Sewer Disposal of Radioactive Material

Introduction

Releasing licensed radioactive material into the sanitary sewer is permitted under 10 CFR 20.2003, but only within strict, well-defined limits. The material must be readily soluble or readily dispersible in water; the monthly average concentration released must stay below the values in Table 3 of Appendix B to Part 20; mixtures of radionuclides must pass a sum-of-fractions test; and the total annual quantity released is capped at 5 curies of hydrogen-3, 1 curie of carbon-14, and 1 curie of all other radioactive materials combined. ^{1, 2}

For a nuclear medicine department, a research laboratory, or any medical-materials licensee, sewer disposal is one of several authorized waste pathways—alongside decay-in-storage, transfer to an authorized recipient, and shipment to a licensed land-disposal facility. It is attractive because it is simple and inexpensive, which is precisely why it is also one of the more commonly mishandled areas in a radiation-safety program. A misunderstanding of the concentration limits, the annual caps, or the medical-excreta exemption can turn a routine disposal into a regulatory finding. ^{1, 3}

This guide explains exactly what 10 CFR 20.2003 requires, the math behind the concentration and quantity limits, the records that make disposal defensible, and the mistakes radiation safety officers (RSOs) most often make. DRPS supports licensees with radiation safety officer consulting and radioactive material license support across its service areas.

Topic Explanation

What 10 CFR 20.2003 actually permits

Sewer disposal is an exception, granted under specific conditions, to the general rule that radioactive waste must be disposed of by an authorized method. The general framework is in 10 CFR 20.2001, which lists the permitted disposal methods; 20.2003 is the provision that allows release into sanitary sewerage when its conditions are met. ^{1, 3}

The rule defines sanitary sewerage as a system of public sewers for carrying off waste water and refuse—but explicitly excluding sewage treatment facilities, septic tanks, and leach fields owned or operated by the licensee. In other words, the discharge must enter a genuine public sewer system, not the facility's own on-site treatment or disposal system. ¹

A facility starting or revising a waste program should review sewer disposal alongside its broader radioactive waste management for nuclear medicine plan and its ALARA program.

The four conditions

Sewer disposal under 20.2003(a) is allowed only when all of the following are satisfied: ¹

If any one of these fails, sewer disposal is not authorized for that material, and another pathway must be used. ¹

Key Technical Principles

The concentration test

The concentration test compares the monthly average concentration of released material to the regulatory limit. The monthly average concentration is the quantity released in a month divided by the average monthly volume of water released into the sewer: ¹

$$\bar{C}=\frac{A_{\text{month}}}{V_{\text{month}}}$$

where $A_{\text{month}}$ is the activity released over the month and $V_{\text{month}}$ is the average monthly sanitary-sewer water volume from the facility. This $\bar{C}$ must not exceed the radionuclide-specific concentration value listed in Table 3 of Appendix B to Part 20. The dilution provided by the facility's normal water usage is therefore part of the calculation—but it cannot be invented or overstated. ^{1, 2}

The sum-of-fractions rule for mixtures

When more than one radionuclide is released, no single Table 3 limit applies. Instead, the licensee must compute, for each radionuclide $i$, the fraction of its limit represented by its discharge, and the sum across all radionuclides must not exceed one: ¹

$$\sum _i\frac{{\bar{C}}_i}{C_{L,i}}\le 1$$

where ${\bar{C}}_i$ is the actual monthly average concentration of radionuclide $i$ and $C_{L,i}$ is its Table 3 concentration limit. This ensures that a mixture of nuclides, each individually below its own limit, cannot collectively exceed the intended overall constraint. ¹

The annual quantity limits

Separate from the concentration test, the rule caps the total annual quantity released into the sewer, regardless of dilution. These limits are facility totals for the calendar year: ¹

A facility must satisfy both the monthly concentration test and the annual quantity test. Meeting one does not excuse the other. ¹

