Water Treatment Methods and Principles (1)

Water treatment is the foundation of all industrial, municipal, and medical water systems. Understanding the core principles and mechanisms of each treatment method ensures stable operation of reverse osmosis (RO) systems, prevents membrane fouling, and protects users from health risks.

This article explains the four most widely used water treatment methods—deionization, water softening, activated carbon adsorption, and sediment filtration—based on their scientific principles and engineering applications.

Deionization (DI) removes dissolved inorganic ions from water using ion exchange resins. Unlike simple filtration, DI eliminates ionic species that pass through physical filters and RO membranes under certain conditions.

Two specialized resins are used:

• Cation exchange resin (H⁺ type) — exchanges calcium, magnesium, sodium, iron, and other cations with H⁺ ions
• Anion exchange resin (OH⁻ type) — exchanges chloride, nitrate, sulfate, fluoride and other anions with OH⁻ ions

The released H⁺ and OH⁻ ions combine to form pure water:

Cation Exchange Reaction:
Mⁿ⁺ + n(H–Resin) → M–Resinₙ + nH⁺

Anion Exchange Reaction:
Aⁿ⁻ + n(OH–Resin) → A–Resinₙ + nOH⁻

H⁺ + OH⁻ = H₂O

Cation affinity:
Ba²⁺ > Pb²⁺ > Sr²⁺ > Ca²⁺ > Ni²⁺ > Cd²⁺ > Cu²⁺ > Co²⁺ > Zn²⁺ > Mg²⁺ > Ag⁺ > Cs⁺ > K⁺ > NH₄⁺ > Na⁺ > H⁺

Anion affinity:
SO₄²⁻ > I⁻ > NO₃⁻ > NO₂⁻ > Cl⁻ > HCO₃⁻ > OH⁻ > F⁻

This explains why weakly adsorbed ions like fluoride can break through if the resin is exhausted.

Once the resin is saturated:

• Cation resin must be regenerated using strong acid
• Anion resin must be regenerated using strong alkali

Monitoring is typically done by conductivity or resistivity meters.

If not maintained:

• Fluoride breakthrough → bone disease (rickets, osteoporosis)
• H⁺ breakthrough → acidic water, corrosion
• Biofilm growth inside resin bed → bacteremia risk in medical applications

Hardness in water comes mainly from calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Excessive hardness causes:

• Scaling in boilers and heat exchangers
• Severe fouling on RO membranes
• Hard-water syndrome in medical applications

Softening uses cation exchange resins (Na⁺ type) to replace Ca²⁺ and Mg²⁺ ions with Na⁺ ions:

Ca²⁺ + 2Na–Resin → Ca–Resin₂ + 2Na⁺
Mg²⁺ + 2Na–Resin → Mg–Resin₂ + 2Na⁺

When Na⁺ is consumed, resin is regenerated using 10% brine solution:

Ca–Resin₂ + 2Na⁺ → Na–Resin₂ + Ca²⁺
Mg–Resin₂ + 2Na⁺ → Na–Resin₂ + Mg²⁺

Without softening:

• Scaling destroys RO membranes
• Energy consumption increases
• Recovery decreases
• Membrane lifespan shortens

• Resin bed promotes bacterial growth → requires periodic backwash
• Hypernatremia risk if brine valve malfunctions in medical systems

Without softening:

• Scaling destroys RO membranes
• Energy consumption increases
• Recovery decreases
• Membrane lifespan shortens

Risks

• Resin bed promotes bacterial growth → requires periodic backwash
• Hypernatremia risk if brine valve malfunctions in medical systems

Activated carbon is essential for removing:

• Chlorine and chloramine
• Organic chemicals (MW 60–300 Da)
• Taste, odor, and color-causing compounds

Through:

• Physical adsorption (porous structure 700–1400 m²/g)
• Chemical reduction (especially chlorine → chloride)

Many RO membranes cannot tolerate chlorine. Activated carbon prevents oxidative damage by removing chlorine before RO.

• Adsorption saturation → downstream contamination if not replaced
• High bacterial growth potential inside pores
• Limited capacity for removing high-molecular organics → must be paired with RO

Check:

• TOC removal efficiency
• Chlorine breakthrough
• Bacterial count increase
• Pressure drop

Sediment filtration removes suspended solids, colloid particles, and large impurities that can damage RO membranes.

• Mesh (screen) filters
• Sand / quartz media filters
• Cartridge filters (PP, melt-blown, pleated)
• Membrane filters (1–5 micron)

Particles larger than the filter pore size are trapped. Dissolved ions, however, cannot be removed.

• Backwash regularly
• Replace cartridges when inlet-outlet pressure difference increases 5×
• Prevent bacterial growth inside filter housing

Sediment filtration is usually the first stage of a complete purification line, protecting carbon filters, softeners, and RO membranes.

Understanding the principles behind water treatment methods—deionization, softening, activated carbon adsorption, and sediment filtration—is essential for designing stable and efficient industrial and medical water systems.

A well-designed multi-stage pretreatment chain prevents RO membrane fouling, improves water purity, reduces operational costs, and extends system lifespan.

For advanced RO and pretreatment systems, explore CHIWATEC’s full product range:
👉 CHIWATEC Water Treatment Equipment & Solutions

DI removes all ions, while softening removes only hardness ions (Ca²⁺, Mg²⁺).

It removes chlorine, which otherwise causes irreversible damage to polyamide RO membranes.

Monitor conductivity, chlorine breakthrough, TOC levels, and pressure drop.

No. It removes only large particles; dissolved ions, organics, or chlorine require additional treatment.

Typically:
Sediment Filter → Activated Carbon → Softener → Cartridge Filter → RO

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

• Water treatment methods and principles (2)
• General water treatment methods and principles and introduction to common water treatment methods(1)
• General water treatment methods and principles and introduction to common water treatment methods(2)