Surface water treatment plants are generally used as a sequence of more or less standard processes. After closing large objects such as fish and sticks, chemical coagulants are added to the water to achieve the tiny particles in suspension that cloud the water attract each other to form "flocs". Flocculation-the formation of larger, smaller, common floc-sized flocs to achieve particles and small flocs so that they "collide" with each other, adhere, and form a larger flock. When the flocs are large and heavy enough to settle, the water moves to the sedimentation or decantation stages. When most solids have settled, there are more forms of filtration and sea through sand or membranes. Disinfection is usually the next step. After disinfection, chemicals can be added to the pH, to prevent corrosion of the distribution system, or to prevent tooth decay. Ion exchange or activated carbon can be used in some parts of this process to eliminate organic or inorganic contaminants. Groundwater sources have a higher initial quality and need less treatment than surface water sources.
Point-of-use and start-point devices are simpler and use a limited number of technologies. In most countries, drinking water that is non-pathogenic and complies with international regulations is available in each customer's tap. In addition to that, a significant number of consumers in the developed world for point-of-entry and exit devices or another access point for the public water supply system. However, the public water supply and public water supply patterns, which provide water for pathogens are not available and success is mainly measured by reducing the risk of diarrheal or other diseases. Therefore, a point-of-use technology that may be appropriate for one location may not be so and is not recommended for another.
Coagulation and flocculation practices are essential pre-treatments for many water purification systems. In the conventional coagulation-flocculation-sedimentation process, a coagulant is added to the source water to create an attraction between the particles in suspension. The mixture is stirred slowly to induce the grouping of particles together to form "flocs". The water is then transferred to a quiet deposit of sedimentation to sediment the solids.
Dissolved air flotation systems also add a coagulant to flocculate the particles in suspension; But instead of using sedimentation, pressurized air bubbles push them towards the surface of the water from where they can be extracted. A flocculation-chlorination system has been developed as a point-of-use technology, especially for developing countries. It uses small packages of chemicals and simple equipment such as buckets and a cloth filter to purify the water.
Finally, lime softening is a technology commonly used to "soften" water - that is, to remove mineral salts of calcium and magnesium. In this case, the material that is decanted is not the sediment in suspension but the dissolved salts.
Filtration systems treat water by passing it through beds of granular materials (eg, sand) that remove and retain contaminants. Conventional, slow, sand and diatomaceous earth filtration systems all do a good job of removing most protozoa, bacteria and viruses (if coagulation is used). Usually, bag and cartridge filters do not eliminate viruses and very few bacteria.
Conventional filtration is a multi-stage operation. First, a chemical coagulant such as iron or aluminum salts is added to the source water. Then, the mixture is stirred to induce the binding of the small particles in suspension to form larger lumps or "flocs" easier to remove. These coagulated masses, or "flocs," are allowed to settle out of the water, so that they carry away many contaminants. Upon completion of these processes, the water is passed through filters so that the remaining particles adhere to the filter material by themselves.
Direct filtration is similar to conventional filtration, except that after adding the coagulant, and after shaking the mixture, there is no separate phase for sedimentation. Instead, the particles in suspension are destabilized by the coagulant and thus adhere more easily to the filter material when the water is subsequently filtered.
Slow sand filtration systems do not have a coagulation phase and usually do not have a sedimentation step either. Slow and descending water is induced through a bed of sand two to four feet (0.6 to 1.2 meters) deep. A biologically active layer forms along the upper surface of the sand bed, trapping small particles and degrading some organic pollutants.
Biological sand filtration (Biosand) is a filtration system at the point of use analogous to slow sand filtration, but its effectiveness is much less established than the latter.
Filtration with diatomaceous earth uses as filter material the fossilized shells of tiny marine organisms through which the water is passed without treatment. The earth physically filters particulate contaminants from water.
Bag and cartridge filters are simple, easy-to-use systems that use a woven bag or rolled filament cartridge or a shirred filter to physically filter microbes and sediment from the source water.
Ceramic filters are mainly used in point-of-use applications. In developing countries, these are manufactured locally, sometimes in self-financed microenterprises.
Most filtration systems use "backwash" to clean the system. This produces wastewater that must be handled properly.
Membrane systems for water treatment were originally used only in desalination projects. But improvements in membrane technology have made them an increasingly popular option for the elimination of microorganisms, particulates and natural organic materials that affect the taste of water and cloud its clarity.
The membranes for water treatment are thin sheets of material that allow contaminants to be separated according to their characteristics such as size or electrical charge. The water passes through a membrane; but depending on their size, larger particles, microorganisms and other pollutants are separated.
Some of these systems are pressure driven, depending on the water pressure to separate the particles according to their size. Microfiltration uses the largest pore size, and can remove sand, silt, clays, algae, bacteria, Giardia and Cryptosporidium. Ultrafiltration can also eliminate viruses. Nanofiltration systems provide almost complete protection against viruses, eliminate most organic pollutants, and can reduce water hardness. Reverse osmosis systems are dense membranes that remove almost all inorganic contaminants and almost everything except the smallest organic molecules.
