Odour Dilution to Threshold and the Reduction a Target Requires

ScopeThe number of dilutions with clean air needed before a trained panel can no longer detect it -- and what it takes to bring that below an objection level at a property line. The arithmetic is simple and the finding is consistent: ODOUR IS REDUCED AT SOURCE OR NOT AT ALL. A source at 2,400 D/T exhausting 15,000 acfm emits 600,000 odour units per second, and that rate disperses exactly as a mass emission rate does. If the atmosphere delivers 400:1 of dilution between the stack and the fence, the receptor sees 6 dilutions, which against a typical ordinance limit of 5 to 15 may or may not comply -- and which ordinance applies is genuinely the question, because limits, methods, and the exceedances allowed vary widely between jurisdictions. Taking that receptor to a D/T of 2 means the source has to fall to 800, a 67% reduction in odour concentration. Getting there by dispersion instead would need three times the dilution, about 1.7 times the effective stack height (the square root of 3), because ground-level concentration falls roughly with the square of it. That is why containment, biofilters, scrubbers, and oxidizers are the answers that work and a taller stack usually is not. The source D/T is entered and comes from dynamic olfactometry, a laboratory panel method with real variability, and one sample represents one operating condition of a source whose odour varies with process state, temperature, and season. This does not model dispersion, address odour character or hedonic tone (unpleasantness is not concentration, and two sources at the same D/T are not equally objectionable), the intensity-concentration relationship, or the frequency and duration terms most modern odour rules weigh alongside concentration. The applicable odour ordinance or nuisance rule, the olfactometry standard used, and the air quality authority govern.

Formula or tableodour emission rate = source dilution-to-threshold x volumetric flow; the receptor D/T is the source D/T divided by the dilution the atmosphere provides; and the source concentration a target receptor D/T requires is that target times the same dilution, from which the reduction follows.

Source and editionSource D/T is ENTERED and comes from dynamic olfactometry on a collected sample -- a laboratory panel method with real variability, where one sample represents one operating condition of a source whose odour usually varies with process state, temperature, and season. The dilution between source and receptor is also entered and comes from a dispersion calculation with all the caveats that carries. It does not model dispersion, address odour character or hedonic tone, the intensity-concentration relationship, or the frequency and duration terms most modern odour rules weigh alongside concentration.

Free accessTwo multiplications and a ratio; no olfactometry standard text is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namessource_dt, airflow_acfm, dilution_factor, limit_dt, target_dt, odour_emission_rate_ou_s, dt_at_receptor, required_source_dt, reduction_pct

• D/T from olfactometry a panel method with real variability, from one operating conditionthe olfactometry standard used
• Which odour unit with the flow in cfm the rate counts cubic FEET of air at threshold; EN 13725's OU_E is a cubic metre, 35.3 times fewer per second, and both are reportedEN 13725
• The dilution is entered it comes from a dispersion calculation, not from this tilea dispersion model
• Concentration is not unpleasantness hedonic tone, frequency and duration are outside this arithmeticthe applicable odour ordinance or nuisance rule