In our daily life, we come across several liquids such as water, oils, beverages, pharmaceutical products, etc., which are considered essential for our survival. It is crucial to monitor the quality of these liquids as their contamination may cause severe effects on human health. Although there are several tests available to monitor such impurities, one simple method is to check for the turbidity in a given sample. Turbidity refers to the cloudiness or haziness caused by the particles suspended in any liquid. It is an expression of the optical property of a medium that causes light to be scattered and absorbed rather than transmitted in a straight line through a sample. For instance, during the rainy season, mud and silt washed into rivers makes it murkier, causing the light to scatter just beneath the water surface rather than being reflected from the river bed. The measurement of turbidity is an important factor in determining the quality of drinking water. Measurements of turbidity can be used in many analytic fields to determine the mass concentration of suspended particles in a sample and for some simple contexts, such as particle size distributions. Apart from liquids, the term Turbidity (also called haze) can also be applied to transparent solids like plastic and glass.
Causes of Turbidity
When light passes through any medium containing inhomogeneities (e.g., solids suspended in the liquid), it gets absorbed and scattered by the constituting impurities, causing turbidity. For water present in the natural resources, these impurities can be:
- Phytoplanktons growing in the open water bodies.
- Sediment erosion caused by several human activities such as construction, mining, agriculture, etc.
- Water pollution.
- Mud and silt erosion caused by fast winds and heavy rains.
- Algal Bloom.
For practical measurements, however, the quantification of turbidity is dependent on several factors including
- the concentration of scattering particles suspended in the medium.
- size distribution of the scattering particles.
- shape, orientation, and surface condition of the scattering particles.
- the refractive index of the scattering particles.
- the refractive index of the suspension medium.
- the wavelength of the light source employed.
Measurement of Turbidity
In 1865, Angelo Secchi, an Italian astronomer, invented the first reference for turbidity measurement. It is a plain white circular disk of 30 cm diameter used to measure water transparency in the water bodies. The disk is mounted on a pole or line and then lowered down slowly in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the Secchi disk has also been used in a modified, smaller 20 cm (8 in) diameter with a black and white design to measure freshwater transparency. Nowadays, several objective analytical instruments are widely available for quantitative turbidity determination in a variety of scientific, natural, industrial, and process management applications, with the choice of instrumentation being largely dependent on the analytical aim (e.g., to derive mass concentration, particle size distribution, molecular dimensions, or crystal/cell growth data). The methods of turbidity measurement are mainly classified into two techniques:
- Turbidimetry: It is the measurement of turbidity achieved by quantifying the degree of attenuation of a light beam of the known initial intensity. It is usually applied to media of fairly high turbidity in which the scattering particles are relatively large. Suspended matter in the light path causes scattering and absorption of some light energy, which reduces the incident illumination falling on the photodetector. These instruments are normally more appropriate for relatively turbid samples in which the scattering particles are large with respect to the light wavelength used because a significant reduction in the intensity of incident light is needed to yield precise results.
- Nephelometry: It is the measurement of turbidity by the direct evaluation of the degree of light scattering taking place in the medium. It is much more appropriate to a media of lower turbidity in which the suspended particles are small. A nephelometer measures directly the intensity of light scattered by the sample, which is proportional to the amount of matter suspended in the light path. However, the influence of size, shape, and refractive index of the scattering particles is very important as these characteristics also control light scattering intensity.
Turbidimetric and nephelometric methods offer considerable time-saving advantages over gravimetric approaches for the determination of particle concentrations.
Units of Measurement
The units for turbidity measurement have long been non-standard. After Secchi’s depth method, the first formal measurement of turbidity was made with the help of something called the Jackson Candle Method. It was essentially a vertical glass tube mounted over a candle. The scale on the tube was calibrated using dilutions of a standard reference solution comprising 1,000 parts per million (ppm) of diatomaceous earth (silica) in distilled water. The calibrated units of measure on the tube were called Jackson Turbidity Units (JTU). Nowadays, the most commonly used unit for turbidity is the Nephelometric Turbidity Unit (NTU), with the occasional use of the Formazin Nephelometric Unit (FNU) that was developed in 1962 by using formazin polymer solution as a standard reference. Due to the toxic nature of formazin, it is advised to use other calibration materials, e.g., Fullers Earth or Hach Gelex ‘fixed’ standards, i.e., metal oxide particles permanently and statically suspended in silica gel media.
Applications of Turbidity Measurement
The methods for measurement of turbidity have found numerous applications in several areas including chemical sciences, pharmaceutical sciences, environmental sciences, food and beverage industries, hydrological and geological sciences, and medical sciences. In addition, turbidimetry and nephelometry are well-established procedures wherever filtration processes have to be carried out, monitored, and controlled. The turbidity measurements of suspended solid concentration in rivers, lakes, reservoirs, nearshore zones, etc., are of prime interest to several disciplines including hydrologists, limnologists, geoscientists, geomorphologists, freshwater ecologists, engineers, oceanographers, glaciologists, and water resource managers. In coastal regions, high-frequency monitoring of turbidity in water via optical backscatter sensors facilitates the determination of critical flow velocities required to move the bed sediments. Knowledge of such threshold conditions is important for improved understanding and management of coastline processes and their associated ecosystems. This is especially true for tidal channel turbidity and sediment transport dynamics. Furthermore, in chemical and biological analysis applications, the calculation of absolute molecular weights and dimensions of polymers in solution as well as particle size determinations of suspended matter is facilitated by turbidity measurements. Chemical profiles can also be obtained by observing turbidity changes induced by the addition of specific substances to a given solution. In microbiology, cell and bacteria growth can be monitored through the media turbidity changes associated with such activity. In foodstuff manufacturing, turbidimetry is often used to monitor product quality and treatment process efficiency, especially to monitor the product quality of fats in dairy products. Turbidity is also a key concern in the petrochemical industry.
Health Impacts of Turbidity in Drinking-Water
With the increasing population (and hence the pollution), the quality of drinking water is of primary concern to several governments. Apart from being aesthetically unappealing, excessive turbidity in drinking water can also become one of the major health concerns. The increase in the turbidity of natural water resources is accompanied by the growth of pathogens that can be harmful to our health. Although turbidity is not a direct indicator of health risk, numerous studies show a strong relationship between the removal of turbidity and the removal of protozoa. The particles of turbidity provide “shelter” for microbes by reducing their exposure to attack by disinfectants. Microbial attachment to particulate material has been considered to aid in microbe survival. Fortunately, traditional water treatment processes can effectively remove turbidity when operated properly. According to the WHO guidelines for drinking-water quality, it is recommended that, for water to be disinfected, the turbidity should be consistently less than 5 NTU or JTU and ideally have a median value of less than 1 NTU.