for Compliance

Determining the correct POD or POG involves more than simply reviewing piping diagrams. Facilities must assess where and how wastewater exits a manufacturing process or control device, whether the stream contains regulated hazardous air pollutants (HAPs), and whether it qualifies as process or maintenance wastewater. Additional layers of complexity arise from determining if a stream is exempt due to classification, such as stormwater, firewater, safety shower runoff, or other excluded categories. Each determination must be supported by defensible technical and regulatory justification.

This process is especially important under the MON, where wastewater streams containing compounds listed in Table 8 (partially soluble HAPs like benzene and toluene) and Table 9 (soluble HAPs like methanol) are subject to flow and concentration thresholds that determine regulatory status. The HON impose similar requirements, albeit with different definitions. Facilities must carefully evaluate whether streams qualify as Group 1 or Group 2, if treatment is required, and whether specific control technologies or work practices apply. While BWON threshold triggers for control are based on a different system, facilities are still required to determine if controls/treatment are required and which work practices apply.

Wastewater characterization, then, becomes a pivotal first step. It is often necessary to thoroughly map all potential drains and transfer points, track where streams are routed (including whether they return to process, are treated on-site, or sent off-site), and document stream-specific concentrations of HAPs. Clear and well-documented POD and POG determinations help facilities stay ahead of inspections and reporting obligations while reducing the risk of compliance errors.

Wastewater Sampling: Generating Data You Can Trust

Of course, none of this can happen without reliable data, which is where wastewater sampling becomes essential. Whether for compliance determination, emissions modeling, or permitting, facilities need a sampling program that reflects actual operating conditions and accounts for both variability and complexity.

Designing an effective sampling program requires careful consideration of the target compounds, expected phases of the waste (aqueous, organic, or sludge), appropriate analytical methods, and the physical conditions of the waste stream. In many cases, samples must be flow-weighted or event-weighted, especially for non-continuous discharges, to accurately reflect chemical loading over time. Sample collection methods must prevent loss of volatiles and preserve chemical integrity—requiring equipment such as cooling coils, appropriate preservatives, and careful handling procedures.

Inadequate sampling can lead to underestimating HAP concentrations or missing key phases of a stream. For instance, if only aqueous samples are taken, benzene which may partition into the free organic phase, will not be captured. Regulatory compliance under programs like BWON hinges on demonstrating a full accounting of benzene-bearing waste, including all relevant phases and flow rates. Similarly, MON and HON evaluations require detailed stream data to determine whether control requirements are triggered. Thorough planning, including the development of regulatory-justified sampling plans, helps ensure that collected data support the intended use—whether for POD/POG determinations, emissions calculations, or permit submittals.

Toxchem Modeling: Understanding Emissions with Confidence

Once sampling is completed and data are in hand, the next critical step is modeling air emissions from wastewater—an increasingly scrutinized aspect of facility compliance for permitting and emission limitations. While EPA’s legacy WATER9 model has served this function for many years, its limitations and lack of support have led many facilities to adopt more advanced tools, such as Toxchem™ by Hydromantis.

Toxchem represents a state-of-the-science modeling platform for estimating volatile organic compound (VOC) emissions from wastewater systems. Unlike WATER9, which often overestimates emissions due to simplified assumptions, Toxchem incorporates detailed fate and transport mechanisms, including volatilization, biodegradation, and partitioning across gas, liquid, and solid phases. The result is a more realistic and defendable emissions profile—critical for permitting and compliance under both state and federal air programs.

Toxchem’s built-in libraries include hundreds of chemical compounds and over 50 unit processes, making it suitable for modeling complex treatment systems and collection networks. Its Excel-compatible interface streamlines input and reporting, while sensitivity analysis and backsolver tools offer insights into how operational changes affect emissions. This modeling power allows facilities not only to quantify emissions more accurately but also to explore scenarios, optimize controls, and assess the impact of process modifications.

These modeling results play a vital role in permitting, particularly under New Source Review (NSR), where accurate VOC emission estimates are used to evaluate applicability thresholds and control technology requirements. They are also important in demonstrating compliance with work practice standards, control efficiency targets, and recordkeeping mandates under various NESHAPs. And because regulatory agencies are increasingly requesting defensible emissions estimates backed by robust modeling, the choice of tool and how it’s used can make all the difference.

Build a Reliable, Defensible Compliance Strategy

Ultimately, accurate POD and POG determination, representative sampling, and defensible air emissions modeling are not just individual tasks but interdependent components of a broader wastewater compliance strategy. Success in this space demands not only regulatory knowledge, but technical rigor and operational awareness. Whether you’re working toward MON and HON compliance, navigating the intricacies of BWON, or preparing for permitting and emissions reporting, an integrated, strategic approach provides the greatest value, and the greatest protection.

Trinity Consultants Chemical Sector Services (CSS) group assist industrial facilities nationwide with POD/POG evaluations, wastewater sampling programs, and Toxchem modeling, supporting a full range of compliance, permitting, and strategic planning needs. Whether your facility is navigating current requirements or preparing for future changes, Trinity can provide expert support every step of the way. For more information, please contact Inaas Darrat at 713.552.1371.