Water Filtration Meaning, Types, Stages and Systems Explained
A water filtration system provides a simple mechanism for purifying water. This article discusses the following concepts with respect to how a water filtration system works;
Why Water Filtration is Carried Out
Requirements for Water Filtration
Types/Techniques of Water Filtration
How a Water Filtration System Works: Stages in Water Filtration
The 2-Stage Water Filtration System
The 3-Stage Water Filtration System
The 4-Stage Water Purification System
Parts of a Water Filtration System
1). Feed-water Supply Connector
Meaning of Water Filtration
Water Filtration is a process which involves the removal of impurities or contaminants in water, by the use of a physical barrier that separates the contaminants from the water based on the difference in their size and density, from the molecules of the water itself.
We may consider water filtration to be one of the most simplified methods of water purification. Contaminants or impurities which are removed from water through filtration, include;
*Algae
*Bacteria
*Fungi
*Viruses
*Insoluble particulate matter
*Suspended solids (which are a kind of insoluble matter)
From another perspective; water filtration may be described as a process by which a filter medium is used to remove insoluble, solid particles from water, using a filter medium or barrier that allows the water to pass through it, while preventing the particles from doing so.
It is important for us to note that filtration can be used for other fluids aside water, through a very similar mechanism.
Based solely on the Mechanism involved, water filtration can be defined as the separation of solid particles from a fluid (water) by the use of a screen, barrier or porous medium, that retains these solid particles and allows the fluid to pass through [4; 5].
In many cases and contexts, filtration is viewed as a purely physical or mechanical process. The porous medium makes use of manly physical properties (size, solubility, density) to remove impurities from the fluid.
Also, the apparatus and technique which is used in water filtration is determined mainly by the nature of the contaminants present in the water that is to be filtered. Contaminants may be large or small in volume or particle size, and so on. These properties also determine the force which will be used to drive the fluid through the filter barrier; which may be pressure, gravity, static force or centrifugal force.
Why Water Filtration is Carried Out
While there may be different reasons for carrying out water filtration, the primary motive is to remove (mainly insoluble, solid) contaminants from the water.
This is needful for various reasons, which may be to produce water for drinking and other domestic purposes; to isolate an important contaminant from the water; or to produce water for industrial and pharmaceutical applications.
Requirements for Water Filtration
In order for water filtration to occur, a number of factors, components and conditions are required.
Based on our definition of water filtration, we can see that it involves removing impurities that may be chemical, biological or physical [2]. In line with this, it means that water filtration requires a medium that can isolate and remove these chemical, biological and physical contaminants.
For drinking water purification, the filtration system usually includes a multi-stage mechanism, whereby the water filtration process will occur in more than one stage. This approach helps to ensure that the different kinds of contaminants are well attended to, and removed. The stages usually include mechanical, sediment, bacterial and mineral contaminant-removal stages.
In a nutshell, the basic requirements for water filtration to take place are;
1). A fluid (in this case; Water) containing insoluble solid, particulate contaminants
2). A filtration medium (such as a barrier through which the fluid may pass)
3). An external force to drive the fluid through the medium
4). A structure to hold the components of the filtration system in place
Contaminants in the water during filtration, are usually isolated through processes like sedimentation, size discrimination, flocculation and coagulation, surface capture, or pressurization.
Types/Techniques of Water Filtration
1). Centrifugal Filtration
The Centrifugal technique of filtration involves the use of an eternal force to rotate the fluid (water) at a high speed for a period of time.
Centrifugal filtration does not require the use of any filter-bed or barrier apparatus. It rather relies solely on the application of centrifugal force; which is a horizontal, inertial force that acts on the fluid while it is being rotated at high speed.
It is important for us to also know that centrifugal filtration makes use of density for the isolation and removal of contaminants from water or any other fluid. It also utilizes insolubility, but density is the primary property used. Through this means, we can also separate two liquids which have different densities from each other.
As a result of the fast rotation of the fluid (water in this case), the contaminants are forced to separate from the fluid according to density (higher-density material moves to the bottom while lower-density material moves to the top). The apparatus or device that is used tor this procedure is called a Centrifuge.
