Potable water, treatment and purification
In a nutshell
The Earth's water resources and treatment must be understood. This summary explores different types of resources, treatment to produce potable and pure water and finally waste water treatment.
Potable water
Definition
Water that is safe for human consumption. Water can be naturally potable or it can be treated. Potable water must have a pH between 6.5 and 8.5, it must not have any pathogens in it and it must not have a high level of dissolved salts.
Water resources
The main water resources are described below.
SOURCE | EXAMPLES | CONTAMINANTS | TREATMENT |
| Freshwater
| Surface water from reservoirs, lakes, rivers
| Leaves, insoluble solids, microbes
| Filtration, sedimentation and chlorination
|
| Sea water
| Sea | Salt, microbes
| Simple distillation (desalination)
|
| Waste water | Bath water, agricultural systems, industrial processes
| Organic matter, microbes | Screening, sedimentation, effluent collection, sludge removal, toxic substance removal. |
Water treatment
Freshwater treatment
Procedure
| 1.
| Sedimentation
| Large insoluble contaminants sink to the bottom of the tank as the flow of the water is slowed down.
|
| 2. | Filtration
| Small insoluble contaminants are removed by filtration through sand beads.
|
| 3. | Chlorination
| Chlorine gas is passed through the water to kill microbes.
|
| 4. | Potable water is produced
| Ready for consumption.
|
Seawater treatment (simple distillation/desalination)
PROCEDURE
| 1.
| Boiling of filtered sea water.
|
| 2. | Cooling of water vapour.
|
| 3. | Condensation to form distilled water.
|
| 4. | Pure water is produced.
|
Seawater can also be treated by using membranes (like reverse osmosis).
Differences between the two treatment types
Freshwater treatment produces potable water but seawater treatment produces pure water. This is because the product of freshwater treatment still contains dissolved salts and chlorine, whereas the product of seawater treatment does not. However, seawater treatment is significantly more expensive.
Tap water
Tap water still contains impurities. These are shown in the table below.
Impurity
| Description |
| Nitrates | Fertilisers can leach into waterways and this can increase the concentration of nitrates in the water. |
| Lead | Lead pipes are a common source of lead in water. This is poisonous. |
| Pesticides | Like fertilisers, pesticides can also leach into waterways and contaminate them. |
Waste water treatment
Waste water comes from many things such as bath water, agricultural systems and industrial processes. Sewage must be treated to remove organic matter and harmful microbes before it can be returned to freshwater sources. Treating waste water is more efficient than desalinating salt water.
Sewage water treatment
Waste water from sewage plants can be treated using the method described below.
PROCEDURE
| 1. | Screening
| To remove large contaminants like twigs.
|
| 2. | Sedimentation
| To remove heavy suspended solids. The lighter liquid waste floats on top. |
| 3. | Collection of the effluent
| To treat by biological aerobic digestion, causing bacteria to break down organic matter and microbes. This is then returned to freshwater supplies.
|
| 4. | Removal of sludge from the bottom of the sedimentation tank
| To treat by biological anaerobic digestion, causing breakdown of organic matter. Methane gas is released in the process, which is then used as an energy source. The waste that remains after treatment can be used in fertilisers.
|
| 5. | Additional stages such as UV or chemicals may be required
| To remove toxic substances.
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