Water Chemistry Basics

Overview

Water treatment is chemistry applied. Understanding what's in water + how to remove it is foundational.

Common contaminants

Hardness (calcium, magnesium):

• Causes scale on fixtures + water heaters
• Soap doesn't lather
• Treated with ion-exchange softener

Iron + manganese:

• Reddish/black staining
• Bad taste
• Treated with oxidation + filtration

Hydrogen sulfide:

• Rotten-egg smell
• From organic decay in groundwater
• Treated with oxidation (chlorine, ozone) + carbon filter

Chlorine + chloramines:

• From municipal disinfection
• Taste + smell
• Treated with activated carbon

Lead:

• Old plumbing or solder
• Health hazard
• Treated with RO or specialty filter

Nitrates:

• Agricultural runoff
• Health hazard (especially for infants)
• Treated with RO, anion exchange, distillation

Arsenic:

• Natural in some geology
• Health hazard
• Treated with RO, specialty filtration

Bacteria/viruses:

• Pathogens
• Treated with UV, chlorine, ozone

Volatile organic compounds (VOCs):

• From industrial pollution
• Treated with activated carbon

Radon:

• Naturally occurring in some areas
• Treated with aeration or activated carbon (special handling)

Measurement units

ppm = parts per million (also mg/L):

• 1 ppm = 1 mg per liter
• Used for most contaminants

GPG = grains per gallon:

• Used for hardness
• 1 GPG = 17.1 ppm CaCO3

μS/cm = microsiemens per cm:

• Conductivity
• Higher = more dissolved ions

TDS = total dissolved solids:

• Measured in ppm
• Sum of all dissolved minerals

pH scale

• 0-14 (7 is neutral)
• < 7 = acidic

• 7 = alkaline (basic)

Most drinking water: pH 6.5-8.5

Effects of pH:

• Low pH (< 6.5): corrosive to copper pipes; pinhole leaks; metallic taste
• High pH (> 8.5): scale; soapy feel; some treatments less effective

Adjusting pH:

• Low pH: calcite filter (raises pH naturally) or soda ash
• High pH: acid injection (CO2, vinegar)

Conductivity + TDS relationship

Conductivity (μS/cm) ≈ 1.5-2 × TDS (ppm)

For drinking water:

• TDS < 100: excellent
• TDS 100-300: typical
• TDS > 500: high; investigate

Hardness chemistry

Calcium + magnesium bicarbonates dissolved in water.

Reaction with heat:

• CaHCO3 → CaCO3 + H2O + CO2
• Limescale forms; piping + water heater clogged

Reaction with soap:

• Ca + soap → soap scum + insoluble compound
• Soap doesn't lather; uses more soap

Ion exchange (softener):

• Resin has sodium ions
• Calcium/magnesium swap with sodium
• Brine recharges resin

Iron chemistry

Ferrous (clear water):

• Dissolved Fe2+ in water
• Invisible in water
• Stains everything after exposure to air

Ferric (rust):

• Already oxidized Fe3+
• Visible in water as orange/red color

Treatment:

• Aerate to oxidize ferrous → ferric
• Filter ferric out
• Or use chlorine to oxidize + filter

Disinfection chemistry

Chlorine:

• Primary disinfectant globally
• Active form: hypochlorous acid (HOCl)
• Effective against most bacteria, viruses
• 1-4 ppm residual maintained in distribution

Chloramine:

• Chlorine + ammonia
• Less aggressive than chlorine
• Lasts longer in distribution
• Used by 25% of US municipalities

UV (ultraviolet):

• Damages DNA of microorganisms
• No chemical residual (good + bad)
• Requires clear water (turbidity blocks UV)

Ozone:

• O3 (3 oxygen molecules)
• Stronger than chlorine
• No residual
• Industrial + premium residential

Removal chemistries

Ion exchange (softeners, deionizers):

• Resin beads
• Sodium for hardness; hydrogen for deionization
• Backwash + regenerate periodically

Activated carbon:

• Granular activated carbon (GAC)
• Adsorbs organics, chlorine, taste/odor
• Doesn't remove most ions
• Bacteria can grow on it; periodic replacement

Reverse osmosis (RO):

• Pressure forces water through semi-permeable membrane
• Removes 95-99% of dissolved solids
• High water consumption (3-4 gallons reject per gallon produced)

Distillation:

• Boil + condense water
• Pure water output
• Slow + energy-intensive

UV disinfection:

• Kills microorganisms
• Doesn't remove anything else

Oxidation:

• Convert dissolved iron, sulfur to filterable particles
• Air, chlorine, ozone, hydrogen peroxide

Standard residential testing

A "comprehensive" water test typically includes:

• Total hardness (GPG)
• pH
• Iron (mg/L)
• Manganese
• Sulfur (qualitative)
• TDS
• Chlorine residual
• Bacteriological (well water)
• Nitrates (well water)
• Lead (some areas)
• Radon (some areas)

Cost: from lab.

Treatment-process flow chart

Typical multi-stage residential treatment (worst-case well water):

1. Sediment filter (5-50 μm): removes particles
2. Iron filter / aerator: removes iron + sulfur
3. Water softener: removes hardness
4. UV disinfection: kills microbes
5. RO at kitchen tap (point-of-use): drinking-grade purity

Most homes don't need all 5; tailor to actual water quality.

Whole-house vs point-of-use

Whole-house (POE - point of entry):

• Treats all water entering the home
• Bigger, more expensive
• All fixtures benefit (clothes washer, dishwasher, shower)

Point-of-use (POU):

• Treats single tap (typically kitchen sink)
• Lower cost
• Used for drinking-grade purity (RO common)

Combination is typical for well water:

• POE softener + iron filter
• POU RO at kitchen

Customer water-quality conversation

Always start with:

1. What's the customer's concern? (Taste, scale, stains, health)
2. Is it well water or municipal?
3. Any recent changes?
4. Test before recommending

Maintenance economics

For a typical softener + carbon filter system:

• Salt:
• Filter replacement:
• Annual service:
• Total: ongoing

vs:

• Bottled water for family:
• Replacement appliances (damaged by hard water): every 5-10 years

ROI: 3-5 years for most installations.

References

• WQA water quality standards
• EPA Safe Drinking Water Act
• NSF 42, 53, 58 (certification standards)
• Manuall internal: Diagnose Hard Water Issues, Water Filtration Technologies Reference