Purification techniques: The Basics Points That You Need To Understand
The water you use, the carbon dioxide in your drinks, the paracetamol you take to prevent you from heaving a headache and the air you breathe have all one thing in common and that is that in every case a purification technique has been involved.
There are thousands of substances that need to be purified, and to get a basic understanding about the common purification techniques involved we share the basics on the classic purification techniques (Sand filtration, activated carbon filtration, Aeration and degassing and Coagulation) and modern purification techniques (molecular sieves, ion exchange, UV, recrystallization and ozone).
The basics that are described will consist of, what is the technique, how does it work, what are the advantages and disadvantages and what can be removed from what.
Table of Contents
2 Rapid and slow sand filtration
2.1 What is a sand filter?
2.2 How does sand filtration work?
2.3 What is it difference between rapid and slow sand filtration?
2.4 What are the advantages of a sand filter?
2.5 What are the disadvantages of a sand filter?
2.6 What can be removed with sand filtration?
3 Activated carbon filtration
3.1 What is activated carbon?
3.2 How is activated carbon made?
3.3 How does activated carbon work?
3.4 What are the advantages of activated carbon?
3.5 What are the disadvantages of activated carbon?
3.6 What can be removed with activated carbon and from what?
4 Aeration and degassing
4.1 What is aeration?
4.2 How does aeration work?
4.3 what are the benefits of aeration?
4.4 What are the disadvantages of aeration?
4.5 What can you remove with aeration and from what?
5 Coagulation
5.1 What is coagulation?
5.2 How does coagulation work?
5.3 What are the advantages of coagulation?
5.4 What are the disadvantages of coagulation?
5.5 What can be removed via coagulation?
6 UV (Ultraviolet) disinfection
6.1 UV, what is it?
6.2 How does UV C work?
6.3 Advantages of UV light
6.4 Disadvantages of UV light
6.5 What can be removed (break off) with UV?
7 Ozone oxidation
7.1 What is ozone oxidation?
7.2 How is ozone produced?
7.3 How does ozone oxidation work?
7.4 What are the advantages of ozone oxidation?
7.5 What can you "remove" via ozone oxidation?
8 Ion exchange
8.1 What is an ion exchanger?
8.2 How does an ion exchanger work?
8.3 What are the advantages of an ion exchanger?
8.4 What are the disadvantages of an ion exchanger?
8.5 What can be removed with an ion exchanger?
9 Recrystallization
9.1 What is recrystallization?
9.2 How does recrystallization work?
9.3 What are the advantages of recrystallization?
9.4 What are the disadvantages of recrystallization?
9.5 What can be purified with recrystallization?
10 Molecular sieves
10.1 What is a molecular sieve?
10.2 How does a molecular sieve work?
10.3 What are the advantages of a molecular sieve?
10.4 What are the disadvantages of a molecular sieve?
10.5 What can be removed and from what with a molecular sieve?
2 Rapid and slow sand filtration
2.1 What is a sand filter?
A sand filter is, as one would expect with this name, a filter that is filled with sand. Sand of different particle sizes is contained in a sand filter and at the bottom of a sand filter is a porous double bottom (collector). You can have sand filters ranging from very large to small, this is depending on the flow rate of liquid (usually water). The sand is often in a steel, plastic or concrete filter.
2.2 How does sand filtration work?
A sand filter works in a way that the water flows from top to bottom through a sand bed, sand of a certain particle size is used. The particle size of the sand depends on what people want to remove from the water, it is often seen that different layers are used with different particle size to remove various floating particles. In the sand bed is a porous layer that ensures the operation of the filter. The removed particles will accumulate in the upper layer of a sand filter and after expiration of time, more pressure will be needed to get the water through the sand filter. In order to prevent a filter from becoming clogged, one will after a while backwash the sand filter, whereby the removed particles are flushed out and the filter can be used again. This back-washing is done by reversing the flow direction of the water.
2.3 What is it difference between rapid and slow sand filtration?
The difference between fast and slow sand filtration is that with a slow sand filter, in addition to the mechanical method of removal, also biologically organic material is broken down, this is because there is a biomass in the filter. You cannot backwash a slow sand filter because you then remove the biomass from the filter and then it loses its biological effect.
