4.5 Softening, Ion Exchange & Membrane Treatment
Key Takeaways
- Hardness is the sum of divalent cations expressed as mg/L CaCO3; carbonate hardness is removed by lime alone, while noncarbonate hardness requires soda ash in addition to lime.
- Practical lime-soda softening limits are about 30 to 40 mg/L of calcium hardness and 10 mg/L of magnesium hardness, and magnesium removal requires raising pH to roughly 10.6 or higher.
- Excess lime softening must be followed by recarbonation with carbon dioxide to drop pH and stabilize the water, otherwise calcium carbonate deposits on filter media and in mains.
- Sodium-cycle ion exchange softening produces essentially zero hardness and is blended back to a target of 80 to 120 mg/L; it adds sodium and generates a high-chloride brine that must be disposed of properly.
- Membrane processes are ranked by what they reject — microfiltration and ultrafiltration remove particles and pathogens, nanofiltration softens, and reverse osmosis removes dissolved salts — and all are controlled by monitoring flux, differential pressure, and salt rejection to detect fouling.
4.5 Softening, Ion Exchange & Membrane Treatment
Under 15A NCAC 18D .0203, lime softening carries some of the highest rating values in the classification table — a lime softening clarifier with coagulation is 12 points and reverse osmosis or electrodialysis is 15 — which tells you how much operational complexity the Board associates with these processes.
1. Hardness fundamentals
Total hardness is the sum of polyvalent cations, dominated by calcium and magnesium, expressed as mg/L as CaCO₃.
| Classification | mg/L as CaCO₃ |
|---|---|
| Soft | 0–75 |
| Moderately hard | 75–150 |
| Hard | 150–300 |
| Very hard | over 300 |
Two splits matter operationally:
- Carbonate hardness (temporary) — hardness associated with bicarbonate and carbonate alkalinity. Removable with lime alone.
- Noncarbonate hardness (permanent) — hardness associated with sulfate, chloride, and nitrate. Requires soda ash (Na₂CO₃) to supply carbonate.
A quick comparison decides the chemistry: if total hardness exceeds total alkalinity, the difference is noncarbonate hardness; if alkalinity equals or exceeds hardness, all hardness is carbonate hardness.
2. Lime-soda ash softening
The core reactions:
CO2 + Ca(OH)2 -> CaCO3(s) + H2O (lime consumed by free CO2 first)
Ca(HCO3)2 + Ca(OH)2 -> 2 CaCO3(s) + 2 H2O (calcium carbonate hardness)
Mg(HCO3)2 + 2 Ca(OH)2 -> Mg(OH)2(s) + 2 CaCO3(s) + 2 H2O (magnesium, needs excess lime)
CaSO4 + Na2CO3 -> CaCO3(s) + Na2SO4 (calcium noncarbonate hardness)
MgSO4 + Ca(OH)2 + Na2CO3 -> Mg(OH)2(s) + CaCO3(s) + Na2SO4
Practical limits. Softening cannot reach zero: the practical floor is about 30 to 40 mg/L of calcium hardness and about 10 mg/L of magnesium hardness, so finished hardness targets of 80 to 120 mg/L are typical — low enough to stop scaling complaints, high enough to leave the water non-aggressive.
pH drives the split. Calcium carbonate precipitates efficiently near pH 10.3; magnesium hydroxide requires pH 10.6 to 11.0 or higher, which is why magnesium removal always means excess lime.
Recarbonation. After excess lime, carbon dioxide is added to convert leftover hydroxide to carbonate and then bicarbonate, dropping pH to roughly 8.5 to 9.5 and stabilizing the water. Skipping or short-changing recarbonation deposits calcium carbonate in the filters (cementing the media), in the clearwell, and in the distribution mains.
Split treatment routes part of the flow around the softening basin and blends it back, saving chemical and carbon dioxide while meeting the magnesium target in the treated stream.
Sludge. Lime softening produces large volumes of calcium carbonate sludge — often 2 to 5 percent solids from the basin, thickening well and dewatering to 30 to 60 percent in a lagoon or press. Some utilities recalcine or land-apply it as an agricultural liming agent.
3. Ion exchange softening
A sodium-cycle cation exchanger passes water through a resin bed that swaps two sodium ions for each calcium or magnesium ion:
Ca2+ + 2 Na-R -> Ca-R2 + 2 Na+ (service)
Ca-R2 + 2 NaCl -> 2 Na-R + CaCl2 (regeneration with brine)
- Product water has essentially zero hardness, so the softened stream is blended with bypassed raw water to hit the 80–120 mg/L target.
- Regeneration uses sodium chloride brine, typically 8 to 16 pounds of salt per cubic foot of resin depending on capacity setting, followed by slow rinse and fast rinse.
- Trade-offs: each 1 mg/L as CaCO₃ of hardness removed adds roughly 0.46 mg/L of sodium, which matters to sodium-restricted customers; the waste brine is a high-chloride stream that can be difficult to discharge to a wastewater plant or receiving stream; and resin is fouled by iron, manganese, and chlorine, so pretreatment matters.
- Hydrogen-cycle exchangers remove hardness while also reducing alkalinity and TDS, and greensand-type exchangers target iron and manganese — all three appear in the 18D .0203 rating table at 5 to 9 points.
4. Membranes
| Process | Typical driving pressure | Removes |
|---|---|---|
| Microfiltration (MF) | Low (5–30 psi) | Particles, turbidity, bacteria, Giardia and Cryptosporidium |
| Ultrafiltration (UF) | Low (10–40 psi) | The above plus most viruses and large organics |
| Nanofiltration (NF) | Moderate (50–150 psi) | Hardness ("membrane softening"), color, DBP precursors |
| Reverse osmosis (RO) | High (150–1,000+ psi) | Dissolved salts, nitrate, arsenic, sodium, most everything |
| Electrodialysis (ED/EDR) | Electrical | Charged dissolved ions; reversal mode limits scaling |
Operating parameters to trend daily: feed, permeate, and concentrate flows; recovery (permeate divided by feed); flux (gallons per square foot per day); transmembrane pressure or differential pressure; salt rejection for NF/RO; and temperature, since flux is strongly temperature dependent and readings must be normalized before they can be compared.
Fouling and scaling. Rising differential pressure with stable rejection points to particulate or biological fouling; falling rejection points to membrane damage or seal leakage. Control measures include cartridge prefiltration, antiscalant dosing, pH adjustment, periodic clean-in-place with acid (mineral scale) and caustic or surfactant (organics and biofilm), and for MF/UF, routine backpulse and air scour. Integrity is verified by pressure decay testing on MF/UF systems.
Concentrate. RO and NF reject streams are concentrated brines. Disposal — to a sanitary sewer, a permitted outfall, deep injection, or evaporation — is a permitting problem that often determines whether a membrane plant is feasible at all.
[!WARNING] Post-treatment is mandatory. RO and NF permeate is aggressive: low pH, low alkalinity, low hardness, and near-zero buffering. It must be stabilized — by blending, by adding lime or caustic and carbon dioxide, or by calcite contactors — before entering a distribution system, or it will attack cement mortar linings and metal plumbing.
A raw water has a total hardness of 240 mg/L as CaCO3 and a total alkalinity of 180 mg/L as CaCO3. How much noncarbonate hardness is present and what does that imply chemically?
Why does magnesium removal in lime softening require excess lime and a higher pH than calcium removal?
On a reverse osmosis skid, differential pressure is climbing steadily while salt rejection stays constant. What does this pattern indicate?