3.4 Aeration, Taste & Odor Control
Key Takeaways
- AERATION is a Level D treatment process in the Missouri Table 1 classification, covering cascade, diffused, packed tower, slat tray and spray designs.
- Aeration adds oxygen and strips volatile compounds: carbon dioxide, hydrogen sulfide, radon, methane and volatile organics.
- Threshold Odor Number is the dilution ratio at which odor becomes barely detectable, calculated as sample volume plus dilution volume divided by sample volume.
- Geosmin and 2-methylisoborneol from blue-green algae and actinomycetes cause earthy-musty taste and odor at nanogram-per-liter levels and are not removed by chlorination.
- Powdered activated carbon dosed ahead of coagulation is the standard seasonal response to algal taste and odor episodes in Missouri reservoirs.
AERATION is a scored objective on Drinking Water A, C and D, and TASTE & ODOR on Drinking Water A, B and D. Missouri's Table 1 classification lists aeration explicitly as a Level D treatment process, naming the designs it recognises: cascade, diffused, packed tower, slat tray and spray.
What Aeration Actually Does
Aeration moves gases across the air-water interface in whichever direction the concentration gradient dictates. Two jobs, opposite directions:
- Adding oxygen — oxidising soluble iron and manganese so they can be filtered, and raising dissolved oxygen in flat, anoxic groundwater.
- Stripping unwanted gases — carbon dioxide, hydrogen sulfide, methane, radon and volatile organic compounds.
The driving force is Henry's law: the amount of gas dissolved is proportional to its partial pressure above the liquid. Aeration works by continuously replacing the air in contact with the water so the partial pressure of the target gas stays near zero.
| Aerator type | How it works | Best suited for |
|---|---|---|
| Cascade | Water falls over a series of steps or weirs | CO₂ removal; simple, no moving parts, low head requirement |
| Slat tray (multiple tray) | Water distributed over stacked trays, often filled with coke or stone media | Iron oxidation; compact; media provides catalytic surface |
| Spray | Nozzles throw water into the air | High oxygen transfer; large area needed; freezing risk in Missouri winters |
| Diffused | Compressed air bubbled through a basin | Retrofit into existing basins; controllable; higher energy cost |
| Packed tower (stripping tower) | Water trickles down through packing against a forced updraft of air | Highest efficiency for volatile organics and radon; high air-to-water ratios |
Carbon dioxide removal is the most common Missouri application. Groundwater from limestone aquifers is often carbonic-acid rich, with a low pH and high CO₂. Aeration strips CO₂, raises pH without chemicals, and reduces the amount of lime or caustic needed downstream for corrosion control.
Limitations to remember: aeration cannot remove non-volatile dissolved solids such as hardness, nitrate, fluoride or most metals. It can also add problems — introducing oxygen into water containing dissolved iron precipitates it, which is desirable ahead of a filter but disastrous if it happens inside a distribution main.
Measuring Odor: Threshold Odor Number
Odor is quantified by dilution. The Threshold Odor Number (TON) is the dilution factor at which odor becomes barely perceptible:
where $A$ is the volume of sample and $B$ is the volume of odor-free dilution water.
Worked example. 25 mL of raw water diluted with 175 mL of odor-free water reaches the detection threshold:
A TON of 1 means no dilution was needed — the odor was barely detectable in the undiluted sample. Larger TON means stronger odor. The secondary standard for odor is a TON of 3.
Taste and odor are secondary standards (SMCLs) — aesthetic rather than health-based — but they generate more customer complaints than any other water quality parameter, and complaints are what bring regulators and city councils to the plant.
The Algal Metabolites: Geosmin and MIB
Missouri's surface water supplies draw from reservoirs that stratify in summer and support blue-green algae (cyanobacteria) blooms. Two compounds dominate the resulting complaints:
| Compound | Source | Odor character | Detection threshold |
|---|---|---|---|
| Geosmin | Cyanobacteria, actinomycetes | Earthy, beetroot | ~4 to 10 ng/L |
| 2-methylisoborneol (MIB) | Cyanobacteria, actinomycetes | Musty, camphor | ~9 to 15 ng/L |
The critical operational fact: these are detectable at nanograms per liter — parts per trillion — which is one thousand times below the µg/L range most instruments report. And they are not removed by chlorination. Adding more chlorine to an earthy-musty complaint makes matters worse, because chlorine reacts with algal organic matter to form additional taste-and-odor compounds and disinfection byproducts.
What actually works
- Powdered activated carbon (PAC) — the standard first response. Dosed at the raw water intake or rapid mix, typically 5 to 30 mg/L depending on severity, and removed with the coagulation floc. Must be applied ahead of chlorination, because free chlorine occupies carbon adsorption sites that would otherwise capture the odor compounds.
- Granular activated carbon (GAC) — used as a filter medium or in post-filter contactors; effective continuously but requires periodic regeneration or replacement.
- Ozone and advanced oxidation — genuinely destroy geosmin and MIB rather than adsorbing them, but require capital investment.
- Potassium permanganate — oxidises some taste and odor compounds and pre-oxidises iron and manganese, but is only partially effective against geosmin and MIB. Overdosing produces pink water and manganese dioxide deposits.
- Source control — intake depth selection to draw from below the algal layer, reservoir aeration or destratification, and copper sulfate algaecide applications. Note that killing algae with copper sulfate releases intracellular geosmin and MIB and any cyanotoxins, so treatment must be ready before the algaecide goes on.
Other Taste and Odor Sources
| Complaint | Likely cause | Response |
|---|---|---|
| Rotten egg | Hydrogen sulfide from anaerobic groundwater or a dead-end main | Aeration, oxidation, flushing |
| Chlorinous, swimming pool | Free chlorine residual too high, or chloramine formation with ammonia | Adjust dose; check for nitrification in distribution |
| Medicinal, phenolic | Chlorine reacting with phenols from industrial contamination | Find the source; chloramination reduces formation |
| Metallic | Iron, zinc or copper from corrosion of unlined pipe | Corrosion control, pH and alkalinity adjustment |
| Musty in one neighbourhood only | Water age in a dead-end or oversized main; biofilm | Targeted flushing programme, main downsizing |
That last row is diagnostically important. A complaint confined to part of the system is a distribution problem, not a treatment problem — check water age and residual before touching plant chemistry.
A laboratory determines that 20 mL of raw reservoir water must be diluted with 180 mL of odor-free water before the odor becomes barely detectable. What is the Threshold Odor Number?
Customers report an earthy, musty taste during a late-summer algal bloom on a Missouri reservoir. The plant already carries a 1.2 mg/L free chlorine residual. What is the appropriate operator response?
A Missouri groundwater system draws water high in carbon dioxide with a pH of 6.3. Which aeration approach most directly addresses this, and what secondary benefit does it produce?