Determining Water Quality

Monitoring and Management‎ > ‎5. Water‎ > ‎

Identify that water quality can be determined by considering:

  • concentrations of common ions
  • total dissolved solids
  • hardness
  • turbidity
  • acidity
  • dissolved oxygen and biochemical oxygen demand  


Perform first-hand investigations to use qualitative and quantitative tests to analyse and compare the quality of water samples

In describing water quality, the following properties are commonly used:

  • Concentrations of Common Ions (generally measured in ppm).
  • Total dissolved solids (TDS): A measure the mass of solids dissolved in a unit volume of water (generally measured in ppm).
  • Hardness: A measure of the concentration of calcium and magnesium ions.
  • Turbidity: A measure of the amount of colloidal suspended matter, which translates into a degree of cloudiness or transparency (measured in relative units called nephelometric turbidity units, or NTUs).
  • Acidity: A measure of the concentration of hydrogen ions (measured in pH).
  • Dissolved oxygen (DO): A measure of the concentration of oxygen (generally measured in ppm).
  • Biochemical oxygen demand (BOD): A measure of the concentration of dissolved oxygen that is needed for the complete breakdown of the organic matter in a sample by aerobic bacteria (generally measured in ppm).

 

Water property
Testing procedure(s) and/or equipment
Other information
Concentrations of common ions
  • Test with ion sensative electrodes (ISEs).
  • For metal ions, atomic absorption spectroscopy (AAS).
  • For anions, gravimetric analysis.
Common guidelines for potable water include:

  • [ Fe2+] less than 0.3 ppm
  • [ Al3+] less than 0.2 ppm
  • [ Mn2+] less than 0.1 ppm
  • [ F–]  less than 1.7 ppm
Total dissolved solids (TDS)
  • Test with conductivity meter and comparison to known standards
  • Evaporation of a known volume of filtered water to dryness, and weighing
  • 0-500 ppm is considered fresh water and is potable
  • Seawater has 35 000 ppm
  • Sydney’s water supply has 40-100 ppm
Hardness
  • Titration with a complexing agent (EDTA), which binds to cations, and indicator
  • Result expressed as the equivalent amount of dissolved calcium carbonate (Ca CO3)
  • Less than 75 ppm is considered soft
  • 150-500 ppm is considered hard
  • Sydney’s water supply has 10-40 ppm
Turbidity
  • Test with turbidity or secchi disk
  • Filtration, drying and weighing of suspended solids
  • Turbidity meter, which measures scattering of light
  • 1-5 NTU is potable
  • Less than 25 NTU is suitable for water life
Acidity
  • Test with pH meter
  • Test with universal pH paper
  • A pH of 6.5-8.5 is potable
Dissolved oxygen (DO)
  • Test with thallium oxygen-sensitive electrode
  • Back titration involving manganese ions (known as Winkler titration)
  • High quality fresh water has about 9 ppm
  • Polluted water has less than 4 ppm
Biochemical oxygen demand (BOD)Collection of two identical samples:

  • Winkler titration immediately on first sample.
  • Winkler titration after five days in dark on second sample.
  • Difference in DO is BOD
  • Unpolluted natural waterway has 1-5 ppm
  • Polluted natural waterway has more than 5 ppm
  • Treated sewerage has 20-30 ppm
  • Raw sewerage has 150-300 ppm