TDS, mg/L, PPM — what the numbers on a water test actually mean

If somebody sold you a filter by waving a TDS meter at your tap and shouting about the number, you were half-sold and half-scammed. The number is useful, but it is one number among several. Here is how to read all of them.

TDS (Total Dissolved Solids)

TDS measures everything dissolved in the water — salts, minerals, metals, some organics — expressed in milligrams per litre or parts per million (same thing). WHO considers anything under 600 mg/L "acceptable" for drinking, under 300 "good taste", under 150 "excellent taste".

Dubai plant output: 80–120 mg/L. Dubai apartment tap typical: 250–500 mg/L. Dubai villa with a private pump and scale build-up: often 400–700 mg/L. A RO-filtered output: under 50 mg/L. A re-mineralised RO-filtered output (what PureOas does in stage 6): around 80–150 mg/L, which is where bottled mineral water sits.

Chlorine (mg/L as Cl₂)

Residual chlorine is how DEWA keeps pipes disinfected. Legal in Dubai up to 5 mg/L; typical apartment reading 0.3–0.8 mg/L. You start tasting it around 0.2 mg/L and smelling it at 0.5. Activated carbon removes more than 99% of it in one pass — that is why even cheap pitcher filters work for taste.

pH

Pure water is 7.0. Dubai tap water typically reads 7.4–8.0 (slightly alkaline). A pure RO output can drop to 5.5–6.5 because stripping minerals makes water more aggressive; we add calcium and magnesium back in stage 6 to bring it to 7.3–7.5.

Hardness (mg/L as CaCO₃)

Measures calcium + magnesium. 0–60 is soft; 60–120 moderate; 120–180 hard; above 180 very hard. Dubai tap water is often 180+, which is why your kettle, iron and dishwasher complain, and why hair goes straight-to-straw without conditioner.