LITERATURE REVIEW: WATER TREATMENT
1.0 Literature Review: Water Treatment.
1.1 Water Quality:
Water quality is the measure of water condition in relation to the requirements of various biotic species, the environment, and/or any human needs and purposes. Water quality is determined by measuring and analyzing it chemical, biological, physical and radiological characteristics. Both natural and human activities influence water quality. Natural influences that affect water quality are mainly climatic, geological and hydrological since they affect both the quality and quantity of water available (Diersing 2009). The highest influence on water quality occurs when its quantity is low, yet the limited resource must be utilized; for example, high salinity is a common problem in coastal and arid areas. In this case, while water may be available in large quantities, its quality limits the number of applicable uses. While the natural ecosystem exists in harmony with the natural quality of water, any significant alterations to water quality are disruptive to the ecosystem (Bartram and Ballance 2007).
Human activities also have a significant impact on water quality and their impacts are widespread and vary in the degree to which they restrict water usage or disrupt the natural ecosystem. For example, water pollution by the human fecal material is attributed to a single source but the reasons, impact and the countermeasures for this type of pollution are varied. The reasons might be a lack of proper waste disposal facilities, failure of waste disposal and sewerage systems, poor operation of waste disposal systems, or direct drainage of sanitation facilities (such as pit latrines) into aquifers. Industrial and agricultural activities also have a major impact on water quality, and pollution from these sources is particularly difficult to control given that the impurities are mostly chemicals such as industrial waste and fertilizers respectively (Bartram and Ballance 2007).
Drinking water quality standards vary around the world and range from simple to complex regulations and controls, and with the wide range of parameters available, it is expected that a broad variety of rules exist. However, based on various global reports on the drinking water and sanitation issues in developing countries, it is evident that international water quality standards are divided into two categories i.e. high water quality standards in developed countries while underdeveloped countries continue to deal with chemical and bacterial contamination. The World Health Organization (WHO) sets the international water quality standards that nations can use to start their own safe drinking water programs, but the international body also acknowledges that most developing countries are unable to conform to the prescribed standards. In order to assist developing countries that are unable to afford modern water treatment facilities and technologies, WHO recommends that these countries use the standards to guide them in simple water filtration and disinfection practices (Radcliff 2003).
In the Kingdom of Saudi Arabia (KSA), SASO (Saudi Standards, Metrology, and Quality Organization) is the regulating agency for drinking water quality standards. SASO water quality standards are broadly categorized standards for bottled drinking water and standards for non-bottled water. The bottled drinking water quality standards are used to specify the desired water quality for public water distribution systems in the KSA. On the other hand, the non-bottled water standards include the overall standards for the biological, physical and microbial characteristics of water (UQU 2010). The general provisions for defining water quality standards in the Kingdom of Saudi Arabia ensure that the general public has access to a reliable drinking water supply (availability). The second priority is to ensure that the chemical and microbial integrity of drinking water by ensuring drinking water is free from microbial and chemical pollution. Finally, water quality standards in KSA strive to ensure that the water supplied has aesthetic value. In this case, water quality standards ensure that the customer’s attitude is geared towards accepting that the quality of drinking water is good, and that there are no doubts regarding the safety of publicly distributed drinking water. If there are aesthetic concerns such as presences of dirt in water, then consumers are likely to have perception of risk regarding drinking water safety (PME 2012).
1.2 Sources of drinking water:
Drinking water is sourced from either surface water such as lakes and oceans or ground water sources such as aquifers (Freedrinkingwater.com 2016). Ground water is as a result of falling precipitation such as snow, and rain that seeps into the ground and fills open spaces (pore space) between layers of gravel or sand underneath the ground surface. Under the ground, there exists a water saturation zone where the subsurface is completely filled with water, and the gravel and sand layers at this zone are known as aquifers. Aquifers are geologic formations that contain water in quantities that are sufficient enough to yield the water into a well. The wells then pump water to the ground surface after which water companies and domestic users treat the water to ensure it is safe for human consumption. After treatment, the water is then pumped into storage tanks and distributed on demand to customers via pipe distribution networks into their homes (Deq.louisiana.gov 2016).
Surface water is water found at the ground level and also originates to due various forms of precipitation. However, unlike ground water, when precipitation such as snow and rain reaches the land surface, it refills surface water bodies such as oceans, wetlands, lakes and oceans directly. When water is sourced from surface sources, it is simply pumped from the water bodies to a water treatment plant and then it follows the same treatment and distribution processes applied for ground water. Since the amount of water on earth is finite, water is generally reused in what is known as the hydrological (water) cycle. The hydrological cycle refers to the continuous circulation of water from ground sources such as oceans and lakes to the air via evaporation, and then back to land as precipitation. The precipitation then flows back into oceans and lakes via rivers and streams, evaporates again and the cycle continues. Some of the precipitations also infiltrate the ground surface to form ground water (Deq.louisiana.gov 2016).
