TEST METHOD: TURBIDITY

Revised 25 Junw 2007 (Palin reference)

RISK ASSESSMENT

Assess Safety, Health, Quality and Environmental aspects of each specific step.

OVERVIEW

Turbidity is a measure of brine/water clarity and reflects the biomass (and therefore nutrient level) or disturbance of the brine or water. Turbidity is measured in the field with a secchi disk, or in the laboratory using a spectrophotometer or LED nephelometer.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory in the field or lab.

SPECIFIC JOB STEPS RISK ASSESSMENT RISK CONTROL
Method One

A secchi disk may be used in the field to measure the relative turbidity of brine or water. A secchi disk is a circular plate of metal with a diameter of 30-40 cm. The surface is painted in 4 segments, alternating black and white. The secchi disk is suspended from its centre point by a length of rope.

   
The secchi disk is lowered into the brine by the rope until the quadrats can no longer be distinguished. Surface is chopped by wind action. Choose a calm day for sampling.
The distance between the surface of the secchi disk and the surface of the brine is recorded in centimetres.    
Method Two
Spectrophotometer - a laboratory method

The spectrophotometer is prepared by turning it on.

   
The sample cell is rinsed with sample, then filled with sample, carefully wiped with a tissue, and placed in the spectrophotometer. Sample may settle prior to return to laboratory Gently agitate the entire sample bottle prior to filling the sample cell.
Readings of Absorbance are taken at 425nm and 580nm.    
The readings are plotted on the Nomogram chart (Palin) to determine colour in Hazens or Colour units, and Turbidity in NTU's. The chart is derived from AT Palin (1955) "Photometric determination of the colour and turbidity of water." Water & Water Engineering, 59:341-345    
Method Three

A calibrated turbidity meter that measures the ratio of transmitted to scattered light using a light emitting diode may be used whether in the field or the laboratory.

   
Turn the meter on and immerse the probe fully in the sample. Record the readings. Inherent uncertainties are present in all methods of turbidity measurement. Only report to the closeness specified in the table below (APHA, Standard Methods for the Examination of Water and Wastewater).

 

Turbidity Reading

Report to Nearest NTU

0-1.0

0.05

1-10

0.1

10-40

1

40-100

5

100-400

10

400-1000

50

> 1000

100

   

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