Turbidity is one of the most common measurements used in qualitative assessment of water suitability. Turbidity data is useful in drinking water treatment and production, wastewater and environmental monitoring, various process applications (e.g. aquaculture) and some pool markets.
“the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample”
Standard Methods for Examination of Water and Wastewater
The concept of turbidity is the ‘cloudiness’ of water due to light being scattered by small particles suspended within the sample. The particles that create turbidity can be suspended inorganic material (silt, sand), organic material and micro-organisms (phytoplankton, cryptosporidium).
The development of turbidity as a qualitative measure has been promoted both by developments in instrumentation and the production of reference materials that can be used for comparison and calibration. The speed of turbidity measurement makes it an excellent tool for general water quality monitoring, requiring no reagents and capable of being determined visually. The human eye is an excellent turbidity meter, capable of determining to approximately 4 NTU.
Turbidity is measured in a number of applications:
- Drinking water - Measurement of raw water, clarification processes, final drinking water quality are all key points for monitoring. Levels can vary significantly but key values to consider are the WHO recommended limit for drinking water (5 NTU), minimum level capable of visual perception/acceptable level at the customer tap (4 NTU), Chinese standard for ‘challenged’ circumstances (3 NTU), maximum level leaving a modern treatment works (1 NTU), expected level leaving a modern treatment works (0.1 – 0.2 NTU).
- Wastewater - Measurement at all stages of treatment processes but the use of Total Suspended Solids is of more importance here. Surface water monitoring (streams, rivers, lakes, oceans) is also of importance due to the effects of turbidity on aquatic ecosystems. Levels of turbidity are very difficult to quantify due to the extremely variable nature of samples and processes e.g. the Blue Nile annually varies from several thousand NTU down to 6 NTU.
- Process Industries - Filtration equipment monitoring, sedimentation studies and aquaculture environment control utilise turbidity measurement for process control.
Continuing our focus on turbidity, we've been looking at how it applies in two very different applications:
Network turbidity testing - A case study looking at the importance of turbidity for a potable supply network. Read the full article online here.
Turbidity in swimming pool management - A technical article on how and why turbidity is measured as part of managing the water quality of swimming pools. Read the full article online here.