The Relationship Between NTU & MG/Ml

The Nephelometric Turbidity Unit (NTU) is term of measurement for the turbidity in a water sample. Turbidity in water refers to how clear the water is. The higher the concentration of Total Suspended Solids (TSS) in the water, the more murky it appears. Conversion from NTU to mg/ml (milligrams per milliliter) is valuable, because the NTU standard is more readily measured in the field.

Importance of TSS

• Studying the levels of TSS in bodies of water is important primarily because it can have substantial impact on fish and their homes. An NTU instrument measures the concentration of particles suspended in a body of water, including such matter as clay, silt, plankton and various microscopic bodies. Turbidity is not a measurement of the mass of these particles, but how light is conducted and scattered through the concentration of the water. Because each environment’s relationship between water content and turbidity is different, measurement instruments should be calibrated prior to use.

Coverting TSS to NTU

• In general, the greater the concentration of TSS per mg/ml in a given sample of water, the greater its turbidity will be. Thus, mg/ml relates to NTU directly, where 1 FTU equates to 7.5 mg/L, or .0075 mg/ml, of SiO2. The practical limit as set by the World Health Organization for water considered fit for human consumption is 5 NTU, though ideally 1 NTU is considered best. Water that is visibly muddy is known to exceed as much as 100 NTU.

Methods of Measurement

• Many methods of NTU measurement.

  There are several different accepted methods for measures NTU values of a sample of water. The Infrared Method (ISO 7027) operates by passing an infrared beam of light through a water sample contained in a vial, then measures how much of the light is scattered by water particles. The USEPA Approved Method shines a pure green LED into water and measures light scattering as well. These readings can then be used to determine the general condition of the body of water.

Turbidity and Life

• Turbidity, by nature, obstructs light from moving in water. The more a body of water’s turbidity rises, the less habitable it is for the growth of marine life, including plants, eggs and larvae. Turbid water can also cause shallower shallow bodies of water to fill up with silt and clay faster, covering over areas that might have been habitable for aquatic life. Turbid water might also cause damage to gills and reduce an organism’s defense against disease.