4.4 Specialty Treatment: Softening, Aeration & PFAS Remediation

4.4 Specialty Treatment: Softening, Aeration & PFAS Remediation

Municipal water systems often encounter raw water contaminants that pass unaffected through conventional coagulation, sedimentation, and granular media filtration. Dissolved hardness minerals, reduced dissolved metals (iron and manganese), dissolved hazardous gases and volatile organics, and persistent synthetic "forever chemicals" (PFAS) demand targeted physical-chemical treatment processes.

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Water Hardness: Concepts, Chemistry & Classifications

Water hardness is defined as the concentration of multivalent metallic cations in solution. In virtually all natural drinking water supplies, total hardness is overwhelmingly dominated by calcium ($ ext{Ca}^{2+}$) and magnesium ($ ext{Mg}^{2+}$). Secondary contributors include strontium ($ ext{Sr}^{2+}$), ferrous iron ($ ext{Fe}^{2+}$), and manganous manganese ($ ext{Mn}^{2+}$).

Hardness Classifications (as mg/L CaCO₃)
┌────────────────────────┬─────────────────────────────────────────────┐
│ Concentration Range    │ Classification & Consumer Impact            │
├────────────────────────┼─────────────────────────────────────────────┤
│ 0 to 60 mg/L           │ Soft (Corrosive to plumbing; low scale)     │
│ 61 to 120 mg/L         │ Moderately Hard (Ideal potable balance)     │
│ 121 to 180 mg/L        │ Hard (Noticeable soap curd, scummy ring)    │
│ > 180 mg/L             │ Very Hard (Severe scale in boilers/heaters) │
└────────────────────────┴─────────────────────────────────────────────┘

Stoichiometric Calculation of Total Hardness

Hardness concentrations are standardly reported as equivalent calcium carbonate ($\text{mg/L as CaCO}_3$). To convert analytical milligram-per-liter concentrations of individual metal cations into equivalent $\text{CaCO}_3$, multiply by the ratio of the equivalent weight of $\text{CaCO}_3$ ($50.04\text{ g/eq}$) to the equivalent weight of the cation:

• Equivalent weight of $\text{Ca}^{2+}$ = $40.08 / 2 = 20.04\text{ g/eq} \implies \text{Multiplier} = 50.04 / 20.04 = \mathbf{2.50}$
• Equivalent weight of $\text{Mg}^{2+}$ = $24.30 / 2 = 12.15\text{ g/eq} \implies \text{Multiplier} = 50.04 / 12.15 = \mathbf{4.12}$

$\text{Total Hardness (mg/L as CaCO}_3) = 2.50 \times [\text{Ca}^{2+},\text{ mg/L}] + 4.12 \times [\text{Mg}^{2+},\text{ mg/L}]$

Carbonate (Temporary) vs Noncarbonate (Permanent) Hardness

Hardness is chemically categorized by the anions associated with the calcium and magnesium cations:

1. Carbonate Hardness (Temporary Hardness): Hardness associated chemically with bicarbonate [$\text{HCO}_3^-$] and carbonate [$\text{CO}_3^{2-}$] anions. It is called temporary hardness because boiling water drives off carbon dioxide, causing the calcium to precipitate naturally as calcium carbonate scale:
   • If $\text{Total Hardness} \le \text{Total Alkalinity} \implies \mathbf{\text{Carbonate Hardness} = \text{Total Hardness}}$ (no noncarbonate hardness exists).
   • If $\text{Total Hardness} > \text{Total Alkalinity} \implies \mathbf{\text{Carbonate Hardness} = \text{Total Alkalinity}}$.
2. Noncarbonate Hardness (Permanent Hardness): Hardness associated with sulfate [$\text{SO}_4^{2-}$], chloride [$\text{Cl}^-$], and nitrate [$\text{NO}_3^-$] anions. It cannot be removed by boiling: $\mathbf{\text{Noncarbonate Hardness} = \text{Total Hardness} - \text{Carbonate Hardness}}$

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Chemical Softening: Lime-Soda Ash & Two-Stage Recarbonation

Large-scale municipal softening precipitates calcium and magnesium ions as insoluble salts: calcium carbonate [$\text{CaCO}_3\downarrow$] and magnesium hydroxide [$\text{Mg(OH)}_2\downarrow$].

