7.1 Iron and Manganese Removal, Softening, and Taste and Odor Control

Iron and Manganese

Groundwater from Willamette Valley alluvium and many basalt aquifers often carries dissolved ferrous iron (Fe2+) and manganous manganese (Mn2+) because the water has little oxygen. Neither is a major health risk at typical levels. But at the secondary MCLs of 0.3 mg/L (iron) and 0.05 mg/L (manganese), they stain fixtures and laundry, cause red or black water, and feed iron and manganese bacteria. Manganese also has an EPA health advisory, so it is getting more attention.

Oxidant demand

Removal starts by oxidizing the dissolved metals to insoluble particles: ferric hydroxide (orange-brown) and manganese dioxide (black).

Worked example. A well has 1.4 mg/L iron and 0.60 mg/L manganese. The theoretical permanganate demand is:

• Iron: 1.4 × 0.94 = 1.32 mg/L
• Manganese: 0.60 × 1.92 = 1.15 mg/L
• Total: about 2.5 mg/L, before any extra demand from organics or sulfide

Real doses are confirmed by jar testing. An overdose sends pink or purple water to customers, and an underdose lets manganese pass and blacken later.

Removal processes

1. Oxidation and filtration. Aerate or dose an oxidant, allow detention time, then filter through sand, anthracite or dual media. High iron loads may need settling first.
2. Catalytic media (greensand and similar). Media coated with manganese dioxide adsorb dissolved manganese and oxidize it on the grain surface. Continuous regeneration (CR) feeds a small permanganate or chlorine dose ahead of the filter. Intermittent regeneration (IR) soaks the bed periodically. Oxide-coated media earn 10 points as pressure or greensand filtration in OHA's Table 51.
3. Sequestration. Polyphosphates or silicates hold small amounts of iron and manganese in solution so they do not precipitate in the mains. This works only at low concentrations, roughly 1 mg/L combined or less. The metals are still delivered, and chlorine contact can eventually release them.
4. Biological filtration. Iron- and manganese-oxidizing bacteria grow on filter media under controlled oxygen and pH, removing the metals with little chemical feed.

Hardness and Softening

Hardness is the sum of calcium and magnesium, expressed as mg/L of CaCO3:

Carbonate hardness is the part balanced by alkalinity. Noncarbonate hardness is the remainder, balanced by sulfate or chloride.

• Lime-soda ash softening: lime (Ca(OH)2) raises pH to about 10.3 to precipitate calcium carbonate, or above 11 to precipitate magnesium hydroxide, removing carbonate hardness. Soda ash (Na2CO3) removes noncarbonate hardness. The water is then recarbonated with CO2 to lower pH and stabilize it before filtration. The process produces large volumes of lime sludge and scores 20 points in Table 51.
• Ion exchange (cation exchange): resin swaps calcium and magnesium for sodium. It is regenerated with brine (sodium chloride) and produces brine waste. Softened water gains sodium, which matters for low-sodium diets, and is often blended back to a moderate hardness.

Most western Oregon surface supplies are soft. Softening comes up mainly with some eastern Oregon groundwater and in industrial or point-of-entry settings.

Taste and Odor (T&O)

PAC is fed as a slurry, typically 5 to 25 mg/L or more. It needs contact time before coagulation and should not be dosed together with chlorine or permanganate, because the carbon consumes the oxidant. GAC filter-adsorbers or post-filter contactors give continuous removal until they are exhausted and need regeneration. Operators confirm T&O with the threshold odor number (TON) test and customer complaint tracking. Copper sulfate algaecide dosed in a reservoir can control the bloom that causes T&O. However, killing a cyanobacterial bloom can release toxins, as discussed in the cyanotoxin section.