Treatment
A number of treatment processes ensure that greater Sydney has a safe and healthy drinking water supply. These processes remove micro-organisms such as pathogens, particulate matter, dissolved metals such as iron and manganese, and other dissolved compounds that cause ‘colour’ in the water.
Prospect water filtration plant
Water treatment involves two main treatment processes along with other supporting processes. The two main processes are:
- removal of particulate matter
- inactivation of microbiological organisms.
The other processes include fluoridisation and adjustments to water pH and hardness where necessary.
Sydney Water operates a water supply system of nearly 21,000 kilometres of water mains, 259 reservoirs, 151 pumping stations and nine water filtration plants.
Sydney Water regularly conducts tests throughout its water supply network to ensure water quality meets the standard required in the National Health and Medical Research Council (NHMRC) Australia Drinking Water Guidelines (2004).
Sydney Water’s operating licence requires compliance with the NHMRC guidelines and any changes that NSW Health makes to these guidelines. Sydney Water’s compliance is audited annually.
Water treatment process
1. Removal of particulate matter
The first step in treating drinking water is removing particulate matter and pathogens through processes including chemical addition and filtration.
One or more ‘coagulants’ are added to the water. The primary coagulant used by Sydney Water is ferric chloride. Sufficient ferric chloride is added to overcome the naturally occurring surface charges of particles in the water and in most cases to also form a ‘floc’. The floc is a ferric hydroxide precipitate. It brings together the neutralised particles into bigger masses, which are then large enough to be trapped and removed by filters. Colour causing compounds are also removed in the floc.
Sydney Water uses a polyelectrolyte (long chain polymers) as a secondary coagulant to assist the ferric chloride in the flocculation process. A small amount of polyelectrolyte is also used to assist filters to remove flocs and particles.
Sometimes the pH of the raw water is adjusted by adding lime or caustic soda to help the coagulation and flocculation before filtration.
Occasionally a pre-oxidant (potassium permanganate or chlorine) is added in small amounts. This oxidises dissolved metals such as iron, manganese and aluminium and/or dissolved organics to a form that can be removed by coagulation and filtration.
Filtration of the coagulated water removes the particulate matter.
2. Inactivation of microbiological organisms
The second major treatment process is disinfection which destroys micro-organisms including pathogens. Disinfection also leaves a residual that protects treated water from recontamination as it travels from the water filtration plant through the distribution system to consumers’ taps.
Sydney Water uses chlorine for disinfection. Various forms of chlorine are used including chlorine gas, liquid sodium hypochlorite, and calcium hypochlorite tablets.
Sufficient chlorine is added to filtered water to ensure effective primary disinfection and enough chlorine residual for the distribution systems. Sydney Water’s distribution systems are extensive, therefore extra chlorine may be added at different times and locations throughout the system. This is necessary to ensure that an effective disinfection residual reaches consumers’ taps.
In some parts of Sydney, ammonia is added after the primary disinfection process. This is done in a fixed ratio to form a less reactive disinfectant called monochloramine. The monochloramine residual persists further into the distribution system because it is less reactive than chlorine.
Other processes
Sydney Water adds fluoride in the form of sodium silicofluoride or hydrofluorosilicic acid to achieve a fluoride content of one milligram per litre. This is done in accordance with the Fluoridation of Public Water Supplies Act (1957).
Lime and carbon dioxide are added at some water filtration plants, where the water is very soft, to adjust and buffer the pH of the treated water. The carbon dioxide reacts with lime to form calcium bicarbonate, which buffers the water (increasing resistance to change in pH), increases hardness and reduces the general corrosivity of the water.
Sodium hydroxide or sulphuric acid may also be used to adjust the pH of drinking water if required.
More information is available from the Sydney Water website at www.sydneywater.com.au/Publications/ (refer to ‘reports’ and ‘typical water analysis’) and at www.sydneywater.com.au/EnsuringtheFuture/WaterSchool/EducationResources/secondary.cfm (refer to ‘information and fact sheets’)
Research task
A research task on water quality treatment is provided to improve your understanding.