Chemical Precipitation Softening Mechanics
┌────────────────────────────────────────────────────────┐
│ Step 1: Neutralize Dissolved CO₂ (Lime Demand)         │
│ CO₂ + Ca(OH)₂ ──► CaCO₃↓ + H₂O                         │
│ (Must be satisfied first; consumes chemical)           │
├────────────────────────────────────────────────────────┤
│ Step 2: Precipitate Calcium Carbonate Hardness (pH 9.5)│
│ Ca(HCO₃)₂ + Ca(OH)₂ ──► 2 CaCO₃↓ + 2 H₂O               │
├────────────────────────────────────────────────────────┤
│ Step 3: Precipitate Magnesium Carbonate Hardness (pH10.8│
│ Mg(HCO₃)₂ + 2 Ca(OH)₂ ──► Mg(OH)₂↓ + 2 CaCO₃↓ + 2 H₂O  │
│ (Requires excess lime to force pH above 10.8)          │
├────────────────────────────────────────────────────────┤
│ Step 4: Precipitate Noncarbonate Hardness (Soda Ash)   │
│ CaSO₄ + Na₂CO₃ ──► CaCO₃↓ + Na₂SO₄                     │
│ MgSO₄ + Ca(OH)₂ + Na₂CO₃ ──► Mg(OH)₂↓ + CaCO₃↓ + Na₂SO₄│
├────────────────────────────────────────────────────────┤
│ Step 5: Two-Stage Recarbonation with CO₂               │
│ • Stage 1 (Pre-filter, pH ~9.5): Stabilizes excess lime│
│ • Stage 2 (Post-filter, pH ~8.4): Prevents main scaling│
└────────────────────────────────────────────────────────┘

The Softening Chemical Equations

1. Carbon Dioxide Consumption: Dissolved carbon dioxide gas reacts with hydrated lime [$\text{Ca(OH)}_2$] immediately. This reaction removes no hardness but must be stoichiometrically satisfied before softening can occur: $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$
2. Calcium Carbonate Removal: Hydrated lime provides hydroxide ions that convert bicarbonate to carbonate, precipitating calcium at pH 9.0 to 9.5: $\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3\downarrow + 2\text{H}_2\text{O}$
3. Magnesium Carbonate Removal: Magnesium hydroxide is substantially more soluble than calcium carbonate. To precipitate magnesium, operators add excess lime (~30 to 50 mg/L beyond stoichiometric demand) to raise the pH to 10.8 to 11.2: $\text{Mg(HCO}_3)_2 + 2\text{Ca(OH)}_2 \rightarrow \text{Mg(OH)}_2\downarrow + 2\text{CaCO}_3\downarrow + 2\text{H}_2\text{O}$
4. Noncarbonate Hardness Removal (Soda Ash): Because noncarbonate hardness lacks carbonate ions, soda ash (sodium carbonate, $\text{Na}_2\text{CO}_3$) must be added to provide $\text{CO}_3^{2-}$: $\text{CaSO}_4 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$ $\text{MgSO}_4 + \text{Ca(OH)}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{Mg(OH)}_2\downarrow + \text{CaCO}_3\downarrow + \text{Na}_2\text{SO}_4$

Two-Stage Recarbonation Dynamics

Water exiting lime-soda softening clarifiers is supersaturated with dissolved $\text{CaCO}_3$ and colloidal $\text{Mg(OH)}_2$ at pH 10.5 to 11.2. If this water is applied directly to sand filters, calcium carbonate plates out on the media grains—a phenomenon known as "growing sand"—eventually cementing the grains into solid rock and clogging distribution pipes.

