3.4 Iron/Manganese Removal, Water Softening & Taste/Odor Control

3.4 Iron/Manganese Removal, Water Softening & Taste/Odor Control

Consumers judge the safety of their drinking water primarily through sensory perception: clarity, color, taste, and odor. Furthermore, excessive mineral hardness causes scale accumulation that degrades piping infrastructure and household appliances. Water operators must master the chemical and physical unit processes that remove dissolved minerals, control hardness, and eliminate organoleptic compounds.

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1. Iron & Manganese: Occurrence, Impact & Secondary Standards

Iron and manganese are abundant elements in the earth's crust. In pristine surface waters exposed to the atmosphere, these metals exist in oxidized, insoluble solid forms that settle into benthic sediments. However, in deep groundwater aquifers or anaerobic lake hypolimnions during seasonal thermal stratification, microbial decomposition depletes dissolved oxygen. Under these reducing conditions, insoluble minerals dissolve into clear, soluble divalent cations:

• Ferrous Iron: $\text{Fe}^{2+}$
• Manganous Manganese: $\text{Mn}^{2+}$

Aesthetic Secondary Maximum Contaminant Levels (SMCLs)

Under the National Secondary Drinking Water Regulations, the EPA establishes non-enforceable aesthetic guidelines (Alabama Rule 335-7-5-.16 requires community and NTNC groundwater treatment facilities to provide water meeting secondary as well as primary standards):

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2. Chemical Oxidation & Removal Technologies

Removing iron and manganese requires converting soluble divalent ions into insoluble precipitates, which are subsequently removed through clarification and media filtration.

Aeration

Aeration introduces atmospheric oxygen ($\text{O}_2$) to oxidize ferrous iron:

$4\text{Fe}^{2+} + \text{O}_2 + 10\text{H}_2\text{O} \longrightarrow 4\text{Fe(OH)}_3\downarrow + 8\text{H}^+$

• Stoichiometry & Kinetics: Theoretically, 1.0 mg/L of oxygen oxidizes approximately 7.0 mg/L of ferrous iron ($0.14\text{ mg } \text{O}_2$ per mg $\text{Fe}^{2+}$). The reaction proceeds rapidly at $\text{pH} \ge 7.2$.
• Limitation on Manganese: Aeration is ineffective for manganese removal under normal water conditions. Oxidizing manganese with dissolved oxygen requires a pH of 9.5 or higher and an impractically long detention time (hours to days). Stronger chemical oxidants are required.

Chemical Oxidants: Potassium Permanganate, Chlorine & Chlorine Dioxide

• Potassium Permanganate ($\text{KMnO}_4$): A powerful crystalline oxidant that reacts rapidly across a wide pH range (6.5 to 8.5): $3\text{Fe}^{2+} + \text{MnO}_4^- + 7\text{H}_2\text{O} \longrightarrow 3\text{Fe(OH)}_3\downarrow + \text{MnO}_2\downarrow + 5\text{H}^+$ $3\text{Mn}^{2+} + 2\text{MnO}_4^- + 2\text{H}_2\text{O} \longrightarrow 5\text{MnO}_2\downarrow + 4\text{H}^+$
  • Stoichiometric demand: $1.0\text{ mg/L } \text{KMnO}_4$ oxidizes 1.06 mg/L ferrous iron and 0.52 mg/L manganous manganese.
  • The Pink Water Hazard: Potassium permanganate has an intense purple color. If dosed in excess of the water's chemical demand, unreduced permanganate passes through downstream filters into the finished clearwell, imparting an objectionable pink-to-purple discoloration to customer tap water. Jar testing and continuous monitoring are mandatory.
• Chlorine ($\text{Cl}_2$): Readily oxidizes iron ($1.0\text{ mg/L } \text{Cl}_2$ oxidizes $1.58\text{ mg/L } \text{Fe}^{2+}$), but requires high pH ($> 8.0$) and extended contact time to oxidize manganese.
• Chlorine Dioxide ($\text{ClO}_2$): Oxidizes both iron and manganese rapidly without creating pink water or chlorinated DBPs, though dosage is limited by chlorite byproduct standards.

Manganese Greensand Filtration

Manganese greensand is processed from glauconite, a natural iron-potassium-aluminosilicate mineral coated with higher oxides of manganese (predominantly manganese dioxide, $\text{MnO}_2$). The coating serves as a continuous electron donor/acceptor catalyst, oxidizing and capturing dissolved iron and manganese on contact.