2.4 What are the advantages of a sand filter?
- Various floating particles can be removed
- Easily and in a simple process
- Effective removal
2.5 What are the disadvantages of a sand filter?
- Disposed water must be treated (extra costs)
- Filters can clog (major disadvantage with slow sand filtration)
- Fine dust in water which is too small to be remove
2.6 What can be removed with sand filtration?
Sand filtration can remove the following; solid particles, sludge, insects, seeds, precipitated iron and manganese particles. This is removed from, for example, surface water, cooling water, waste water, process water, swimming pool water and drinking water.
3 Activated carbon filtration
3.1 What is activated carbon?
Activated carbon (activated coal, activated charcoal) is a special type of carbon that can adsorb various molecules, and these are adsorbed in the large surface area by van der Waals forces. A gram of activated carbon has a surface that can be as large as a football field. This surface is divided into micro, meso and macro pores of the activated carbon. Activated carbon occurs in different particles size and different forms: powder, extrudates, granules and spheres.
3.2 How is activated carbon made?
Activated carbon can be made through various processes and from various carbon-containing raw materials. The best-known process is the steam activation process which involves a carbon source which is carbonized and activated at around 1000 degrees with steam. Another process is chemical activation involving chemicals, for example phosphoric acid, a raw material is impregnated and then activated at around 500 degrees. Often raw materials such as, coal, brown coal, charcoal or coconut shells are used. There are also not natural resources that are used to make synthetic activated carbons.
3.3 How does activated carbon work?
Activated carbon has through the micro, meso and macro pores have a selective adsorption capacity which works by the van der Waals forces. Activated carbon therefore works on the basis of adsorption (ie not absorption). When using activated carbon in, for example, water or gas or air the molecules that one wants to remove eventually end-up via diffusion in the pore structure of the activated carbon where the adsorption takes place.
3.4 What are the advantages of activated carbon?
- Very high removal capacity for organic components
- Cost-effective product, good price performance
- Reuse, in some cases activated carbon can be reactivated and reused
3.5 What are the disadvantages of activated carbon?
- Non-selective removal for certain molecules
- Possible impurities (e.g. Metals or minerals) can leach into a liquid
3.6 What can be removed with activated carbon and from what?
Activated carbon is the champion in the amount of purification applications. Think of applications as;
- Medicine, removal of colors or removal of toxic substances.
- Water purification
- Removal of mineral oil
- Removal of BTEX
- Removal of poly aromatic hydrocarbons (PAHs) and removal of phenols.
- Process industry, removal of dioxins and polychlorinated biphenyls.
- Removal of colors and removal of odors.
- Fruit juices, removal of phenols and removal of patulin
- Sugar, removal of color
- Biogas, removal of VOCs and removal of H2S or siloxanes
- Flue gas, removal of dioxins
- Flue gas, removal of mercury
- Water, Removal of contaminants of emerging concern (CECs)
4 Aeration and degassing
Aeration is contacting a liquid phase with air, this happens in the water purification industry, for example through the use of a turbine aerator that pumps air through the water. Aeration of water can also be done via for example cascades, sprays and plate aerators.
4.2 How does aeration work?
With aeration substances dissolved in water are brought into contact with the gas phase (air). By the differences in concentration in the water and in the air, substances can be introduced in the water, for example, oxygen that enters the water or from water substances can be transferred to the gas phase (e.g. removal of carbon dioxide). The last of the two is purification technique aeration. A purification technique which also uses the difference in concentrations of liquid and air are vacuum installations or vacuum degassers, these ensure, by creating a vacuum, that the solubility of an unwanted product is lowered in the water phase and therefore easier to remove.
4.3 what are the benefits of aeration?
- Air does not need to be purchased
- Natural process
- Effective, with correct conditions effective removal possible
- Addition, oxygen is added to the water.
- Mix, aerate ensures that the water is well mixed and remains homogeneous
4.4 What are the disadvantages of aeration?
- Costs, aeration costs energy
- Clogged pipes, higher mechanical stress and damage to aerators, for example calcium layers that form (due to the aerobic purification, the calcium carbonate settles)
- Odor, possible odor emissions to the environment
- Used Air often needs to be purified
- Sensitive, low (outside) temperature can reduce the efficiency of air stripping
4.5 What can you remove with aeration and from what?
Via the purification technology aeration, it is possible to remove or reduce volatile compounds, volatile organic compounds (VOCs) and carbon dioxide. One can remove these components from, for example, sewage, waste water and groundwater. A very positive one is that the technique also adds components such as oxygen.