Worked example: annual quantity tracking

Suppose a nuclear medicine clinic disposes of small residual activities of several soluble, short-lived radionuclides (excluding patient excreta, which are exempt). Over a calendar year, the clinic logs each release and totals them, finding 35 mCi of assorted "all other" radionuclides released to the sewer. The fraction of the annual "all other" limit is:

$$f=\frac{35\text{mCi}}{1000\text{mCi}}=\frac{35\text{mCi}}{1\text{Ci}}=0.035$$

This is 3.5% of the 1 Ci annual limit for all other radioactive materials combined—comfortably within bounds. If the clinic also released, say, 0.4 Ci of tritium from labeled compounds, that would be $0.4/5=0.08$, or 8% of the separate 5 Ci tritium limit. Tracking each category against its own annual cap, every month, is what keeps the program defensible. ¹

A practical caution on numbers

The Table 3 concentration values and the annual quantity limits are fixed by regulation and must be read directly from the current text of 10 CFR 20.2003 and Appendix B to Part 20 (or the equivalent Agreement State rule) at the time of disposal. Values in Appendix B are given in "E" notation (for example, 6E-02 means $6\times 10^{-2}$). Always compute against the current published limit for each specific radionuclide rather than from memory or a legacy table. ^{1, 2}

Clinical Impact

For medical-materials licensees, sewer disposal questions arise most often around therapy and labeled-compound work, where the line between exempt patient excreta and regulated facility waste matters. ^{1, 4, 5}

Several practical realities shape how facilities use the rule:

• Patient excreta are exempt, but framing matters. Under 20.2003(b), excreta from patients undergoing diagnosis or therapy are not subject to the paragraph (a) limits. A patient treated with I-131 who uses the restroom is not a "release" the facility must count against its annual limit. However, this exemption is specific to the excreta of treated patients—not to vials, residual doses, contaminated water, or other facility-generated waste. ^{1, 4}
• Sewer authorities may impose their own limits. Independent of NRC rules, the local publicly owned treatment works may set conditions or request notification, particularly for I-131 therapy programs. Studies of I-131 in hospital wastewater have shown that storage/decay tanks can substantially reduce activity reaching the public system and the dose to downstream sewage workers. ⁵
• Newer therapies raise newer questions. As Lu-177–labeled radiopharmaceuticals have grown in use, facilities increasingly need to measure and document activity concentrations in therapy-ward wastewater to demonstrate releases are below applicable limits; practical wastewater-counting methods have been published for mixed I-131/Lu-177 samples. ⁶

These issues connect directly to therapy radiation-safety practice; see our guides to patient release after radiopharmaceutical therapy and I-131 thyroid cancer therapy.

Practical Optimization Tips

1. Confirm solubility first

Before any sewer release, confirm the material is readily soluble or readily dispersible biological material. If it is not, sewer disposal is off the table—use decay-in-storage, transfer, or licensed disposal instead. ¹

2. Establish your dilution volume honestly

Document the facility's average monthly sanitary-sewer water volume from a defensible source (utility records or engineering estimates). The concentration test depends on this denominator, and an unsupported volume is a common audit weakness. ¹

3. Track both tests, every month

Maintain a running log that checks (a) the monthly average concentration (with the sum-of-fractions calculation for mixtures) and (b) the cumulative annual quantity by category. A simple spreadsheet that flags both is far better than a year-end reconstruction. ¹

4. Separate exempt excreta from regulated waste

Make the distinction explicit in procedures so staff do not either over-count (logging patient excreta) or under-count (dumping facility waste under the excreta exemption). ¹

5. Keep records inspection-ready

Record radionuclides, activities, dates, water-volume basis, and the compliance calculation. These records are reviewed during inspection and are part of a defensible program. For the broader audit picture, see common radiation safety violations and how to avoid them.

Common pitfalls to avoid

• Treating sewer disposal as a default. It is one authorized pathway, not the easy answer for all soluble waste; ALARA still applies.
• Forgetting the annual cap. A facility can satisfy every monthly concentration test and still exceed the 1 Ci "all others" annual limit.
• Misapplying the excreta exemption. It covers patient excreta, not residual doses, contaminated supplies, or rinse water.
• Using a septic system or on-site treatment. These are explicitly excluded from the definition of sanitary sewerage.
• Relying on a legacy concentration table. Always check the current Appendix B values.

