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 like sand, organic material, and micro-organisms. 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 can determine up to approximately 4 NTU (Nephelometric Turbidity Units). 

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 because of turbidity on aquatic ecosystems. Levels of turbidity are 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.

Regulatory bodies such as the United States Environmental Protection Agency (EPA) and the International Organization for Standardization (ISO) provide standardized methods for measuring turbidity. These methods are widely accepted and used for regulatory compliance and environmental monitoring. Here are some of the key methods:

EPA Method 180.1: Determination of Turbidity by a Nephelometric Method

This method outlines procedures for the nephelometric measurement of turbidity in water. It specifies the use of a turbidimeter and provides details on calibration, measurement, and quality control.

ISO 7027: Water quality – Determination of turbidity

ISO 7027 is an international standard that provides a method for the determination of turbidity using nephelometric measurements. The standard defines units such as Nephelometric Turbidity Units (NTU) and Formazin Nephelometric Units (FNU). 

EPA Method 2130B: Turbidity, Inorganic (Silica)

This method is specifically designed for the determination of inorganic turbidity (silica turbidity) in water. It involves the use of a spectrophotometer and is applicable to clear, colourless solutions.

When conducting turbidity measurements for regulatory compliance, it is essential to adhere to the specific methods recommended by the relevant authority.

In summary, measuring turbidity of water plays a key role ensuring safe drinking water, in safeguarding environmental bodies like lakes, rivers and supporting various water-related activities.