• First-Stage Recarbonation: Gaseous carbon dioxide ($\text{CO}_2$) is bubbled into the water prior to filtration to lower the pH to ~9.5. This neutralizes excess lime and precipitates supersaturated calcium: $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}$
• Second-Stage Recarbonation: After filtration, additional $\text{CO}_2$ is injected to lower the pH to ~8.2 to 8.6. This converts insoluble carbonate ions ($\text{CO}_3^{2-}$) into stable, soluble bicarbonate ions ($\text{HCO}_3^-$): $\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2$

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Ion Exchange Softening & Brine Regeneration

Ion exchange (IX) softening is an automated, non-chemical-precipitation process primarily utilized by groundwater utilities.

Ion Exchange Reversible Equilibrium Mechanics

  Exhaustion (Softening Cycle):
  Ca²⁺ / Mg²⁺ (Raw Water) + 2 R-Na (Resin Bed) ──► R₂-Ca / R₂-Mg (Resin) + 2 Na⁺ (Effluent)

  Regeneration (10–15% NaCl Brine Cycle):
  R₂-Ca / R₂-Mg (Exhausted Resin) + 2 Na⁺ (High-Concentration Brine) ──► 2 R-Na + Ca²⁺ / Mg²⁺ (Waste)

Synthetic Cation Exchange Resin & Selectivity

Ion exchange softeners utilize bead-form Strong Acid Cation (SAC) synthetic resins made of sulfonated polystyrene-divinylbenzene copolymers. Each bead contains millions of active sulfonate functional groups ($-\text{SO}_3^-$) charged with exchangeable sodium ions ($\text{Na}^+$).

The resin exhibits distinct thermodynamic affinities (selectivity) based on ionic charge and hydrated atomic radius:

$\text{Fe}^{3+} > \text{Al}^{3+} > \mathbf{\text{Ca}^{2+}} > \mathbf{\text{Mg}^{2+}} > \mathbf{\text{Na}^+} > \text{H}^+$

Because divalent calcium and magnesium have a higher affinity than monovalent sodium, the resin binds $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$, releasing two $\text{Na}^+$ ions into the water for every divalent cation captured.

Operational Cycle & Regeneration Protocol

1. Service (Softening) Run: Produces water with 0 mg/L total hardness until the exchange sites approach exhaustion. The run endpoint is detected by continuous on-line hardness titrators or totalized gallon meters.
2. Backwash: Upward flow of filtered water expands the resin bed by 50%, removing filtered sediment and re-stratifying resin beads.
3. Brine Regeneration: A concentrated solution of 10% to 15% sodium chloride (NaCl) is slowly introduced downward through the bed. The overwhelming mass action of sodium ions ($[\text{Na}^+] > 100,000\text{ mg/L}$) reverses the thermodynamic equilibrium, forcibly stripping $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ from the resin and restoring it to the $R\text{-Na}$ form.
4. Slow & Fast Rinse: A slow rinse pushes remaining brine through the bed, followed by a high-rate fast rinse to purge all residual chloride before returning the vessel to service.

Split-Stream Blending

Because IX produces zero-hardness water, distributing unblended softener effluent would cause extreme corrosivity (leaching lead and copper from customer pipes) and impart a flat taste. Utilities employ split-stream blending: a calculated fraction of filtered, unsoftened raw water bypasses the softener and mixes with the zero-hardness effluent to produce a stabilized, non-corrosive finished water with 60 to 80 mg/L hardness as $\text{CaCO}_3$.

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Iron and Manganese Oxidation & Removal

Dissolved iron and manganese occur naturally in anaerobic groundwater formations (such as New Jersey's Kirkwood-Cohansey and PRM aquifers) and in deep stratified reservoirs during summer hypolimnetic anoxia.

