PFAS inventory: The critical link between monitoring and reduction in Illinois NPDES permits

As Illinois wastewater treatment plants (WWTPs) navigate new per- and polyfluoroalkyl substances, or PFAS, requirements under updated National Pollutant Discharge Elimination System (NPDES) permits, one step stands out as especially critical: the PFAS inventory.

While PFAS sampling and monitoring identify what per- and polyfluoroalkyl substances are present in a wastewater system, the PFAS inventory answers an important question: What are the potential contributors of PFAS? That source information ultimately drives regulatory compliance, operational planning and long-term cost control for publicly owned treatment works.

Under new Illinois Environmental Protection Agency (EPA) NPDES permit conditions, major WWTPs are required to complete an initial PFAS inventory within 12 months of the permit's effective date, with annual updates required thereafter. These deadlines are mandatory, and timelines are not negotiable.

PFAS monitoring: Setting the stage

Before a PFAS inventory can be effective, facilities must first understand what PFAS are already present in their system.

New Illinois NPDES permits require immediate PFAS sampling, monitoring and analytical testing using EPA-approved methods, which is EPA Method 1633. Sampling is required for:

Influent (incoming wastewater).
Effluent (final treated discharge).
Biosolids/sludge.

Sampling frequencies are specified in the permit and generally include quarterly sampling for influent and effluent and semiannual sampling for biosolids. Testing must begin immediately upon permit issuance.

For most WWTPs, PFAS detections are expected. Detecting PFAS does not indicate permit noncompliance. Instead, these results establish a baseline that informs future PFAS evaluation, source identification and reduction strategies.

Because PFAS testing involves extremely low detection and quantification limits, sampling is sensitive to contamination. Strict field protocols, experienced oversight and strong quality control procedures are essential to ensure defensible data that can be relied upon during PFAS inventory development and regulatory review.

What PFAS inventory is (and what it isn't)

The PFAS inventory is not a list of confirmed violators. Rather, it is a structured evaluation of potential PFAS contributors within a WWTP's collection system, with a particular focus on industrial and commercial users.

NPDES permits include lists of high-risk SIC and NAICS codes commonly associated with PFAS use, such as:

Metal finishing and electroplating.
Textile manufacturing.
Chemical manufacturing.
Landfills and landfill leachate.
Firefighting and firefighting training activities.

Facilities must evaluate all industrial users in these categories. However, one of the most important, and often overlooked, aspects of the PFAS inventory requirement is this: Even contributors that are not classified under high-risk SIC or NAICS codes might still be discharging PFAS to the treatment plant.

Location matters

A facility near an airport, firefighting training center, landfill or landfill leachate discharge point might receive PFAS regardless of the contributor's primary business classification. Commercial laundries, transport wash facilities and businesses handling PFAS-treated materials can also be sources, even when PFAS are not central to their operations.

For this reason, the Illinois EPA requires permittees to include any activity the facility knows or reasonably expects to be a PFAS source, not just those listed in permit tables..

Building a PFAS inventory

Most WWTPs develop their PFAS inventory using a combination of:

Industrial user surveys.
Review of SIC and NAICS codes.
Safety Data Sheet (SDS) collection and evaluation.
Follow-up questions, site inspections or targeted PFAS sampling, as warranted.

Surveys typically ask about PFAS-related activities such as firefighting foam use, surface coatings, nonstick or water-repellent treatments, chemical cleaning agents and PFAS-containing process aids. SDS reviews focus on chemical composition, CAS numbers and fluorinated compounds that might indicate PFAS presence.

For each industrial user of concern, the PFAS inventory must document:

Facility name and address.
Justification for inclusion.
Potential PFAS-containing waste streams.
Estimated wastewater discharge volume.

The PFAS inventory is the bridge between monitoring and reduction. Without it, wastewater utilities are left reacting to PFAS data without context. With it, facilities can prioritize sources, collaborate with contributors and focus PFAS reduction efforts where they will have the greatest impact.

Just as importantly, a well-documented PFAS inventory creates a defensible and transparent regulatory record, demonstrating due diligence, system understanding and proactive compliance – long before treatment technologies or discharge limits enter the conversation.

In Illinois' evolving PFAS regulatory landscape, the PFAS inventory is not just a paperwork requirement. It is the foundation for long-term compliance, cost control and environmental protection.

Fehr Graham's environmental experts stay current on Illinois NPDES requirements and PFAS best practices. Contact our team today to keep your wastewater facility's NPDES permitting process running smoothly. Contact us or call 630.897.4651.

Karoline Qasem, PhD, PE, PMP, CFM, is a water engineer specializing in water quality, wastewater regulatory compliance and watershed management. Her work focuses on NPDES permitting, PFAS inventories and reduction initiatives, nutrient management planning, watershed modeling and stormwater management to help communities navigate complex environmental regulations and protect water resources. Reach her at This email address is being protected from spambots. You need JavaScript enabled to view it..