Anthracite Filter Media, Water Filtration and Wastewater Treatment
African Pegmatite is a leader in the mining, processing and supply of anthracite optimised for filtration and water treatment applications.
Anthracite media is a filtration medium mainly used for water filtration and wastewater treatment. It has benefits such as higher service flow rates, longer filter runs, reduced backwash rates, reduced head loss compared to single media filter beds and generally extends the life of the filter beds.
What is Anthracite?
Anthracite is a type of coal, and is the coal with the highest carbon content, in excess of 95% by weight. Like most minerals, anthracite is formed over millions of years of deposited plant layers, combined with pressure and temperature.
Of all the carbon materials, this black and rather shiny mineral is the hardest of them all. Long used in water filtration applications, the anthracite used for water filtration is usually specially selected from deeper mines, where, due to higher pressures, the carbon content is typically even greater.
Why Anthracite Is Used as Water Filter Medium?
In simple terms, filter media are materials that facilitate the passing of raw untreated water through them, while retaining any impure particle matter, allowing now cleaner water to pass through. Each filter medium has specific characteristics such as size, weight, whether they are activated or non activated, gravity and more. All of these characteristics give rise to differing operational performance at certain filtration tasks - oftentimes a compromise between them is sought to provide the most robust system, or a system at a lower price point. Anthracite’s broad tolerance of contaminants and ease of forming it into a filtration grade product make it a popular choice.
For drinking water, wastewater filtration and water for industrial use, anthracite has ideal properties for purifying and clarifying this type of water - especially when utilised in combination with other sand filters, or as a component of a dual- or multi-media filtration system.
Because of its ideal properties, relative ubiquity and ease of use, anthracite as a water filter medium is one of the most widely used around the world.
Benefits of Anthracite Water Filter Media
Here’s a quick rundown of the benefits of using Anthracite as a Water Filter Media:-
● The shape of particles allows suspended particles to be retained over the depth of the filter bed
● It complements mixed filters very well, particularly Green and Manganese Sand filter media
● Allows for higher flow rate compared to sand
● Smaller pressure drop compared to sand
● Faster backwash rate compared to sand
● High resistance to a variety of chemicals
● Greater physical strength for use in industrial water purification.
● Prevents filtration blockage due to Synedra, Microcystin and more.
● Prevention of mad ball forming at the surface of the sand layer.
● Can easily transform from Single Media Filtration to Dual Media Filtration (sand)
● An ideal carrier for BioFilm Treatment
Physical Properties of Anthracite
| Colour | Black |
| Dry Bulk Density | 800kg/m3 |
| Specific Gravity | 1.6+ .05 g/cm3 |
| Effective Size | 1.2 - 2.4mm |
| Uniformity Coefficient | 1.5 - 1.7 |
| Bondwork Index | 20 - 22 BBW (KwH/mt) |
Why Anthracite Is Used for Deep Bed Water Filtration
Deep bed filtration is a fast and very efficient method for removing small particles from liquid, and works particularly well with Water. This method of particle dispersion in liquids and water are common in a wide range of different industries and in particular the civil purification industries concerning water and wastewater purification.
Anthracite is perfect for this type of industry as the shape of the particles lend themselves extremely well to deep bed filtration uses. Their strength and other properties - as outlined above, make them an ideal source for these applications.
What Exactly Is Multilayer Filtration?
Multilayer simply means there are a variety of layers in deep beds that provide filtration at different levels. In contrast to methods like cartridge filtration, multi-layer filtration - which includes gravel in various sizes and forms as well as anthracite - represents a deep-bed filtration. Multilayer, also known as dual- or multi-media filtration, and is by far the most robust and effective filtration set up.
With multi-layer filtration, the water passes through various layers of filter material with increasing fineness in the direction of filtration. Gravel filtration or multilayer filtration, in combination with flocculation, is particularly effective in the treatment of river water. When this process is followed, it usually also involves a process stage of sedimentation. Gravel and multi-layer filters are regularly cleaned by back-flushing.
Why Anthracite Media Is Good at Multilayer Water Filtration
Anthracite’s strength and durability has already been mentioned, and how its particle shapes lend themselves well for deep bed filtrations. With multi-layer filtration, any water requiring purification passes through a number and variety of layers of filter material, with increasing fineness of media along the direction of filtration. Having multiple layers allows the extraction of a variety of particle sizes and types passing through the layers. Thereby ensuring a purer outcome at the end of the process. Multilayer filters can cover a large number of solids to be extracted.
