Perth's Water Sources And Water Treatment PDF

Perth’s Water Sources And Water Treatment Ground Water Groundwater originates from precipitation (rain or snow) that infiltrates the ground, and as water seeps downward, it accumulates in underground reservoirs called aquifers*. The groundwater is stored in aquifers and moves slowly through intercon...

Perth’s Water Sources And Water Treatment Ground Water Groundwater originates from precipitation (rain or snow) that infiltrates the ground, and as water seeps downward, it accumulates in underground reservoirs called aquifers*. The groundwater is stored in aquifers and moves slowly through interconnected spaces within the rock or soil. It flows horizontally and vertically, following the natural gradient. *Aquifers are layers of permeable rock or sediment that can hold and transmit water. They vary in depth and capacity. Some aquifers are shallow, while others are deep underground. Extraction Methods In Perth boreholes are drilled into aquifers to access groundwater and are used to provide water for drinking, irrigation, industrial processes, and other purposes. The bores have steel shafts which have one section which is screened to allow water to enter the borehole with some bores there is enough hydraulic pressure in the aquifer to reach the surface without pumping - this is called artesian flow. Most bores however require pumping (using submersible pumps) to extract the water from the aquifer and send it to treatment. Treatment Methods 1. Coastal water Use air to oxidise manganese and iron then filter → then chlorination and fluoridation Not softened anywhere else Neerabup Lime softening reactors are used before filtration that use upflow crystallisation pellet reactors. In this case lime is added to increase the pH which causes the calcium to want to come out of the solution. The pellets act as a surface for the calcium to crystallise rather than precipitate grow from 0.2 mm → 01mm, Anything that does precipitate gets filtered out and pellets are removed Other water sources blending is used to achieve hardness reduction. →mixes hard and soft water together to create neutral Alkimos 2. Inland groundwater (Lexia, Wanneroo, Mirrabooka water treatment plants) - Inland groundwater comes from bores in swamp and lake areas, and therefore the water must undergo an aeration process to release H2S from the water. - When water is influenced by swamps and lakes; air does not have a high enough oxidising effect to liberate the organically bound iron and manganese, so Chlorine is used instead (ie. It has a higher standard reduction potential). This is the reaction used: Cl2 + 2H₂O ⇔ HCl + HOCl - The HOCl oxidises any iron, manganese and organic matter in the water Flocculation Furthermore, because the iron and manganese are organically bound, coagulation is required, and alum is dosed to destabilise the organically bound iron and manganese and bind it up. Alum provides Al³⁺ ions in solution and these combine with hydroxide ions to form a precipitate of aluminium hydroxide: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) - Aluminium hydroxide is produced in the form of a gelatinous (jelly-like) precipitate called the floc, which oxidises iron and manganese in the water, neutralising their charge which allows the charged particles to bond to one another. Sedimentation - Because significant alum doses are required the solids load is too great for filters to manage so a sedimentation step is required first. - Other fine particles, microorganisms and colours from the water adsorb to the floc and a polymer is added so that it begins to clump together more effectively. - The floc eventually becomes so heavy that it sinks to the bottom of a clarifier in a process of sedimentation. The large clumps of floc are transported to a thickener where some additional water is recovered and then it is sent to sludge drying beds. Meanwhile, liquid coming from the clarifier goes to filters where the pH is adjusted by adding lime in order to push the pH of the water up. (formula) This is raised from a pH 6.3 (ie. the optimal pH for coagulation at the clarifiers) to 7.5 at which the filters can operate better and minimise the presence of dissolved aluminium. Filtration Water is then allowed to filter through a bed of anthracite, fine sand, and gravel. Chlorination Gaseous chlorine is pumped into the water: Cl2 + 2H₂O ⇔ HCl + HOCl ⇔ ClO⁻ + 2H⁺ + Cl⁻ This is done with the main purpose of chlorination being able to kill bacteria and biological contaminants through oxidation properties of the HOCl produced (comes in contact with bacteria and then reacts with the cellular components). The water previously was more alkaline, but this gets reduced during the chlorination process **Some Cl will react with naturally occurring organic matter in the water so a higher dose is required to achieve the target free chlorine residual Fluoridation