2). Vacuum Filtration
Vacuum filtration is a filtration technique in which the fluid to be purified is drawn through the filter medium using a vacuum pump [1].
Usually, vacuum filtration is carried out with an apparatus that includes a Buchner funnel fitted with filter paper, through which the fluid is passed, to purify it. The vacuum pump supplies static or suction pressure to drive the fluid in the desired direction.
On passing through the filter paper, the fluid (which may be water) is collected in a flask [6]. Pressure from the vacuum pump makes the collection of filtrate, to be performed in a relatively short period of time.
3). Gravity Filtration
As the term implies; in Gravity filtration, the force of gravity is the main factor which is applied to separate the contaminants from the fluid. This technique of water filtration (and filtration of other fluids) is very effective to remove insoluble solid particles that are suspended in the fluid.
In gravity filtration, the fluid is poured (that is; made to fall) from a relatively high point to a relatively low point. By this approach, the force of gravity is able to act intensively on the fluid as it falls.
The set-up used in gravity filtration is very simple. It usually includes a funnel, in which a filter paper is placed, and beneath which is a beaker to collect the filtrate. As the fluid passes through the filter paper, the contaminants are retained and do not pass through alongside the filtrate. In the case of water filtration, this helps to produce filtrate which is relatively pure.
4). Hot Filtration
As a technique, hot filtration is best suited for fluids that contain crystalline compounds.
In water purification by hot filtration, the water can be heated till it evaporates and can be condensed and collected in a separate vessel, leaving the crystallized impurities behind. This method is useful when separating compounds such as salts, from water.
In another context, hot filtration is used for fluids that have crystalline properties, or for substances which are crystalline under normal conditions. By heating these substances, the formation of crystals which may interfere with the filtration process, is prevented.
5). Cold Filtration
Just as hot filtration involves raising the temperature of the fluid, cold filtration involves lowering the temperature of the fluid that is to be purified.
The purpose of cold filtration is to remove contaminants that tend to congeal under cold conditions. Examples of contaminants removed using this technique, are proteins, and fatty acids. At low temperatures these materials become easier to isolate and remove.
Cold filtration is often utilized in the food manufacturing industry, during the production of beverages and other food materials.
6). Mechanical Filtration
As the term implies, mechanical filtration depends on the directional movement of the fluid for it to be effective.
To produce this directional movement, pressure is supplied by a pump, which forces the fluid through the filter barrier, thereby separating the contaminants. Mechanical filtration is used in operations where the filtrate is required to be highly pure; such as in industrial processes. It is a relatively modern method and is still undergoing improvement and modification.
7). Multilayer Filtration
The idea behind multilayer filtration, is to allow the fluid to pass through different filter beds or barriers, each of which has different characteristics, composition and filtration capacity. This approach is a very brilliant way to deal with the different kinds of contaminants that may be present in the fluid.
The different filter barriers (or layers) which could be used include gravel, sand, and activated carbon. In the case of water filtration by the multilayer technique, each of these layers help to remove contaminants, and to improve the taste and smell of the water.
8). Granular Filtration (also called ‘Granular-Media Filtration’)
We can consider granular filtration to be very similar to the multilayer filtration technique. However, it does not necessarily involve the use of multiple layers.
As the term implies, granular filtration is carried out by passing the fluid through a filter bed or barrier that is composed mainly of granular material, which may be activated carbon; sand or gravel, among others.
Granular filtration is effective in removing mainly sizeable solid particles in water, and is a very common technique. However, it is being replaced by more efficient and effective methods of filtration.
9). Depth Filtration
In depth filtration, the filter barriers are placed within the container itself, rather than on the surface; as in other filtration techniques.
This means that the contaminants in the fluid are retained as they settle at different depths in the fluid itself. Depth filtration usually involves the use of filter beds that have varied size categories and are arranged in a graded manner, so that the finer particles can be retained at the upper part of the fluid column and the larger particles can be retained at the lower part of the column.
10). Surface Filtration
This is the direct opposite of the depth filtration technique.
Surface filtration involves placing the filter barrier at the surface of the water column, so that the contaminants are retained as the fluid passes through the barrier.
We may choose to view most of the conventional filtration techniques as types of surface filtration.
