Reading a borehole water analysis before specifying an RO pre-treatment train

A borehole water analysis that reports TDS, pH and total hardness looks complete, and for a lot of applications it is. For specifying reverse osmosis pre-treatment, it usually isn't — the numbers that determine whether a standard pre-treatment package will work, or foul the membranes within months, are often the ones left off a basic analysis.

Iron and manganese, not just their oxidised state

A lab result reporting "iron: below detection limit" on a sample that sat in a bottle for two days before testing is not the same as iron-free water at the wellhead. Dissolved ferrous iron oxidises to a solid ferric precipitate on contact with air, and if that happens before the sample reaches the lab, the result understates what the RO membranes will actually see. The number that matters is dissolved iron measured on site, ideally with a field test kit at the point of sampling — not a lab certificate for a sample that travelled.

The same applies to manganese, and the practical consequence is the same either way: a pre-treatment train specified against a low-iron lab result, installed against a wellhead that actually carries meaningful dissolved iron, fouls its cartridge filters and membranes far faster than the maintenance schedule assumes.

Silica, and whether it's reactive or colloidal

Silica limits achievable RO recovery independently of TDS. A feed with moderate TDS but high reactive silica can hit its recovery ceiling well before a TDS-based estimate would suggest, because silica scaling on the membrane doesn't respond to standard antiscalants the way calcium carbonate or sulfate scaling does. Distinguishing reactive silica from colloidal silica matters here — colloidal silica is addressed by pre-treatment (filtration, coagulation), reactive silica by recovery limits and antiscalant chemistry, and treating one as the other wastes either capital or chemical budget.

Sulfate, barium and strontium together, not separately

Barium and strontium sulfate scales are among the least reversible fouling mechanisms on an RO membrane — once precipitated, they don't respond to a standard cleaning-in-place cycle. They only become a risk in combination with sulfate, so a report that lists barium and sulfate on separate lines, without flagging the combination, can miss a scaling risk that neither number raises on its own.

What this changes in practice

None of this requires an unusual analysis — it requires asking for the right one before the pre-treatment package is specified, not after a membrane autopsy explains a fouling problem that a complete borehole analysis would have flagged at the design stage.