In the Kingdom of Saudi Arabia (KSA), drinking water is collected from rainfall, dams, desalination of sea water and recycling of waste water. The KSA is a desert country, and thus, heavy rains occur in short durations and cause flash floods that fill up rivers that are otherwise dry the rest of the time when there is no rain. Part of water from surface runoff infiltrates through the sedimentary rock layers in the valleys and refills the ground water aquifers. This water can then be pumped by digging wells and treating the raw water collected to meet drinking water standards. The largest amount of surface runoff in KSA occurs in the western region and represents about 60 percent of the total surface runoff experienced even though it only represents about 10 percent of the total area in the whole country. The other 40 percent of surface runoff occurs in a region known as Tahama in the far south of the west coast in KSA, and this only represents about 2 percent of the total area of the country. The total surface water sources are estimated to be about 2.2 cubic kilometers annually (km3/year), and most of this water infiltrate the ground and refills the aquifers. Only about 1 cubic kilometer recharges the usable water aquifers. In total, the ground water reserves in Saudi Arabia stand at an estimated 500 cubic kilometers and about 340 of this amount can be abstracted at acceptable costs given the Gulf nation’s economic conditions. Another source of water in Saudi Arabia is water dams, and in 1993, there were around 185 dams of various sizes constructed to control floods and recharge groundwater. At the times, the dams had a combined storage capacity of 475 million cubic meters, and yet another 45 dams were planned to be built (Panikkar 2016).
Another major source of drinking water in Saudi Arabia is desalinated sea water. In fact, Saudi Arabia is the world’s largest producer of desalinated sea water and by 1992, the country had 18 desalination plants on the west coast. These plants had a total capacity of 700,000 cubic meters of water per day, and on the east coast, the daily production stood at over 1.1 million cubic meters. These desalination plants also doubled up as power plants. By the end of 1992, the production of desalinated water stood at about 675 cubic meters (Panikkar 2016).
1.3 Drinking Water Treatment Processes:
The conventional water treatment process for drinking water involves some basic steps which include coagulation/flocculation, sedimentation, filtration, post-chlorination and corrosion control water treatment. These steps are required to produce water that meets drinking water quality standards, pathogen free, and suitable for human consumption.
Most particles suspended in raw (untreated) water entering the treatment plant are so small that it becomes impractical to remove directly them via sedimentation and filtration processes. Usually, most of the soil particles are negatively charged which is the main reason why they remain stable while suspended in the raw water (Mazille and Spuhler 2012). The soil particles that may otherwise settle are mutually repelled by the negative charges and thus, remain in suspension. In order to destabilize the suspended particles, a chemical technique known as coagulation is applied (Water quality.de 2016). The coagulation process involves adding alum (liquid aluminum sulfate) and/or polymer into the raw water. The chemical coagulant (alum) has positive charges and this causes it to attract the negatively charged suspended particles in water and neutralize them. This attraction and neutralization of charges is the one that causes the suspended dirt particles in water to stick together or coagulate (SDWF 2011).
Over the years, the coagulation process followed by flocculation have been used to aid in water sedimentation and filtration and are by far the most widely used techniques for eliminating substances that cause water turbidity (muddiness). If the raw water is highly turbid, sedimentation follows after flocculation in order to reduce that quantity of suspended material prior to filtration (Mazille and Spuhler 2012). Flocculation is a slow mixing technique that promotes the aggregation of coagulated dirt particles (Water quality.de 2016). During this process, the coagulated particles stick together to form larger, and heavier particles known as flocs and these are easily removed in the sedimentation and filtration stages (Baltimorecity.gov 2002; Hunter Water 2016).
Sedimentation:
The water and floc particles progress through to the next stage in the treatment process known as the sedimentation stage. In this stage, water is moved slowly into the sedimentation basins also known as clarifiers, and this causes the large floc particles to settle slowly to the bottom. The sedimentation clarifiers are usually designed in circular or rectangular shapes but in most cases, the raw water is usually introduced at the bottom of the clarifier to give the flocs a better chance at settling completely and in lesser time than if the water was introduced from the top. After the flocs have settled at the bottom, they are collected by mechanical racks and channeled on to sludge treatment sections or drying lagoons (UH 2013). In water treatment plants that employ direct filtration, the sedimentation step is skipped, and the floc is removed via filtration (Hunter Water 2016).