This is because these techniques involve passing the fluid through the filter barrier, so that the contaminants are retained on the contact surface between these two components (filter barrier and fluid) as the filtrate passes.
How a Water Filtration System Works: Stages in Water Filtration
Water filtration systems are designed to remove contaminants from water, through a well laid-out process that usually involves the use of different filter layers. In general, the goal of the various filtration stages is to remove all the chemical; physical and biological contaminants that can be removed from water, since no single filter barrier is able to remove all these contaminants on its own [9].
There are different kinds of filtration systems, which have different capabilities with regards to removing contaminants in water. These filtration systems can be categorized based on the number of filtration stages, into;
* 2-Stage Filtration Systems
* 3-Stage Filtration Systems
*4-Stage Filtration Systems
It is important to know the difference between these stages in order to make good decisions when choosing a water filtration system to purify water for any purpose.
The 2-Stage Water Filtration System
When the main contaminants to be removed from the water are solid particles (sediments), the 2-stage water filtration system becomes the best option to use.
In a 2-stage water filtration system, there are two main filter layers or beds. These are the granular filter bed (also known as ‘sediment’ filter bed) and the carbon filter bed.
As its name implies, the granular filter bed is usually made of granular material like sand or gravel, which serves the purpose of removing particulate material from water that flows through it.
Similarly, the carbon filter layer is composed mainly of activated carbon. Its main function is to remove chlorine or chloramine (and any other similar compound) from the water, thereby improving its taste and smell.
The 2-stage water filtration system is good for very fundamental purification applications. Where the contaminant-composition of the water is more complex, the more advanced and elaborate filtration techniques/systems are usually required.
Below is a simple outline of the stages involved in a 2-stage filtration system operation;
| Stage | Filter layer | Filter layer Composition | Contaminant(s) removed |
| Stage 1 | Granular (or Sediment) filter layer | Sand, Gravel, other suitable granular materials | Rust, silt, algae, insoluble organic and inorganic particles |
| Stage 2 | Carbon filter | Activated Carbon (charcoal) | Chlorine, Chloramine, similar compounds |
The 3-Stage Water Filtration System
While the 2-stage water filtration system can be referred to as the basic filtration system, another term which is often used to describe the 3-stage water filtration system is the ‘Reverse Osmosis System‘.
This is because the apparatus and approach used in the 3-stage system are always similar or even identical to those used for reverse osmosis. Therefore, a 3-stage water filtration system can be called a reverse osmosis system, and vice-versa.
What makes this technique a unique one, is the use of a membrane as one of the filter layers for purifying the water (or any other fluid to be purified).
Also known as the reverse osmosis membrane; this layer is designed to remove both inorganic and organic contaminants from the water, in an effective and efficient manner [7]. It is semi-permeable, meaning that it is very selective with regards to which materials are allowed to pass through it.
Along with the other two layers; the 3-stage water filtration system provides one of the most effective water purification techniques. Below is summarized the main stages involved in this system;
| Stage | Filter layer | Filter layer Composition | Contaminant(s) removed |
| Stage 1 | Granular (or Sediment) filter layer | Sand, Gravel, other suitable granular materials | Rust, silt, algae, insoluble organic and inorganic particles |
| Stage 2 | Carbon filter | Activated Carbon (charcoal) | Chlorine, Chloramine, similar compounds |
| Stage 3 | Reverse Osmosis Membrane | Polymers; such as polyamides, polysulphones and polyacetates | Lead, Iron, Sulphides, Carbonates, Arsenic, Copper, Dissolved Solids, and Microbial Parasites, among others |
For several domestic and industrial applications, the 3-stage water filtration system is a very safe way to purify or treat water. However, there are cases where it is very needful to treat the water for its ionic content. By doing so, we usually produce a filtrate that has a very high level of purity. In the case of water purification, this filtrate is called ‘deionized water’ (or demineralized water).
To produce such deionized, highly-purified water, the 4-stage water purification system is needed.
The 4-Stage Water Purification System
As we have already stated; the 4-stage system produces what is known as deionized water. This is because it includes equipment that remove unwanted ions from water in the process of filtration. It is therefore also referred to as the deionization (DI) system, or the Reverse Osmosis-and-Deionization (RODI) system.
