Odor Control Technician cheat sheet

Olfactory receptor neurons in the olfactory epithelium number roughly 6 million per human nasal cavity; they regenerate about every 30-60 days, unlike most neurons.

Odor travels two routes: orthonasal (sniffing, outside air) and retronasal (from the mouth via the nasopharynx); retronasal perception dominates flavor complaints in food and beverage odor work.

Odor threshold types: detection threshold (first perceptible), recognition threshold (identifiable quality), and difference threshold (smallest concentration change detectable, roughly 10-50% above baseline).

Weber-Fechner law: perceived intensity grows with the logarithm of concentration, so a 10x concentration increase is felt as roughly a doubling of intensity, not 10x.

Stevens' power law: I = k*C^n, where n is typically 0.2-0.7 for odorants; n<1 means perceived intensity rises slower than concentration.

Adaptation: continuous exposure reduces sensitivity within seconds to minutes; recovery requires minutes of clean air, so panelists must rest 1-2 minutes between samples.

Anosmia to specific compounds is common and genetic (e.g., inability to smell androstenone); a single panelist's null response is not evidence of no odor.

Mixtures rarely smell like the sum of components: masking, synergy, and counteraction occur, and a mixture can smell weaker than its strongest single component (hypoadditivity).

Key odorant families: reduced sulfur compounds (H2S rotten egg, mercaptans, dimethyl sulfide), amines and ammonia (fishy, urine), fatty acids (rancid, sweaty), aldehydes (grassy, pungent), and VOCs (solventy).

Anaerobic decomposition produces reduced sulfur and amine odors; aerobic decomposition trends toward earthy, musty, and fatty-acid notes.

H2S odor threshold is in the low parts-per-billion range (roughly 0.5-5 ppb); above roughly 10 ppm the sense of smell is rapidly fatigued, so odor is not a safe exposure indicator.

Wastewater: septic conditions (low dissolved oxygen, high sulfide loading, long retention) drive sulfide generation; grease and solids build-up in collection lines is a primary source.

Composting and animal operations emit ammonia and amines when nitrogen-rich feedstock is under-aerated, and sulfides when material turns anaerobic.

Mold and musty odors are associated with microbial volatile organic compounds such as geosmin and 2-methylisoborneol, detectable at parts-per-trillion levels in water.

Indoor sources include off-gassing from new materials (formaldehyde, styrene), occupant bioeffluents, damp building materials, and dry floor drains allowing sewer gas entry.

Industrial and combustion sources: solvent vapors, paint and coating emissions, diesel and smoke, and process vents; source identification precedes any treatment selection.

Four measurement routes: sensory (human panel), instrumental (analytical), field monitoring (portable meters, badges), and olfactometry (dilution-to-threshold).

Odor concentration is expressed in odor units (OU or OUE/m3): the number of dilutions of sample with neutral air needed to reach the panel's detection threshold.

EN 13725 is the European standard for dynamic olfactometry; it specifies panel selection, n-butanol reference, and a laboratory accuracy criterion.

Dynamic olfactometry uses a forced-choice method with multiple panelists and dilutions; results are reported as the geometric mean of individual thresholds.

Field olfactometry (portable dilution devices) gives rapid screening values but is less reproducible than laboratory olfactometry; use it for trends, not compliance proof.

Hedonic tone (pleasant-unpleasant, typically a -4 to +4 or -5 to +5 scale) and intensity (e.g., 0-5 or 0-10 scale) are rated separately from concentration.

Total reduced sulfur (TRS) and H2S are measured with gold-film or electrochemical sensors; colorimetric tubes give spot checks, and sorbent-tube GC/MS gives speciation.

Sampling rules: use inert (PTFE, Tedlar) containers, minimize holding time, avoid light and heat, and run field blanks; sorption and leakage bias results low.

Four control strategies: removal (capture and destroy), dilution (dispersion), masking (overpowering with another odor), and counteraction/neutralization (pairing agents that reduce perceived intensity).

Mass balance governs design: emission rate (mass per time) divided by air flow gives concentration, and dilution to a target threshold sets the required stack height or airflow.

Capture before treatment: containment and local exhaust reduce the volume to be treated, which is usually cheaper than treating a larger diluted stream.

