5.9 Taste & Odor Control and Algal Toxin Management

Two Blueprint Entries, One Topic

The treatment Expected Range of Knowledge lists Taste and Odor under Water Treatment Processes and Taste and Odor Analysis separately under Laboratory Procedures. The distribution blueprint reaches the same material through Water Quality and through customer complaint response. California adds a regulatory hook: under 22 CCR 64449 the secondary MCL for odor is a threshold odor number (TON) of 3 and for color is 15 units.

Taste and odor is not a health issue in most cases, but it is the issue customers judge the utility on. As the classic operator formulation goes, if the water tastes bad, customers assume it is unsafe - and then they stop trusting the utility about things that genuinely are safety issues.

---

Where Taste and Odor Comes From

The Two Compounds That Matter Most

Geosmin and MIB are semi-volatile metabolites produced by cyanobacteria and actinomycetes. Two properties make them uniquely difficult:

1. Detection thresholds of roughly 5 to 10 nanograms per liter - parts per trillion. A concentration that a laboratory struggles to quantify is one that customers can taste easily.
2. They are not oxidized effectively by chlorine, chloramine, or permanganate at practical doses, and they are not removed by coagulation, sedimentation, or conventional filtration.

That combination means the standard treatment train does nothing about them, and the operator needs a dedicated tool.

---

Treatment Options

Powdered Activated Carbon (PAC) - the episodic tool

• Fed as a slurry, typically 5 to 30 mg/L, at the rapid mix or just ahead of it to maximize contact time before the floc envelopes the carbon particles.
• Requires contact time before coagulant addition if possible; carbon that gets swept into floc early adsorbs less.
• Removed with the sludge in the sedimentation basin and filters.
• Never feed PAC and an oxidant at the same point - the carbon consumes chlorine, ozone, or permanganate that was intended for disinfection or iron and manganese oxidation.
• Increases sludge volume and can carry over as black turbidity if overdosed or poorly settled.

Granular Activated Carbon (GAC) - the continuous tool

• Used as a filter cap replacing anthracite (adsorption plus filtration) or as a post-filter contactor (adsorption only, longer empty bed contact time).
• Empty bed contact time (EBCT) of 10 to 20 minutes is typical for taste and odor control.
• Exhausts over months to years and must be reactivated or replaced; breakthrough is monitored by influent/effluent comparison.
• Also removes many synthetic organic chemicals and DBP precursors, which is why GAC is a common answer to more than one treatment problem.

Ozone and Advanced Oxidation

• Ozone oxidizes geosmin and MIB far more effectively than chlorine, though not completely; typical geosmin and MIB removal is partial and improves with ozone plus hydrogen peroxide (peroxone), an advanced oxidation process that generates hydroxyl radicals.
• Ozone increases assimilable organic carbon, so it should be followed by biologically active filtration to remove the biodegradable products before they feed distribution biofilm.
• Ozone also oxidizes iron, manganese, and color, and provides excellent Cryptosporidium inactivation - it solves several problems at once, which is why large California plants adopted it.

Aeration

Effective for volatile odor compounds - hydrogen sulfide, some VOCs, dissolved gases. Ineffective for geosmin and MIB, which are semi-volatile and do not strip well.

Source Control

The cheapest solution is often to not draw the problem in: multi-level intakes, reservoir destratification or hypolimnetic aeration to prevent the anoxic conditions that release sulfide and metals, watershed nutrient control to limit blooms, and short-term intake depth changes during a bloom.

---

Cyanobacteria and Cyanotoxins

California reservoirs experience harmful algal blooms (HABs) of Microcystis, Anabaena (Dolichospermum), Aphanizomenon, and Cylindrospermopsis, producing:

[!WARNING] Do not pre-chlorinate a heavy cyanobacteria bloom at the intake. Chlorine, permanganate, and ozone all lyse the cells, releasing intracellular toxin into the water in one step. Intact cells can be removed physically by coagulation, sedimentation, and filtration, which takes the toxin out with the cell. The correct sequence in a bloom is: remove the cells intact first, then treat any dissolved extracellular toxin with PAC, GAC, ozone, or advanced oxidation after solids removal. Pre-oxidation in a bloom converts a removable particulate problem into a dissolved one.

California's Harmful Algal Bloom program uses a tiered advisory posting - Caution, Warning, Danger - for recreational waters. For drinking water sources, DDW expects a system with a bloom-prone source to have a monitoring and response plan covering cell counts, toxin analysis, intake management, and treatment adjustments.

---

Tracing a Complaint

A complaint is data. Work it systematically:

1. Where and when? Get the address, the time of day, hot or cold tap, first draw or after running.
2. Is it at the plant? Compare plant effluent TON and any geosmin/MIB results against the complaint. If the plant is clean, the problem is downstream.
3. Is it clustered? Multiple complaints along one main or downstream of one storage tank points to distribution stagnation, nitrification, or biofilm. A single address points to customer plumbing.
4. Hot water only? Water heater - anode rod reacting with sulfate-reducing bacteria produces the classic rotten egg odor in hot water only.
5. Only the first draw? Stagnation in the service line or premise plumbing.
6. Chlorinous or swimming pool? Check the chloramine ratio - dichloramine and trichloramine are the usual culprits, and the fix is at the plant.
7. Confirm with a threshold odor number run at 60 °C by a trained panel, and record the result. TON of 3 is the California secondary MCL.