Some examples of applications which require deionized water, include
-Aquarium Usage:
To preserve the health of organisms in an aquarium, it is important to limit their exposure to some of the hazardous ions in regular tap water. Deionized water also ensures that the conditions within the micro-ecosystem in an aquarium, are balanced and optimal.
Algae growth is very limited when deionized water is used in an aquarium, as well as potentially harmful micro-organisms like bacteria.
-Industrial Processing and Manufacturing:
These include petrochemical processing, chemical manufacture, food processing. power and electrical processes, and pharmaceutical manufacturing
-Chemical Laboratory Applications:
Here, deionized water is used for carrying out chemical simulations, reactions and tests. This is important because deionized water is highly pure and does not usually have any significant impurities to affect the results of chemical reactions
-Cooling Systems:
As in fire extinguishing, deionized water is good for cooling applications, because it does not have any significant level of conductivity. This limits the ability to transfer the heat from one medium another while cooling. Also, because deionized water does not contain dissolved ions, it does not tend to form scales while cooling, thereby prolonging the lifespan of the equipment involved.
-Fire Extinguishers:
Especially for electrically-induced fire outbreaks, deionized water is a very good component for extinguishing fire. This is because it does not contain any charges or conductivity to reduce the effectiveness of the extinguishing process.
-Automotive Engine Applications
This falls within the same context as the industrial cooling applications of deionized water. Using water which has been purified by a 4-stage filtration system, prevents the build-up of scales when cooling automotive engines. It also does not introduce chemical constituents that can cause corrosion of the engine equipment.
-Feed-water for Boilers
The water which is fed into the boiler drum of a steam engine and heated to produce steam, is called feed-water. Using deionized water for such purposes is helpful because it prevents corrosion and buildup of scale on the steam engine parts, which can damage them.
Using deionized water also optimizes the energy consumption by the heat-supply unit of the steam engine, because it does not contain impurities that can alter the boiling point of the water. On the overall, deionized water prolongs the life of the steam-boiler system, and improves its performance.
The following table summarizes the stages involved in a 4-stage water filtration system;
| Stage | Filter layer | Filter layer Composition | Contaminant(s) removed |
| Stage 1 | Granular (or Sediment) filter layer | Sand, Gravel, other suitable granular materials | Rust, silt, algae, insoluble organic and inorganic particles |
| Stage 2 | Carbon filter | Activated Carbon (charcoal) | Chlorine, Chloramine, similar compounds |
| Stage 3 | Reverse Osmosis Membrane | Polymers; such as polyamides, polysulphones and polyacetates | Lead, Iron, Sulphides, Carbonates, Arsenic, Copper, Dissolved Solids, and Microbial Parasites, among others |
| Stage 4 | Deionisation fliter | Ion-exchange resins
The resin may be any of two kinds; i). Anion exchange resin (which will release negatively-charged ions like hydroxyl OH-, in exchange for anionic contaminants in the water)
ii). Cation exchange resin (which releases positively-charged ions like hydrogen H+; in exchange for cationic contaminants in the water |
Remainder of dissolved solids which were not removed by the other filter layers; unwanted ions present in the water |
4-stage water filtration systems are effective for removing a wide range of ions that are commonly found in ground water from the tap, or surface water in streams and rivers.
Some examples of these commonly-found ions in water are listed as follows [3; 8];
| Common Anions found in Water | Common Cations found in Water |
| Bicarbonate (HCO3-) | Potassium (K+) |
| Chloride (Cl-) | Magnesium (Mg2+) |
| Carbonate (CO3-) | Sodium (Na+) |
| Nitrate (NO3-) | Calcium (Ca2+) |
| Sulfate (SO4-) | Iron (Fe2+) |
Parts of a Water Filtration System
1). Feed-water Supply Connector
The feed-water supply connector (also simply called; ‘water supply connector’) is the part of the water filtration system which connects the system to the main supply point for the water which is to be purified.
This component usually consists of a flow line or pipe, which is fitted with an adapter that is compatible with the plumbing of the system.
2). Filter Layers
These are the media, or barriers through which the water is passed, in the process of filtration. Different types of filter layers exist, as briefly described below;
-Sediment Filter Layer
This filter layer specifically removes particulate solids like iron oxide, silt, and debris.
