2.12 Iron and Manganese, Taste-and-Odor Control, GAC & PFAS

Iron and Manganese, Taste-and-Odor Control, GAC & PFAS

The ABC Water Treatment outline names "iron/manganese treatment" under Treatment Process, "activated carbon contactors" among the technologies that drive Virginia's waterworks classification, and "emerging contaminants (e.g., microplastics, PFAS)" as an analysis the operator must be able to perform, record, and interpret.

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1. Iron and Manganese

Why they matter

Neither is a health risk at the concentrations found in most water supplies. Both are secondary contaminants regulated for aesthetic reasons:

Complaints typically begin below the SMCL - manganese at 0.02 mg/L will stain laundry over time, and manganese deposits accumulate in mains for years before a flow disturbance releases them all at once as black water.

Where they come from

• Anoxic groundwater - reduced Fe2+ and Mn2+ are soluble and colorless. The water is clear at the tap and turns red or black in the sink as it oxidizes.
• Reservoir hypolimnion during summer stratification - the same reduction chemistry, then delivered to the plant intake at turnover.

Oxidation rates - the key operational difference

Stoichiometry worth remembering: roughly 1 mg/L KMnO4 per 1 mg/L Fe and roughly 2 mg/L KMnO4 per 1 mg/L Mn. Chlorine requires about 0.62 mg/L per mg/L Fe and about 1.3 mg/L per mg/L Mn.

Treatment options

1. Oxidation and filtration. Oxidize to insoluble Fe(OH)3 or MnO2, allow adequate reaction time in a detention basin, then filter. The most common approach and the most robust.
2. Manganese greensand or catalytic media. Glauconite or engineered media coated with manganese dioxide catalyzes oxidation on the grain surface. Operated either continuously regenerated (a low permanganate or chlorine dose fed ahead of the filter) or intermittently regenerated (batch permanganate soak). The classic failure is the pink water breakthrough that follows an intermittent regeneration that was not fully rinsed.
3. Sequestration. Polyphosphate or silicate binds Fe and Mn as a soluble complex so it never precipitates.
   • It does not remove anything. The metal is still in the water delivered to the customer.
   • Practical limit is roughly 1 mg/L combined Fe plus Mn; above that, sequestration fails.
   • The complex breaks down on heating, so the water discolors in a water heater or a kettle even though it is clear at the tap.
   • Polyphosphate must be applied before any oxidant, or the metal oxidizes first and cannot be sequestered.
   • Polyphosphate can also mobilize lead deposits, so it must be evaluated against corrosion control objectives.
4. Ion exchange or lime softening - both remove iron and manganese incidentally, but iron fouls cation resin badly unless the water is truly anoxic.
5. Biological filtration - engineered filters cultivating Gallionella and Leptothrix to oxidize iron and manganese, used in Europe and increasingly in North America.

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2. Taste and Odor

The dominant compounds

• Geosmin - earthy, musty. Produced by cyanobacteria and actinomycetes.
• 2-methylisoborneol (MIB) - musty, camphor. Same producers.

Human detection thresholds are approximately 4 to 10 nanograms per liter (parts per trillion), which is one thousandth of the concentration at which most contaminants are even measurable in a plant lab. Complaints therefore arrive long before routine analysis flags anything.

Other taste-and-odor sources: hydrogen sulfide (rotten egg, groundwater), chlorophenols (medicinal, from chlorine reacting with phenolic industrial discharge), decaying vegetation (grassy, swampy), and chloramine off-flavors.

Treatment

PAC application rules: apply as far upstream as contact time allows (presedimentation or rapid mix), never simultaneously with chlorine or permanganate (the carbon consumes the oxidant and the oxidant is wasted), and expect PAC to increase sludge volume and to darken filter backwash.

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3. Activated Carbon Fundamentals

Activated carbon works by physical adsorption onto an enormous internal surface area - 500 to 1,500 square meters per gram.

Empty bed contact time = (Bed volume) / (Flow rate). A 500 cubic foot bed at 250 cubic feet per minute has an EBCT of 2 minutes - far too short for organics removal.

Exhaustion and breakthrough. A GAC bed does not fail suddenly; a mass-transfer zone migrates down the bed until the target compound appears in the effluent. Two operational consequences:

• Monitor the effluent, not the calendar. Bed life varies enormously with loading.
• Competitive adsorption means a strongly adsorbed compound can displace a weakly adsorbed one already on the carbon, so effluent concentration can briefly exceed influent concentration - a phenomenon called chromatographic peaking.
• GAC beds are warm, wet, and organic-rich, so they support biological growth. That is a benefit in intentional biofiltration and a problem if it produces heterotrophic plate count increases downstream.
• Spent GAC is either reactivated thermally off site or landfilled. Reactivation loses about 5 to 10 percent of the carbon per cycle.

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4. PFAS

Per- and polyfluoroalkyl substances are synthetic fluorinated compounds used in firefighting foam, non-stick and stain-resistant coatings, and many industrial processes. The carbon-fluorine bond makes them extremely persistent - hence "forever chemicals."

The current federal standards

In April 2024 EPA finalized the first enforceable National Primary Drinking Water Regulation for PFAS:

Systems must complete initial monitoring and, where levels exceed the standards, install treatment. Note that PFOA and PFOS also carry MCLGs of zero, reflecting the absence of an identified safe threshold.

What works and what does not

That last row is the exam-relevant trap. Adding more chlorine or ozone does nothing for PFAS, and oxidizing polyfluorinated precursors can convert them into the very perfluorinated compounds being measured.

Analysis

PFAS are measured at parts per trillion by liquid chromatography with tandem mass spectrometry - EPA Methods 533 and 537.1. Sampling requires PFAS-free field practice: no PTFE-lined caps, no waterproof field clothing, no adhesive notes, no fast-food wrappers, and specified bottle types. Contamination during sampling is the most common cause of a false positive.

Microplastics

Also listed in the ABC criteria as an emerging contaminant. There is currently no federal MCL for microplastics in drinking water and no standardized regulatory analytical method. Conventional coagulation, sedimentation, and filtration remove a substantial fraction of larger particles, and membranes remove more, but operators should describe the status accurately: monitored and researched, not regulated.