Potabilization is a process to purify water to make it drinkable for human or industrial use.
Freshwater for drinking comes from surface basins (rivers, streams or lakes, springs) and/or deep basins (via the draining-off of wells). The origin of the water determines its chemical/physical characteristics and therefore the treatment processes.
The type of treatment depends on the source of water: surface water (lake, rivers) and ground waters (deep wells) but also on turbidity (suspended mineral or organic matters).
The main problems of surface water are:
- Presence of varying degrees of turbidity, depending on the characteristics of the surface stream from which the water is drawn and the morphology of the ground it crosses along its path. In the majority of cases it is a finely dispersed turbidity, consisting of silica in the colloidal state, with only a minimum part settling spontaneously.
- Presence of micro pollutants (traces of solvents, weed killers, pesticides, etc.) due to the industrial and agricultural activities carried out along the river.
- Presence of high bacterial load.
Pollution of the aquifers can derive from easily identifiable point sources (e.g. waste pipes) or from sources widespread throughout the territory (e.g. release into the ground from agricultural activity, rainwater flow in urban or industrial areas etc.).
The pollution is generally due to reducing substances, such as: ammonia, hydrogen sulphide and methane, with the addition of iron, manganese, carbonic acid and humic substances.
Deep water doesn’t have turbidity so it will just need to be filtered before a final disinfection. Filtration treatments remove organic matters and some micro pollutants through sand filters, adsorbent media (activated carbon) and membranes (ultrafiltration, nano filtration) which provide a barrier against bacteria and virus.
WATER TREATMENT TECHNOLOGIES
We may organize water treatment technologies into three general areas: Physical Methods, Chemical Methods, and Energy Intensive Methods.
Physical methods of wastewater treatment represent a body of technologies that we refer largely to as solid-liquid separations techniques, of which filtration plays a dominant role. Filtration technology can be broken into two general categories – conventional and non-conventional. This technology is an integral component of drinking water and wastewater treatment applications. It is, however, but one-unit process within a modern water treatment plant scheme, whereby there are a multitude of equipment and technology options to select from depending upon the ultimate goals of treatment. To understand the role of filtration, it is important to make distinctions not only with the other technologies employed in the cleaning and purification of industrial and municipal waters, but also with the objectives of different unit processes.
The following technologies are among the most commonly used physical methods of purifying water:
The use of ultrafiltration technology for municipal drinking water applications is a relatively recent concept, although in the beginning, it is already commonly used in many industrial applications such as food or pharmaceutical industries. Ultrafiltration is proven to be a competitive treatment compare with conventional ones. In some cases, the combination of ultrafiltration with conventional process is also feasible particularly for high fouling tendency feed water or for removal of specific contaminants. Recently, ultrafiltration has been recognized as competitive pre-treatment for reverse osmosis system. Ultrafiltration (UF) is a low-pressure operation at transmembrane pressures of, typically, 0.5 to 5 bars. This is not only allowing nonpositive displacement pumps to be used, but also the membrane installation can be constructed from synthetic components, which has cost advantage.
Ultrafiltration (UF) is proven to be a competitive treatment compare with conventional ones. The production of clear and sparkling water that is safe as far as disease is concerned usually require chemical precipitation, adsorption, sedimentation, and filtration . Each step of this process has to be controlled to get an optimal performance of the overall process, which results in a complex control system. Nowadays, UF is used to replace clarification step in conventional water treatment plant, i.e., coagulation, sedimentation, and filtration and can be defined as a clarification and disinfections membrane operation. UF membranes are porous, however, all particulate contaminants such as viruses and bacteria, including macromolecules are rejected. The main advantages of low-pressure UF membrane processes compare with conventional clarification (direct filtration, settling/rapid sand filtration, or coagulation/sedimentation/filtration) and disinfection (post chlorination) processes are no need for chemicals, size-exclusion filtration as opposed to media depth filtration, good and constant quality of treated water in terms of particle and microbial removal regardless of raw feed water quality, process and plant compactness, and simple automation.
Source water quality directly impacts UF membrane performance. Therefore, in practice, depending on the quality of raw water, UF can be operated as single operation or combination with other process (coagulation, adsorption, etc.) or hybrid membrane system (UF/MF). In water application, UF can be the main process or as pre-treatment for example in RO system..