• Continuous Regeneration (CR): The standard process when raw water iron concentrations exceed manganese concentrations. A continuous chemical feed of potassium permanganate (or chlorine followed by a small trim dose of $\text{KMnO}_4$) is injected directly ahead of the greensand filter. Dissolved metals oxidize in the influent piping and the filter media, while excess permanganate continuously regenerates the greensand catalytic coating.
• Intermittent Regeneration (IR): Utilized when raw water contains primarily manganese and low iron ($< 5\text{ mg/L}$). The greensand bed treats water until its catalytic oxidation capacity is depleted. The filter is then taken offline, backwashed, and soaked in a potassium permanganate solution at the media manufacturer's recommended dose before being rinsed to waste and returned to service.

Sequestration (Chemical Masking)

When combined iron and manganese concentrations are low (total $\text{Fe} + \text{Mn} < 1.0\text{ mg/L}$) and the utility lacks filtration infrastructure, operators may dose polyphosphates (such as sodium hexametaphosphate) or sodium silicates. Sequestering agents bind divalent iron and manganese into clear, soluble coordination complexes, preventing oxidation and precipitation in customer plumbing. Crucial rule: Sequestration chemicals must be injected before the water is exposed to chlorine or air; once oxidation occurs, polyphosphates cannot reverse precipitation.

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3. Water Softening: Chemical Precipitation & Ion Exchange

Hardness is defined as the concentration of multivalent metallic cations in water, dominated by calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$). Hardness is quantitatively reported in milligrams per liter as calcium carbonate (mg/L as $\text{CaCO}_3$):

$\text{Total Hardness} = \text{Calcium Hardness} + \text{Magnesium Hardness}$

• Soft Water: 0 to 60 mg/L as $\text{CaCO}_3$
• Moderately Hard: 61 to 120 mg/L as $\text{CaCO}_3$
• Hard: 121 to 180 mg/L as $\text{CaCO}_3$
• Very Hard: > 180 mg/L as $\text{CaCO}_3$

Carbonate vs. Non-Carbonate Hardness

Hardness is categorized based on associated chemical anions:

• Carbonate Hardness (Temporary Hardness): Hardness chemically associated with bicarbonate ($\text{HCO}_3^-$) and carbonate ($\text{CO}_3^{2-}$) alkalinity. It precipitates upon heating and is calculated as: $\text{Carbonate Hardness} = \min(\text{Total Hardness}, \text{Total Alkalinity})$
• Non-Carbonate Hardness (Permanent Hardness): Hardness associated with sulfate ($\text{SO}_4^{2-}$), chloride ($\text{Cl}^-$), and nitrate ($\text{NO}_3^-$) anions. It cannot be precipitated by heating. It is calculated as: $\text{Non-Carbonate Hardness} = \text{Total Hardness} - \text{Carbonate Hardness}$

Lime-Soda Ash Precipitation Softening

Precipitation softening removes hardness by converting soluble calcium and magnesium into insoluble precipitates: calcium carbonate ($\text{CaCO}_3\downarrow$) and magnesium hydroxide ($\text{Mg(OH)}_2\downarrow$). Hydrated lime ($\text{Ca(OH)}_2$) raises pH, while soda ash ($\text{Na}_2\text{CO}_3$) provides the carbonate ions needed to precipitate non-carbonate hardness:

1. Neutralization of Free Carbon Dioxide: $\text{CO}_2 + \text{Ca(OH)}_2 \longrightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$ (Consumes lime without reducing hardness).
2. Precipitation of Calcium Carbonate Hardness (at pH about 10.3): $\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \longrightarrow 2\text{CaCO}_3\downarrow + 2\text{H}_2\text{O}$
3. Precipitation of Magnesium Carbonate Hardness (at pH about 11): $\text{Mg(HCO}_3)_2 + 2\text{Ca(OH)}_2 \longrightarrow 2\text{CaCO}_3\downarrow + \text{Mg(OH)}_2\downarrow + 2\text{H}_2\text{O}$
4. Precipitation of Calcium Non-Carbonate Hardness with Soda Ash: $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \longrightarrow \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$
5. Precipitation of Magnesium Non-Carbonate Hardness with Lime + Soda Ash: $\text{MgSO}_4 + \text{Ca(OH)}_2 \longrightarrow \text{Mg(OH)}_2\downarrow + \text{CaSO}_4$ $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \longrightarrow \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$

Recarbonation Stabilization

Water exiting lime softening clarifiers is supersaturated with calcium carbonate at a high pH (10.5 to 11.2). If discharged directly onto downstream sand filters, calcium carbonate precipitates onto the media, cementing the sand grains together into large conglomerates and causing rapid head loss. Furthermore, post-precipitation scales distribution piping.

