@strange-steve and @Argentum Following a number of discrepancies, I asked Murphy's to let me know if it is possible the test was erroneous. They retested and there were some irregularities. To their credit they have requested another sample. Be interesting to see if there is a small or big difference.
The supply comes from Hanningfield Treatment Works. This is fed by the River Blackwater and River Chelmer. The supply changes between the rivers for operational purposes. I've used 2018 -19 data for calcium and magnesium for a broader range.
Now I await Murphy and Son's retest with baited breath!
The mean values balance for cation and anion mEq's if you presume roughly 62 ppm sodium.
9.5 mL of your 75% phosphoric acid will treat 30 Liters to a pH of between about 5.4 and 5.45.
The extremes are not as extreme as I would initially have expected. Your water should be usable.
I just mailed off a sample of my well water to Ward Labs today for analysis. Your water is great compared to my well water. A budget line GH/KH test kit from API told me that my total hardness is 757 ppm, and my alkalinity is 437 ppm. My TDS meter reads 876 ppm. It will be interesting to see what Ward Labs says.
BTW: TDS is roughly equal to Ca+Mg+Na+Cl+SO4+Bicarb/2 (I.E, half of the bicarb)
I don't sparge, but if I did I would add non-alkaline minerals to both mash and sparge water to achieve the same ppm's for both, and add alkalizing minerals (only if called for) exclusively to mash water. Calcium in the mash is beneficial, so many choose to add minerals to mash only, and treat sparge water only to drop it to ~pH 5.4-5.5. With your alkalinity you may never find it necessary to add additional alkalinity to your mash water. And yet others add non-alkaline minerals to the kettle post run-off. The choice is yours...
I've made an initial estimate that CRS/AMS has an acid strength of ~3.66 mEq/mL. It's not available here. Would someone more versed in its actual use please let me know if this initial estimate sounds about right.
As I understand it, Calcium in the mash has at least this list of benefits (to which there may be additional benefits):
1) Stabilizes the Alpha Amylase enzyme so it does not get degraded as rapidly at mash temperature.
2) Precipitates malt oxalates in the mash that may lead to downstream haze issues and potentially even bottle gushers (if not precipitated in the mash).
3) Liberates H+ from malt phosphates and thereby lowers mash pH.
One big difference that I see between US and UK home brewing is that in the US we tend to presume that around 50 mg/L of Ca++ ions is generally sufficient in the mash, whereas in the UK the perception (or rather, my perception) is that it may be more like 100-150 mg/L as the desired range for Ca++ ions. That plus we don't have CRS/AMS here, and this is perhaps because (IPA's excluded) we don't typically target as much chloride or sulfate in our beers. And lastly in the USA we generally advise to not add magnesium, or at least to keep it on the very low ppm side, with the perception that the only thing it can contribute is an undesirably nasty/bitter flavor. I doubt if there are any hard and fast rights or wrongs here. Our differences in approach may stem from a more German influence to US brewing. ???