• Secondary Maximum Contaminant Levels (SMCLs):
  • Iron (Fe): $\mathbf{\le 0.30\text{ mg/L}}$ (causes reddish-brown staining of laundry and fixtures, yellow/brown water, and metallic taste).
  • Manganese (Mn): $\mathbf{\le 0.05\text{ mg/L}}$ (causes black staining, astringent taste; NJDEP enforces strict health-based monitoring due to neurological concerns in infants).

Chemical Oxidation Kinetics

In subsurface aquifers devoid of oxygen, iron exists as soluble ferrous ion ($\text{Fe}^{2+}$) and manganese exists as soluble manganous ion ($\text{Mn}^{2+}$). To be removed by filtration, they must be chemically oxidized into insoluble precipitates: ferric hydroxide [$\text{Fe(OH)}_3\downarrow$] and manganese dioxide [$\text{MnO}_2\downarrow$].

Manganese Greensand Filtration

Manganese greensand is processed glauconite sand coated with active higher-valence manganese oxides ($\text{MnO}_2$). The coating serves as an active chemical buffer that adsorbs and oxidizes dissolved $\text{Fe}^{2+}$ and $\text{Mn}^{2+}$ on contact.

• Continuous Regeneration (CR): Potassium permanganate (or chlorine followed by a trim dose of $\text{KMnO}_4$) is continuously injected into the raw water pipeline immediately upstream of the greensand filter. The oxidant oxidizes the bulk of the metals in the water column, while the greensand captures precipitates and removes any remaining dissolved traces. CR is preferred for high-iron, high-manganese waters.
• Intermittent Regeneration (IR): The filter processes unoxidized raw water directly; the greensand media itself supplies the oxidation capacity until its active $\text{MnO}_2$ coating is exhausted. The bed is then backwashed and regenerated by soaking with a concentrated $\text{KMnO}_4$ solution ($0.1\text{ lb }\text{KMnO}_4\text{ per ft}^3\text{ media}$). IR is restricted to low iron/manganese waters ($<1.0\text{ mg/L}$ combined).

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Aeration, Degasification & VOC Stripping

Aeration transfers volatile substances between the gas phase and liquid phase, governed by Henry's Law ($P_g = K_H \times C$).

Countercurrent Packed Tower Aeration (PTA)

         Off-Gas Exhaust ──► [Vapor-Phase GAC Carbon Vessel] (NJDEP Air Quality Limit)
              ▲
              │
       ┌──────┴──────┐
       │             │ ◄── Raw Water Feed (Sprayed over packing)
       │   Packed    │
       │   Tower     │ Random Dumped Packing (Tri-Packs / Pall Rings)
       │   Bed       │ High surface-area-to-volume ratio
       │             │
       │             │ ◄── Forced-Draft Clean Air Blower (Upward Flow)
       └──────┬──────┘
              │
              ▼
       Treated Water Sump (Stripped of VOCs, Radon, CO₂, H₂S)

• Packed Tower Aerators (PTA): Water trickles downward over high-surface-area polypropylene packing media while forced-draft blowers push air upward in countercurrent flow.
  • Dissolved Gas Removal: Carbon dioxide ($\text{CO}_2$) stripping raises raw water pH from 5.5 to >7.0 without chemical addition, dramatically reducing downstream lime and caustic consumption. Hydrogen sulfide ($\text{H}_2\text{S}$) stripping removes rotten egg odors.
  • Radon-222: Stripped effectively at air-to-water ratios of 25:1 to 50:1.
  • Volatile Organic Compounds (VOCs): Highly effective for chlorinated solvents common in industrial groundwater plumes across New Jersey (Trichloroethylene [TCE], Tetrachloroethylene [PCE], Benzene). Requires air-to-water ratios of 50:1 to 150:1.
• NJDEP Air Permitting & Off-Gas Carbon: Under N.J.A.C. 7:27 (Air Pollution Control), public water facilities stripping VOCs cannot vent hazardous air pollutants freely into the atmosphere. Exhaust air from the packed tower must pass through vapor-phase Granular Activated Carbon (GAC) adsorbers before discharge.