To stabilize the water, utilities bubble carbon dioxide gas ($\text{CO}_2$) through the water in a recarbonation basin: $\text{Ca(OH)}_2 + \text{CO}_2 \longrightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$ $\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \longrightarrow \text{Ca(HCO}_3)_2$

Recarbonation lowers pH to approximately 8.5–8.8, converting unstable carbonate ions back into soluble, non-scaling bicarbonate ions.

Ion Exchange Softening (Zeolite Softening)

Ion exchange softening passes hard water through a pressure vessel containing synthetic polystyrene divinylbenzene cation exchange resin loaded with sodium ions ($\text{Na}^+$). The multivalent calcium and magnesium ions possess a higher affinity for the resin matrix and displace the sodium ions:

$2\text{R-Na}^+ + \text{Ca}^{2+} \rightleftharpoons \text{R}_2\text{-Ca}^{2+} + 2\text{Na}^+$ $2\text{R-Na}^+ + \text{Mg}^{2+} \rightleftharpoons \text{R}_2\text{-Mg}^{2+} + 2\text{Na}^+$

• Performance: Ion exchange removes 100% of hardness, producing water with zero hardness. Because zero-hardness water is corrosive, utilities use split-treatment blending: a portion of raw, filtered water bypasses the softening beds and is blended with the softened effluent to produce finished water with 70 to 90 mg/L hardness.
• Regeneration Cycle: When the resin's exchange sites are exhausted, the bed is taken offline and regenerated in four stages:
  1. Backwash: Upward flow expands the resin bed by 50%, removing trapped particulate matter.
  2. Brine Injection: A concentrated solution of sodium chloride (10% to 14% $\text{NaCl}$) is injected. By mass action, the overwhelming concentration of sodium ions displaces calcium and magnesium off the resin into waste brine.
  3. Slow Rinse (Displacement): Pushes the remaining brine through the bed.
  4. Fast Rinse: Flushes residual salt brine from the vessel before returning to service.

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4. Taste and Odor (T&O) Control: Geosmin, MIB & Activated Carbon

Taste and odor events in drinking water are the leading cause of consumer complaints. While rarely toxic, unpleasant tastes and odors undermine public confidence in municipal water safety.

Organic Metabolites: Geosmin & 2-Methylisoborneol (MIB)

The vast majority of seasonal earthy and musty odors in surface reservoirs are caused by two semi-volatile secondary organic metabolites:

• Geosmin (trans-1,10-dimethyl-trans-9-decalol): Distinct earthy / dirt odor.

• 2-Methylisoborneol (MIB): Distinct musty / moldy / woody odor.

• Biological Origin: Produced naturally by cyanobacteria (blue-green algae such as Anabaena, Aphanizomenon, Oscillatoria, and Microcystis) and filamentous actinomycetes bacteria in shallow, warm lake sediments.

• Extreme Sensitivity: The human olfactory system is extraordinarily sensitive to these compounds. The odor threshold concentration for Geosmin is 4 to 10 ng/L (nanograms per liter, or parts per trillion, ppt), and for MIB is 5 to 15 ng/L. Conventional coagulation, sedimentation, and granular filtration achieve negligible removal of dissolved Geosmin and MIB.

• Aesthetic Benchmark: Evaluated using the Threshold Odor Number (TON) (Standard Methods 2150 B). The secondary maximum contaminant level for odor is $\text{TON} \le 3$.

Treatment Technologies: Powdered & Granular Activated Carbon

Adsorption onto activated carbon is the most effective operational solution for dissolved organic taste and odor compounds. Activated carbon is processed from coal, wood, or coconut shells, yielding an immense internal porous network with surface areas ranging from 600 to 1,200 m²/gram.