One of the first membrane applications for the utilization of membrane technology was the conversion of seawater into drinking water by reverse osmosis (RO). RO system separate dissolved solutes (includes single charged ions, such as Na+, Cl-) from water via a semipermeable membrane that passes water in preference to the solute. RO can be described as diffusion-controlled process in which mass transfer of ions through RO membranes is controlled by diffusion. Physical holes may not exist in an RO membrane, which distinguishes RO membrane with other filtration system. RO membrane is very hydrophilic; therefore, water will be able to readily diffuse into and out of the polymer structure of the membrane. RO membrane is capable of rejecting contaminants as small as 0.001 μm
Reverse osmosis (RO) membranes filters frequently are used the level of total dissolved solids and suspended particles within water. Some contaminant treated effectively by RO are Arsenic, Barium, Cadmium, Calcium, Chloride, Mercury, Sodium, Chloride, Iron etc.
In reverse osmosis, pressure is applied to the concentrated side of the membrane (the contaminated side). This forces the osmotic process into reverse so that, with adequate applied pressure, pure water is forced from the concentrated (contaminated) side to the dilute (treated) side. Treated water is collected in a storage container. The rejected contaminants on the concentrated side of the membrane are washed away as wastewater.
Drinking water treatment using RO is one option to treat drinking water problems. RO is an effective method to reduce certain ions and metals, such as nitrate and arsenic. It is often used in combination with AC filtration.
Reverse osmosis can remove microorganism. However, it is not recommended for that use (i.e., only coliform-free water should be fed to the system) because membrane deterioration can occur due to the bacteria, and the contamination may occur through pinhole leaks.
Contaminants not removed from water by RO filters include dissolved gases such as CO2, hydrogen sulfide, a common nuisance contaminant with characteristic rotten egg odor, which passes through the RO membrane. Some pesticides, solvents and volatile organic chemicals (VOCs) are not removed by RO.
No one piece of treatment equipment manages all contaminants. All treatment methods have limitations and often situations require a combination of treatment processes to effectively treat the water. Activated Charcoal filters and /or sediment filter is commonly used in conjunction whit RO filters. Sediment filters help remove silt particles that may. AC filters remove chlorine and certain pesticides and organic solvents that the RO membrane is not as effective in removing. Typical RO systems consist of a pretreatment filter, the RO membrane, flow regulator, post-treatment filter, storage tank and dispensing faucet.
AC or sediment filters before the RO membrane and AC filters after the RO membrane are commonly used. Pre-filters help extend the life of the system by removing silt and other large particles and/or chlorine that may be harmful to the RO membrane. If the feed water is not chlorinated, AC filters should not be used for pre-filtration because they can encourage microbial growth on the membrane surface. In this case, only a sediment pre-filter is recommended. AC post-filters can also remove certain pesticides and organic solvents that the RO membrane does not remove. The AC treatment process is also improved since the RO membrane removes compounds that may hinder adsorption by the carbon.
Microfilters are small-scale filters designed to remove cysts, suspended solids, protozoa, and, in some cases, bacteria from water. Most filters use a ceramic or fiber element that can be cleaned to restore performance as the units are used. Microfilters are the only method, other than boiling, to remove Cryptosporidia. Microfilters share a problem with charcoal filter in having bacteria grow on the filter medium. Som advise against storage of a filter element after it has been used. Many microfilters may include silt prefilters, activated charcoal stages, or an iodine resin. Most filters come with a stainless steel prefilter, but other purchased or improvised filters can be added to reduce the loading on the main filter element. Allowing time for solids to settle, and/or prefiltering will also extend filter life.
Slow Sand Filter
Slow sand filters pass water slowly through a bed of sand. Pathogens and turbidity are removed by natural die-off, biological action, and filtering. Typically, the filter will consist of a layer of sand, then a gravel layer in which the drain pipe is embedded. The gravel doesn’t touch the walls of the filter, so that water can’t run quickly down the wall of the filter and into the gravel.