In Perth and other cities in Australia, fluoride is added to drinking water in a process called fluoridation before it is released from storages. Fluoride in drinking water helps to reduce the incidence of tooth decay. It does this by interacting with tooth enamel (Ca₁₀(PO₄)₆(OH)₂). The fluoride ion replaces the hydroxyl ion in tooth enamel, forming fluorapatite (Ca₁₀(PO₄)₆F₂), which is stronger and more resistant to decay. In Perth, fluoride is added to the water in the form of compounds including, fluorosilicic acid (H₂SiF₆) or sodium fluoride (NaF) in the country; however, sodium hexafluorosilicate (Na,₂SiF₆) is another fluorine compound used elsewhere. Each of these compounds releases fluoride ions when it dissolves in water. The water obtained from these procedures is suitable for drinking and is described as potable. Oxidation loses electrons Reduction gains Iron in organic bound in +2 (ferrous) or 3 (ferric) form when dose chrome, bust up carbon chains which free ions in some cases oxides Fe2+ (happy to give up electrons and therefore unstable) to Fe3 (more stable fully oxidised, given up all electrons easy to lose)) Coastal has lots of Fe2+ → reduced over time due to lack of O2 Fe2 loses one electron to form Fe3 Evaluate reliability It supports the lakes, wetlands, bushland and urban trees that make our city green. Groundwater is also used in local community parks and recreation areas, school grounds, local businesses and 1 in 4 household gardens through bores. It's also under pressure due to decreasing streamflow into dams and increasing demand. Water has been from the coastal superficial aquifers that would have otherwise discharged into the ocean. Use of groundwater from the superficial aquifers has reduced, lessening impacts on some of our wetlands and lakes that are supported by the aquifer. Overuse of groundwater can result in a system out of balance. If garden bores draw water faster than groundwater is recharged by rainfall, groundwater levels can drop. This has a serious impact on Perth’s lakes, wetlands, parks and bushland. Falling groundwater levels can also lead to water quality problems, including acid sulphate soils and saltwater intrusion. Yarragadee aquifer (confined) - the oldest and deepest aquifer that provides a robust supply even in dry years and droughts because of its vast storage and limited connection to the surface environment or environmentally sensitive ecologies Leederville aquifer (confined) - below the Superficial aquifer, and is separated by confining layers which minimises vertical water movement, because it is mostly confined it can withstand short-term drought periods allowing the aquifer to be pumped with few consequences. The Leederville aquifer is often several hundred metres thick, and in some areas it connects with the surface Superficial aquifer. Superficial aquifer is the shallowest aquifer located closer to the surface and often expresses itself as wetlands and for this reason we are now required to reduce abstraction from the aquifer to protect surface ecology ○ Water corporation treats wastewater and injects it into the superficial aquifer to recharge it Groundwater has also taken centre stage in Perth's water mix, accounting for 30-40% of our scheme water. However, even groundwater isn't immune to the effects of dwindling rainfall. Surface Water Surface water refers to any body of water found above the ground, including streams, rivers, lakes, wetlands, reservoirs, and creeks. It encompasses both freshwater and saltwater. Precipitation (rainfall and snowmelt) combined with water runoff from higher elevations are the primary sources of surface water, it flows downhill due to gravity, eventually filling water bodies. Extraction Methods The major surface water source for Perth is Artificial Surface Water, found in man-made structures like dams and constructed wetlands. Perth’s artificial surface water supply is captured during primarily Autumn, Winter and Spring rain flows. The dams have a very large storage capacity in general which mitigates turbidity events and microbiological contamination (settling and ultra violet disinfection from the sun). Dams are not run when they become turbid, they are turned off to allow settling to occur. Treatment Methods Treatment of surface water in Perth is only disinfection (chlorination) and fluoridation with the exception of the Samson Brook Water Treatment Plant which incorporates a filtration process. Chlorination Gaseous chlorine is pumped into the water: Cl2 + 2H₂O ⇔ HCl + HOCl ⇔ ClO⁻ + 2H⁺ + Cl⁻ This is done with the main purpose of chlorination being to kill bacteria and remove biological contaminants through oxidation properties of the HOCl produced (comes in contact with bacteria and then reacts with the cellular components). The water previously was a more alkaline, but this gets reduced during the chlorination process **Some Cl will react