Water Treatment Processes

Study Notes

Approximately 400 people die every day worldwide due to pathogens in unsafe drinking water.
Clean drinking water is crucial for good health.

Water is pumped from the ground or a reservoir and brought to water treatment plants.
Contaminant removal agents are added, including chemical, physical, and biological processes to remove contaminants and pathogens from the water.

Flocculants: to remove suspended bacteria and fungus (e.g., water-soluble polymers).
Neutralizing agents: to balance pH (e.g., sodium hydroxide, calcium carbonate, and hydrochloric acid).
Corrosion inhibitors: to prevent oxidation with metal surfaces (e.g., cations, precipitation inducers, and sulfite).
Coagulants: to remove suspended solids (e.g., aluminum sulfate, ferric sulfate, and alum).
Antifoam: to reduce the amount of foam present (e.g., a blend of oil and silica).
Algaecides: to kill algae (e.g., benzalkonium chloride, copper sulfate, and iron salts).
Disinfectants: to kill microorganisms (e.g., chlorine, ozone, hydrogen peroxide, UV light, silver, copper, and radiation).

Sedimentation: water becomes still, allowing suspended particles to settle to the bottom using gravity.
Filtration: water is passed through a semipermeable barrier of various pore sizes.
Common types of water filters:
- Screens: used at the source to remove debris and large objects.
- Sand filters: slow or fast sand filters move water through beds of sand and gravel to remove suspended solids.
- Membrane filters: water runs through a porous cloth or organic material.
- Reverse osmosis: forces water under pressure through a semipermeable membrane to remove suspended solids, bacteria, ions, chemical contaminants, and viruses.

Biological filters rely on plants, bacteria, or microorganisms to purify the water and remove organic compounds.
Examples of biological filters:
- Slow sand filters: require a large area of layered rock, gravel, and sand, with micro-organisms naturally building up on these layers.
- Aerobic microorganisms: require oxygen to break down organic matter into carbon dioxide.
- Anaerobic microorganisms: do not require oxygen to break down organic matter into methane or carbon dioxide.
- Anoxic microorganisms: use another chemical, other than oxygen, to break down organic matter.

Water treatment systems are used to clean drinking water and process wastewater to prevent the spread of disease.
Types of water treatment systems:
- Central treatment centers: used in developed countries like the United States.
- Point-of-use treatment: used in developing countries, including solar disinfection and sand filters.

Potable water is water that is safe to drink.
In developed countries, potable water is delivered to homes.
In rural areas, people often get their water from wells, which may contain dissolved ions affecting color and taste.
Individual water filters in a house can be placed on a tap or a whole house water line.
Examples of home water purification methods:
- Boiling water before drinking.
- Chlorination: adding bleach to water for disinfection.
- Solar radiation: leaving water bottles in the sun for at least six hours to kill bacteria.

Proper handling of wastewater is crucial in reducing the spread of disease.
Wastewater treatment plants process sewage before returning it to the environment.
Types of wastewater treatment:
- Central wastewater treatment plants: used in developed countries.
- Septic systems: used in rural areas, processing wastewater through a septic tank and drain field.

A water treatment plant is a facility that improves the quality of water to make it free of impurities and contaminants.
There are two main types of water treatment plants:
- Potable water treatment plants: purify water to make it safe to drink.
- Wastewater treatment plants: process sewage before returning it to the environment.
The five main steps to treating water:
1. Coagulation and flocculation: adding chemicals to remove suspended solids.
2. Sedimentation: allowing water to settle in tanks to remove flocs.
3. Filtration: passing water through filters to remove particles, bacteria, viruses, and chemicals.
4. Degasification: mixing air and water to reduce dissolved gases and eliminate odor.
5. Disinfectant: adding a disinfectant to kill any remaining parasites, bacteria, or viruses.
Wastewater treatment involves three steps:
1. Primary treatment: physical treatment to remove solids and suspended matter.
2. Secondary treatment: biological treatment to remove biological components.
3. Tertiary treatment: removing fine grain suspended solids, salts, nitrates, or phosphates.