How do we get clean water?
After knowing more about what water is, we come to an aspect of water which is important to all of us, yet not known enough about. Every day, we use water for various reasons. How does our water become clean and safe enough for usage? Having a better understanding of this can help us develop a greater appreciation for the water we use.
Water from Local Catchment
Singapore is a relatively small country and does not have natural water resources. The country collects rainwater that falls on two thirds of the land mass. It is being transported to the 17 reservoirs by rivers, canals and drains. With the new Marina, Punggol and Serangoon Reservoirs, the water catchment area has increased from half to the current two third. In the near future, there are plans to increase it to 90% of the land mass.
Figure 3.1. Singapore water catchment distribution (Public Utilities Board, 2017).
Imported Water
Singapore imports water from her neighbouring country, Malaysia. The water is transported from the Linggiu Reservoir in the State of Johor across the causeway to Singapore. The current agreements in place, signed in 1962 and 2000, will end in 2061. The clauses include limited volume of water to be brought into the country per day. Singapore has been finding ways to reduce the dependency of imported water through other alternative like NEWater and desalinated water. There are recent reports that Linggiu Reservoir is drying out in 2017.
Figure 3.2. Imported water in Singapore (Public Utilities Board, 2016).
Water Treatment
Raw water from various sources, such as local catchment area, is transported to the waterworks where it is chemically treated, filtered and disinfected. Harmful bacteria and suspended particulate matters as small as microns are removed, making the water clear, odourless, colourless, and safe for consumption.
The coagulation and flocculation method is preliminarily used to allow sedimentation to occur before easily filtering them out. Aluminium sulphate is the main coagulant that is mixed into the raw water. The coagulant neutralises the fine particles’ charges and allow them to clump together. These particles are not large enough to be seen by the naked eye. Thus, the process of slow mixing called flocculation is needed. The particles collide into one another to form larger clumps. Hydrated lime is added to adjust the pH level for best flocculation results. A coagulant aid like polyelectrolyte is used to bind and strengthen the clumps, and improve settling rates.
Next, the unwanted particles are allowed to settle by gravity in the sedimentation tank to be removed. Finer particles of suspended matter are further removed by sand and membrane filtration.
The last stage of treatment is to disinfect the water by adding chlorine to destroy any bacteria or viruses. To remove any odours, activated carbon is mixed in. Before the water reaches the clear water tanks, sodium silicofluoride is added from the filters. Water fluoridation (addition of sodium silicofluoride) helps to prevent tooth decay, which is a chronic disease in industrialised countries like Singapore.
Once the water treatment process is complete, this water is sent to households and industries for domestic and nondomestic uses.
Figure 3.3. Water treatment process (Public Utilities Board, 2016).
NEWater
Used water is collected from households and industries and sent via pipelines to water reclamation plants for the following treatment:
- Debris is removed physically using mechanical screens. Sandy materials are settled and removed as well.
- Smaller solid pollutants are then allowed to settle in another tank as primary sludge. Lighter materials such as grease float instead, and are removed together with primary sludge.
- The remaining water is sent to aeration tanks containing microbes that help absorb and break down organic pollutants. The microbes are then settled to the bottom of a tank and then removed. Water at this stage can then be used for other purposes.
Part of this treated water is then further treated at NEWater plants to produce even cleaner water (for potable and non-potable use) via three steps:
- Microfiltration
Treated used water is filtered using membranes. Small particles as well as some bacteria are retained in the filter and thus removed from the water.
2. Reverse osmosis
The water is then forced through membranes with even smaller pores. At this stage, water is free from most disease-causing microorganisms and already almost pure.
3. Ultraviolet (UV) Disinfection
A safe-guard process where UV rays are used to eliminate disease-causing microorganisms, if any left, present in the water.
Figure 3.4. How NEWater is produced (Public Utilities Board, 2017).
NEWater currently meets 40% of total water demand in Singapore, and is projected to meet 55% by 2060.
Desalination
Seawater is directed into desalination plants and chlorine is added to kill off bacteria. The water is then treated in the same manner as described in ‘Water Treatment’ above, with the addition of the Reverse Osmosis step between sedimentation and addition of chlorine/fluorine.
Figure 3.5. Desalination process (Public Utilities Board, 2017).
Other means of managing water use
Desalination consumes a lot of energy, and will consume even more in the future as Singapore plans to increase desalination capacity in time to come. Efforts are underway to utilise up-and-coming research and technology that utilise less energy, such as biomimicry where certain biological processes are mimicked in technology to purify water. Such technology with low energy requirements could potentially help supplement current methods and thus help reduce energy usage in producing freshwater.
Also, seawater could be used instead of fresh water to cool factories close to sea, to reduce the usage of fresh water.
References
Coagulation-Flocculation. (n.d.). Retrieved March 19, 2017, from http://www.sswm.info/content/coagulation-flocculation
Ministry of Health Singapore. (n.d.). Retrieved March 19, 2017, from https://www.moh.gov.sg/content/moh_web/home/pressRoom/Parliamentary_QA/2010/Water_fluoridation_to_prevent_dental_caries.html
P. (n.d.). PUB, Singapore’s National Water Agency. Retrieved March 19, 2017, from https://www.pub.gov.sg/
P. (2016, November 28). PUB, Singapore’s National Water Agency. Retrieved March 16, 2017, from https://www.pub.gov.sg/usedwater/treatment/usedwatertreatmentprocess
P. (2016, October 19). PUB, Singapore’s National Water Agency. Retrieved March 17, 2017, from https://www.pub.gov.sg/research/industrialwatersolutions
P. (2017, January 19). PUB, Singapore’s National Water Agency. Retrieved March 17, 2017, from https://www.pub.gov.sg/watersupply/fournationaltaps/newater
P. (2017, February 7). PUB, Singapore’s National Water Agency. Retrieved March 17, 2017, from https://www.pub.gov.sg/watersupply/fournationaltaps/desalinatedwater
P. (n.d.). PUB, Singapore’s National Water Agency. Retrieved March 17, 2017, from https://www.pub.gov.sg/Documents/NEWater%20Technology.pdf