Water treatment

Question 1

Water treatment objectives

Answer 1

• Production of a safe and aesthetically water that is protective of public health and in compliance with current water quality standards
• Primary goal of a public or private water utility or purveyor is to provide tretaed water without interruption and at reasonable cost to the consumer.
• PRotection and management of the watershed (cuenca)
• Effective water treatment
• effective management of the water distribution system to ensure water quality at the point of use

Question 2

Characteristics can affect treatability

Answer 2

• Physical characteristics: turbidity and particulates
• Microbial constituents: protozoa , viruses, bacteria
• Organic characteristics: natural organic matter and chlorine demand
• Chemical parameters: pH, alkalinity, hardness
• Inorganic constituents: iron, manganese, arsenic and bromide
• Aesthetic concerns: colour, taste and odour.

Question 3

Conventional water treatment

Answer 3

Conventional treatment trains are typically used to treat surface and groundwater with water quality issues such as high turbidity (typically > 20 NTU), high colour (>20 c.u.), or high TOC (>4 mg/L)

A conventional process treatment train consists of coagulation, flocculation, sedimentation, granular media filtration, and disinfection

Question 4

Classification of Water treatment methods

Answer 4

• Phisical unit operations
• Chemical unit processes
• Biological unit processes

Question 5

Coagulation and Flocculation

Answer 5

The first step involves the addition of a chemical coagulant to react with impurities in the raw water. These impurities typically take the form of dissolved material and suspended particles. The coagulant, when thoroughly mixed with the raw water, reacts with these impurities. Within minutes the suspended material in the raw water begins to group together into small pin-head sized particles called flocs

Question 6

Coagulation

Answer 6

Coagulation is the desalination of colloids by addition of chemicals that neutralize the negative charges.

• the chemicals are known as coagulant, usually cationic salts (Al 3+, Fe3+)
• A rapid mixing occurs to dissolve the chemicals and distribute it evenly throughout the water.

Question 7

Flocculation

Answer 7

Flocculation provide slow mixing and bring the small particles together to form larger particles called flocs.

• rapid mixing is also the start of the flocculation process
• If the mixing is too fast, the flocs will break apart into small particles.
• The particles have neutral charge and can stick together

Question 8

Sedimentation

Answer 8

Within approximately 30 minutes, the water passes from the flocculation tanks though to sedimentation basins where, after several hours, most of the floc material settles to the bottom to form a slude layer. This sluge is removed for further treatment to recover the water and the dried sludge is disposed of appropriately. Following sedimentation, the water now contains only small amounts of impurities and subsequently passes on to the next stage of the treatment process-filtration.

Question 9

Filtration

Answer 9

Rapid gravity filtration is the process of passing water through a bed of material to remove impurities

• to ensure impurities are removed as it is the final step in removing suspended matter.

Question 10

Disinfection

Answer 10

Disinfection is achieved by adding chlorine to the filtered water to destroy any micro-organisms not removed during the sedimentation and filtration stages.

• Chlorination is currently the most frequently used form of disisnfection in the water treatment field.
• Afetr disinfection the water passes to covered storage tanks ready for distribution