Electrodialysis combines membrane technology with the application of electrical current, to separate the pollutants according to their electrical charge. Unlike other membrane processes, spring water never passes through the membranes during electrodialysis. This option is not used in both large-scale water treatment facilities and some of the other technologies described in this document. On the contrary, it is used mainly in medical and laboratory applications that need ultrapure water.
Membranes, especially reverse osmosis membranes and nanofiltration, can be a good option for smaller-scale water treatment systems that face a wide range of contaminants. However, they often produce larger volumes of wastewater (or "concentrate") than most other treatment systems (up to 15 percent of the total volume of treated water) and can be clogged with clay or organic materials If the source water rich in particles does not filter first.
Usually, maintenance is not difficult, but it can be very expensive since the first necessary action is to replace the membrane as necessary. Maintenance problems tend to involve leaking and contaminated membranes.
Disinfection systems are used to combat diseases spread in water and caused by bacteria or viruses. These processes neutralize pathogens by treating water from sources with chemical additives, or by exposure to ultraviolet light. These treatment systems are often inexpensive and can easily reduce their capacity for low volume treatment facilities.
Free chlorine, chloramines and chlorine dioxide are some of the most common disinfectants. Chlorination is the most popular (and oldest) class of chemical additives. Chlorine is also an oxidant, so it helps eliminate iron, hydrogen sulfide and other minerals.
Ozone, a colorless gas, treats organic and inorganic contaminants in much the same way as chlorination but is even more effective against bacteria and other germs. Ozone systems are not common worldwide because they require a lot of infrastructure, and their implementation can have a high cost.
Ultraviolet light is an invisible part of the electromagnetic spectrum that kills bacteria and viruses in water exposed to its rays, and is typically produced by mercury lamps. The UV process is inexpensive and is often used in small-scale facilities, but is not as effective as other disinfectants in surface water supply sources that contain many suspended particles.
Adsorption systems treat water by adding a substance, such as activated carbon or alumina (aluminum oxide), to the water supply source. Adsorbents attract contaminants through chemical and physical processes that cause them to 'adhere' to their surfaces for later disposal.
By a large margin, the most commonly used adsorbent is activated carbon - a substance similar to common but highly porous carbon. Activated carbon powder is often used when temporary quality problems arise; it can simply be added to the water and disposed of with the waste sludge. The activated granular carbon is often distributed in a tray through which the source water is slowly passed or percolated.
Activated alumina treatment is used to attract and remove contaminants, such as arsenic and fluoride, that have negatively charged ions. However, this option can be expensive and may require complicated maintenance of the system. In addition, water may require pH adjustment before the adsorption column, and often the problem of excessive aluminum waste arises. For regeneration acids and bases are required.
The ion exchange uses a resin that removes charged inorganic contaminants such as arsenic, chromium, nitrate, radium, uranium and excess fluoride by exchanging them for innocuous charged ions on its surface. It works best with water without particles and you can modify its scale to adapt it to any size of treatment facility. Ion exchange is most often used to remove hardness (cationic resin) or nitrate (anionic resin). In both instances, it can be regenerated with salt water. The use of ion exchange to eliminate radionuclides is complicated by the fact that these materials accumulate in the resin and occur at high levels in the regenerant, to greatly complicate operations.
Activated carbon is usually preferred to remove organic contaminants, while ion exchange is often better to remove soluble inorganic molecules.
Air extraction systems, also known as aeration systems, mix air with a water supply. The objective is to generate the maximum possible air-water contact area for volatile organic chemicals and dissolved gases such as radon and hydrogen sulfide to pass from water to air.
Rectification tower or drained bed systems use a distributor to introduce water evenly through the top of a tower equipped with plastic beds, ceramics or metal objects designed to maximize air-water contact. The air is pushed or pulled up through the tower in a counter-current direction to the water.
Tray aeration systems distribute the binder materials in vertical trays and drain water through them.
Diffuse aeration systems force compressed air through diffusers in the lower part of a tank. Mechanical aeration systems work by vigorously shaking the surface of the water with a mixer.
Although it is simple in principle, air extraction systems tend to suffer from obstructions due to particulate bacteria that produce corrosion and precipitation of calcium carbonate. The costs of the treatment increase significantly if it is necessary to pre-treat the water or if the air in the system must be purified before discharging it into the atmosphere.
None of the air extraction systems is designed to be effective against microorganisms. All require a reliable power source, except tray aerators, which are designed to use natural air convection and the force of gravity, and therefore can often be operated without electrical power.
Water treatment by solar energy takes advantage of the natural cleaning processes found in nature and improves them to obtain more efficient results. Compact units and even laptops are popular in homes. These can be a good treatment option in developing nations with plenty of sunny days because they are inexpensive and investment and infrastructure are almost nil.
Solar distillation involves placing unpurified water in a container, to evaporate it by means of the sun's rays, and then condensing it in a separate container. Most contaminants such as salts, heavy metals and microbes are left in the container of unpurified water, which can be discarded periodically.
Disinfection by solar radiation uses the ultraviolet rays of the sun to eliminate pathogens. A plastic or glass container containing untreated water is placed on a roof or on a corrugated iron surface. With enough time and sunlight, ultraviolet light combined with high temperatures will eliminate most viruses, bacteria and protozoa.
