Turbidity Measurement
Turbidity is used a lot when talking about water quality and the performance of water treatment plants (WTP). But what is turbidity and what do the results mean?
Simply put turbidity is the amount of light scattered by water. Light passing through water is scattered by suspended and colloidal matter.
Turbidity measurements are made using electronic nephelometers, which report results as nephelometric turbidity units (NTU). These instruments use light detectors at a right-angle (90O) to a light source to measure the amount of scattered light. These can either be online (measures continuously) or bench top instruments (grab sample measurement).
As a guide:
<1 NTU water looks crystal clear
>5 NTU water in a glass looks slightly muddy or milky
>60 NTU and the water is not possible to see through
Why Measure It?
In terms of water treatment, turbidity only informs operators of the amount of suspended and colloidal matter in the water. This measurement alone does not imply that water is safe or unsafe for human consumption. Results must be understood and interpreted correctly to provide these types of insights. The location of where the sample are taken in the treatment train and an understanding of the water treatment process are vitally important to understanding what the results mean.
Raw Water
There are no set levels of turbidity that a WTP can cope with, this differs from WTP to WTP. A particular plant will have a maximum turbidity that it can treat successfully (i.e. produce the desired water quality). As turbidity is monitored operationally to identify changes in raw water turbidity. WTP coagulation processes usually need to be adjusted if there is a change in water quality, therefore rate of change is vitally important. Where rapid changes in source water can occur, turbidity should be monitored more often to ensure WTP chemical dosing is optimised.
Settled Water
It is important to determined the level of performance of the coagulation/flocculation/settling processes prior to filtration. If there is an increase in turbidity there is potential problem with the upstream process that needs to be investigated. Every WTP is different but ideally settled water should have a turbidity low enough not to overload the filters.
Filtered Water
Turbidity is measured post filtration to determine the performance of the filtration process. Filters play an important role in the removal of suspended matter for aesthetic purposes, removal of large filterable pathogens (e.g. protozoa, which are resistant to chlorine) and pathogens associated with suspended matter and to provide particulate free water required for disinfection.
Disinfection Process
Disinfection process generally require low turbidity to ensure efficacy. Particles in the water can hide pathogens, use up available chlorine and form disinfection by-products. In some process trains chemicals such as lime are added, which could increase turbidity after filtration, and need to be taken into consideration.
Supplied to the Custom
Measured for aesthetic purposes (e.g. the look/smell/taste of the water).
Turbidity Limits
The following limits specified in the Australian Drinking Water Guidelines (ADWG) for some of the above scenarios:
Aesthetic purposes:
<5 NTU
Measured at the point of supply
Removal of protozoa (required if protozoa is a hazard in the raw water)
Never above 0.5 NTU
Target < 0.2 NTU
Measured on individual filters
Chlorine disinfection:
< 1 NTU
Measured at a representative point for disinfection