NABL Accredited · IS 10500 Compliant Testing

Turbidity Test of Water

Understanding turbidity in water — what it means, how it is measured, IS 10500 limits, health implications, and when laboratory turbidity testing is needed.

Turbidity is one of the most important basic parameters in water quality assessment — and one of the most visible. Turbid water appears cloudy, hazy, or coloured because suspended particles scatter light passing through the sample. High turbidity indicates the presence of particulate matter that can shelter pathogenic microorganisms, interfere with disinfection, and signal potential contamination of the water source.

Whether you are testing drinking water from your tap or bore well, checking water quality for an FSSAI food business licence, monitoring industrial process water, or verifying packaged drinking water compliance, turbidity measurement is a fundamental starting point. Auriga Research's NABL-accredited water testing laboratory provides accurate, IS 10500-aligned turbidity testing using calibrated nephelometric instruments.

What Is the Turbidity of Water?

Water contains suspended solids of varying sizes. Larger particles settle under gravity — these are settleable solids. Smaller colloidal particles remain suspended and cause water to appear turbid. When light passes through turbid water, particles scatter the beam in multiple directions; a nephelometer measures the intensity of this scattered light at 90 degrees to the incident beam.

Turbidity is expressed in NTU (Nephelometric Turbidity Units). Values below 1 NTU are typical of well-treated drinking water; values above 10 NTU indicate significant turbidity that is visible to the naked eye and potentially unsafe for drinking without treatment.

Turbidity Level (NTU) Interpretation
< 1 NTU Excellent — meets IS 10500 and WHO drinking water standard
1–5 NTU Acceptable — IS 10500 permissible limit (no alternative source); monitor and treat
5–10 NTU Marginal — exceeds IS 10500 limit; treatment required before consumption
10–50 NTU High turbidity — significant particulate load; treatment essential
> 50 NTU Very high — not suitable for drinking; indicates major contamination

Causes of Turbidity in Water

  • Soil erosion and agricultural runoff — clay, silt, and fine soil particles
  • Urban storm water from roads, construction sites, and paved areas
  • Algal growth in reservoirs, lakes, and open water bodies
  • Industrial effluent containing suspended solids
  • Resuspension of bottom sediments during heavy rainfall or flooding
  • Sewage or septic system contamination entering water sources
  • Bore well contamination — defective casing or drilling residues
  • Decaying organic matter and leaf litter in surface water

Turbidity Testing Method — Nephelometric Measurement

The standard method for turbidity measurement is the nephelometric method per IS 3025 Part 10 (Indian Standards for Water Quality). A turbidimeter (nephelometer) illuminates the water sample with a light beam and measures the intensity of scattered light at exactly 90 degrees to the incident beam.

Instrument: A nephelometer with a light source, photoelectric detector, and readout device. The instrument is calibrated using formazin standards (Formazin Attenuation Units, FAU) or equivalent AMCO-AEPA polymer standards traceable to primary formazin. The range for the method is 0–40 NTU; higher turbidity samples require dilution before measurement.

Sample requirements: 10–25 mL of clear, undisturbed water in a clean borosilicate glass cell. Samples should be measured within 24 hours of collection. Avoid introducing air bubbles, as these cause false high readings.

When Is Turbidity Testing Required?

  • ✓ Drinking water quality monitoring for households, apartment complexes, and institutions
  • ✓ Bore well and groundwater assessment before commissioning a new water source
  • ✓ FSSAI food business licence applications — water used in food processing must meet quality standards
  • ✓ Packaged drinking water compliance testing per IS 14543
  • ✓ Industrial process water monitoring — cooling towers, boilers, textile, and food manufacturing
  • ✓ Pharmaceutical water systems (purified water, WFI) per IP/USP/EP requirements
  • ✓ Post-rainfall assessment of surface water sources and water treatment plant performance
  • ✓ Wastewater and effluent monitoring for CPCB/SPCB discharge compliance

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Frequently Asked Questions — Turbidity Testing

What is turbidity in water?
Turbidity is the cloudiness or haziness of water caused by suspended particulate matter — including sediment, clay, silt, finely divided organic material, algae, and microscopic organisms. Turbidity is measured by the degree to which a beam of light passing through the sample is scattered by suspended particles. The unit of measurement is NTU (Nephelometric Turbidity Units) or FTU (Formazin Turbidity Units). High turbidity makes water appear cloudy, brown, or green depending on the particulate type.
What is the permissible turbidity limit in drinking water per IS 10500?
IS 10500:2012 (Indian Standard for Drinking Water Specification) sets the acceptable limit for turbidity at 1 NTU for drinking water. The permissible limit in the absence of an alternative source is 5 NTU. The World Health Organisation (WHO) guideline value is also 1 NTU. For swimming pools, CPCB and BIS standards typically require turbidity below 0.5–1 NTU. Higher turbidity is permitted for industrial and non-potable applications depending on end use.
How is turbidity measured in the laboratory?
The standard laboratory method for turbidity measurement is the nephelometric method, in which a calibrated turbidimeter (nephelometer) measures the intensity of light scattered at a 90-degree angle relative to the incident beam. The sample is placed in a clear glass or plastic cell; the instrument illuminates the sample and measures the scattered light. Results are reported in NTU. The method conforms to IS 3025 Part 10 for Indian water quality testing. Turbidimeters are calibrated using formazin standards traceable to NIST.
Why is turbidity a health concern in drinking water?
Turbidity itself is not a direct health risk, but it is a critical indicator of potential microbial contamination. Suspended particles in turbid water can shelter bacteria, viruses, and protozoa (like Cryptosporidium and Giardia) from disinfection by chlorine or UV treatment. High turbidity significantly reduces disinfection efficacy — at turbidities above 1–5 NTU, even standard chlorination may fail to inactivate all pathogens. The IS 10500 turbidity limit of 1 NTU provides a safety margin to ensure effective disinfection.
What causes high turbidity in water?
High turbidity in water can be caused by: soil erosion and agricultural runoff (clay, silt, sand); urban storm water runoff from roads and construction sites; algal blooms in reservoirs; industrial effluent discharge; resuspension of settled sediment by flooding or high flow; sewage contamination; decomposition of organic matter; and, in groundwater, bore well contamination or improper casing. Seasonal variation is common — turbidity peaks during monsoon in surface water sources.
When should I test water for turbidity?
Turbidity testing is recommended: before using a new bore well or water source; after heavy rainfall or flooding (surface water contamination risk); if water appears cloudy, brown, or has an earthy/musty odour; for FSSAI food business licence applications (water used in food processing must meet quality standards); before installing or after servicing water treatment equipment; for packaged drinking water compliance per IS 14543; for industrial process water quality monitoring; and for pharmaceutical water systems (purified water, WFI).

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