with naturally occurring organic matter in the water so a higher dose is required to achieve the target free chlorine residual Fluoridation In Perth and other cities in Australia, fluoride is added to drinking water in a process called fluoridation before it is released from storages. Fluoride in drinking water helps to reduce the incidence of tooth decay. It does this by interacting with tooth enamel (Ca₁₀(PO₄)₆(OH)₂). The fluoride ion replaces the hydroxyl ion in tooth enamel, forming fluorapatite (Ca₁₀(PO₄)₆F₂, which is stronger and more resistant to decay. In Perth, fluoride is added to the water in the form of compounds including, fluorosilicic acid (H₂SiF₆) or sodium fluoride (NaF) in the country; however, sodium hexafluorosilicate (Na,₂SiF₆) is another fluorine compound used elsewhere. Each of these compounds releases fluoride ions when it dissolves in water. The water obtained from these procedures is suitable for drinking and is described as potable. Evaluate the Reliability In the South West corner of Western Australia, climate change is significantly affecting water supply as we face the ongoing challenge of downward trending rainfall. The problem with surface water is that dams don’t really act as bowls that catch rainwater. When the sky opens up, rain first falls onto the natural landscape. The dam catchment areas soak up rainfall like a sponge. Only after the landscape is soaked does rainfall then flow into the dams which is called streamflow. Due to less rainfall over the years, catchment areas absorb most of the water. Therefore dams don't receive the volume of streamflow that they once did, and it doesn’t look like this will improve anytime soon. Winter and spring rainfall is projected to decrease around 15% by 2030. Traditionally, Perth relied heavily on streamflow into our dams as a water source, but these days surface water sources are primarily used as storage reservoirs for our desalinated seawater and groundwater. They provide only 26% of Perth’s drinking water and store this water during periods of low demand so it's available when we need it most, during the hot, dry summer months. Because of this our dam levels now go up and down independent of rainfall patterns. Desalination Seawater desalination is the process of removing salt and impurities from seawater to produce drinking water. It’s a crucial method for obtaining fresh water, especially in areas where other water sources are scarce, as we experience declining rainfall and a growing population. The most common technique used in desalination plants is reverse osmosis, in which semipermeable membranes filter out the dissolved mineral salts from seawater under high pressure, leaving behind clean, potable water. Extraction Methods Extraction starts 500 metres out to sea with two eight-metre high intake towers which have screens to prevent fish and sharks from entering. The intake flow is very slow, which allows fish and marine life to easily swim away from them. At this point, the ocean is 12 metres deep. The seawater flows via gravity through 2.4-metre wide pipes to the wet well, ten metres below sea level. The seawater then passes through mechanical screens that filter out seaweed and other ocean debris (molluscs). Processing Methods - The ultrafiltration process consists of pumps that push the seawater through a series of filters consisting of more than 7000 tiny straws with holes that are 700 times smaller than a human hair. These can reject viruses and bacteria.  - Reverse osmosis or RO requires extremely high pressure (52-57 bar). The seawater is pressurised by very large pumps and energy recovery devices; the latter capture the hydraulic energy from the high pressure RO reject stream (called brine or concentrate). - The energy recovery device makes desalination a cost effective option. Now that the pressure's high enough, the water can pass through the reverse osmosis racks. - There are 16 RO racks in the plant, each with 244 vessels that contain seven membranes. Each membrane is about 440 square feet and consists of layers that are spiral wound. Treatment Methods Potabilisation Following the seawater separation process (reverse osmosis), the product water (permeate) undergoes a process known as potabilisation to increase the pH which stabilises the water (increases the water’s buffering capacity and decreases its corrosivity to metals and cement-lined pipes). The stabilisation process uses lime and carbon dioxide to add calcium and carbon species to the water. The buffering process commences with the addition of CO₂ which leads to the formation of carbonic acid (40%) and bicarbonate (60%). (insert formula) H₂O + CO₂ ⇔ H₂CO₃ ⇔ H⁺ + HCO₃⁻ (diagram equilibrium) Lime is then dosed to increase the pH and change the carbonate equilibria: Ca(OH)₂ + H⁺ + HCO₃⁻ → H₂O + Ca⁺ + CO₃ With a target pH of 8 this leads to a speciation as follows: HCO3- at around >92%, H₂CO₃ ~