What is Turbidity Definition

Turbidity is a parameter which measures the content of particulate foulants in the reverse osmosis feed water source. Turbidity can also be defined as the measurement of the light scattering properties of particles in suspension. The instrument used to measure turbidity is called a turbidimeter or turbidity meter.

The principle of turbidity meters is that suspended particles will block or reflect light. There are several different types of turbidity meters, but they are all based on the principle of particles blocking or reflecting light. Let’s look at reflected light first.

If there are no particles suspended in a sample, light will pass through the sample in a straight line. If a photocell (photoreceptor) is located at 90 degrees to the light path, no light will reach the photocell and the turbidity will read zero.

If particles present in a sample, the light will be reflected off of the surfaces of the particles and will be scattered at different angles. A photoreceptor at 90 degrees from the source of light will pick up light that is scattered and a turbidity measurement greater than 0 will be obtained.

The unit of measurement for turbidity is the NTU or the JTU or the FTU. There is no correlation between NTU and JTU and FTU. The term nephelometric indicates that this is a light-interference analytical technique. NTU is the measurement unit for light scattering. Some on-stream turbidity meters also use the light-scattering principle. A common one is called the surface scatter turbidimeter.

For a surface-scatter turbidimeter, the sample with suspended particles is passed up through a tube. A light source is shining on the surface of the liquid as it flows over the top of the tube. A photocell is located above the surface of the water. If there are no particulates in the water, the light beam is absorbed by the black tube and the black interior of the enclosure. If they are present, they scatter light as they surface (This is why this technique is called surface-scatter). The photocell picks up the scattered light and displays the reading in NTU.

There is another type of turbidimeter which doesn’t measure the light scattered but rather the reduction in transmitted light. In this case the full beam is received by the photocell. Full-beam reception equates to clean water and no turbidity.

  

If particles are present, less light reaches the photocell. This, then, is displayed as a turbidity reading, in JTU (Jackson Turbidity Unit, named after the man who created the standard candle which was originally used as the source of the light). Figure below illustrates particulates in the sample scattering, and therefore blocking, the light beams.

The turbidity level in the source of water is indicative of the content of clay, silt, suspended organic matter, and microscopic aquatic life. Such as phyto- and zooplankton. It is expressed in Nephelometric Turbidity Units (NTU) and Formazin Nephelometric Unit (FNU). The turbidity of open ocean and surface brackish water can vary between 0.1 and several hundred NTU, although under normal dry weather conditions, it is typically between 0.5 and 2.0 NTU. Rain events, algal blooms, storms, snow melt, river discharges, and human activity (such as wastewater discharges, ship traffic, etc.) can cause significant turbidity increases and variations. Usually, water that is saline with a turbidity below 0.05 NTU causes very low particulate fouling of the reverse osmosis membrane. Most RO membrane manufacturers have a maximum feed raw water turbidity of 1.0 NTU, although this level is relatively high in practical terms. Usually, filtered water turbidity below 0.1 NTU is desirable.

Although turbidity is a good measure of the overall content of particulates in the source water, on its own it is not an adequate parameter to characterize water’s potential for particulate or other fouling. Turbidity measurement does not provide information regarding the type and particle size in the source feed water and does not measure the constitution of dissolved organic and inorganic foulants. The size of particles contained in the source water matters because RO membrane feed and concentrate spacers, through which the saline raw water is distributed inside the membranes, are of limited width (typically 0.7 to 0.9 mm).

Even with these problems, turbidity measurements can be valuable trending indications for monitoring RO unit feed water. Most manufacturers require that the feed water to an RO unit be less than 1.0 NTU. Turbidity is almost always measured on feed waters which are processed through a clarifier. Clarifiers are usually found at facilities with a surface water source and no prior municipal treatment. In this case turbidity is frequently measured before and after the clarifier to monitor the performance of the clarifier. In a few cases, turbidity is measured before and/or after a multimedia filter to monitor performance of this piece of equipment. Turbidity or TSS measurements alone are insufficient to tell us about the fouling potential of our feed water. They must be used in conjunction with SDI (Silt Density Index).