Introduction to Water Treatment

Introduction to Water Treatment

• Two main reasons for water treatment: remove contaminants harmful to health and improve aesthetic qualities (appearance, color, taste).
• Contaminants affecting taste and odor include hydrogen sulfide gas (rotten egg odor), turbidity (scattering of light), and hardness (calcium and manganese ions).

Most Common Contaminants

• Giardia (6-18 micrometers): excreted by animals.
• Cryptosporidium (4-6 micrometers): also excreted by animals.
• Total Trihalomethanes (TTHM): formed when natural organic substances from decaying vegetation react with chlorine.
• Five Haloacetic Acids (HAA5): also formed from reactions between chlorine and natural organic matter.

Basic Water Treatment Unit Processes

• Biological Activated Carbon: removes contaminants from potable water.
• Oxidation: removes inorganic contaminants like iron, manganese, and arsenic. This process also improves removal of particulates, taste, and odor.
• Chlorine is commonly used in gas, solid, and liquid form. Potassium permanganate is a granular solid, dissolved in water. Factors affecting oxidation speed include pH, oxidant type, and contaminant concentrations. Lower pH or temperature slow oxidation.
• Chlorine use can produce disinfection byproducts like total trihalomethanes (TTH) or Haloacetic acids (HAA5). Ammonia may form chloramines. Chloramines are a weaker oxidant and lead to slower oxidation.

Coagulation

• Destabilizes or decreases stability of colloids (hydrophobic and hydrophilic) in water. Common coagulants include aluminum sulfate, sodium aluminate, ferric sulfate, ferrous sulfate, ferric chloride, polyaluminum chloride, and cationic polymers.
• Dissolved Organic Carbon (DOC): fraction of organic material smaller than 0.45 micrometers.
• Various factors affect coagulation, including pH, turbidity, temperature, and alkalinity. Effective treatment often requires coagulants to be thoroughly dispersed within 1-2 seconds.
• Common types of flash mixers include pump diffusion systems, static inline systems.

Jar Test Apparatus

• Used to determine the correct chemical dosage of coagulants.
• Multiple samples of water, along with various dosages, are used in the process.
• The correct dosage is determined by observing the best flocculation (floc formation). It is best to record pH and turbidity levels.
• Flocculation, disinfection, and sedimentation are additional steps in the treatment process.
• Flocculation involves gently stirring the water. Disinfection controls pathogenic organisms.
• Sedimentation involves removal of solid particles through settling.
• Adsorption is the process of a substance gathering on the surface of a solid.
• Precipitation occurs when a solid substance is formed from a liquid phase.