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PFAS Remediation Technologies: New Jersey Standards & Best Practices

Per- and polyfluoroalkyl substances (PFAS) are synthetic organofluorine compounds containing exceptionally strong carbon-fluorine bonds that resist natural environmental degradation.

New Jersey's Landmark Drinking Water Standards

New Jersey was the first state in the nation to establish strict, legally binding Maximum Contaminant Levels (MCLs) for specific PFAS compounds under N.J.A.C. 7:10:

• Perfluorooctanoic Acid (PFOA): $\mathbf{14\text{ ng/L}}$ (14 parts per trillion, ppt)
• Perfluorooctanesulfonic Acid (PFOS): $\mathbf{13\text{ ng/L}}$ (13 ppt)
• Perfluorononanoic Acid (PFNA): $\mathbf{13\text{ ng/L}}$ (13 ppt)

The Three Treatment Technologies

PFAS Remediation Process Comparison
┌────────────────────────────────────────────────────────────────────────┐
│ 1. Granular Activated Carbon (GAC)                                     │
│ • Adsorption via hydrophobic interactions inside carbon micropores     │
│ • Deployed in Lead/Lag (series) configuration                          │
│ • EBCT: 10 to 20 minutes (high footprint)                              │
│ • High affinity for long-chain PFAS; shorter life on short-chain PFAS  │
├────────────────────────────────────────────────────────────────────────┤
│ 2. Synthetic Anion Exchange (IX) Resin                                 │
│ • Single-use, macroporous Strong Base Anion (SBA) resin                │
│ • Dual mechanism: ionic exchange + hydrophobic sorption                │
│ • Shorter EBCT: 2 to 5 minutes (compact footprint)                     │
│ • Non-regenerable on-site; spent resin thermally incinerated           │
├────────────────────────────────────────────────────────────────────────┤
│ 3. High-Pressure Reverse Osmosis / Nanofiltration (RO/NF)              │
│ • High rejection (>98%) across all short- and long-chain PFAS          │
│ • High capital & energy cost; complete demineralization                │
│ • Major Challenge: Generates 15–25% reject brine stream containing     │
│   concentrated PFAS with extreme NJPDES discharge restrictions         │
└────────────────────────────────────────────────────────────────────────┘

Granular Activated Carbon (GAC) Engineering: Lead/Lag Carousel

Municipal drinking water systems deploying GAC utilize pressurized vessels filled with bituminous coal- or virgin coconut-shell-based carbon possessing a high Iodine Number (>900 to 1,000 mg/g), signifying extensive microporosity.

GAC Lead/Lag Carousel Operation Protocol

Raw Water Feed ──► [Lead Contactor] ──────► [Lag Contactor] ──────► Finished Water
                         │                       │
                   Sampling Port 1         Sampling Port 2 (Effluent: Non-Detect)
                   (Midpoint Break)

  WHEN SAMPLING PORT 1 SHOWS PFAS BREAKTHROUGH (e.g., > 2 ng/L):
  1. Take Lead Contactor off-line.
  2. Shift Lag Contactor into the LEAD position.
  3. Evacuate spent carbon from old Lead Contactor; reload with virgin GAC.
  4. Reintroduce fresh vessel into the LAG position.

• Empty Bed Contact Time (EBCT): The calculated duration water spends inside the carbon media volume, calculated as media volume divided by flow rate: $\text{EBCT (minutes)} = \frac{\text{GAC Media Volume (ft}^3) \times 7.48\text{ gal/ft}^3}{\text{Flow Rate (gpm)}}$
  • New Jersey design guidelines require a minimum EBCT of 10 to 20 minutes (typically 10 minutes per vessel with two vessels in series, totaling 20 minutes overall EBCT).
• Upstream Pre-Treatment: Feedwater entering GAC vessels must have dissolved iron and manganese removed ($<0.1\text{ mg/L}$) and pass through 5-micron cartridge bag filters. Iron precipitation and suspended silt foul carbon pore openings, reducing carbon lifespan by more than 50%.