Building the walls with a rough surface also helps. A typical loading rate for the filter is 5-7 meters/hour. The filter can be cleaned several times before the sand has to be replaced. Slow sand filters should only be used for continuous water treatment. If a continuous supply of raw water can’t be insured (say, using a holding tank), then another method should be chosen. It is also important for the water to have as low turbidity (suspended solids) as possible. Turbidity can be reduced by changing the method of collection (for example, building an infiltration gallery, rather than taking water directly from a creek), allowing time for the material to settle out (using a raw water tank), pre-filtering or flocculation (adding a chemical, such as alum to cause the suspended material to floc together.) The SSF filter itself is a large box. The walls should be as rough as possible to reduce the tendency for water to run down the walls of the filter, bypassing the sand.
The sand for a SSF needs to be clean and uniform, and of the correct size. The sand can be cleaned in clean running water, even if it is in a creek. The ideal specs on sand are effective size (sieve size through which 10% of the sand passes) between 0.15 and 0.35 mm, uniformity coefficient (ratio of sieve sizes through which 60% pass and through which 10% pass) of less than 3; maximum size of 3 mm, and minimum size of 0.1 mm. The sand is added to a SSF to a minimum depth of 0.6 meters. Additional thickness will allow more cleanings before the sand must be replaced. 0.3 to 0.5 meters of extra sand will allow the filter to work for 3-4 years. An improved design uses a geotextile layer on top of the sand to reduce the frequency of cleaning. The outlet of a SSF must be above the sand level, and below the water level. The water must be maintained at a constant level to insure an even flow rate throughout the filter. The flow rate can be increased by lowering the outlet pipe, or increasing the water level. While the SSF will begin to work at once, optimum treatment for pathogens will take a week or more. During this time the water should be chlorinated, if at all possible (iodine can be substituted). After the filter has stabilized, the water should be safe to drink, but chlorinating of the output is still a good idea, particularly to prevent recontamination. As the flow rate slows down the filter will have to be cleaned by draining and removing the top few mm of sand. If a geotextile filter is used, only the top 12 mm may have to be removed. As the filter is refilled, it will take a few days for the biological processes to re-establish themselves.
Dual Media Pressure Filters
Dual Media Pressure Filter is contained under pressure in a carbon steel tank, which may be vertical or horizontal, depending on space available. The pressure sand filters contain multiple layers of coarse and fine sand or anthracite in a fixed proportion. It is a kind of a deep filter bed with adequate pore dimensions for retaining both large and small suspended solids and un-dissolved impurities like dust particles. As compared to conventional sand water filter, this multigrade filtration system works on higher specific flow rates. It is also a low cost pre-treatment system for ion exchangers (deionizer and softener) and membrane systems such as reverse osmosis etc. With high throughputs, high dirt-holding capacity and capacity to reduce turbidity and TSS (< 5ppm) from water, it protects ion-exchange resins and membranes from physical fouling due to suspended impurities present in the water.
The working principle of a multigrade filter is quite straight forward. In a multigrade filter or pressure sand filter, water is passed through multi layers of filter media consisting graded sand, pebbles and gravels layers. The contaminants in the water are captured in the media bed and filtered water passes into the discharge manifold at the bottom of the tanks. The next and last step is backwashing, a process of effectively removal of captured contaminants from the media bed. After back-washing the filter is rinsed with raw water and after the required quality of water is achieved the filter is put back into service.
Groundwater is first aerated to oxidize the iron or manganese, and then pumped through the filter to remove the suspended material.
Activated Charcoal Filter
Activated charcoal filters water through adsorption; chemicals and some heavy metals are attracted to the surface of the charcoal, and are attached to it. Charcoal filters will filter some pathogens, though they will quickly use up the filter adsorptive ability, and can even contribute to contamination, as the charcoal provides an excellent breeding ground for bacteria and algae. For this reason, Activated charcoal can be used in conjunction with chemical treatment. The chemical (iodine or chlorine) will kill the pathogens, while the carbon filter will remove the treatment chemicals. In this case, as the filter reaches its capacity, a distinctive chlorine or iodine taste will be noted. The more activated charcoal in a filter, the longer it will last. The bed of carbon must be deep enough for adequate contact with the water. Production designs use granulated activated charcoal (effective size or 0.6 to 0.9 mm for maximum flow rate). Home or field models can also use a compressed carbon block or powered activated charcoal (effective size 0.01) to increase contact area. Powered charcoal can also be mixed with water and filtered out later. As far as life of the filter is concerned, carbon block filters will last the longest for a given size, simply due to their greater mass of carbon. A source of pressure is usually needed with carbon block filters to achieve a reasonable flow rate.