Filtration:
After sedimentation, water is passed through a filter to remove the fine particles. The filters used to remove suspended dirt particles comprise several layers of gravel and sand, fabrics, or synthetic granular material such as crushed anthracite. The most common types of filters are rapid sand filters used in tanks. In these filtration units, the filter material is held in place by gravity and unfiltered water flows downwards. Cartridge filters made from paper, fabrics or plastic screens are also commonly used since they are smaller, cheaper and easier to dispose of. Filters are also rated differently depending on the size of particles they filter out (Water quality.de 2016). The filtration process removes the suspended impurities in the water thus enhancing the effectiveness of the disinfection process. The water filters are cleaned routinely by reversal of flow, also known as backwashing (Hunter Water 2016).
Disinfection/Pre-chlorination:
In most water treatment plants, chlorine is added to raw water as it enters the plant. The chlorine kills various microorganisms such as protozoa, viruses, and bacteria that cause illnesses. Chlorination also prevents algae growth in the treatment plant systems since the algae may bring odor and taste issues. Chlorine is added in excess such that a residual 1 part per million is left in the water after the filtration process. The residual chlorine prevents the regrowth of bacteria in the distribution system (piping network) (Baltimorecity.gov 2002; Hunter Water 2016).
Other stages in the water treatment process:
There are other stages in the water treatment process, and these include sludge drying, fluoridation, and pH correction. In the sludge drying process, the solids collected from the sedimentation and filtration stages are removed and transferred to drying lagoons. The fluoridation process involves adjusting free fluoride ion concentrations in water to an optimum level in order to reduce incidences of fluorosis and dental caries. Finally, the pH correction stage involves adding lime to already filtered water to adjust its pH and also stabilize the naturally soft water thus minimizing corrosion in the pipe networks (distribution systems) and also in the consumer’s plumbing systems (Hunter Water 2016).
References:
Baltimorecity.gov,. 2002. "How Is Water Treated?". Cityservices.Baltimorecity.Gov. http://cityservices.baltimorecity.gov/dpw/waterwastewater02/waterquality5.html.
Bartram, Jamie, and Richard Ballance. 2007. Water Quality Monitoring - A Practical Guide To The Design And Implementation Of Freshwater Quality Studies And Monitoring Programmes. 2nd ed. London: Taylor & Francis.
Deq.louisiana.gov,. 2016. "Where Does Drinking Water Come From?". Deq.Louisiana.Gov. http://www.deq.louisiana.gov/portal/PROGRAMS/DrinkingWaterProtectionProgram/Wheredoesdrinkingwatercomefrom.aspx.
Diersing, Nancy. 2009. Water Quality: Frequently Asked Questions. Ebook. 1st ed. Key West, FL.: Florida Brooks National Marine Sanctuary. http://floridakeys.noaa.gov/scisummaries/wqfaq.pdf.
Freedrinkingwater.com,. 2016. "Water Quality Information - Sources Of Our Drinking Water | APEC Water". Freedrinkingwater.Com. http://www.freedrinkingwater.com/water-education/water-info-safe-drinking-source.htm.
Hunter Water,. 2016. "Water Treatment Processes - Hunter Water". Hunterwater.Com.Au. http://www.hunterwater.com.au/Water-and-Sewer/Water-Supply/Water-Treatment-Processes.aspx.
Mazille, Félicien, and Dorothee Spuhler. 2012. "Coagulation-Flocculation | SSWM". Sswm.Info. http://www.sswm.info/content/coagulation-flocculation.
Panikkar, Avanish K. 2016. "Water Profile Of Saudi Arabia". Eoearth.Org. http://www.eoearth.org/view/article/156986/.
PME,. 2012. Kingdom Of Saudi Arabia National Environmental Standard Drinking Water Quality. Ebook. 1st ed. Presidency of Meteorology and Environment - PME Reference. http://www.pme.gov.sa/en/En_EnvStand10.pdf.
Radcliff, Richard. 2003. "International Drinking Water Regulations: The Developed World Sets The Standards". Nesc.Wvu.Edu. http://www.nesc.wvu.edu/ndwc/articles/ot/sp03/inter_dwregs.html.
SDWF,. 2011. Conventional Water Treatment: Coagulation And Filtration. Ebook. 1st ed. Safe Drinking Water Foundation (SDWF). http://www.safewater.org/PDFS/knowthefacts/conventionalwaterfiltration.pdf.
UH,. 2013. Drinking Water Treatment: Coagulation, Flocculation, And Sedimentation. Ebook. 1st ed. University of Houston (UH). http://gk12.egr.uh.edu/sites/gk12.egr.uh.edu/files/docs/lessons/Coagulation-Flocculation-Sedimentation.pdf.
UQU,. 2010. Water Quality And Standards - Chapter 3. Ebook. 1st ed. Umm Al-Qura University (UQU). https://uqu.edu.sa/files2/tiny_mce/plugins/filemanager/files/4230066/numan/(ch)3.pdf.
Waterquality.de,. 2016. "Coagulation, Flocculation, Sedimentation, And Filtration". Waterquality.De. http://www.waterquality.de/hydrobio.hw/TWAFLOCK.HTM.