5 Coagulation
5.1 What is coagulation?
To understand coagulation, it is important to know what a colloid is and what coagulation is. Coagulation in water purification, it is destabilizing colloidal particles by neutralizing their charge with an added chemical, a so-called coagulant. A colloid is a small particle that is slightly larger than a molecule and has a diameter between 1 and 1000 nanometers. These particles can be solid, liquid or gaseous. A colloidal system where the particles are solid and the media is liquid, or in other words a colloidal suspension, is in a state that is midway between a solution and a precipitate. These particles are therefore often very difficult to remove.
5.2 How does coagulation work?
Coagulation is all about destabilizing colloidal particles by adding chemicals. This will cause these particles to clump together (by van der Waals forces) to larger agglomerates (particles or flakes). In the water industry people, for example, use a coagulant such as FeCl3 together with the water that needs to be treated, the FeCl3 ensures that the process of coagulation can take place in a filter or vessel. Often there is yet another purification technique used to remove the formed particles, the particles settle or float in the water. Settled particles removal is usually through sedimentation and the floating particles are removed via flotation, or the water passes over a filter and this is called filtration. Intervention of a coagulant should be done as quickly as possible for effective operation. High short-term mixing energy is a requirement here. (The flake formation (particle formation) is with low mixing energy).
Important when applying coagulation is determining the right coagulant, the optimal pH value, and the correct dosage. To determine this, practical tests are done, these tests are called Jar tests.
5.3 What are the advantages of coagulation?
- Small installations, high flow rate possible with a relatively small installation
- Robust process, security
- Good return, high achieve removal efficiencies for particles that are difficult to remove
5.4 What are the disadvantages of coagulation?
- Precipitation from calcium carbonate (lime) due to possible pH changes in the filter or pipes
- Use of chemicals required compared to, for example, aeration
- Costs, contaminated sludge often still has to be processed.
5.5 What can be removed via coagulation?
With coagulation you can remove various small particles such as, phosphates (orto and total P), humus and fulvic acids. In addition, it is also possible to reduce fats, carbohydrates, proteins, microorganisms and (heavy) metals. Coagulation is applied in the drinking water, textile, water treatment, food, and metal treatment industry.
6 UV (Ultraviolet) disinfection
6.1 UV, what is it?
UV means Ultraviolet (sometimes this is also called black light or UV light) and this is electromagnetic radiation that is just outside the part of the spectrum that the human eye still can see. The wavelength of ultraviolet radiation is between 100 and 400 nm (nanometer). Ultraviolet light has a shorter wavelength than visible light, the shorter wavelength makes it more energy-rich. Because UV is more energy-rich, the ionization potential of organic molecules can be reached, and chemical reactions can be triggered. In connection with the effects of ultraviolet light on people and the environment a distinction is made between UV A, UV B and UV C. See the difference per UV type below:
- UV A is ultraviolet radiation with a wavelength between approx. 315 and 400 nm ("long waves ").
- UV B has a wavelength between 280 and 315 nm.
- UV C has a wavelength between 100 and 280 nm ("short-wave radiation") UV C is the short-wave radiation used in disinfection and sterilization applications.
6.2 How does UV C work?
UV C disinfection is based on the absorption of UV C radiation by, for example, micro-organisms. With the absorption of the UV-C, the DNA and RNA become damaged in the micro-organisms, so that the microorganisms can no longer reproduce. Just like with a normal lamp, the power is expressed in UV lamps in Watts (W). The more Watts the UV lamp has, the more power and effect for disinfection or sterilization this lamp will have.
6.3 Advantages of UV light
- UV has no addition of chemicals that may affect taste, odor and color or that are there have to be purified again later on
- UV is environmentally friendly. In principle, no harmful intermediates arise and only cost energy
- UV light has many application areas and is an effective disinfection method.
