Water treatment and water treatment technologies are a primary line of defense to remove contaminants and bacteria before delivering clean drinking water for consumption. Water sources may be subject to contamination and therefore require appropriate treatment to remove disease agents. Public drinking water systems use a variety of methods to provide safe drinking water.
What is purification?
Water purification consists of several stages. This may include initial pretreatment by settling or by using a coarse media, filtration followed by chlorination, called the multiple barrier principle. The latter enables efficient water purification and allows each stage to process and prepare the water to a suitable quality for the next process in the chain. For example, filtration can prepare the water to ensure it is suitable for UV (ultraviolet), disinfection.
How does the purification process work?
Coagulation, flocculation, and sedimentation are processes used to remove color, turbidity, algae, and other microorganisms from surface water.
Chemical coagulants can be added to the water to form a precipitate or floc to trap these impurities. After sedimentation and/or filtration, the floc is separated from the purified water.
Aluminum and iron sulfate are two of the most commonly used coagulants. The quality of the raw water near the inlet of a mixing tank or flocculator determines the rate at which coagulants are dosed into the solution. By adding the coagulant at a point of high turbulence, it disperses quickly and completely during dosing. The next stage is the settling tank. This is where the aggregation of the flocs takes place, which settle to form a precipitate that will have to be removed. One of the advantages of coagulation is that it reduces the time required for suspended solids to settle. In addition, it can be very effective in removing fine particles that are otherwise very difficult to remove.
Cost and the requirement for accurate dosing, thorough mixing, and frequent monitoring are often cited as the primary disadvantages of using coagulants for small-volume processing. As a result, to remove color and turbidity, coagulation and flocculation are considered the most effective treatment techniques. However, for small water sources they may not be suitable. This is due to the level of control required and the volumes of sludge generated.
Various water treatment technologies are required to work together, in sequence, to treat raw water before it can be distributed. Here is a list of the main technologies that are often used in water treatment facilities.
Grids
Screens are used on many surface water intakes to remove particles and debris from the raw water. Weeds and debris can be removed using coarse sieves, while smaller particles, including fish, can be removed using belt sieves and micro sieves. Before coagulation or subsequent filtration, microscreens are used as a pretreatment to reduce solids loading.
Gravel filters
Turbidity and algae can be removed using gravel filters, which consist of a rectangular channel or tank divided into several sections and filled with graded gravel (size 4 to 30 mm). An inlet manifold allows the raw water to enter and flow horizontally through the tank, meeting first the coarse and then the finer gravel. An outlet chamber collects the filtered water, with the solids removed from the raw water accumulating on the filter floor.
Sand filters
Turbidity, algae and microorganisms can also be removed using slow sand filters. A simple and reliable process, slow sand filtration is often suitable for processing small stockpiles, provided there is sufficient land available. Slow sand filters typically consist of tanks containing sand of different grain sizes (size range 0.15-0.30 mm) at a depth of between 0.5 and 1.5 m.
Activated carbon
Using physical adsorption, pollutants can be removed using activated carbon. This will be affected by the amount and type of carbon, the nature and concentration of the contaminant, the residence time of the water in the unit, and the overall quality of the water (temperature, pH, etc.).
Aeration
Aeration is designed to transfer oxygen to the water and remove gases and volatile compounds through deaeration. A common method is packed tower aerators as a result of their compact design and high energy efficiency. A variety of techniques can be used to achieve deaeration, including countercurrent cascade aeration in packed towers, diffuse aeration in basins, and spray aeration.
Membrane processes
Reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF), and nanofiltration (NF) are the most commonly used membranes for water purification processes. Previously applied to the production of water for industrial or pharmaceutical applications, the membranes are applied to the treatment of drinking water. Membrane processes can provide adequate removal of pathogenic bacteria, Cryptosporidium, Giardia and potentially human viruses and bacteriophages.
UV disinfection
Ultraviolet (UV) light, invisible to the human eye, can be used to disinfect microorganisms in water purification processes. The wavelengths of UV light range between 200 and 300 nanometers (billionths of a meter). Ultraviolet radiation is produced at 254 nm by special low-pressure mercury lamps. This is the optimal wavelength for disinfection and ozone destruction. Categorized as bactericidal, this means that they are capable of inactivating microorganisms such as bacteria, viruses and protozoa. It is important to note that UV lamps never come into contact with water; they can be mounted outside the water flowing through UV transparent Teflon tubes or placed in a quartz glass sleeve in the water chamber.
How it works? The wavelength of UV light renders bacteria, viruses and protozoa unable to reproduce and infect.
UV disinfection can be used for primary drinking water disinfection technology. Additionally, the process can also be used as a secondary form of disinfection. For example, against microorganisms such as Cryptosporidium and Giardia, which can be resistant to chlorine. In addition, ultraviolet light (alone or in combination with hydrogen peroxide) can destroy chemical contaminants such as pesticides, industrial solvents, and pharmaceuticals through a process called UV-oxidation.
Under ideal conditions, UV technologies can provide more than 99% reduction of all bacteria. However, even with this performance, UV disinfection has two potential limitations: “spot” disinfection and cells that are not removed.
Treatment of water with ozone – ozonation
Ozone can be used during water treatment, for example during pre-oxidation, intermediate oxidation or final disinfection, as it has excellent disinfection and oxidation properties. It is generally recommended to use ozone for pre-oxidation, before a sand filter or an activated carbon (GAC) filter. After ozonation, these filters can remove remaining organic matter (important for final disinfection).
Ozonation is carried out by an electric discharge field, as in CD type ozone generators, or by ultraviolet radiation (UV type ozone generators). Ozone can also be produced by electrolytic and chemical reactions, in addition to commercial methods. In general, an ozonation system involves passing dry, clean air through a high-voltage electrical discharge, ie. corona discharge, which creates an ozone concentration of approximately 1% or 10,000 mg/L. In the treatment of small quantities of waste, UV ozonation is the most common, while large-scale systems use either corona discharge or other methods of mass production of ozone.
Types of Water Treatment Chemicals (and Why They’re Used)
Chemical disinfection of drinking water includes any chlorine-based technology, such as chlorine dioxide, as well as ozone, some other oxidants, and some strong acids and bases. With the exception of ozone, proper dosing of chemical disinfectants is intended to maintain a residual concentration in the water to provide some protection from post-treatment contamination during storage.
Disinfection of drinking water for households in developing countries is mostly done with free chlorine, or in liquid form such as hypochlorous acid (commercial household bleach or a more dilute solution of sodium hypochlorite between 0.5% and 1% hypochlorite marketed for domestic use for water purification), or in dry form as calcium hypochlorite or sodium dichloroisocyanurate. This is because these forms of free chlorine are convenient, relatively safe to handle, cheap and easy to dispense.
Chlorine is the most widely used primary disinfectant and is often used to provide residual disinfection in the distribution system. Monitoring the level of chlorine in drinking water entering a distribution system is generally considered a high priority (if possible) because monitoring is used as an indicator that disinfection has taken place. Residual chlorine concentrations of about 0.6 mg/l or more may cause acceptability problems for some consumers based on taste.
Chlorine dioxide breaks down, leaving the inorganic chemicals chlorite and chlorate. They are best managed by controlling the dose of chlorine dioxide applied to the water. Chlorite can also be found in a hypochlorite solution that has been allowed to age.
Correct dosage of chlorine for household water treatment is critical to ensure sufficient free chlorine to maintain a residual during storage and use. Recommendations are to dose with free chlorine at about 2 mg/l for clear water (< 10 nephelometric turbidity units [NTU]) and twice as much (4 mg/l) for cloudy water (> 10 NTU).
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