Carbon, due to its excellent adsorptive properties combined with a “neutral” (harmless) impact on human health is widely used in water filtration systems in both – commercial as well as individual household scales (for example popular Brita filters used in pitchers). In the past, mostly coal, wood (charcoal), and peat were used as the source of carbon for water filtration. These days, due to superior characteristics, coconut shells are dominating the production of carbon filters.
How the carbon filters are made:
In practice, two structures of carbon filters are commercialized: Granular Activated Carbon (GAC) filters and Block Activated Carbon (BAC) filters.
Source: “Preparation of Activated Carbon from Green Coconut Shell and its Characterization” by Dipa Das, Debi Prasad Samal1, Meikap BC (Journal of Chemical Engineering & Process Technology)
First, the carbon is ground down to a fine powder, then in the absence of oxygen, it is subjected to high temperatures (about 1,000 oC /1,832 oF) to bake off impurities. The final processing stage consists of exposure of the carbon to the high-temperature steam (about 1,600 oC / 2,912 oF) to “activate” the carbon by creating cracks and pores.
The higher the level of porousness, the larger the active (exposed to water penetration) surface of the carbon and the more effective filtration (adsorption) process. The hot steam “treatment” also makes atoms of carbon slightly “charged”. For most contaminants, this electrostatic “attraction” acts similarly to the magnetic force for metals, which increases carbon’s effectiveness in capturing contaminants. This effect, combined with immensely expanded “reactive surface” is referred to as “activation” of carbon, hence the name Activated Carbon (note, that it has nothing to do with harmful radioactivity!).
Granulated, active carbon charcoal from coconut shells
Source: American Water Solutions
Activated carbon particle. Adapted from Culp, G.L., and R.L. Culp. 1974. “New Concepts in Water Purification”, (Van Nostrand Reinhold Co., New York)
The mentioned above carbon’s “activation” process gives stunning results. Just 1g (0.0022 lbs) of properly processed, granulated active carbon, thanks to zillions of micropores has an incredibly large active (adsorptive) surface of about 3,000m2 (32,000 sq ft). Such enormously large “trapping area” guarantees that a huge amount of contaminants can be trapped and stored by the carbon lattice, eliminating them from the flowing-through water. In practice, it means a high level of water purification and a longer lifespan of the filter (a higher volume of water that can be purified before the filter has to be replaced or re-generated).
Note that during the Adsorption Process, the molecules of contaminants bond with the carbon. In other words, they adhere (stick) to the carbon lattice becoming a quasi-permanent part of the core material (adsorbent). In contrast, Absorption is the process when the contaminants are “contained” (detained) by the core material, but do not interact with it at the molecular level. The sponge is a good example of an absorber.
In practice, the filtering process in Active Carbon Filters (ACFs) consists of two independent mechanisms:
Eliminates “minuscule” solid contaminants, soluble chemical components, gases, etc… The physical mechanism is based on the molecular reaction between contaminants and carbon (binding).
Eliminates larger solid contaminants and microorganisms, that due to their size cannot pass through micrometer-sized carbon pores. It works pretty much the same way as a traditional screen filter.
The difference between GAC and Block Carbon Filters.
In GAC filters, carbon granules are less compact. As the result, the natural water flow is faster and so the filtration time is shorter. However, faster water flow means the process of capturing impurities is less efficient. In other words, the water leaving the filter may still carry some contaminants. For these reasons, GAC filters are mainly used in gravity-based systems (the best examples are Brita filters for table pitchers).
Carbon used in Block Filters is ground down into much finer particles than that for GAC filters, and then it is compressed to form a solid block. As a result, the water flow through the block carbon filter is much slower. However, a longer time of contact of water with atoms of carbon leads to a much higher level of the purification process. Block Carbon filters can capture much smaller, micron-size particles including even some bacteria. Note, that Block Carbon filters are rated by the size of contaminants that will be absorbed (typically ranging from 50 μm to 0.5 μm).
From coconut-shell to activated block carbon filter:
Source: DLPAC Series Carbon Block Filter from Dairly (China)
Usually, the choice of the carbon filter (GAC or Block) is determined by the compromise between two contradictory effects:
a. Required water flow rate (quantity of water/minute)
Which is inversely proportional to:
b. Achieved level of water’s purity
Regardless of which type of filter is selected for a given Water Treatment System, its size (amount of carbon) is determined by the task you want to accomplish. In other words – expected demand for the water (in gallons) and filter’s lifespan (determined by its saturation by contaminants).
