Water Softening & Ion Exchange Flashcards

Free chlorine residual in the water supply can damage strong acid cation resin primarily by:

Answer: Oxidizing and degrading the polymer matrix, causing cracking and loss of capacity

Chlorine attacks the styrene-divinylbenzene resin matrix through oxidative degradation, causing bead cracking, fines generation, and irreversible capacity loss.

After completing a regeneration cycle, a technician tests the softener effluent and finds 3 gpg hardness. The most likely cause is:

Answer: Insufficient brine contact time or under-dosing of salt during regeneration

Post-regeneration hardness indicates the resin was not fully recharged, commonly due to insufficient salt dose, incorrect brine concentration, or inadequate brine contact time.

What happens to Total Dissolved Solids (TDS) in water after passing through a sodium-cycle water softener?

Answer: TDS increases slightly because two sodium ions replace each divalent hardness ion

For each Ca²⁺ or Mg²⁺ ion removed (atomic weight ~40 or 24), two Na⁺ ions are added (atomic weight ~23 each), resulting in a slight net increase in TDS by weight.

A dealkalizer uses which type of ion exchange resin and regenerant to reduce alkalinity?

Answer: Strong acid cation resin regenerated with hydrochloric or sulfuric acid

A split-stream dealkalizer (sodium cycle decarbonator) or H-cycle dealkalizer uses SAC resin in hydrogen form (acid-regenerated) to convert bicarbonate alkalinity to carbonic acid, which degasses out.

Silica fouling on strong base anion (SBA) resin used in demineralization systems is most effectively treated with:

Answer: Warm (120–140°F) sodium hydroxide regeneration at increased dose

Silica polymerizes on SBA resin and is best removed by warm caustic (NaOH) regeneration, since elevated temperature and high pH dissolve polysilicic acid deposits.

Osmotic shock damage to ion exchange resin beads most commonly occurs when:

Answer: The resin undergoes rapid and extreme changes in ionic strength, such as contact with concentrated brine after being in dilute service water

Rapid swelling and shrinking of resin beads caused by sudden ionic strength changes creates internal stress that cracks beads, especially in low-crosslinked resins.

Which water quality parameter is NOT directly reduced by a properly functioning sodium-cycle water softener?

Answer: Nitrate concentration

Sodium-cycle softeners exchange cations (Ca²⁺, Mg²⁺, Ba²⁺, etc.) but do not remove anions; nitrate (NO₃⁻) is an anion and passes through unchanged.