- UV light works quick, safe and is easy to use and maintain
- UV light has low energy costs
6.4 Disadvantages of UV light
- UV light needs the right amount of energy to be effective
- UV light is effective for microorganisms not for chemicals
- UV light does not delete, but only "breaks"
6.5 What can be removed (break off) with UV?
With UV disinfection and sterilization microorganisms, such as bacteria, viruses, fungi and traces on or in all kinds of materials or substances are killed or greatly reduced in number. The sterilization and disinfection of air and water is mainly done with use of UV light. Ultraviolet light is often combined with peroxide for organic waste and makes organic micro-pollutants harmless. It is more about breaking down than removing with UV light.
7 Ozone oxidation
7.1 What is ozone oxidation?
To understand what ozone oxidation is, it is important to first know what ozone is. Ozone is a singular substance of the element oxygen, with gross formula O3. At room temperature and normal pressure, ozone is a gas and is colorless to light blue. Ozone has an unpleasant odor and is irritating. Unlike normal oxygen (O2), Ozone is a strong oxidizer.
The next point to understand is what oxidation is. Oxidation is a chemical process substance (reduction) releases electrons to another substance (oxidizer), the oxidation number of the reducer increases. Ozone decays in water, OH radicals react. These radicals are very short-lived compounds that can oxidize even more strongly than the ozone itself.
7.2 How is ozone produced?
Ozone is a gas that comes from air, more often dry air because gives better returns. By using high electrical voltage (Corona discharge), oxygen (O₂) from the air is converted into ozone (O₃).
7.3 How does ozone oxidation work?
As above, the effect of ozone oxidation is known, so it is not really removal because components are not removed out of the process. The oxidation can be done via reaction with ozone or via indirect reactions with OH Radicals. The components in fresh water are oxidized. In the case of micro-genes, the cell walls and the genetic material from the cells will be oxidized.
7.4 What are the advantages of ozone oxidation?
- Widely applicable for oxidation of various components.
- Oxidized substances are easier to remove via other purification techniques.
- No transport needed, is produced on location
What are the disadvantages of ozone oxidation?
- No deletion, it is oxidized and not removed.
- Short disinfectant operation
- Safety, Ozone reacts with bromide to bromate (carcinogenic substance)
- Solid particles may adversely affect the operation of ozone
- Safety, strong Oxidizing agents are dangerous and harmful to health
7.5 What can you "remove" via ozone oxidation?
Ozone is often used for the disinfecting effect (against pathogenic organisms, pathogens, bacteria, viruses and endospore), but also for components such as unwanted color, odor, and flavors (organic components). Also, organic micro-contaminants, such as pesticides and medicines can be oxidized by ozone. The oxidized substances are often less soluble in water so that they remove better with other purification techniques. Ozone is often applied in potable water production, swimming pool water and cooling water purification, control from odor and disinfecting waste water.
8 Ion exchange
8.1 What is an ion exchanger?
Ion exchangers (IEX) are spheres of sulphonated synthetic resin that can take ions from liquid, water, process water, condensate water, swimming pool water) by changing them out against other ions. The liquid will flow over a with beads filled with ion exchanger resin column. The beads are solid, and this is often referred to as adsorption resin.
There are also non-ionic resins, these are capable of removing organic contaminants such as for example trichlorethylene and tetrachlorethylene.
8.2 How does an ion exchanger work?
An ion exchanger works on the basis of exchange of ions (as the name implies). A little bit of background information about what an ion is; an ion is an electric charged atom or molecule that is present in, for example, water as K + Na +, CA 2+ or Cl-, positive ions are called cations and negative ions anions. If negative ions are removed, then an anion exchanger is used, if it is desired to remove positive ions then one can use a cation exchanger. In some cases, you can do both in one system, this is called a mixed bed (mixed bed system). After a certain time, an ion exchanger is saturated, it can be regenerated with a regenerant usually this is an acid or a base. The resin particles have often a uniform particle size. A uniform particle size has several advantages over a large spread of the particle size of the resin particles. Benefits include; better water quality, better separation layered beds and mixed beds, higher efficiency and greater mechanical stability.
