Environmental Chemistry: Water
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Water is one of the most important molecules on Earth. We need it for survival, washing, industrial needs, drinking, electricity... etc. In this section we study the importance of water, including its solubility, treatment, pollution, composition and its role in sewage treatment. [1]
Contents |
Hardness of Water
- Hard water is water which will not readily form lather with soap due to the presence of dissolved calcium or magnesium salts in the water.[2]
- Permanent hardness is caused by the presence of calcium sulfate, CaSO4, or magnesium sulfate, MgSO4, in the water.
Methods of Removing Hardness
- Washing soda
- Ion exchange resins
- Cation exchange resin
- Mixed bed resin
- Distillation
Water Treatment
- Screening
- Flocculation
The process in which small particles in water are made to coagulate (clump together) and form a precipitate is known as flocculation. It is done by adding a flocculating agent such as aluminium sulphate Al2(SO4)3.
- Settlement (Sedimentation)
- Filtration
- Chlorination
- Fluoridation
- pH adjustment [3]
Water Pollution
Dissolved Oxygen
Dissolved oxygen in water is vital for the survival of fish life. Approximately 10ppm O2 dissolve in water and fish need at least 5ppm to survive. The trouble arises in the conversion of food and oxygen to CO2 and H2O by bacteria. If too much organic matter is in the water, bacteria will use dissolved oxygen to break it down and there is a chance that air will not be replenished from the atmosphere quick enough. If the dissolved oxygen drops to almost 0ppm, anaerobic bacteria dominate and the river will smell due to H2S being produced. [4]
- Biochemical oxygen demand is a measure of the amount of organic pollution in water. It is defined as the amount of dissolved oxygen consumed by biochemical action, when a sample of water is kept in the dark at 20 C for five days.
- The B.O.D. is the difference in the levels of dissolved oxygen between two samples of water, with one tested immediately and the other subjected to the aforementioned conditions.
- A high value indicates a large amount of organic matter.
Eutrophication
- Eutrophication also consumes dissolved oxygen in the water.
- Eutrophication is the enrichment of natural waters by nutrients (nitrates and phosphates in particular). It causes the very rapid growth of algae which, when they die and decay, consume large amounts of oxygen. The water is then depleted of oxygen, leading to the death of fish and aquatic life. [5]
Heavy Metal Pollution
Water pollution is also caused by the release of toxic ions into the water. Cations such as Pb2+, Hg2+ and Cd2+ are found in water supplies due to illegal dumping and are extremely dangerous to the human body. The EU has set limits [6] for some of these chemical species in water.
Sewage Treatment
- Primary Treatment
- Screening
- Sedimentation
- Secondary Treatment
- Activated Sludge Process (aeration tank and settling tank)
- Tertiary Treatment
- Removal of phosphorus and nitrogen compounds [7]
Analysis of Water
- pH Meter
- Colorimetry
- Atomic Absorption Spectrometry
Mandatory Experiments
- To determine the total hardness in a water sample
- To determine (a) the total suspended solids of a sample of water by filtration, (b) the total dissolved solids by evaporation and (c) the pH
- To measure the amount of dissolved oxygen in a sample of water by means of a redox titration
- To estimate the concentration of free chlorine in swimming-pool water or bleach using (a) a comparator or (b) a colorimeter [8]
References
- ↑ http://en.wikipedia.org/wiki/Water
- ↑ http://en.wikipedia.org/wiki/Water_hardness
- ↑ http://www.excelwater.com/eng/b2c/about_8.php
- ↑ http://www.k12science.org/curriculum/dipproj2/en/fieldbook/bod.shtml
- ↑ http://en.wikipedia.org/wiki/Water_pollution#Causes_of_water_pollution
- ↑ http://ec.europa.eu/environment/water/water-dangersub/76_464.htm
- ↑ http://www.euwfd.com/html/sewage_treatment.html
- ↑ http://chemistry.slss.ie/ch_mandatoryexperiments.html
