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Water Purification

  • Physical processes: filtration, sedimentation, distillation
  • Biological processes: slow sand filters, biologically active carbon
  • Chemical processes: flocculation, chlorination, electromagnetic radiation (ultraviolet light)
  • Potable water purification
  • Drinking water treatment: pre-chlorination, aeration, coagulation for flocculation, coagulant acids (polyelectrolytes), sedimentation, filtration, desalination, disinfection
  • Industrial water treatment: boiler water treatment, cooling water treatment
  • Reverse osmosis
  • Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids and gases from contaminated water.
  • The goal of this process is to produce water fit for a specific purpose. Most water is disinfected for human consumption (drinking water) but water purification may also be designed for a variety of other purposes, including meeting the requirements of medical, pharmacological, chemical and industrial applications.
  • In general the methods used include physical processes such as filtration, sedimentation, and distillation, biological processes such as slow sand filters or biologically active carbon, chemical processes such as flocculation and chlorination and the use of electromagnetic radiation such as ultraviolet light.
  • The purification process of water may reduce the concentration of particulate matter including suspended particles, parasites, bacteria, algae, viruses, fungi and a range of dissolved and particulate material derived from the surfaces that water may have made contact with after falling as rain.
  • The standards for drinking water quality are typically set by governments or by international standards. These standards will typically set minimum and maximum concentrations of contaminants for the use that is to be made of the water.
  • It is not possible to tell whether water is of an appropriate quality by visual examination. Simple procedures such as boiling or the use of a household activated carbon filter are not sufficient for treating all the possible contaminants that may be present in water from an unknown source. Even natural spring water – considered safe for all practical purposes in the 19th century – must now be tested before determining what kind of treatment, if any, is needed. Chemical and microbiological analysis, while expensive, are the only way to obtain the information necessary for deciding on the appropriate method of purification.
  1. Groundwater: The water emerging from some deep ground water may have fallen as rain many tens, hundreds, or thousands of years ago. Soil and rock layers naturally filter the ground water to a high degree of clarity and often it does not require additional treatment other than adding chlorine or chloramines as secondary disinfectants. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep ground water is generally of very high bacteriological quality (i.e. pathogenic bacteria or the pathogenic protozoa are typically absent), but the water may be rich in dissolved solids, especially carbonates and sulphates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bicarbonate. There may be a requirement to reduce the iron or manganese content of this water to make it acceptable for drinking, cooking, and laundry use. Primary disinfection may also be required. Where groundwater recharge is practised (a process in which river water is injected into an aquifer to store the water in times of plenty so that it is available in times of drought), the groundwater may require additional treatment depending on applicable state and federal regulations.
  2. Upland lakes and reservoirs: Typically located in the headwaters of river systems, upland reservoirs are usually sited above any human habitation and may be surrounded by a protective zone to restrict the opportunities for contamination. Bacteria and pathogen levels are usually low, but some bacteria, protozoa or algae will be present. Where uplands are forested or peaty, humic acids can colour the water. Many upland sources have low pH which require adjustment.
  3. Rivers, canals and low land reservoirs: Low land surface waters will have a significant bacterial load and may also contain algae, suspended solids and a variety of dissolved constituents
  4. Atmospheric water generation is a new technology that can provide high quality drinking water from the air by cooling the air and thus condensing water vapor.
  5. Rainwater harvesting or fog collection which collects water from the atmosphere can be used especially in areas with significant dry seasons and in areas which experience fog even when there is little rain.
  6. Desalination of seawater by distillation or reverse osmosis.
  7. Surface water: Freshwater bodies that are open to the atmosphere and are not designated as groundwater, are termed surface waters.
  • Water treatment describes industrial-scale processes that make water more acceptable for an end-use, which may be drinking, industrial, or medical.
  • Water treatment is unlike small-scale water sterilization that campers and other people in wilderness areas practice. Water treatment should remove existing water contaminants or so reduce their concentration that their water becomes fit for desired processes such as settling and filtration, and chemical processes such as disinfection and coagulation.
  • Biological processes are employed in the treatment of wastewater and these processes may include, for example, aerated lagoons, activated sludge or slow sand filters.
  • Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for the most critical of its intended uses, usually for human consumption.
  • Substances that are removed during the process of drinking water treatment include suspended solids, bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and other chemical pollutants such as fertilisers.
  • A combination selected from the following processes is used for municipal drinking water treatment worldwide
  • Pre-chlorination – for algae control and arresting any biological growth
  • Aeration – along with pre-chlorination for removal of dissolved iron and manganese
  • Coagulation – for flocculation
  • Coagulant aids, also known as polyelectrolytes – to improve coagulation and for thicker floc formation
  • Sedimentation – for solids separation, that is, removal of suspended solids trapped in the floc
  • Filtration – removing particles from water
  • Desalination – process of removing salt from the water
  • Disinfection – for killing bacteria
  • There is no unique solution (selection of processes) for any type of water. Also, it is difficult to standardise the solution in the from of processes for water from different sources.
  • Treatability studies for each source of water in different seasons need to be carried out to arrive at the most appropriate processes.
  • Technologies for potable water treatment are well developed, and generalised designs are available that are used by many water utilities (public or private).
  • In addition, a number of private companies provide patented technological solutions. Automation of water and wastewater treatment is common in the developed world.
  • Capital costs, operating costs available, quality monitoring technologies, locally available skills typically dictate the level of automation adopted.
  • Two of the main processes of industrial water treatment are boiler water treatment and cooling water treatment.
  • A lack of proper water treatment can lead to the reaction of solids and bacteria within pipe work and boiler housing. Steam boilers can suffer from scale or corrosion when left untreated, leading to weak and dangerous machinery, scale deposits can mean additional fuel is required to heat the same level of water because of the drop in efficiency.
  • Poor quality dirty water can become a breeding ground for bacteria such as Legionella causing a risk to public health.
  • With the proper treatment, a significant proportion of industrial on-site wastewater might be reusable.
  • This can save money in three ways: lower charges for lower water consumption, lower charges for the smaller volume of effluent water discharged and lower energy costs due to the recovery of heat in recycled wastewater.

Reverse Osmosis

Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane. This membrane technology is not properly a filtration method. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential, a thermodynamic parameter. Reverse osmosis can remove many types of molecules and ions from solutions, and is used in both industrial processes and producing potable water. The result is the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side.

To be “selective”, this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.