Filter’s Saturation (Lifespan)
It’s a tricky definition because commercial household filters do not have built-in “intelligence” to shut off themselves when the time comes. From the physical point of view, the degradation of filtering efficiency is a continuous process. Captured contaminants effectively decrease carbon’s “active surface”. At the beginning of the operation, the filter’s active area is still far larger than necessary for trapping contaminants, so, its shrinking cannot affect the effectiveness of the filtration process (after all, water filters are designed with a large operating margin). However, with the time (and quantity of captured contaminants), the size of the “active surface” will drop below the level necessary to guarantee the required purification level. And from that moment, the degradation of water quality quickly drops.
Sample of GAC’s filtering efficiency as a function of filtration time (water volume).
Source: “Performance of AC in water filters” by J.K Siong, M.M Atabaki and J. Idris;
Unfortunately, we may not be able to clearly identify this moment (known as “Filter’s Breakthrough”) till it’s too late. That’s why we should strictly follow the recommendations of the filter’s manufacturer.
It may be easy in the case of popular on our kitchen tables Brita filters (often the pitcher counts the number of water filling cycles and displays a warning). It is much more difficult in the case of “in-line” Water Treatment Filters, usually hidden below the countertop and so often forgotten.
Connecting several filters “in-series” does not change the “physics”, it may only extend the system’s lifespan, because if the first filter will reach saturation, the next in line will take over.
Unfortunately, there are no ideal “multipurpose” filters able to remove all contaminants. That’s why to achieve the required purification goal, typically several specialized filters (typically 2 or 3) are assembled in a “Multi-stage” Filtering System, where each filter is designed for the specific task, like for example adsorbing chemical compounds, or heavy metals, or chlorine, etc…). As the result, the failure (saturation) of any filter will affect the operation of the overall water treatment system.
Typical Contaminants that can be removed from water by Active Carbone Filters (ACF)
– Volatile Organic Compounds (VOC)
Sector share of non-methane VOCs emissions
Source: European Environment Information and Observation Network (Eionet), EEA
There are thousands of man-made chemicals widely used in agriculture, industry, transportation. Most of them are if not highly toxic, then at least “unhealthy”. Some of VOCs, due to their widespread use are often classified into separate groups. VOCs also include chemical compounds daily used in our homes like paints, varnishes, cleaners, bleaches, sanitizers, deodorants, detergents, insect repellents, preservatives, skin lotions (and the list can go on and on). Most VOCs dissolve in water ending their “voyage” in freshwater sources. The list of VCOs is too long, but fortunately, due to the sort of chemical “compatibility” with carbon, they tend to bond to it!
Granulated chlorine powder (Source: Indiamart)
It’s standing out among other chemicals due to its widespread use for the disinfection of municipal water.
– Petroleum and its derivatives
Gasoline, diesel, etc…), are obvious sources of water pollution. Less known are numerous chemicals added to improve the performance of petroleum products. In the past, it was lead (still present in groundwater). However, In the early ’80s, the Methyl tert-butyl Ether (MtBE) replaced Lead as an octane-boosting agent in gasoline. While less toxic than lead, it is also harmful and unfortunately not easily bio-degradable, so it dissolves and accumulates in the water.
– Pesticides and herbicides
Since long, widely (and often “too generously”) used for protection of crops, fruits, veggies but also often to our garden’s grass.
– Trihalomethanes (THMs)
Among them, the most common is chloroform which is a product of interaction between chlorine and dissolved organic matter (for example killed microbes), THMs belong to carcinogens and may be present in municipal water systems.
– Heavy metals
Lead (although for improved performance, there are specially “modified” Active Carbon Filters).
The most typical due to its widespread presence in Earth’s crust is Radon. Note, that the natural concentration of Radon is rather negligible. However, due to the accumulation of radon in AC (or any other) filters, the level of radiation will increase. For this reason, the filter capturing radon (if any), must be installed in the main water pipe outside of the house. And the used filter must be deposited according to the law.
These days, the largest group represents antibiotics (massive use by the livestock industry).
Typical Contaminants that CANNOT be removed from water by Carbone filters
– Metals (cadmium, chromium, copper, Aluminum…..)
Magnesium, salts (including Nitrates NO3), fluoride…. (In general, ACFs do not soften water)
Belong to Perfluoroalkyl Substances widely used in consumer and industrial products to make them more resistant to stains, grease, and water…
– Microbes (bacteria, viruses, fungal spores … in general submicron-size pathogens).