Chemical methods of treatment rely upon the chemical interactions of the contaminants we wish to remove from water, and the application of chemicals that either aid in the separation of contaminants from water, or assist in the destruction or neutralization of harmful effects associated with contaminants. Chemical treatment methods are applied both as stand-alone technologies, and as an integral part of the treatment process with physical methods.
For the chemical treatment of drinking water, a great variety of chemicals can be applied. Below, the different types of water treatment chemicals are summed up.
Chlorine is a strong oxidant commonly used in water treatment for oxidation and disinfection. As an oxidant, chlorine is applied to control biological growth and to remove color, taste and odor compounds, iron and manganese, and other dissolved inorganic contaminants such as arsenic. As a primary disinfectant, chlorine is applied to disinfect and to control microbial activity in the distribution system. It is also used as a secondary disinfectant after chlorine, ozone, UV irradiation, or chlorine dioxide. Chlorine is commonly applied at one or two points during treatment. Downstream residual chlorine concentrations make chlorination concurrent with other treatment processes. Chlorine residuals are common during filtration to inhibit microbial (biofilm) growth on filter media that could increase filter head loss (pressure) build up.
Chlorine is available as compressed elemental gas, sodium hypochlorite solution (NaOCl) or solid calcium hypochlorite (Ca(OCl)2). All forms of chlorine, when applied to water, form hypochlorous acid (HOCl). Gaseous chlorine acidifies the water and reduces the alkalinity, whereas the liquid and solid forms of chlorine increase the pH and the alkalinity at the application point. The pH of the water will affect the dominating chlorine species such that HOCl dominates at lower pH, while the hypochlorite ion (OCl-) dominates at higher pH. Of the two species, HOCl is the stronger oxidant. Therefore, chlorine is more effective as an oxidant and a disinfectant at lower pH. Both forms, HOCl and OCl-, are referred to as free chlorine.
The concentration (C), contact time (T), pH and temperature affect the effectiveness of chlorine application. The product of concentration and time (CT) is the most important operational parameter in disinfection and inactivation. Although increasing the dose increases the ability of chlorine to oxidize and disinfect, it may also lead to taste and odour issues and to the formation of disinfection by-products (DBPs) by chlorine’s reaction with natural organic matter (NOM). The dose is also affected by the application point, chlorine demand of the water, and desired residual concentration. Total organic carbon (TOC) and ultraviolet absorbance (UV) are two measures of DBP-reactive NOM and of chlorine demand.
Polymers used in water treatment are generally low molecular weight (<500,000) and may be used as primary coagulants, coagulant aids, flocculent aids or as filter aids. Cationic, anionic and nonionic compounds are available. Polymers used for primary coagulants, coagulant aids are generally cationic compounds. Flocculant aids will typically be anionic or nonionic and slightly higher molecular weight. Those used as filter aids may be slightly cationic or nonionic. Polymers fed as primary coagulants are typically dosed at 0.1 to 2 mg/l. Polymers fed as coagulant aids are typically dosed at 0.1 to 0.5 mg/l and those used as flocculant or filter aids might be dosed at less than 0.1 mg/l.
Chemical treatment typically is applied prior to sedimentation and filtration to enhance the ability of a treatment process to remove particles. Two steps typically are employed: coagulation and flocculation. Flocculation and coagulation treatment chemicals are used in effluent water treatment processes for solids removal, water clarification, lime softening, sludge thickening, and solids dewatering.
Coagulants neutralize the negative electrical charge on particles, which destabilizes the forces keeping colloids apart. Water treatment coagulants are comprised of positively charged molecules that, when added to the water and mixed, accomplish this charge neutralization. Inorganic coagulants, organic coagulants or a combination of both are typically used to treat water for suspended solids removal.
When an inorganic coagulant is added to water containing a colloidal suspension, the cationic metal ion from the coagulant neutralizes the negatively charged electric double layer of the colloid. Much the same occurs with an organic coagulant, except the positive charge most commonly comes from an amine (NH4+) group attached to the coagulant molecule.
Flocculants gather the destabilized particles together and cause them to agglomerate and drop out of solution. Flocculation is gentle stirring or agitation to encourage the particles thus formed to agglomerate into masses large enough to settle or be filtered from solution.
