Ferric chloride is not added in any concentrations that would corrode piping. Also just enough is added to cause the precipitation reaction and it gets removed with the precipitate, so it doesn't stay in the water after it's added. Ferric chloride works a little more efficiently as a clarifying agent than ferrous sulphate but they both perform essentially the same function.
I'm confused about carbon dioxide "increasing the alkalinity" of the water. Is this a mis-translation? Carbon dioxide is an acid gas and will decrease alkalinity/increase acidity. I think perhaps they increase the pH with lime water and decrease the pH with CO2. That would give them control in both directions and would fit the chemistry better.
Fair enough, just my gut feling - it is true that it would be quite a soup if FeCl was aded in PCB etching concentrations

As to carbon dioxide - no mistranslation, the same wording can be found on the original Finnish page. In fact the case is not that simple. Some dissolved CO
2 in water turns partly into carbonic acid H
2CO
3 and from there it dissociates at least partially into ionic forms HCO
3- (the bicarbonate ion) and CO
32- (carbonate ion) the proportions depending on the original pH of the water. The bicarbonate ion at least is an amphoteric compound that can act either as an acid or a base, depending on the acidity of the original solution. So i think they knew what they were writing. At the same time there is a diagram of the process here:
http://www.hsy.fi/vesi/Documents/Juomavesi_ja_veden_laatu/Vedenpuhdistusprosessi_HSY.pdf describing how they add CO
2 ("Hiilidioksidi") and lime water ("Kalkkivesi") as the last stages of the treatment, in that order.