• Powdered Activated Carbon (PAC):
  • Applied during seasonal taste and odor episodes as a dry powder or pre-slurried suspension at the raw water intake or rapid mix basin.
  • Typical dosages range from 5 to 25 mg/L depending on TON severity.
  • CRITICAL OPERATIONAL BAN: Never inject PAC and chlorine (or potassium permanganate) at the exact same location! Activated carbon is a strong reducing agent; chlorine adsorbs directly onto the carbon surface, which neutralizes the disinfectant into inactive chloride ions while simultaneously blinding the carbon adsorption pores and wasting taste/odor capacity. PAC must be allowed at least 10 to 15 minutes of contact time before chlorine addition, or fed downstream.
• Granular Activated Carbon (GAC):
  • Installed in gravity filter basins as a permanent 24- to 36-inch media bed (or as an anthracite replacement cap). Water passes through GAC with an Empty Bed Contact Time (EBCT) of 10 to 20 minutes, providing continuous passive adsorption of Geosmin, MIB, and synthetic organic chemicals.
• Oxidation Alternatives: Ozone ($\text{O}_3$) and Advanced Oxidation Processes ($\text{UV}/\text{H}_2\text{O}_2$ or $\text{O}_3/\text{H}_2\text{O}_2$) break down the saturated bicyclic ring structures of Geosmin and MIB. Aeration effectively strips volatile hydrogen sulfide ($\text{H}_2\text{S}$, rotten-egg odor), but is ineffective on semi-volatile Geosmin and MIB.

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5. Practical Engineering Worked Calculations

Worked Example 1: Potassium Permanganate Oxidation Feed Rate

A municipal groundwater well produces 2.4 MGD. Laboratory analytical testing reveals that the raw water contains 1.8 mg/L of soluble ferrous iron ($\text{Fe}^{2+}$) and 0.5 mg/L of soluble manganous manganese ($\text{Mn}^{2+}$). The operator must calculate the required potassium permanganate ($\text{KMnO}_4$) dosage and daily chemical feed rate.

Stoichiometric oxidation ratios:

• $1.0\text{ mg/L } \text{KMnO}_4$ oxidizes $1.06\text{ mg/L } \text{Fe}^{2+}$ (Requirement: $1.8 / 1.06 = 0.943\text{ mg } \text{KMnO}_4$ per mg $\text{Fe}$).
• $1.0\text{ mg/L } \text{KMnO}_4$ oxidizes $0.52\text{ mg/L } \text{Mn}^{2+}$ (Requirement: $0.5 / 0.52 = 1.923\text{ mg } \text{KMnO}_4$ per mg $\text{Mn}$).

Step 1: Calculate the theoretical permanganate demand for iron. $\text{Demand}_{\text{Fe}} = \frac{1.8\text{ mg/L}}{1.06} = 1.698\text{ mg/L } \text{KMnO}_4$

Step 2: Calculate the theoretical permanganate demand for manganese. $\text{Demand}_{\text{Mn}} = \frac{0.5\text{ mg/L}}{0.52} = 0.962\text{ mg/L } \text{KMnO}_4$

Step 3: Calculate the total required dosage. $\text{Total Dose} = 1.698\text{ mg/L} + 0.962\text{ mg/L} = 2.66\text{ mg/L } \text{KMnO}_4$

Step 4: Calculate the chemical feed rate in pounds per day. $\text{Feed Rate (lb/day)} = 2.4\text{ MGD} \times 2.66\text{ mg/L} \times 8.34\text{ lb/gal} = 53.24\text{ lb/day}$

The dry chemical feeder should deliver 53.2 lb/day of potassium permanganate.

Worked Example 2: Hardness Fractions Calculation

A raw surface water sample yields the following laboratory titrations:

• Total Hardness = $240\text{ mg/L as }\text{CaCO}_3$
• Total Alkalinity = $175\text{ mg/L as }\text{CaCO}_3$

Step 1: Calculate Carbonate Hardness. $\text{Carbonate Hardness} = \min(\text{Total Hardness}, \text{Total Alkalinity}) = \min(240, 175) = 175\text{ mg/L as }\text{CaCO}_3$

Step 2: Calculate Non-Carbonate Hardness. $\text{Non-Carbonate Hardness} = \text{Total Hardness} - \text{Carbonate Hardness} = 240 - 175 = 65\text{ mg/L as }\text{CaCO}_3$

Hydrated lime must be applied to remove the 175 mg/L of carbonate hardness, and soda ash must be applied to remove the remaining 65 mg/L of non-carbonate hardness.