Regulatory Considerations

Sewer disposal sits inside the broader waste-disposal and radiation-protection framework, and compliance with 20.2003 does not stand alone. ^{1, 7}

Key frameworks to reference:

• 10 CFR 20.2003 — Disposal by release into sanitary sewerage. The controlling rule: solubility, concentration, mixture, and annual-quantity conditions, plus the patient-excreta exemption. ¹
• 10 CFR Part 20, Appendix B, Table 3. The radionuclide-specific monthly average concentration limits for release to sewers. ²
• 10 CFR 20.2001 — General requirements for waste disposal. Lists the authorized disposal methods within which sewer release is one option. ³
• 10 CFR 20.1301 — Dose limits for individual members of the public. The public-dose limits and ALARA obligations that continue to apply regardless of the disposal pathway. ⁴
• 10 CFR Part 35 and NUREG-1556, Volume 9. Medical-use requirements and program-specific licensing guidance that frame waste handling for medical licensees. ^{7, 8}

Agreement States administer their own equivalent programs. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that adopt compatible rules under their own radiation-control programs, while Washington, DC and Delaware are regulated directly by the NRC for radioactive material. A licensee must verify which authority issues its license and confirm the exact concentration and quantity limits, records, and any state or local sewer-authority conditions that apply. Always confirm requirements with the authority having jurisdiction. For the dose-limit context, see NRC occupational dose limits under Part 20.

Frequently Asked Questions (FAQs)

Can a licensee dispose of radioactive material in the sanitary sewer?

Yes, but only under the specific conditions in 10 CFR 20.2003. The material must be readily soluble (or readily dispersible biological material) in water, the monthly average concentration released must not exceed the values in Table 3 of Appendix B to Part 20, mixtures must satisfy a sum-of-fractions test, and the total annual quantity released must stay within the limits in the rule. Agreement States have equivalent requirements.

What are the annual sewer-release quantity limits?

Under 10 CFR 20.2003, the total quantity of licensed and other radioactive material a licensee releases into the sanitary sewer in a year may not exceed 5 curies (185 GBq) of hydrogen-3, 1 curie (37 GBq) of carbon-14, and 1 curie (37 GBq) of all other radioactive materials combined. These are facility totals, not per-discharge values.

Are patient excreta counted against the sewer limits?

No. Under 10 CFR 20.2003(b), excreta from individuals undergoing medical diagnosis or therapy with radioactive material are not subject to the limitations in paragraph (a). However, this exemption applies to the patient's excreta themselves; other radioactive waste a facility generates is still subject to the rule, and many facilities still manage therapy excreta carefully for ALARA and local sewer-authority reasons.

What does "readily soluble" mean for sewer disposal?

The material released must be readily soluble, or readily dispersible biological material, in water. Insoluble solids, sealed sources, and material that would not disperse in water do not qualify for sewer disposal and must be handled by another authorized pathway such as decay-in-storage, transfer to an authorized recipient, or shipment to a licensed disposal facility.

How do I handle a mixture of radionuclides?

For mixtures, you compute, for each radionuclide, the ratio of its actual monthly average concentration to its Table 3 limit, and the sum of those fractions over all radionuclides must not exceed one. This sum-of-fractions approach ensures that no single nuclide and no combination exceeds the overall concentration constraint.

What records does sewer disposal require?

Licensees must maintain records of radioactive material disposed of by release into sanitary sewerage, including the activity, the radionuclides, the dates, and the basis for demonstrating compliance with the concentration and annual-quantity limits. These records support license compliance and are reviewed during NRC or Agreement State inspections.

Does sewer disposal still require ALARA and public-dose compliance?

Yes. Meeting the 10 CFR 20.2003 limits does not relieve a licensee of the obligation to keep doses as low as reasonably achievable and to comply with the public-dose limits in 10 CFR 20.1301. Sewer disposal should be part of a documented waste-management program, not a default for convenience.

