Part 3. Turbidity
Turbidity of water is an optical property that causes light to be scattered and absorbed, rather than transmitted. This is primarily caused by suspended solids. The higher the turbidity, the greater the amount of scattered light. Even a very pure fluid will scatter light to a certain degree; no solution will have zero turbidity.
The measurement of turbidity is used to indicate water quality and filtration effectiveness. Increases in suspended and colloidal matter increase turbidity. These levels are often associated with higher levels of disease-causing microorganisms such as viruses, parasites and some bacteria that can cause symptoms such as nausea, cramps, diarrhea, and associated headaches.
Although microbiological testing is also done, turbidity measurements are used with regularity to detect changes immediately.
Turbidity is an important parameter in numerous industries. In the following sections we briefly describe some of the applications of turbidity.
Natural Water Supplies
In natural water, turbidity measurements are taken to gauge
general water quality and its compatibility in applications where
there are aquatic organisms. It has been found that there is a strong correlation between turbidity and BOD (Biochemical Oxygen Demand) value – which we discussed last week. Moreover, by definition, turbidity obstructs light, thus reducing the growth of marine plants, eggs and larvae, which are usually found in the lower levels of an aquatic ecosystem.
Historically, turbidity is one of the main parameters monitored in wastewater. In fact, the monitoring and treatment process was once solely based on the control of turbidity. Currently, the measurement of turbidity at the end of the wastewater treatment process is necessary to verify that the values are within regulatory standards.
By monitoring the turbidity level, it can be determined if the different stages of the process, particularly in the filtration and purification stages, have been completed correctly.
Drinking Water Purification
Turbidity is one of the most important parameters used to determine the quality of drinking water and public water suppliers are required to treat their water to remove turbidity. According to Australian Drinking Water Guidelines, turbidity cannot be higher than 1.0 nephelometric turbidity units (NTU) at the plant outlet.
Adequately treated surface water does not usually present a turbidity problem. The World Health Organisation indicates 5 NTU as the reference turbidity value of water for trade. This value has been established based on the aesthetic characteristics of water. From a hygienic point of view, 1 NTU is the recommended value. Many drinking water utilities strive to achieve levels as low as 0.1 NTU.
Turbidity is an indicator and will not give results for a specific pollutant. It will, however, provide information on the degree of overall contamination.
Turbidity Measurement Units
There are different measurement standards used based on applications, and with these standards are applied units. The ISO standard adopted the FNU (Formazin Nephelometric Unit) while the EPA uses the NTU (Nephelometric Turbidity Unit). Other units include the JTU (Jackson Turbidity Unit), FTU (Formazin Turbidity Unit), EBC (European Brewery Convention Turbidity Unit) and diatomaceous earth (mg/L SiO₂).
|JTU||FTU (NTU/FNU)||SiO2 (mg/L)|
The Hanna Testing Solutions
There are three analytical test methods for turbidity:
- ISO 7027 “Water Quality: Determination of Turbidity”
- USEPA Method No. 180.1, “Turbidity”
- Seawater and Wastewater No. 2130, “Turbidity”
Specific wavelengths are recommended for each method. For the USEPA and Standard Methods, the wavelength in the visible range of the spectrum is recommended, where the European ISO method requires an infrared light source.
Click on the links below to explore Hanna’s turbidity measurement solutions.