Dunedin's water treatment plants use a variety of treatment methods to make the water clear and safe to drink.
Relatively coarse screens are placed at the intake to the treatment plant. They remove large objects such as sticks, leaves and stones that have come through the supply pipes. At Mt Grand and Southern Water Treatment Plants, parallel plate separators and a rotary inlet screen respectively are used to reduce solids in the water.
Coagulation and flocculation
Water always contains very small lightweight particles of silt, algae and micro-organisms, which can cause colour, taste, smell and health problems. The addition of chemicals in tiny accurately controlled doses assists in removing most of these tiny particles.
- Coagulation involves the rapid mixing of a chemical (such as Aluminium Sulphate) into the water.
- Flocculation happens because the small particles are attracted to the chemical to take larger particles that can be more easily filtered out of the water.
After flocculation, the particles are clumped together and look like little bits of jelly, called flocs. The flocs are removed from the water through clarification and filtration or directly through filtration. Highly specialised machinery is needed to control this process, as it requires particular pH, alkalinity, coagulant concentration and mixing energies, which vary considerably between different types of water.
Clarification (or separation)
This is done to remove the main solids in the water prior to filtration. Methods used at our water treatment plants are:
- Dissolved air flotation (DAF) at Mt Grand. The DAF unit injects tiny air bubbles into flocculated water, which causes the floc to float to the top of the rectangular tanks. The concentrated floc is scraped off the top using mechanical brushes.
- Upflow sedimentation tank at Port Chalmers.
Filters are used to remove the fine particles and flocs remaining in the water. There are two basic types of filters used at our treatment plants:
- Rapid sand filters: Water passes through sand or alternative medium (can drain through gravity or be pumped under pressure), and particles are trapped within the medium.
- Membrane filtration: Water passes through the walls of tubes of membranes and leaving the dirt to remain on the upstream side of the membrane. At Southern Water Treatment Plant, the membranes are called submerged type membranes and water is sucked through from the outside to the inside of the tube.
It is necessary to clean all filters regularly as they clog up. Backwashing is done by passing water through the filters in the opposite direction to dislodge the trapped particles. Most treatment plants have a number of filters so backwashing can be done on one filter at a time without interrupting the water supply.
Fluoride is added to the water to reduce tooth decay. The decision to add fluoride to the water is based on recommendations from the Ministry of Health. The council adds fluoride to water at a target dose rate of 0.75 mg/l in order to achieve dosing at the lower end of the Ministry of Health’s recommended range of 0.70 to 1.0 mg/l.
The water treated by the two main plants (Mt Grand and Southern) is fluoridated, for the areas serviced please downloaded the map below.
The purpose of disinfection is to make sure that all of the harmful organisms in the water are killed. We use:
- Chlorination: is achieved by dosing chlorine into the water following filtration. In Dunedin, chlorine gas or liquid Sodium hypochlorite are used. To ensure the chlorine has had time to kill the bugs, the dosed water is passed through storage tanks (contact tanks) before entering the reservoirs.
- After the holding tanks, the water goes on to the distribution system. The advantage of using chlorine is that there is always a little bit of chlorine left in the water after it has left the treatment plant. This means that if any bugs get into the water through leaks in the distribution system, there is still some chlorine left to kill them.
- Ultraviolet disinfection: ultraviolet light kills bugs in the water by altering their DNA structure. Filtered water is passed through special reactors surrounded by lamps.
pH and alkalinity control
Prior to the distribution of water to consumers, the water is dosed with lime and carbon dioxide to adjust the pH and alkalinity. This prevents the water corroding the inside of the pipe system so the water stays as clean as possible when travelling to your tap.
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