NTU is the unit for measuring

Correct — option (4), Turbidity of water.

NTU stands for Nephelometric Turbidity Unit. Turbidity is the cloudiness or haziness of water caused by many small particles that are generally invisible to the naked eye.

The name describes the method. A nephelometer shines a light beam through the sample and places its detector to the side; the more particles there are, the more light they scatter towards the detector.

India's drinking water standard uses this unit. The Bureau of Indian Standards' IS 10500:2012, Drinking Water — Specification (Second Revision), has a row "Turbidity, NTU, Max" with an acceptable limit of 1 and a permissible limit of 5 in the absence of an alternate source.

So NTU is the unit for Turbidity of water.

The idea to remember: NTU measures turbidity (cloudiness); acidity is indicated by the pH value, temperature in degrees, pressure in pascal or bar.

Why the others are wrong

• (1)
  Pressure of water — Pressure is force per unit area and has its own units: the pascal (Pa), bar, torr and atmosphere (atm). None of them is a turbidity reading.

  A pressure gauge on a water line reads in such units, while NTU comes from a nephelometer measuring scattered light, a different instrument for a different property.

• (2)
  Temperature of water — Temperature is read in degrees Celsius or kelvin with a thermometer, not in NTU.

  IS 10500:2012 groups turbidity with colour, odour, pH, taste and total dissolved solids in its table of organoleptic and physical parameters, each with its own unit or test; turbidity is the one reported in NTU.

• (3)
  Acidity of water — How acidic water is, is indicated by its pH value, a number on a scale rather than a reading in NTU.

  IS 10500:2012 lists pH value as a separate requirement from turbidity: 6.5-8.5 for drinking water, with no relaxation. Turbidity, two rows further down the same table, is the parameter given in NTU.

Turbidity is a measure of how cloudy water is. Particles in the water, such as sediment washed in by runoff or phytoplankton, scatter light, so turbid water looks hazy.

The measurement of turbidity is a key test of water clarity and water quality. The way particles scatter a focused light beam is now considered a more meaningful measure; ISO 7027 measures the light scattered at right angles from the sample.

Two standard units report it: Nephelometric Turbidity Units (NTU), from the US Environmental Protection Agency's Method 180.1, Determination of Turbidity by Nephelometry, and Formazin Nephelometric Units (FNU), from ISO 7027.

Key facts

• NTU = Nephelometric Turbidity Unit, the unit of US EPA Method 180.1, Determination of Turbidity by Nephelometry.
• A nephelometer measures turbidity from light scattered by particles, with its detector set to the side of the light beam.
• IS 10500:2012 turbidity limits: 1 NTU acceptable, 5 NTU permissible in the absence of an alternate source.
• IS 10500:2012 sets pH value for drinking water at 6.5-8.5, with no relaxation.
• IS 10500:2012 colour limits: 5 Hazen units acceptable, 15 permissible; total dissolved solids: 500 mg/l acceptable, 2 000 mg/l permissible.

Common traps

• pH is not a unit of measure like NTU; it is a value on a scale. IS 10500 lists pH value and turbidity as separate rows with separate limits.
• Colour and turbidity are different parameters. IS 10500 reports colour in Hazen units and turbidity in NTU.
• IS 10500 gives two numbers for turbidity. 1 NTU is the acceptable limit; 5 NTU is permissible only in the absence of an alternate source.

• practice — not a real PYQ

  As per IS 10500:2012, the acceptable limit of turbidity in drinking water is:

  • (a)1 NTU
  • (b)5 NTU
  • (c)10 NTU
  • (d)25 NTU
  Answer(1) — Table 1 of IS 10500:2012 gives turbidity an acceptable limit of 1 NTU. Option (2), 5 NTU, is the permissible limit in the absence of an alternate source, and options (3) and (4) are not the limits the standard gives for turbidity.
• practice — not a real PYQ

  In IS 10500:2012, the Hazen unit is used to express which characteristic of drinking water?

  • (a)Turbidity
  • (b)Colour
  • (c)Total hardness
  • (d)pH value
  Answer(2) — The standard lists "Colour, Hazen units, Max" with limits of 5 and 15. Turbidity is given in NTU, total hardness (as CaCO3) in mg/l, and pH as a value without such a unit, so options (1), (3) and (4) are wrong.