Anthracite Use in Multilayer Beds
A technique widely used for the purification of water and wastewater is the use of multilayer beds. The main applications for this area - and for anthracite use within this area tend to be in urban wastewater purification stations, drinking water treatment plants and desalination plants. Desalination plants tend to proceed using a reverse osmosis technique, with anthracite used as a filter system prior to desalination (see later).
Filtration is a process flowing through a single filter bed, or several that overlapping filter beds. In multilayer beds, usually the less dense filter media (like anthracite) is placed on the upper layer of the bed. Before passing through to other layers such as finer sand, coarse sand, fine gravel, and coarse gravel. Each layer plays it’s part to remove particulate matter from the water. To preserve a uniform filtration depth through each layer, the layers are compacted when added to the filter. Additionally, flow and backwashing rates should be optimised so as not to adversely disturb the filter bed.
One of the major reasons that anthracite is used is that it provides reliable improvements in turbidity extraction, mostly due to its capacity to effectively retain solids.
Selected Applications
Some of the most widely used and highly specialised and valued uses for anthracite filtration are in the areas of water treatment by reverse osmosis and in the solvent extraction-electrowinning process.
Anthracite In Reverse Osmosis Filtration For Water Treatment
Reverse osmosis is the leading pathway for seawater desalination to produce a viable potable water supply, this process relies on a partially permeable membrane across which dissolved ions cannot pass, leaving drinkable water. Crucial to the efficacy of the membrane is that it doesn’t get mechanically blocked, for example by microplastics or organic material in the source water supply. Anthracite is used as a vital component in pre-RO filters. Though it is rarely used alone, it is often used as part of a dual media system alongside sand(1) or alongside both sand and ground garnet in a mixed media filter. Collectively, these filters provide effective coverage across the coarse to ultra-fine size range. Anthracite’s primary contribution in this space is the removal of dissolved organic material (such as residual oils) and suspended solids(2). As such, it can be said that most of anthracite’s filtration utility is derived from its excellent size exclusion properties and desirable porosity.
Anthracite-containing dual and multi media filters have proven highly effective at treating raw, polluted seawater in pre-RO applications in tropical waters and in the eastern Mediterranean Sea, producing in excess of 50 m3 per day of drinking water that is WHO standards compliant(3).
The two major issues with RO for water production are energy requirement and overall water recovery. Examples using anthracite as a component in their pre-RO filtration systems can achieve over 35% water recovery over the course of one year. When used with granular activated carbon, anthracite has been used as a highly effective component to remove biological materials(4). Activated carbon and anthracite share many properties. In a different multi-media filtration setup, researchers have shown that anthracite alongside chromate-containing green sand and activated charcoal was able to remove significant quantities of iron from aqueous solutions before being subjected to a RO process(5).
Typical particle sizes for anthracite in pre-RO filtration environments range from 0.35 to 0.8 mm, and have bed depths in excess of 0.8 m. Up to 40 m3 per hour of salt water can be processed by the most modern RO plants equipped with anthracite multi media filters(6). Unlike in other filtration apparatus, anthracite used in RO settings is discarded after use.
Anthracite In Dual Media Filtration For Solvent Extraction-Electrowinning Processes
Solvent extraction-electrowinning is a robust and reliable technique to isolate metals from their ores through sequential steps of extraction and back extraction into and out of suitable solvents, followed by an electrolysis process where the desired metal will be deposited in a highly pure form at one electrode. Filters are essential to remove unwanted insoluble debris from reaching the electrolysis cell.
Sequential deionised beds containing anthracite and garnet are used in modern SXEW plants to protect the electrolysis cell. In these beds, anthracite is used as a coarser, top-layer filter media, and is responsible for the removal of the vast majority of organic residues and coarse to fine graded insoluble contaminants or artifacts like salts from the extraction phases. Finely ground garnet follows the anthracite, and acts to remove all remaining solids down to the micron scale. SXEW’s use is widely seen in the production of copper and cobalt, and zinc, iron(7) and nickel(8) have also been demonstrated in this process with anthracite dual media filters, especially when their ores have been co-located with copper or cobalt.
Heavy Metals Removal
One of the most prevalent issues in drinking water supply is the presence of heavy metals such as iron, aluminium and manganese. Removal is possible via chemical means, but this can be financially intensive. Anthracite filtration - much like its use in the aforementioned pre-RO filtration scenarios - is an effective method of immobilising these heavy metal ions, chiefly by exploiting its impressive porosity profile. 100% removal of manganese has been demonstrated in a study on polluted groundwater(9), and its performance was equivalent to much more expensive granular activated carbon (GAC). Such filters performed well at ambient temperatures - meaning no temperature treatment of filter or inbound effluent was required to ensure optimal filtration. Other studies point towards the comparison between combination anthracite and sand filtration for the removal of manganese, and chemical methods which require the use of strong oxidising agents. In a real study on contaminated lake water (120 μg L-1 Mn), the simple and low maintenance anthracite-led filter ensured concentrations reduced to just 10 μg L-1(10). The authors noted that the filters were additionally desirable as they are not adversely impacted by higher than normal background concentrations of ammonia and iron. Furthermore, anthracite-led filtration pathways for manganese (and biomaterial) removal for municipal potable supplies have been shown to show good longevity profiles(11). Over more than a year, the anthracite filtration performed as well as traditional chlorination treatment.
