On May 29, the Minnesota Pollution Control Agency (MPCA) hosted a webinar providing their latest ideas around the framework for the odor rulemaking required by Minnesota statute 116.064 subdivision 7. The MPCA has solicited feedback on the rulemaking this summer and has tentatively planned on releasing draft language in 2027, with a final adoption of rules in 2028. This article steps through the required elements and preliminary framework for the odor rule.
Required Elements
An important thing to understand with regard to the odor rulemaking is that Minnesota state law dictates required elements of the odor rule. These required elements are the minimum items expected in the rulemaking. The statute on its own is not an odor rule and so these items do not become applicable until the final rule is issued.
The rule will only apply to the 7 county metropolitan area (Anoka, Carver, Dakota, Hennepin, Ramsey, Scott, and Washington counties), but there are a number of exempt facilities/activities:
- On-farm animal and agricultural operations;
- Motor vehicles and transportation facilities;
- Municipal wastewater treatment plants;
- Single-family dwellings not used for commercial purposes;
- Materials odorized for safety purposes;
- Painting and coating operations that are not required to be licensed;
- Restaurants;
- Temporary activities and operations;
- Refineries; and
- Metropolitan Council wastewater systems.
The driver for the MPCA to investigate and facilities to take action is the receipt of odor complaints; the statute specifies 10 complaints within 48 hours requires an investigation.
MPCA is required to include a standard for objectionable odor, a process to determine whether an odor is objectionable, and a process to investigate/address odor complaints. MPCA is also required to provide guidance on odor management plan development and the procedure sand criteria for determining the success or failure of an odor management plan.
To inform their proposed rulemaking, the Minnesota Management Analysis and Development group reviewed odor rules in over 20 jurisdictions and interviewed local regulators in states and municipalities in the United States. Many of these rules include a dilution-to-threshold (D/T) which is commonly utilized for off-property odors. A numerical D/T is the unbiased strength of an odor and is measured with an instrument called a field olfactometer. The most commonly utilized field olfactometer in the United States is the Nasal Ranger. Trained odor observers use the field olfactometer to sniff odorous ambient air and compare it to “clean” air to help assign a strength of the odor in D/T (i.e., at a given dilution level, can the odor observer notice an odor: does it smell different from the “clean” carbon-filtered air?). A threshold of 7 or 8 D/T is commonly used to define an objectionable odor, but some jurisdictions use thresholds as low as 2 D/T. Ultimately, the unbiased strength of an observed odor is only one dimension of the odor, as the frequency, intensity, duration, and offensiveness of the odor will also contribute to whether it is objectionable. The frequency, intensity, duration, and offensiveness are the “FIDO” characteristics, and they are important to odor assessments but not fully codified in odor regulations. MPCA stated its intent is to utilize a combined approach of measuring unbiased odor strength with a precision instrument, but to also consider the FIDO characteristics when developing their rule.
Preliminary Framework
The MPCA has developed 3 standards that they have considered implementing under the rule (i.e., all 3 standards would apply, and exceeding any single standard could be considered an objectionable odor):
| Standard | Odor Strength | Odor Frequency | Is the level considered to negatively impact human health, welfare, or enjoyment of life? |
| 1 | 2 D/T | 3 times in 1 hour | Yes |
| 2 | 2 D/T | 3 times in 30 days | Yes |
| 3 | 7 D/T | Once | Assumed to based on the strength of the odor |
Under this preliminary framework, MPCA staff would visit the facility it believes is the cause of the odor complaints to interview the owner/operator and conduct a physical examination of the facility. MPCA staff would also conduct inspections with a field olfactometer to determine whether the odor strength and frequency conditions are met beyond the property line. As noted above, an investigation is triggered by odor complaints, so with all three criteria, it is assumed that an odor measured at the above criteria would negatively impact human health, welfare, or enjoyment of property and therefore be deemed an objectionable odor.
If an objectionable odor is found, MPCA will require the facility to develop an odor management plan. Once adopted, if MPCA determines objectionable odor continues, the facility will be asked to revise the odor management plan. MPCA stated during its presentation that they understand some changes may take time to implement and will work with facilities to understand timelines.
The above framework may change and there will likely be more substance in the draft rule.
Next Steps
It may be another year or two before draft rule language is proposed and adopted. If you are a facility in the 7 county metropolitan area, it would be worthwhile to consider how odors are currently perceived by the area surrounding the facility and assess how complaints are received. Trinity is experienced in conducting odor assessments and authoring odor management plans for industrial facilities.
If you have any questions, please reach out to Angie Wanger.
