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Iron & Hardness

Dissolved iron is naturally found in groundwater as a result of mineral deposits and chemically reducing underground currents. Iron can also be found in surface water as a result of anaerobic decay in sediments and complex soil formations.

The aesthetic objective in the Ontario Drinking Water Standards is set at 0.3mg/l. Higher levels of iron in drinking water can cause discoloration of laundry and plumbing fixtures as well as impair the taste of water and beverages made from that water. It can also support the growth of iron reducing bacteria in water mains and in house plumbing.

Treatment for High Iron Levels

  • Aeration and settling or filtration (for low levels of iron)
  • Greensand / potassium permanganate ion exchange.
  • Polyphosphate feeders.
  • Water softeners (for low levels of iron)
  • Chlorination / filtration units.
  • Filtration alone (for removal of precipitated iron only)

Note that high levels of iron in drinking water can render ozone treatment ineffective.


Hardness in drinking water is caused by dissolved calcium and magnesium. The Ontario Drinking Water Standards have set an operational guideline of 80 -100 mg/l (measured as calcium carbonate) for hardness. This provides the best balance between corrosion (water too soft) and scaling (water too hard). Higher levels of hardness can lead to mineral scale deposits in kettles, plumbing, and water heaters, and form excessive soap scum. Water with hardness above 200 mg/l is considered poor but tolerable. Hardness above 500 mg/l is not acceptable for most domestic purposes.

Treatment for Hardness

  • 1. Water Softeners.
  • 2. Soluble phosphate additives.
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