TDS, Hardness and Iron: A Water Quality Guide

Behaviour is set by which ions carry the TDS, not by the total. Sodium and chloride are monovalent, stay soluble and raise conductivity without scaling. Calcium and magnesium are divalent, precipitate as carbonate above roughly 60 °C, and consume soap. Sulphate adds a scaling path as calcium sulphate above about 1,800 mg/L at 25 °C. Silica polymerises above 100–150 mg/L and is difficult to remove once baked onto a membrane.

Temperature and pH decide the outcome. Water at 500 mg/L TDS as calcium bicarbonate may be stable at 20 °C and deposit scale at 70 °C, because calcium carbonate solubility falls as temperature rises. The same concentration as sodium chloride stays clear at any domestic temperature. That is why a softener decision needs hardness, alkalinity and pH, not a TDS pen alone.

Softening is justified by the combination of hardness, temperature and downstream equipment. Below 60 mg/L as CaCO₃, most commercial systems run without scale. Between 60 and 120 mg/L, scale forms slowly above 60 °C, and the decision depends on the heater, the steam generator and the RO recovery target. Above 120 mg/L, softening is the default for any system that heats water or runs an RO above 65% recovery.

Iron above 0.3 mg/L stains fixtures and laundry, and manganese above 0.05 mg/L leaves black deposits. Both are aesthetic thresholds used by the World Health Organization, and both are the point where an RO membrane begins to foul, because iron oxide plugs the feed spacer and is not removed by normal cleaning. Dissolved ferrous iron passes a 0.45 µm filter and looks clear; oxidised ferric iron is caught on the filter and looks orange.

A laboratory analysis costs far less than a wrong skid. Take the sample after the pressure tank and before any treatment, and record whether the iron and manganese samples were field-filtered, because filtered and unfiltered results describe different fractions of the same metal. A single snapshot is not enough for a well: test again after heavy rain and at the end of the dry season.

The order is fixed by what each step can tolerate. Oxidation and filtration come first because iron and manganese foul every downstream surface, including softener resin and RO membranes. Softening comes next because it protects heaters and allows higher RO recovery. Reverse osmosis follows, and carbon plus UV polish the permeate.

RO is justified when TDS drives taste, a process limit or a regulated limit, not when it merely reads high. For drinking water, many people notice a mineral or salty edge above 600 mg/L and reject the water above 1,000 mg/L. For process water the limit comes from the equipment: boiler make-up, humidifiers, ice machines and beverage recipes each set their own number.

A single-stage RO removes 95–99% of dissolved salts and, from a feed of 300 µS/cm at 25 °C, delivers permeate at or below 10 µS/cm. Below 600 mg/L, carbon filtration and UV usually solve taste and safety problems at lower cost and without a concentrate stream. Between 600 and 1,000 mg/L, blending permeate with raw water is often the cheapest way to hit a taste target.

Concentrate disposal belongs in the same decision. A small RO at 50% recovery doubles the salt load sent to drain, and where discharge is metered or sewer rules limit TDS, that cost changes the comparison. Where the building also needs soft water, a softener may solve the scale problem at lower capital cost and leave the TDS untouched. The two devices are not interchangeable, and buying one to fix the other problem is how projects end up with a plant that meets the spec sheet and not the requirement.

No single cartridge does all three. A sediment filter removes particles down to its rating, typically 5–100 µm, and passes dissolved salts, hardness and dissolved iron. A softener removes hardness by ion exchange and leaves TDS almost unchanged; sodium rises by about 46 mg/L for every 100 mg/L of hardness removed. Activated carbon removes chlorine, taste and some organics but not salts or hardness.

RO rejects all three dissolved groups at 95–99%, but it needs iron removed first, because iron oxide fouls the membrane and is not recovered by cleaning. The practical order is oxidation and filtration for iron, softening for hardness, then RO for salinity, with carbon and UV as polishing steps. A vendor who proposes one vessel for all three is describing a cartridge, not a treatment train.