homeburrero wrote:Magnesium carbonate is relatively insoluble, and like calcium carbonate the solubility goes down as temperature increases, but magnesium carbonate is much more soluble than calcium carbonate. The LSI calculations are based on calcium, so replacing calcium with magnesium should reduce the scaling rate.

I assume you are saying 2 drops KH/alkalinity and 8 drops GH/total hardness with the API kit. If so, then given the accuracy/precision of this drop test I'd call that somewhere in the ballpark of 15 - 50 mg/l (1-3 gpg) alkalinity, and 125 - 160 mg/l (7 -10 gpg) total hardness (both as CaCO3, of course). That is well in agreement with the St. Louis water report. Much of your total hardness would be magnesium, maybe 40-50% based on the St. Louis report, so you may have a calcium hardness closer to the 90 mg/l ballpark.

Given your measurements and the St. Louis water report, I think it's a good bet that your alkalinity and calcium numbers are low enough that you might never need to descale even if you used a simple carbon and particulate filter. And the guidance you got from Reiner Stanelle at BWT certainly implies that the filter would be a waste of money when the carbonate hardness is below 3 grains (as yours is.)

The BWT perhaps improves your odds that you would never need to descale, but it's hard to make that argument based on any real evidence. As far as flavor improvement with higher mg in the incoming water, I think the jury is still out on that one. Keep in mind that the water flowing through the coffee bed is a soup that includes dissolved minerals that were in the coffee itself, which includes magnesium and especially potassium far above whatever was in the incoming water. (The final espresso shot will contain about 800 mg/l of magnesium, which is equivalent to 3400 mg/l of hardness as CaCO3.)

I think you're probably right that something about using this filter with moderately low alkalinity water makes it difficult or impossible to see any reduction in total hardness that you'd normally expect. Let us know if you learn anything more from WLL. They appear to have some water expertise - their water and scale videos by Marc are pretty good - he's a salt water aquarium guy.

P.S.
If you want to get a little better precision out of that API KH test you can cheat it up to a 10 ml sample rather than 5 ml, then for each drop you have 0.5 °dH or 8.9 mg/l alkalinity.