Source: “Introduction to Microbes” by Shirley Bonnici Spiteri (Telescola.mt)
Note, that special types of active carbon filters can have an increased ability to trap specific contaminants. For example, “Catalytic Carbon Filters” (filters with modified active carbon) are designed specifically for removing hydrogen sulfide from the water (“rotten-egg” smelling gas). Another example is mentioned above filters for trapping lead.
Summarizing the effectiveness of Active Carbon Filters
The above estimations represent expected performance, however:
a. No single filter is 100% efficient in terms of the elimination of contaminants. Combining “specialized” filters in a multi-filter system greatly helps to achieve the required purity of the water
In practice, the efficiency of individual activated carbon filters depends on:
b. Speed of the water flow
It all comes to the simple equation: the more time contaminants spend in the proximity of adsorber’s molecules, the higher is the probability of trapping them. In general, ACFs will perform better in gravity-based systems (usually low flow rate) than in pressurized water systems (like municipal, hydrophore-based, etc..)
c. Filter’s Saturation
With time, the continuous build-up of captured contaminants within the carbon pores decreases its trapping capabilities (however, more carbon, longer its lifespan). Most filters do not have “warning mechanisms”, so it is solely the responsibility of the user to monitor them and replace them whenever necessary.
d. Quality of the water source (initial concentration of contaminants)
Manufacturers specify the lifespan of filters based on the typical level of water contamination (some “generic” numbers) and the volume of filtered water. “Dirtier” is the water source, shorter will be ACF lifespan. Especially, temporary increase of sediments (silt) after rains, and chemical pollution after seasonal agricultural activities may significantly shorten the lifespans of AC filters.
e. Other factors
The two most important factors affecting the purification of water by ACFs are water temperature and pH level. For both – higher levels make active carbon filters less efficient.
Is water filtered by ACFs ready for drinking?
The answer is: NO unless it is already disinfected, or proven to be free from microorganisms, like municipal (and presumably hauled) water.
To make it clear, ACFs cannot remove (or kill) pathogens from the water source. It’s because:
– fungal spores can be as small as 0.3 to 1.5 microns.
– human bacteria are in the range of 0.2 to 2 microns
– viruses are in the range of 0.02 to 0.2 microns
Conclusion: active carbon filters (especially Block ones) may remove most fungi and some bacteria from the water; however, they cannot eliminate pathogens.
Risks related to the use of water filters
10-micron Silver Impregnated Carbon Block Filter (AguaSafe Water Filtration Systems)
Operating AC filters can trap larger microorganisms (including some bacteria). When left “idling” (weekend or seasonal houses), they will provide an excellent environment for the growth of microorganisms. For safety, before the next use of the water, filters must be flushed for several seconds to make sure that these potentially “deadly” microbes end-up in the sink rather than in our organism.
Some ACFs are dopped (impregnated) with silver, which seems to be slowing down (or even inhibiting) the growth of microorganisms. Such filters if approved by EPA (and correspondingly labeled), are safe for humans (in other words the silver or other approved by EPA agent) will not be harmful to humans. It does not mean, however, that such filters perform as they may be advertised.
New Technologies: Carbon Nanotube (CNT) water filter
Recent advances in nanotechnologies may offer solutions to problems that so far cannot be resolved by traditional tools and production methods. One of good examples breaking existing technological barriers are graphite nanotubes (for information – graphite is the purest form of carbon). They seem to offer a water filtering structure with “regularity” and “pores-size” that cannot be achieved in the traditional steam-activation process.
The truth is that CNTs, thanks to sub-micron dimensions of carbon” tubes” offer the largest active surface per gram of the raw material and higher chemical reactivity compared to the traditional active carbon material. Also, thanks to possibilities created by nanotechnology, CNTs can be easily altered (modified) to improve the adsorption of specific (targeted) contaminants. As a result, CNT-based water filters can remove a wide-ranging spectrum of pollutants, including organic, inorganic, and biological contaminations. The latter means, removal of bacteria, viruses, and any other submicron-sized pathogens (size of viruses ranges from 0.2 micron (200 nm) to 0.02 microns (20 nm))!
Unfortunately, at this stage of development, there is evidence that the graphite nanotube production process (as well as the possibility of leaks of fragments of nanotubes to the drinking water) may be harmful to human health. Researchers point to similar problems of our respiratory systems to those caused by infamous asbestos.
The bottom line – CNT-based water purification filters are not ready yet for commercialization.
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