Key Takeaways

• Sewer disposal is conditional, not automatic. 10 CFR 20.2003 permits it only for readily soluble or dispersible material meeting concentration, mixture, and annual-quantity limits. ¹
• Two independent tests apply. The monthly average concentration must stay below Appendix B Table 3 values, and the annual quantity must stay within the curie caps. ^{1, 2}
• Annual caps are 5 Ci H-3, 1 Ci C-14, and 1 Ci all others combined. These are facility-year totals. ¹
• Mixtures use sum-of-fractions. Each nuclide's concentration ÷ its limit, summed, must be ≤ 1. ¹
• Patient excreta are exempt—but only patient excreta. Facility-generated waste is still regulated. ¹
• ALARA and public-dose limits still apply, and complete records are essential for inspection. ⁴

Conclusion

Sewer disposal of radioactive material is a legitimate, useful pathway—but a narrow one. The rule in 10 CFR 20.2003 is built around a simple logic: only soluble material, only below defined concentrations, only within annual quantity caps, and only into a genuine public sewer system. The patient-excreta exemption removes the largest medical source from the count, but everything else a facility generates must be tracked against both the concentration and the quantity limits.

For an RSO, the path to a defensible program is straightforward: confirm solubility, establish an honest dilution basis, run both tests every month, separate exempt excreta from regulated waste, and keep inspection-ready records. Treated this way, sewer disposal is a routine, low-risk part of a well-run waste-management program rather than a source of findings.

How DRPS Can Help

Diagnostic Radiation Physics Services helps medical and research licensees build defensible radioactive-waste programs. This may include sewer-disposal procedure development, concentration and annual-quantity tracking tools, waste-pathway decision support, records and audit preparation, radiation safety officer consulting, and radioactive material license support and medical physicist consulting aligned with NRC and Agreement State requirements.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A strong waste program is not just about passing inspection. It is about making the compliant pathway the obvious, easy choice for the clinical and laboratory staff who use it every day.

Related Resources

• Radioactive waste management for nuclear medicine
• NRC occupational dose limits under Part 20
• Building an ALARA program
• Patient release after radiopharmaceutical therapy
• Common radiation safety violations and how to avoid them
• I-131 thyroid cancer therapy
• Radiation Safety Officer consulting
• Radioactive material license support

References

1. U.S. Nuclear Regulatory Commission. 10 CFR 20.2003: Disposal by release into sanitary sewerage. nrc.gov
2. U.S. Nuclear Regulatory Commission. Appendix B to Part 20 — Annual Limits on Intake (ALIs) and Derived Air Concentrations (DACs); Effluent Concentrations; Concentrations for Release to Sewerage (Table 3). nrc.gov
3. U.S. Nuclear Regulatory Commission. 10 CFR 20.2001: General requirements for waste disposal. nrc.gov
4. U.S. Nuclear Regulatory Commission. 10 CFR 20.1301: Dose limits for individual members of the public. nrc.gov
5. Andrés C, Barquero R, Tortosa R, et al. 131I activity in urine to the sewer system due to thyroidal treatments. Health Physics. 2011;101 Suppl 2:S110-S115. doi:10.1097/HP.0b013e318209459c. PubMed
6. Kupitz D, Einspänner E, Wissel H, et al. Technical note: Quantifying radionuclide residues in hospital wastewater: A case study on 131I and 177Lu/177mLu. Medical Physics. 2024;51(10):7574-7579. doi:10.1002/mp.17272. PubMed
7. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. nrc.gov
8. U.S. Nuclear Regulatory Commission. NUREG-1556, Volume 9, Revision 3: Consolidated Guidance About Materials Licenses — Program-Specific Guidance About Medical Use Licenses. nrc.gov
9. U.S. Nuclear Regulatory Commission. Disposal of Radioactive Material by Release Into Sanitary Sewer Systems; Withdrawal of Advance Notice of Proposed Rulemaking. Federal Register, 70 FR 68512; 2005. federalregister.gov
10. U.S. Nuclear Regulatory Commission. Health Physics Position (HPPOS-158): Disposal by Release Into Sanitary Sewerage Systems. nrc.gov