Biofiltration For The Removal Of Trace Organics
Whilst heavy metals are perhaps a more broadly spread problem, the presence of organic compounds in groundwater is equally problematic as it can easily lead to biological regrowth in a distribution system, conditions pending(12). Removal of such compounds is carried out by biofiltration. The removal of haloacetic acids - to which extensive exposure has been linked to an increase in cases of bladder cancer in humans - for example is a facile application for anthracite-led groundwater filter systems. Again, relying on surface porosity, a rugged and reliable simple filtration system was designed to remove in excess of 89% of the haloacetic acids present(13) in a short amount of time, at ambient temperatures and easily attained pressures.
Tolerance to backwashing is broad with anthracite filtration - an important factor considering how important backwashing is to biofiltration more broadly(14). Backwashing ensures removal of the organic pollutants and manages any build up of disinfection byproducts. It should be noted, however, that studies have shown that whilst aged anthracite is equally as effective at organics removal compared to virgin or new anthracite, it has a much greater tendency to release aromatic organic compounds, which can react with chlorine (from water chlorination processes) to form undesirable disinfection byproducts(15). Care should therefore be taken to use a high quality and new source of anthracite. Overall, however, anthracite-led filtration is viewed as an excellent component of multi stage treatments for ensuring a safe and secure municipal water supply, with a broad tolerance to other in-line treatment methods such as ozonation(16).
Why Is Gravel Used Alongside Anthracite?
In multi layer sand filters, the Anthracite is usually deployed on the top, above a number of sand layers that are designed to filter out finer particles from the water. Gravel normally forms part of the base layer and facilitates the purified water passing through to outlets. If sand were the base layer, these outlets would probably be clogged. Gravel helps prevent clogging from occurring.
Summary
- Anthracite is a highly pure form of coal, is known for its harness, ubiquity and is a good source of carbon
- In filtration media, anthracite is used often times alongside sand or gravel for simple water purification
- For reverse osmosis, anthracite is used alongside other filter media to ensure highly efficient operation at the osmosis membrane, for water desalination applications
- In solvent extraction electrowinning, anthracite filters (often alongside garnet) are used in several stages of the process to remove residual organic and insoluble materials so as to increase extraction and electrolysis efficiencies
- Anthracite is an effective medium for the removal of both heavy metal and biological contaminants from groundwater - either on its own or alongside other contemporary filtration techniques
African Pegmatite specialises in the mining, milling and processing of a wealth of ores and minerals, including anthracite optimised for water treatment and filtration applications - among many other use cases. With the ability to process anthracite to virtually any particle size and a truly global reach, African Pegmatite is the go-to partner for the highest quality anthracite.
References
1 S. Jeong and S. Vigneswaran, Chem. Eng. J., 2013, 228, 976
2 S. Vigneswaran et al., Separation and Purification Tech., 2016, 162, 171
3 C. P. Teo et al., Desalination and Water Treat., 2009, 3, 183
4 S. Vigneswaran et al., Desalination, 2009, 247, 77
5 S. Chaturvedi and P. N. Dave, Desalination, 2012, 303, 15
6 Department of the Army, Water Desalination Technical Manual, Washington, D.C., 1986
7 G. Cote, Solvent Extr. and Ion Exch., 2000, 18, 703-727
8 G. Bacon and I. Mihaylov, J. S. Afr. Inst. Min. Metall., 2002, 102, 435-443
9 A. N. Kent et al., AWWA Water Sci., 2022, 4, 1314
10 M. R. Earle et al., Sci. Rep., 2023, 13, 9020
11 P. To, N. L. Fahrenfeld et al., AWWA Water Sci., 2023, 5, 1334
12 L. G. Terry and R. S. Summers, Water Res., 2018, 128, 234
13 M. Vines and L. G. Terry, Water, 2023, 15, 1445
14 M. Turan, AQUA - Water Infra. Ecosyst. Soc., 2023, 72, 274
15 L. Wang et ai., Env. Sci. Tech., 2023, 57, 1103
16 P. Sinha and S. Mukherji, Proc. Ind. Nat. Acad. Eng., 2022, 7, 1069
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