The Catchment Area:
Sydney's water supply is made of 5 main catchments: Warragamba, Woronara, Hawkesbury Nepean, Shoalhaven and Blue Mountains. Warragamba Catchment covers 9,051km². My local area is Blacktown, in an image shown below; all the catchment areas are linked to Warragamba Catchment Area.
Possible Sources of Contamination:
Water quality in Warragamba catchment is affected by:
* Wastes and runoff from industry and urban development
* Poor sewage and storm water management
* Agricultural activities
* Chemical fuel spills
* Uncontrolled bush ﬁres
* Fallen Tree Branches
Chemical tests available to determine levels and types of contaminants:
Turbidity test is used to indicate microbial contamination such as algal growth and faecal contamination, and to detect insoluble solids such as sludge suspended in the water.
Dissolved oxygen (DO) is used to determine possible increase in organic wastes. Low DO indicates activity of aerobic microbes which use up the oxygen.
BOD is a measure of organic matter in water. High BOD is due to paper and food processing industries.
AAS is used by adding Na2S to test quantitatively for heavy metals and metal ions.
Physical and Chemical Processes used to purify water:
Screening: Water from the dam is screened by removing large contaminants. E.g.: Fallen Trees.
Sedimentation: Allowing floc particles to settle out to form a sludge, which can be removed from settling tanks periodically.
Filtration: Water from the settling tanks is piped to filtration tanks to be filtered. The water at this stage should be clear.
Aeration: The screened water is then sprayed into the air to increase oxygen levels so that iron salt becomes oxidised and forms insoluble salts oxides which can be removed.
Flocculation: Removes colloidal impurities. Alum (Aluminium Sulfate) is a chemical added to cause the impurities within the water to stick/thicken together forming a precipitate or floc of Al(OH)3 which would be large enough to be filtered out.
Al3+(aq)+ 3H2O(l) →Al(OH)3 (s)+ 3H+(aq)
All other compounds and iron oxides adhere to the flocs.
Chlorination: The filtered water is then disinfected with chlorine gas to kill microorganisms such as bacteria.
UV light: Emitted to kill bacteria.
pH adjustment: Buffering chemicals such as carbonates are added, to achieve the required pH between 7 and 8.5.
Fluoridation: Fluoride is added to help prevent tooth decay.
Chemical Additives in the water and the reasons for the presence of these additives:
* Chlorine added for disinfectant purposes as it destroys bacteria and some viruses (1-2 ppm). If the starting water was reasonably clean and if the right amount of chlorine is used, there is no odour when it reaches the households. * Fluorine (sodium ﬂuoride) added to the water after it has passed the chlorination plant. Fluoride is added (1ppm) to strengthen tooth enamel & to help prevent tooth decay. * If needed, lime water, sodium carbonate is added to lower the acidity of the water. * Sulfuric acid is added to break down organic wastes and aids coagulation.
Describe and assess the effectiveness of methods used to purify and sanitise mass water supplies
Solid objects are removed by screening devices. Water treatment consists of removing very small particles by flocculation. The water is sand filtered to remove bigger particles. A 2m deep bed of sand is used. Finally, it is treated with chlorine to kill microorganisms as disinfection is needed to ensure concentrations are acceptably low. Water testing is carried out for ions, colour, pH, hardness, turbidity, conductivity, micro-organisms. Alternative methods of filtration such as membrane filtration may be both more efficient in...