TDS: total dissolved solids
TDS measures the total mineral content of the water in parts per million. It is the most-quoted number and the most-misunderstood. Low TDS does not mean clean water. It just means dilute. High TDS does not mean dirty water. It means concentrated.
For drinking, TDS sets the treatment: below 200 ppm, ultrafiltration with UV is sufficient; above 500 ppm, reverse osmosis is the right answer; between 200 and 500, the call depends on hardness, iron, and what the household is actually drinking. For bathing and washing, TDS matters less and hardness matters more.
Hardness: calcium plus magnesium
Hardness is the calcium and magnesium concentration in the water, also measured in ppm. It is the single largest factor in how the water feels. Below 60 ppm is soft. 60 to 120 is moderately hard, 120 to 180 is hard, and anything above 180 is very hard. Most Indian metropolitan supply runs 60 to 180 ppm. Most borewell-fed homes run 250 to 500 ppm.
Hardness causes the scale on the geyser, the dry-skin feel after a shower, the soap that will not lather, and the orange grout (in combination with iron). Softening removes it. Nothing else does.
Iron
Iron is measured in ppm. Anything above 0.1 ppm will, given time, stain tile grout, sanitaryware, and porous stone. Anything above 0.3 ppm requires dedicated iron pre-treatment before softening. Anything above 1.0 ppm, which is common in Guwahati borewells and some Ranchi residential boreholes, requires a substantial iron filter sized to the household draw.
pH
pH measures acidity or alkalinity on a 0 to 14 scale, with 7 being neutral. Indian residential supply usually runs 6.8 to 7.6, slightly alkaline and well within range. Outside 6.5 to 8.5, the water can corrode pipes (low pH) or scale aggressively (high pH). If your pH is outside this range, the survey will flag it and propose neutralisation.
FRC: free residual chlorine
FRC measures the chlorine still in the water at the tap, in ppm. Municipal supply usually arrives with 0.2 to 1.0 ppm. That is enough to disinfect along the way, not enough to taste obviously. Above 1.5 ppm, the chlorine becomes noticeable and starts to age CP fittings and rubber seals. Activated carbon removes it.
Five numbers. Each one decides a stage of the system. None of them are optional.
What to ask the tester
Two questions matter. First: is this test from the borewell directly, or after the overhead tank? The numbers can differ, because sediment settles in the tank and iron oxidises in storage. Second: is the report from the past month or the past year? Borewell chemistry shifts seasonally, especially in monsoon-fed regions. A test from last May is not the same as a test from this November. The right test is taken at the time the system is being sized.