Partitioning drives scrubbing: Henry's law constant (H = partial pressure / liquid concentration) predicts whether a compound is strippable or absorbable; high H favors stripping, low H favors absorption.

Biological treatment relies on microbial oxidation; design parameters include empty bed residence time (EBRT), media moisture, pH buffering, and nutrient supply.

Adsorption capacity depends on compound polarity and molecular size; activated carbon favors higher-molecular-weight, low-polarity VOCs, and moisture competes for sites.

Dispersion modeling (Gaussian plume) uses stack height, exit velocity, and atmospheric stability class; stable conditions and low wind produce the worst ground-level impact.

Odor impact is assessed with frequency, intensity, duration, offensiveness, and location (FIDOL); a single strong event can outweigh a low average concentration.

Wet scrubbers: packed-bed (absorption, low-Henry compounds), spray/venturi (particulate and high-Henry compounds), and caustic or hypochlorite stages for H2S and mercaptans.

Chemical scrubber reagents: sodium hydroxide for H2S and acids, sodium hypochlorite for mercaptans and amines, and hydrogen peroxide or permanganate for other reduced sulfur species.

Biofilters (organic media, 15-60 s EBRT) suit large low-concentration streams; biotrickling filters use inert packing and recirculated water for better control of pH and nutrients.

Activated carbon adsorbers: virgin or impregnated (caustic-impregnated for H2S); monitor breakthrough and replace or regenerate before saturation to avoid odor slip.

Thermal oxidation: regenerative thermal oxidizer (RTO) or catalytic oxidizer destroys VOCs and odors at high temperature; check for NOx formation and fuel use.

Ultraviolet/photocatalytic oxidation and ozone injection are used for low-concentration polishing; ozone itself has an odor and must not be over-dosed.

Vapor-phase monitoring: electrochemical and metal-oxide sensors for H2S and ammonia, photoionization detectors (PID) for total VOCs, and colorimetric tubes for spot checks.

Fogging and misting systems deliver masking or neutralizing agents; droplet size and coverage determine contact efficiency, and over-wetting creates its own problems.

Oxidizers: hydrogen peroxide, sodium hypochlorite, potassium permanganate, and ozone convert sulfides and mercaptans to sulfate; dosing must be controlled to avoid odorant by-products.

Iron salts (ferrous/ferric chloride or sulfate) precipitate sulfide in collection systems and reduce H2S release; they also affect pH and corrosion.

Nitrate addition controls sulfide in force mains by favoring nitrate-reducing bacteria over sulfate reducers; it is a preventive, in-sewer option.

Acids and bases shift equilibria: lowering pH keeps H2S in the liquid (less stripping) but increases corrosion; raising pH keeps amines and ammonia in the liquid.

Masking agents add a pleasant or stronger odor; they do not remove the original compound, and the mixture can smell worse to some observers.

Counteractants (neutralizers) are designed to reduce perceived intensity of the target odor; efficacy is compound-specific and must be verified by panel testing.

Enzyme and bacterial additives target grease and organic buildup; they need adequate contact time, moisture, and appropriate temperature to work.

Safety and compatibility: never mix hypochlorite with acids (chlorine gas) or with ammonia (chloramines); check material compatibility and secondary by-products.

Wastewater treatment: cover and ventilate headworks and clarifiers, dose iron or nitrate in collection systems, and scrub or biofilter the collected air.

Landfill and transfer station: gas collection and flaring, daily cover, and perimeter misting or neutralizing systems for fugitive odor.

Composting: maintain aerobic conditions with adequate porosity and aeration; biofilter the exhaust and control leachate, which is a strong odor source.

Food processing: control grease and organic loading in drains, use enzymatic or biological drain maintenance, and scrub or oxidize cooking and rendering exhaust.

Animal facilities: ventilation and manure management, acidification or additives to reduce ammonia, and biofilters or scrubbers on exhaust air.

Industrial and paint/coating operations: capture at the source, use carbon adsorption or thermal oxidation, and verify with PID and odor panel checks.

Pulp and paper and rendering: control reduced sulfur compounds with caustic or hypochlorite scrubbing and thermal oxidation of high-concentration streams.

Confined space and sewer entry: test for H2S and oxygen before entry, ventilate, and treat odors at the source; odor fatigue makes smell an unreliable warning.