-Carbon Filter Layer
Composed mainly of activated carbon, this filter layer is particularly effective at removing chlorine and chloramine from water. These compounds are often introduced into water for disinfection purposes, however, in large concentrations they are capable of causing illnesses and mortality. Removing chlorine and other toxic chemical contaminants also helps to protect subsequent filter layers from being damaged due to contact with such chemicals.
-Reverse Osmosis Membrane
As the name implies, this filter layer is found only in systems that filter water by the reverse osmosis method. These include the 3-stage and 4-stage water filtration systems.
The elaborate structure as well as the composition of the membrane make it capable of removing a vast number of inorganic, organic, insoluble and soluble contaminants. However, external pressure us usually required to force the water through the membrane.
-Ion-Exchange Resin
Also known as Water Softener because it is able to remove magnesium and calcium, the ion-exchange resin layer in a water filtration system. is typically made up of a bed of spherical materials which are able to exchange the ions which they carry, for the ions in the water.
As the water flows through the ion-exchange layer, the cations and anions in the water are exchanged for hydroxyl (OH-) and hydrogen (H+) ions from the resin. This produces water which is chemically pure; with most of its ions being hydrogen and hydroxyl.
3). Pressure -Control Valve
The function of this component is to control the supply and distribution of pressure through the water filtration system. By doing so, it protects the pressure-sensitive parts of the system like the filter beds; from being damaged. It also ensures that the overall process of filtration is optimal.
4). Flow-Control Valve
Like the pressure-control valve, flow-control valves regulate the supply and distribution of water through the water filtration system. This is mainly with regards to the main water supply (coming from the feed-water supply connector). The reasons for regulating water supply to the system include;
-To ensure that the filtration process is effective and optimal
-To conserve water especially by stopping flow when the internal storage tank has been filled
-To protect the membrane and other filter layers from damage due to water pressure
5). Backflow-Control Valve
Also known as the ‘Check Valve,’ the backflow-control valve prevents water from flowing back from the storage tank to the filter layers. It is especially important in reverse osmosis (3, 4-stage water filtration systems, where it prevents purified water from flowing toward the filter membrane when the feed-water is no longer being pumped into the system. This protects the membrane and other filter layers from potential damage.
6). Storage Tank and Faucet
In a water filtration system, the storage tank is used to hold a significant volume of purified water. This water can be dispensed through the faucet (or faucet connector) which serves as the main outlet of the system.
In the storage tank, water is usually stored under high pressure. This pressure helps to force the water out through the faucet.
Conclusion
In a water filtration system, water is made to pass through two or more stages of barriers, including;
-Sediment filter layer
-Charcoal filter layer
-Semi-permeable membrane (for reverse osmosis)
-Deionization resin layer
These filter layers each have their peculiar composition, characteristics and capabilities. They are usually used in combination in order to ensure that the different contaminants in water can be removed.
Pressure is often supplied by a pump, to drive the water through the filter layers in the system. This pressure is controlled by valves; which protect the membrane and other parts of the system from being damaged by excessive pressure.
The water filtration system uses a feed-water supply control adapter to regulate the flow of water into the system. When the storage tank is filled with filtered water and the water supply is closed, the backflow-control valve prevents water in the tank from flowing back toward the filter layers. Regular maintenance of the water filtration system is important, to keep it in optimal working condition.
In summary, contaminants which you should expect to remove using the different types if water filtration systems are as follows;
| Water Filtration System | Main Contaminants that are Removed |
| 2-stage water filtration system | Sand, silt, mineral particles, iron oxides, algae, chlorine, chloramine |
| 3-stage water filtration system | Sand, silt, mineral particles, iron oxides, algae, chlorine, chloramine, parasites, microbial organic particles, heavy metals, other inorganic elements |
| 4-stage water filtration system | Sand, silt, mineral particles, iron oxides, algae, chlorine, chloramine, parasites, microbial organic particles, heavy metals, other inorganic elements, ions |
Based on the contaminant content of the water which is to be filtered, and the desired level of purity, as well as the desired use of the filtered water, any of the available kinds of water filtration systems can be used.
References
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