Water and Wastewater Engineering : Water Quality

Water and Wastewater Engineering

Water Quality Introduction

The topic of water quality focuses on the presence of foreign substances in water and their effect on people or the aquatic environment. Water of good quality for one purpose may be considered to be of poor quality for some other uses. For example water suitable for swimming may not be of good enough for drinking, But even drinking water may not be suitable for certain industrial purposes that require pure water. However it Is necessary to determine what substance are in the water and In what concentrations they are present. Finally, some standards against which to compare the result of the water quality analysis and thereby judge the suitability of the water for a particular use are needed. .

Water has a remarkable tendency to dissolve other substances. Because of this it Is rarely found in the nature in a pure condition. Even water In a mountain stream, far from civilization, contains some natural impurities in solution and suspension. The changes in water quality begin with precipitation. As rain falls through the atmosphere it picks up. dust particles and such gases as oxygen and carbon-dioxide. Surface runoff picks up silt particles, bacteria, organic materials and dissolved minerals. Ground water usually contains more dissolved minerals than surface water because of its longer contact with soil and rock. Finally water quality is very much affected by human activities including land use (agriculture) and the direct discharges of industrial or municipal waste water to the environment.

Impurities in the Natural Water

• Suspended particles
• Colloidal particles
• DIssolved particles & gases

Suspended Particles (settleable)

Size is 1 pm —200 pm. These are large enough to be removed by filtration or sedimentation. Suspended impurities are bacteria, algae (These are small weeds like plants growing in water which is exposed to sunlight), fungi (These are small plants growing in water in.the absence of sunlight), clay, silt, & sand.

ColloIdal Particles (fine particles)

Size is 0.001 µm - 1 µm. Clay. Silica are examples. These are not possible to be removed by ordinary
sedimentation. These impurities remain in continuous movements as these are electrically charged due to presence of absorbed iron on the surface of the soil particles.

Dissolved Particles & Gases .

Size is less than 0.001 pm. These Impurities mainly contain organic compounds, salts (Calcium/Magnesium/Sodium Carbonates/Sulfates/ Chlorides etc.) and gases (Oxygen, Carbon Dioxide, Hydrogen Sulfide, Nitrogen etc.).

Characteristics of Water

There are three broad type of characteristics needed In analysis of the quality of water required for water supply.

They are

• Physical characteristics
• Chemical characteristics
• BiologIcal characteristics