8.3 What are the advantages of an ion exchanger?
- Selective removal is possible by choosing the right ion exchanger
- High removal efficiency is possible for cations and anions
- Regeneration of the bed gives a very long service life (disadvantage of acid and base required = costs)
- Ion exchangers can also work well with low water pressure
8.4 What are the disadvantages of an ion exchanger?
- Sensitive to pollution, think, for example, of suspended particles
- Resin can be sensitive for oxidizing substances
- Cost of acids and bases for regeneration
- Often one pre-purification step is needed such as aeration of the water, sedimentation and sand filtration.
- Disposal costs of the acids and bases used for regeneration
8.5 What can be removed with an ion exchanger?
With an ion exchanger, cations can be removed (e.g., K +, Na +, CA2 +, Fe3 +, Cu2 +, Mg +) and anions can be removed (e.g., Cl, SO3 2-, NO3-, CO3 2-) from for example drinking water, spring water, process water and condensate water.
9 Recrystallization
9.1 What is recrystallization?
In chemistry, and especially with organic synthesis and analytical chemistry, recrystallization is an often-applied purification technique. With recrystallization a material is dissolved and re-crystallized under other conditions to reach higher purity levels.
9.2 How does recrystallization work?
In the case of recrystallization, one solidifies crystallized material, and this is finally crystallized again to obtain the final product; crystals with the desired size, shape, purity and yield. The underlying mechanisms, dissolution and recrystallization can also minimize the internal energy of the crystal so that a better energy balance is achieved, resulting in a stable polymorphic product. Usually, recrystallization is intentionally applied to crystals and processes to optimize these. The method works based on the principle that the recrystallizing substance is low soluble in a given solvent at a low temperature (e.g. ethanol or acetone), but much more soluble at higher temperature.
9.3 What are the advantages of recrystallization?
- To achieve high efficiency, the purification technique works especially well when the difference in solubility between hot and cold large is.
- Organic chemistry, very good to apply technique in organic chemistry.
9.4 What are the disadvantages of recrystallization?
- Unwanted or by-products, uncontrolled recrystallization can cause the undesired production of hydrates and solvents or polymorphic transformations.
- Much research needed (high costs and a lot of time), choosing the right solvent is therefore important and this usually requires a lot research.
9.5 What can be purified with recrystallization?
Various products become, through recrystallization, purified in the chemical and pharmaceutical industry in particular;
- Salicylic acid (painkiller)
- Benzoic acid (food additive, E210)
- Cocaine (stimulant that belongs to the alkaloids)
- Potassium perchlorate (very powerful oxidizer)
- Melting is a variant of recrystallization and this is mainly used as a purification technique for silicon and other semiconductors for making various electronic components.
10 Molecular sieves
10.1 What is a molecular sieve?
A molecular sieve is a solid material with a pore distribution that is very uniform and is used for adsorption in both the liquid and the gas phase. There are various types of molecular sieves such as carbon molecular sieves (CMS), zeolites, limestone and silica gels. With molecular sieving there is made a distinction between the types based on the micro (smaller than 2 nanometers), meso (between 2 and 50 nanometers) and macro pores (larger than 50 nanometers) they have.
10.2 How does a molecular sieve work?
A molecular sieve works on the basis of adsorbing molecules that are small enough to enter the pore structure through diffusion. Molecules that are larger than the pores cannot enter the molecular sieve. This mechanism of adsorption works both in the liquid phase as well as in the gas phase.
10.3 What are the advantages of a molecular sieve?
- Selective removal - the uniform pore size makes it possible to selectively remove molecules.
- High removal efficiency possible - the large amount of pores ensures high capacity.
- Reuse - Regeneration is common as a result of which the total operating costs can remain low.
10.4 What are the disadvantages of a molecular sieve?
- Costs - molecular sieves are often more expensive than other techniques in the purchase price
- Fragile - molecular sieves can break which can result in particles that can get into a process
- Selective - not always applicable at removal of various molecules
10.5 What can be removed and from what with a molecular sieve?
Molecular sieves are used in both liquid and gas / air, see some examples below.
- Removal of water from LNG
- Removal of moisture from air
- Removing acid gases such as CO2 (carbon dioxide), H2S (hydrogen sulfide) for example from biogas
- Removal of mercaptans from gas or air
- Removal of hydrocarbons such as BTX (benzene, toluene and xylene)
We have now described the basics of 9 purification techniques and trust that we provided useful information.