Synthetic Ion Exchange (IX) Resin Mechanics

Specialized PFAS-selective IX systems utilize single-use, macroporous Strong Base Anion (SBA) resins. Because PFAS molecules are negatively charged anions at environmental pH (perfluoroalkyl acids with sulfonate or carboxylate heads), they bind tenaciously to positively charged quaternary ammonium sites on the resin bead.

• Footprint Advantage: IX kinetics are significantly faster than GAC, requiring an EBCT of only 2 to 5 minutes. A treatment facility can install IX vessels with less than one-third the physical footprint of an equivalent GAC system.
• Disposal: Unlike water-softening resins, PFAS resins are non-regenerable in potable applications. When breakthrough occurs, the spent resin is extracted and transported to specialized licensed facilities for high-temperature thermal destruction (>1,100°C) or hazardous incineration.

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Water Treatment Plant Residuals Management

Every drinking water process that removes something has to put it somewhere. Residuals disposal is an explicit WPI job task and one of the largest recurring operating costs at a conventional surface water plant.

Residual Streams

Handling and Disposal Options

• Equalization and recycle: SFBW is settled or clarified in a washwater recovery basin and the supernatant is recycled to the head of the plant. The Filter Backwash Recycling Rule requires recycle to return ahead of the primary coagulant addition point so the recycled Cryptosporidium load is treated by the full process train, not injected downstream of it.
• Gravity thickening ahead of any mechanical dewatering, typically to 2 to 4 percent solids for alum sludge.
• Sludge drying beds and lagoons: low-cost, land-intensive, weather-dependent; standard at small New Jersey plants.
• Mechanical dewatering: plate-and-frame presses, belt filter presses, or centrifuges, usually with polymer or lime conditioning.
• Discharge to a sanitary sewer: common, but requires the receiving POTW's approval and may require a significant indirect user permit, because alum sludge can upset a wastewater plant's solids handling.
• Direct discharge to surface water requires a NJPDES permit and is limited by TSS, aluminum, iron, and pH limits.
• Land application and landfill for dewatered cake, and on-site monofills at larger plants.

Exam Trap Alert: Alum sludge is notoriously hard to dewater because the aluminum hydroxide floc is a hydrated gel that binds water. Lime softening sludge, by contrast, is a crystalline calcium carbonate that thickens and dewaters easily and can be recalcined to recover lime. Answers that treat all water plant residuals as equivalent are wrong.

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Practical Operational Scenarios & Exam Traps

[!WARNING] Exam Trap: Potassium Permanganate Overdose ("Pink Water") Potassium permanganate ($\text{KMnO}_4$) has an intense, unmistakable purple/pink color. When dosing $\text{KMnO}_4$ upstream of greensand filters, the dosage must be matched precisely to stoichiometric iron and manganese demands (0.94 mg/L per mg Fe; 1.92 mg/L per mg Mn). Overfeeding permanganate by as little as 0.05 to 0.10 mg/L sends unreacted permanganate into the distribution system, producing widespread consumer complaints of bright pink tap water.

[!CAUTION] Exam Trap: Calculating Carbonate vs Noncarbonate Hardness A standard exam question provides: Total Hardness = 240 mg/L as $\text{CaCO}_3$; Total Alkalinity = 180 mg/L as $\text{CaCO}_3$. It asks for Noncarbonate Hardness. Remember the rule: Carbonate Hardness is equal to Alkalinity or Total Hardness, whichever is less. Here, Carbonate Hardness = 180 mg/L. Noncarbonate Hardness = Total Hardness - Carbonate Hardness = $240 - 180 = 60\text{ mg/L as CaCO}_3$.