13.3 Softening, Iron, Manganese, and Chemical Feed

Why These Processes Are Tested Together

Hardness, iron, manganese, and chemical feed sit at the boundary between water chemistry and plant operation. The PE Civil WRE exam may ask a chemistry concept, then turn it into a design or operations calculation: pick a process, compute a feed rate, or explain why a finished water is scaling, staining, or generating complaints.

Hardness is mainly calcium and magnesium. For exam work, hardness and alkalinity are reported as mg/L as CaCO3 (equivalent weight 50 g/eq). Reporting as CaCO3 lets you add and subtract species by equivalents instead of raw mass. To convert an ion to as-CaCO3, multiply its mg/L by 50 / (equivalent weight of the ion). For calcium, equivalent weight is 40/2 = 20, so mg/L Ca x (50/20) = mg/L as CaCO3. Carbonate hardness is the portion of hardness matched by alkalinity; the excess is noncarbonate hardness.

Softening Process Selection

Lime-Soda Stabilization and Metals

Lime-soda softening leaves a high-pH water (often pH 10.5-11) and a CaCO3/Mg(OH)2 sludge. Practical finished hardness floors near 30-40 mg/L (CaCO3) and 10 mg/L (Mg(OH)2) because of solubility limits - you cannot soften to zero. The high-pH water is unstable and must be conditioned: recarbonation with carbon dioxide drops pH and converts excess carbonate chemistry toward a stable, slightly scale-forming water (positive Langelier Saturation Index) that protects pipe.

Ion exchange is operationally simpler but adds sodium, needs salt regeneration, and produces a concentrated waste brine; blending part of the raw water past the resin is a common way to hit a hardness target economically.

Iron and manganese occur as soluble reduced forms (Fe2+, Mn2+) in groundwater. Once exposed to oxygen or chlorine they oxidize to insoluble Fe(OH)3 and MnO2 that cause red/brown (iron) and black (manganese) staining. The Secondary MCLs are 0.3 mg/L iron and 0.05 mg/L manganese. Standard removal is oxidation then filtration: oxidants include aeration, chlorine, potassium permanganate (KMnO4), or ozone, with enough oxidant, pH, and contact time before the filter. Manganese needs higher pH and stronger oxidant than iron and is the harder of the two.

Sequestration with polyphosphates holds low metal concentrations in solution to limit staining but removes nothing. If a question asks for actual removal, choose oxidation plus filtration, not sequestration alone.

Chemical-Feed Calculation Workflow

The master relation is lb/day = flow (MGD) x dose (mg/L) x 8.34, where 8.34 lb/gal is the weight of water. Steps:

1. Identify target active dose (mg/L) and design flow (MGD).
2. Active chemical lb/day = MGD x mg/L x 8.34.
3. Adjust for purity: product lb/day = active lb/day / active fraction.
4. For liquids, convert product lb/day to gal/day using solution density (lb/gal).
5. Confirm whether the stated dose is dry product, active ingredient, or neat solution.
6. Compare with feeder capacity and chemical storage duration if asked.

Worked Example

A plant wants a 2.0 mg/L active oxidant dose at 3.0 MGD. Active requirement is 3.0 x 2.0 x 8.34 = 50.0 lb/day. If the chemical is only 25% active, neat product is 50.0 / 0.25 = 200 lb/day. If the solution weighs 10 lb/gal, that is 20 gal/day. The classic wrong answer stops at 50 lb/day and forgets product strength.

Operations Checks

Feed must track actual flow. Flow-paced feed avoids underdosing at peak and overdosing at low flow. Plants also need standby feed pumps, calibration (drawdown) columns, day tanks, secondary containment, ventilation for incompatible chemicals (never store chlorine and ammonia or acid together), and residual monitoring. The correct operational answer usually closes the mass balance and keeps the chemical compatible with the process objective.

Equivalent-Weight Reference (as CaCO3)

To move any species to an as-CaCO3 basis, multiply its mg/L by 50 divided by the ion's equivalent weight. Memorizing the common equivalent weights speeds every softening problem:

• Calcium, Ca2+: eq wt 20, factor 2.50
• Magnesium, Mg2+: eq wt 12.2, factor 4.10
• Bicarbonate, HCO3-: eq wt 61, factor 0.82
• Carbon dioxide, CO2: eq wt 22, factor 2.27
• Quicklime, CaO: eq wt 28, factor 1.79
• Soda ash, Na2CO3: eq wt 53, factor 0.94

A classic lime-soda stoichiometry sequence: dose lime for CO2, then for carbonate hardness (the bicarbonate portion), then add excess lime for magnesium, and finally soda ash for noncarbonate calcium hardness. Skipping the CO2 demand or the magnesium step underdoses lime and leaves the question's hardness target unmet.

Stability and Corrosion

Finished water should be neither aggressively corrosive nor heavily scaling. The Langelier Saturation Index (LSI) compares actual pH to the pH of calcium carbonate saturation: a slightly positive LSI deposits a thin protective CaCO3 film, while a negative LSI dissolves pipe and can mobilize lead and copper. Softened, high-pH water is typically supersaturated and must be recarbonated; soft, low-alkalinity surface water is often corrosive and may need corrosion control under the Lead and Copper Rule. The exam may ask you to interpret an LSI sign rather than compute it - positive means scaling tendency, negative means corrosive.