- Objective 1
- Objective 2
Drinking Water Treatment
The process of water treatment to provide safe drinking water for their communities. There are various combinations and types of processes in water treatment depending on the scale of the plant and quality of the raw (source) water. Today, the most common steps in water treatment used by community water systems (mainly surface water treatment).
Coagulation and Flocculation
Before these processes, raw water is run through a screen to separate visible solids from the liquid. Next, Aluminum sulfateAl2(SO4)3 and Calcium hydroxide Ca(OH)2 are added to the liquid. The aluminum based compound is often called alum and induces clumping of suspended particules. This process is called flocculation. Calcium hydroxide (also known as lime) raises the pH of the mixture and produces more charged species (electrolytes) on the suspended particles. These charges are extracted and increase the size of the solid (coagulate) which simplify the filtration process.
The slightly basic water causes Al(OH)3, Fe(OH)3 and Fe(OH)2 to precipitate, bringing the small particulates with them and the water becomes clear. Some records have been found that Egyptians and Romans used these techniques as early as 2000 BC.
Suspension of iron oxide particulates and humic organic matter in water gives water the yellow muddy appearance. Both iron oxide particulates and organic matter can be removed from coagulation and flocculation. The description given here is oversimplified, and many more techniques have been applied in the treatment of water. Coagulation is a major application of lime in the treatment of wastewater.
Other salts such as iron sulfates Fe2(SO4)3 and FeSO4, chromium sulfate Cr2(SO4)3, and some special polymers are also useful. Other ions such as sodium, chloride, calcium, magnesium, and potassium also affect the coagulation process. So do temperature, pH, and concentration.
During sedimentation, floc settles to the bottom of the water supply, due to its weight. This settling process is called sedimentation. The floc particles are then removed from the bottom of the basins.
Bringing air into intimate contact with water for the purpose of exchanging certain components between the two phases is called aeration. Oxygenation is one of the purposes of aeration. Others are removal of volatile organic substances, hydrogen sulfide, ammonia, and volatile organic compounds
Once the floc has settled to the bottom of the water supply, the clear water on top will pass through filters of varying compositions (sand, gravel, and charcoal) and pore sizes, in order to remove dissolved particles, such as dust, parasites, bacteria, viruses, and chemicals.
After the water has been filtered, a chemical disinfectant (chlorine and/or ozone based) or ultraviolet radiation will be applied in order to kill any remaining parasites, bacteria, and viruses, and to protect the water from germs when it is piped to homes and businesses.
This process is not mandatory for water processing plants in the United States. Around 72% of public water systems have this chemical added in order to reduce dental issues.
Content taken and edited from: chem.libretexts.org/Bookshelves/Introductory_Chemistry/Map%3A_Chemistry_for_Changing_Times_(Hill_and_McCreary)/14%3A_Water/14.6%3A_Making_Water_Fit_to_Drink
Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)