Particles in water smaller than about 10 microns are difficult to remove by simple settling or by filtration. This is especially true for particles smaller than 1 micron – colloids.
When referring to coagulants, positive ions with high valence are preferred. Generally, aluminum and iron are applied, aluminum as Al2(SO4)3 and iron as either FeCl3 or Fe2(SO4)3-. One can also apply the relatively cheap form FeSO4, on condition that it will be oxidised to Fe3+ during aeration.
Coagulation is very dependent on the doses of coagulants, the pH and colloid concentrations. To adjust pH levels Ca(OH)2 is applied as co-flocculent. Doses usually vary between 10 and 90 mg Fe3+/ L, but when salts are present a higher dose needs to be applied.
ENERGY INTENSIVE METHODS
Among the energy intensive technologies, thermal methods have a dual role in water treatment applications. They can be applied as a means of sterilization, thus providing high quality drinking water, and/or these technologies can be applied to the processing of the solid wastes or sludge, generated from water treatment applications. In the latter cases, thermal methods can be applied in essentially the same manner as they are applied to conditioning water, namely to sterilize sludge contaminated with organic contaminants, and/or these technologies can be applied to volume reduction. Volume reduction is a key step in water treatment operations, because ultimately there is a trade-off between polluted water and hazardous solid waste.
Energy intensive technologies include electrochemical techniques, which by and large are applied to drinking water applications. They represent both sterilization and conditioning of water to achieve a palatable quality.
All three of these technology groups can be combined in water treatment, or they may be used in select combinations depending upon the objectives of water treatment. Among each of the general technology classes, there is a range of both hardware and individual technologies that one may select from. The selection of not only the proper unit process and hardware from within each technology group, but the optimum combinations of hardware and unit processes from the four groups depends upon such factors as:
- How clean the final water effluent from our plant must be;
- The quantities and nature of the influent water we need to treat;
- The physical and chemical properties of the pollutants we need to remove or render neutral in the effluent water;
- The physical, chemical and thermodynamic properties of the solid wastes generated from treating water; and
- The cost of treating water, including the cost of treating, processing and finding a home for the solid wastes.
Ozone is used for disinfection of drinking water r. Ozone, a molecule composed of 3 atoms of oxygen rather than two, is formed by exposing air or oxygen to a high voltage electric arc. Ozone is an unstable gas comprising of three oxygen atoms, the gas will readily degrade back to oxygen, and during this transition a free oxygen atom, or free radical form. The free oxygen radical is highly reactive and short lived; under normal conditions it will only survive for milliseconds.
Ozone is a colorless gas that has an odor similar to the smell of the air after a major thunderstorm. Ozone has a greater disinfection effectiveness against bacteria and viruses compared to chlorination. In addition, the oxidizing properties can also reduce the concentration of iron, manganese, Sculpture and reduce or eliminate taste and odor problems. Ozone oxides the iron, manganese, and Sculpture in the water to form insoluble metal oxides or elemental Sulphur. These insoluble particles are then removed by post-filtration. Organic particles and chemicals will be eliminated through either coagulation or chemical oxidation. Ozone is unstable, and it will degrade over a time frame ranging from a few seconds to 30 minutes. The rate of degradation is a function of water chemistry, pH and water temperature. The formation of oxygen into ozone occurs with the use of energy. This process is carried out by an electric discharge field as in the CD-type ozone generators (corona discharge simulation of the lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultraviolet rays from the sun). In addition to these commercial methods, ozone may also be made through electrolytic and chemical reactions. In general, an ozonation system includes passing dry, clean air through a high voltage electric discharge, i.e., corona discharge, which creates and ozone concentration of approximately 1% or 10,000 mg/L. In treating small quantities of waste, the UV ozonation is the most common while large-scale systems use either corona discharge or other bulk ozone-producing methods.
The raw water is then passed through a venturi throat which creates a vacuum and pulls the ozone gas into the water or the air is then bubbled up through the water being treated. Since the ozone will react with metals to create insoluble metal oxides, post filtration is required.
Ultraviolet light has been known to kill pathogens for a long time. A low pressure mercury bulb emits between 30 to 90 % of its energy at a wave length of 253.7 nm, right in the middle of the UV band. If water is exposed to enough light, pathogens will be killed.