I noticed (while replacing a flush valve), that iron oxide is precipitating in the flusg tahk. Where is this coming from? The water lines into the house are copper, and the water lines are asphalt-lined cement. So where does the rust come from? And, why does it precipitate out? Is this the same mechanism that forms “bog” iron?
It is probably present in your water supply. You can have as much as 4 ppm iron and not see regular staining. Most states don’t have a maximum limit for iron in water supplies.
Iron in the water can percipitate out for a few different reasons. If water is left sitting the iron can come out of solution. If mousture left on the side of the tank evaporates while the tank is refilling it would leave the iron behind.
When I have to deal with toilets the bolts holding the tank to the bowl are usually rusted over. I think there are also rusting parts in flapper chains and some filler linkages.
But you are right, the amount of rust seems hard to explain.
How do you know it is precipitating, and not present as suspended particles in the incoming water?
Because he didn’t expect it.
There is a federal non-enforceable secondary standard for iron of 0.3 mg/L (0.3 ppm).
Water from public water supplies may have dissolved iron present in the source water, but this is typically removed at the water treatment plant. Iron may also be picked up in any of the pipes or fittings between the treatment plant and your tap. The chlorinated water typically supplied is naturally corrosive. Some public water supplies increase the pH of the water to minimize corrosion of pipes and fittings.
My house water is supplied by an individual groundwater well. Dissolved iron is naturally present at about 4 ppm. When I built the house, I had to install an expensive water treatment system to remove the iron before we could move in. Before installing the system, my toilet tanks were bright orange. We haven’t had a problem since installing the system (knock on wood).