3.4 Taste and Odor Control, Powdered Activated Carbon & Algae Management

3.4 Taste and Odor Control, Powdered Activated Carbon & Algae Management

Taste and odor is a secondary standard — no one gets sick from geosmin — but it is the parameter customers judge a utility by, and the Needs to Know composition tables list "Taste/Odor Control" as a scored category on the A, B, and C Surface examinations.

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1. Where the compounds come from

Geosmin and MIB are detectable at 5 to 10 ng/L (parts per trillion) by sensitive consumers, so a reservoir can produce a flood of complaints while every regulated parameter remains in compliance.

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2. Measuring taste and odor

• Threshold Odor Number (TON). Sample is diluted with odor-free water until a panel can just detect odor. TON is the dilution ratio: if 50 mL of sample diluted to 200 mL total is the weakest detectable dilution, TON = 200/50 = 4. Testing is done at a controlled temperature (commonly 60 °C for the hot test) using odor-free glassware.
• Flavor Profile Analysis (FPA). A trained panel rates intensity and assigns descriptors (earthy, musty, chlorinous, grassy), which is far more useful operationally than a single number.
• Instrumental confirmation. Geosmin and MIB are quantified by gas chromatography–mass spectrometry, usually at a contract laboratory, and results in ng/L should be trended against reservoir temperature and chlorophyll.

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3. Oxidation — what works and what backfires

[!WARNING] Pre-chlorination is a taste-and-odor gamble. Adding chlorine to a raw water containing phenols or algal metabolites can make the finished water smell worse while also multiplying DBP formation. If an oxidant is needed ahead of the plant, jar test it — and remember that under the Stage 1/Stage 2 DBP rules moving the chlorine application point downstream is a recognized control strategy.

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4. Powdered activated carbon (PAC)

PAC is the workhorse of episodic taste-and-odor control because it can be started and stopped as a bloom comes and goes.

• Dose range. Typically 1 to 20 mg/L, with higher doses during severe episodes. The correct dose is determined by jar tests that measure the residual geosmin/MIB or the threshold odor number, not by guesswork.
• Feed point. As early as possible to maximize contact time — commonly at the raw water intake or the rapid mix, ahead of the coagulant.
• Never with chlorine. PAC and chlorine consume each other. Feeding them at the same point wastes both the carbon and the disinfectant and destroys CT. If pre-oxidation is required, allow the oxidant to be consumed, or feed PAC well upstream.
• Interference with coagulation. PAC adds solids load and can interfere with polymer if fed at the same injection point; it also increases sludge volume and, at high doses, can carry through to filters and cause black specks in finished water.
• Handling. Carbon dust is a respiratory nuisance and an explosion concern in confined slurry rooms; keep it away from oxidizers, use dust collection, and ground the slurry equipment.

Granular activated carbon (GAC) is the continuous alternative: anthracite is replaced with GAC in the filter (a "filter-adsorber") or a post-filter contactor is added. GAC gives excellent geosmin and MIB removal until the bed exhausts, and it is also a Stage 2 DBP control. It must be regenerated or replaced on a monitored schedule, and biological growth in the bed is normal in warm weather.

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5. Reservoir and intake management

1. Profile the reservoir weekly in summer for temperature and dissolved oxygen, and withdraw from the depth with the best combination of low algae, adequate oxygen, and low iron and manganese.
2. Track the bloom, not the calendar. Cell counts, chlorophyll a, and taste-and-odor compound trends predict a PAC start-up better than a fixed seasonal schedule.
3. Algaecides. Copper sulfate and chelated copper products control algae but lyse the cells, releasing stored MIB, geosmin, and any cyanotoxins at once. Treat early in a bloom, treat portions of the reservoir at a time, and increase finished water monitoring afterward. Copper applications are also subject to permitting and label restrictions.
4. Cyanotoxins. During harmful algal blooms, monitor for microcystins with ELISA screening. EPA has issued health advisories for microcystins and cylindrospermopsin; conventional treatment removes intact cells well but dissolved toxin requires PAC, GAC, ozone, or advanced oxidation — which is another reason not to oxidize cells at the intake without a plan.
5. Finished-side causes. Before blaming the reservoir, rule out distribution causes: nitrification in chloraminated systems, dead-end stagnation, biofilm in unlined cast iron, and customer-side plumbing such as water heater anode rods (a classic source of sulfur odor at one house only).