Water Treatment Technologies

Treatment Selection

Treatment technology selection depends on specific water quality issues identified through testing. Each contaminant class has preferred treatment methods:

Chlorination Systems

Chlorination is the most widely used disinfection method worldwide:

Chlorine Types

• Liquid sodium hypochlorite (bleach): 5-12% concentration — easy to handle, most common for residential
• Calcium hypochlorite (granular): 65-70% available chlorine — longer shelf life, requires careful handling
• Chlorine gas: 100% available chlorine — used in large municipal systems, requires gas detection and safety systems
• Electrolytic generation: Salt + electricity → chlorine on-site — eliminates chemical storage and handling

Chlorination Design

• Contact time: Minimum 30 minutes (C × T ≥ 15 for 3-log virus kill at pH <7.5)
• Free chlorine residual: 0.2-0.5 mg/L at point of use (maintains disinfection in distribution)
• pH effect: Above pH 7.5, chlorine becomes less effective — may need pH adjustment
• Breakpoint chlorination: Adding 1.5× the ammonia concentration to achieve free chlorine
• Dechlorination: Required before discharge — use activated carbon or sodium bisulfite

Iron & Manganese Removal

Aeration + Greensand Filtration

• Aeration: Introduces oxygen to convert dissolved Fe²⁺ to insoluble Fe³⁺ — spray aerator, cascade, or diffused air
• Oxidation time: 15-30 minutes in a contact tank before filtration
• Greensand filter: Manganese-coated sand catalyzes oxidation — backwash every 24-72 hours
• Regeneration: Potassium permanganate or chlorine — restores filter capacity
• Capacity: 5-10 gallons per minute per square foot of filter area

Oxidation Filtration Alternatives

Ion Exchange

Water passes through resin beads that exchange undesirable ions for desirable ones:

• Water softening: Na⁺ exchanges for Ca²⁺/Mg²⁺ — removes hardness, adds sodium
• Nitrate removal: Cl⁻ form resin selects NO₃⁻ over SO₄²⁻ — regenerates with NaCl
• Radium removal: Cation exchange captures Ra²⁺ — highly effective (95-98%)
• Capacity: 20,000-30,000 grains per cubic foot of resin
• Regeneration: 8-15 lbs NaCl per regeneration cycle — typically 3-5 days between regenerations

Reverse Osmosis

Pressure-driven membrane filtration removes dissolved solids, pathogens, and most contaminants:

• Operating pressure: 150-600 psi depending on TDS and membrane type
• Recovery rate: 25-75% (rest becomes concentrate/waste stream)
• Rejection rate: 95-99% of dissolved salts, 99.99% of bacteria and viruses
• Pre-treatment: Sediment filter, carbon filter, antiscalant — protects membrane
• Post-treatment: Remineralization, UV, or carbon — improves taste and safety

UV Disinfection

• Wavelength: 254 nm (UV-C) — damages DNA of pathogens
• Dose: 40-100 mJ/cm² — achieves 4-log (99.99%) virus inactivation
• Advantages: Chemical-free, no residual taste, effective against chlorine-resistant pathogens
• Limitations: No residual disinfection — must be combined with chlorine for distribution
• Lamp replacement: Every 12 months or 9,000 hours — UV output degrades over time

System Design Principles

• Treat sequence: Sediment → iron/manganese → hardness → disinfection → polishing
• Redundancy: Critical systems should have backup treatment for each stage
• Bypass: Allow untreated water for irrigation (salt-sensitive crops don't need softening)
• Monitoring: Continuous chlorine residual, periodic water quality testing
• Maintenance: Filter replacement, resin regeneration, lamp replacement — schedule all on calendar