13.3 Softening, Iron, Manganese, and Chemical Feed
Key Takeaways
- Hardness calculations should be kept on an as CaCO3 basis so calcium, magnesium, alkalinity, lime, and soda ash demands can be compared consistently.
- Lime softening removes carbonate hardness first and can remove magnesium hardness at higher pH, while soda ash is used when noncarbonate calcium hardness must be precipitated.
- Ion exchange softening swaps hardness ions for sodium or hydrogen ions and requires regeneration, waste brine handling, and finished-water blending checks.
- Iron and manganese treatment usually requires oxidation followed by filtration; sequestration may control staining at low levels but does not remove the metals.
- Chemical-feed math is a mass-rate problem: required product feed must account for flow, target dose, percent active ingredient, solution strength, and density.
Why These Processes Are Tested Together
Hardness, iron, manganese, and chemical feed all sit at the boundary between water chemistry and plant operation. The PE Civil WRE exam can ask a chemistry concept, but it often turns that concept into a design or operations calculation. You may need to choose a treatment process, compute a chemical feed rate, or identify why a finished water is causing scale, staining, or customer complaints.
Hardness is mainly caused by calcium and magnesium. For exam calculations, hardness and alkalinity are commonly reported as mg/L as CaCO3. That common basis matters because it lets you compare ions by equivalent weight rather than by raw mass. If one value is given as calcium ion and another as CaCO3, convert before drawing conclusions.
Softening Process Selection
| Problem condition | Likely process | Design idea | PE trap |
|---|---|---|---|
| Carbonate hardness | Lime softening | Convert bicarbonate alkalinity to carbonate and precipitate CaCO3 | Ignoring alkalinity limit |
| Magnesium hardness | Excess lime, higher pH | Precipitate Mg(OH)2 | Forgetting recarbonation or pH control |
| Noncarbonate calcium hardness | Soda ash plus lime | Add carbonate to precipitate CaCO3 | Assuming lime alone removes all hardness |
| Small system hardness | Ion exchange | Exchange Ca and Mg for sodium | Ignoring regeneration and brine waste |
| High-purity water | Membranes or demineralization | Remove dissolved ions broadly | Treating all membranes as particle filters |
Lime-soda softening produces sludge and high-pH water that usually needs stabilization before distribution. Recarbonation with carbon dioxide can lower pH and convert excess carbonate chemistry to a more stable finished water. Ion exchange can be simpler operationally, but it introduces sodium, requires regeneration, and creates a waste brine stream.
Iron and Manganese
Iron and manganese are often present in reduced soluble forms in groundwater. Once exposed to oxygen or chlorine, they can oxidize and form particles that cause red, brown, or black staining. The usual removal logic is oxidation followed by filtration. Oxidants include aeration, chlorine, potassium permanganate, ozone, or other site-specific choices. The process must provide enough oxidant, pH, and contact time before the filter.
Sequestration is different. Polyphosphates or similar chemicals can hold low concentrations of iron or manganese in solution and reduce staining, but they do not remove the metals. If the question asks for actual removal, choose oxidation plus filtration or another removal process, not sequestration alone.
Chemical-Feed Calculation Workflow
- Identify the target active dose in mg/L and the design flow.
- Convert flow to MGD if using the PE shortcut.
- Compute active chemical required: lb/day = MGD x mg/L x 8.34.
- Adjust for product purity or percent active ingredient: product lb/day = active lb/day / active fraction.
- For liquid products, convert product lb/day to gallons per day using solution density and active fraction.
- Check whether the dose is for dry product, active ingredient, or neat solution.
- Compare the result with feeder capacity and chemical storage duration if asked.
Example Logic Without Memorized Recipes
Suppose a plant wants a 2.0 mg/L active oxidant dose at 3.0 MGD. The active requirement is 3.0 x 2.0 x 8.34 = 50.0 lb/day. If the chemical delivered is only 25 percent active, the neat product requirement is 50.0 / 0.25 = 200 lb/day. If the solution weighs 10 lb/gal, that is 20 gal/day. A common wrong answer stops at 50 lb/day and forgets strength.
Operations Checks
Chemical feed must match actual flow. Flow-paced feed reduces underdosing at peak flow and overdosing at low flow. Operators also need standby feed pumps, calibration columns, day tanks, secondary containment, ventilation for incompatible chemicals, and residual monitoring. On PE questions, the correct operational answer is often the one that closes the mass balance and keeps the chemical compatible with the process objective.
A plant treats 4.0 MGD and needs an active polymer dose of 0.80 mg/L. The dry polymer product is 40 percent active by weight. What dry product feed rate is required?
A groundwater supply has soluble iron and manganese that cause staining after chlorination in the distribution system. Which treatment train best addresses actual removal?