What counts as high
The Indian standard, IS 10500:2012, gives an acceptable TDS limit of 500 mg/L and a permissible limit of 2000 mg/L where no better source exists. Above 500, then, the water is officially high, and above 2000 it is outside what the standard tolerates even as a last resort. On a meter those figures are 500 and 2000 ppm.
In practice there are two thresholds that matter more than the legal ones. Around 500 the water starts to taste of its minerals — salty, bitter or chalky depending on which ones — and RO moves from optional to sensible. Around 2000 the water starts to damage the membrane you fit to treat it, and RO on its own is no longer the answer. The full chart of bands and what each calls for is on the TDS reference page.
What is actually dissolved in it
TDS is the sum of every dissolved ion, and in high-TDS Indian water the bulk of that sum is usually a handful of familiar ones. Calcium and magnesium, from limestone and from the weathering of most rock, which together make the water hard. Sodium and chloride, from salt in the ground and, near the coast, from seawater working its way into the aquifer. Sulphate and bicarbonate, from the same rocks. Together these make up most of the reading and most of the taste.
The reading cannot say how much of the total is which. Two supplies at 1200 ppm can be quite different water: one mostly calcium bicarbonate, which is hard and scales a kettle but is otherwise harmless; the other sodium chloride, which is salty, soft and hard on a membrane in a different way. And riding along in a share that is far too small to show on the meter may be the things that actually decide safety: fluoride, nitrate, arsenic. The TDS vs hardness guide separates the first two ions from the total; the guide on what TDS cannot see covers the trace contaminants.
Where high TDS comes from
Geology. Groundwater dissolves the rock it moves through. Water that has sat for a long time in mineral-rich rock, with little fresh recharge, picks up more. That is the ordinary reason a borewell reads higher than a river-fed municipal supply, and the borewell guide goes into where in India it is worst.
Evaporation. In a hot, dry climate, surface water and shallow groundwater lose water to the air and keep their salts. The same amount of mineral in less water is a higher reading.
Over-extraction. When a well draws faster than the aquifer refills, the water table drops and the pump reaches older, deeper, saltier water. Readings rise year on year, not just season on season.
Saline intrusion. Near the coast, drawing down a freshwater aquifer lets seawater move in underneath. This is the usual explanation for readings well past 2000 in coastal districts.
Contamination. Sewage, fertiliser run-off and industrial waste all add dissolved solids. A high reading in an area with no obvious geological reason is worth a lab test for nitrate.
What it does: taste, health, appliances
Taste is the first effect and the reason most people notice. Past about 500 the water tastes of what is in it. Sodium chloride is salty; magnesium sulphate is bitter; calcium bicarbonate is chalky and leaves a film on tea.
Health, for the common ions, is mostly a question of taste and of what the standard tolerates. The permissible ceiling of 2000 exists because water that salty is unpleasant and adds a meaningful amount of sodium to the diet, not because the calcium in it is harmful. The health risks that matter are the trace contaminants that often accompany high TDS in groundwater, and those need their own tests.
Appliances take the visible damage. Hard water scales kettles, geyser elements and the inside of pipes. Salty water corrodes. An RO membrane fed high-TDS water works harder, clogs sooner and, above 2000 ppm on hard supply, scales within months. Sediment cartridges upstream of it, which take the silt that often comes with borewell water, shorten too.
At what point RO becomes necessary
Between 300 and 500, RO is a good idea for taste and for summer headroom, and a UV purifier is still a defensible choice if the lab report is clean. Above 500, RO is the only domestic method that brings the reading down; boiling concentrates dissolved solids rather than removing them, and carbon and sediment cartridges do not touch them. Above 2000, RO is still necessary but no longer sufficient: fit a membrane rated for the feed, and put a resin softener ahead of it if the hardness is high, or the membrane will scale before it has earned its price.
In the LUXGEN range, the membrane for feed above 2000 ppm is the 100 GPD molded high-TDS membrane, rated to 3000 ppm with 96–98% salt rejection. It needs pressure to reject that much salt, which is what the 140 PSI LUX-PUMP-100 is for. Ahead of both, keep the sediment stage honest: the two-candle pre-filter kit is rated for feed to 2000 ppm and takes the silt, and a 5-micron spun cartridge changed on time is the cheapest protection the membrane will ever get.
Questions
Is 1000 ppm TDS water safe to drink?
It is above the 500 mg/L acceptable limit but inside the 2000 mg/L permissible limit of IS 10500:2012, so the standard tolerates it where no better source exists. It will taste of its minerals, and it should be lab-tested for fluoride and nitrate, which the TDS figure cannot show. An RO purifier with a standard 1812 membrane handles 1000 ppm comfortably.
Does boiling reduce TDS?
No. Boiling kills microbes but leaves dissolved solids in the water, and as some water evaporates the reading goes up slightly. Only reverse osmosis, distillation or deionisation lower TDS.
Does a carbon filter lower TDS?
No. Activated carbon removes chlorine, odour and organic taste, none of which register on a TDS meter. Sediment cartridges remove suspended particles, which do not register either. A membrane is the only stage in a domestic purifier that lowers the reading.