Filtration Media In Desalination Plants: Expanded Clay Vs Anthracite
As desalination via reverse osmosis requires a lot of contaminants to be removed for it to be a viable process, pre-treatment filtration strategies are important considerations. African Pegmatite supplies anthracite and lightweight expanded clay aggregate that can both provide effective, long-term filtration solutions to ensure the optimal running of the reverse osmosis process - ensuring a safe and potable supply for residents and businesses alike.
Modern-day desalination is a process essential to provide water for both agriculture and potable supply in regions where adequate rainfall is rare or highly unpredictable. The contemporary desalination plant takes in sea water and subjects it to a reverse osmosis process. Reverse osmosis is the mechanism where a partially permeable membrane is used to remove ions and unwanted molecules from water.
Efficiency of the reverse osmosis (RO) process chiefly lies on the efficacy of the membrane itself and of the pretreatment of the source water. As such, the modern desalination plant employs various types of filters, often in sequence, to remove a large proportion of contaminants before they can reach the membrane. Sea water often has concentrations of dissolved solids in excess of 35 grams per litre(1).
Two such widely used filter technologies are light expanded clay aggregate (LECA, process expanded clay) and anthracite, and both will be discussed here. The basic idea is that sea water passes through these materials set up as filters before it reaches the membrane. When it reaches the membrane, it is largely devoid of dissolved solids, making the overall RO process more efficient.
Many of the causes of membrane fouling from untreated seawater are not easily visible to the human eye. Leading causes include dissolved silica compounds, adsorbed organic molecules, microorganisms, metallic oxides and various colloids of iron and aluminium(2). Most effective are dual media filtration set-ups, and it is these that are used most extensively worldwide(3). Direct filtration (i.e. a simple filtration through just sand or through a mesh filter) is regarded generally as ineffective, especially against organic contaminants(4).
Imperative for RO operations, due to the immense scale on which they operate, any filter needs to be reliable for long periods of time and relatively inexpensive. LECA and anthracite fit these requirements. Pretreatment of seawater is the most effective method of ensuring non-fouling of the RO membrane(5). It should be noted that neither anthracite nor LECA are able themselves to remove salt from salt water - they are only used as a filter that preempts the reverse osmosis process.
Light Expanded Clay Aggregate (LECA)
LECA is a lightweight aggregate formed by heating standard clay to ca. 1,200 °C in a rotary kiln. During the heating and rotating process, the clay dries out, forms itself into ball shapes and becomes highly porous. The pores are reminiscent of a honeycomb structure, with the shape often being described as “potato like” owing to the constant tumbling through the rotary kiln. This porosity leads it to possessing good absorbent properties, having been utilised previously to remove heavy metals such as cadmium from paint factory waste streams(6).
Modern LECA is produced to a range of standards and sizes, commonly ranging from 0.1 to 25 mm in diameter. Bulk densities tend to fall in the range of approximately 250 to 500 kg m-3.
It comes into its own, however, in the water purification space, where research has shown that LECA is able to adsorb fluoride compounds from aqueous solution, either on its own or modified with magnesium chloride or hydrogen peroxide(7). Furthermore, dual media filters containing sand and MgCl2/H2O2-modified LECA are able to remove chromium compounds from water(8) - and these doped LECAs are able to be regenerated via treatment with dilute acid.
One of the most pernicious pollutants is arsenic. Should this get into the water system, fatalities could occur. Researchers have demonstrated the use of Fenton’s reagent-modified LECA has been effective at removing 99% of arsenite (As2+) and arsenate (As4+) ions from an aqueous solution at a concentration of 150 μg per litre, in 60 minutes(9). Fenton’s reagent is a mixture of hydrogen peroxide and iron, and is an inexpensive material to use at scale.
Organic materials have shown to be removed from water by LECA(10), with highly effective sorption of phenanthroline, fluoranthroline and pyrene from water at a 0.02 mg L-1 individual concentration. LECA was able to remove between 70 and 72% of these organics on a single pass over a maximum 21 hour period when just 0.2 g of LECA was used. Researchers reported increased sorption (i.e. filtration) rates with additional LECA addition - with in excess of 92% of organic contaminants removed when 4 g of LECA was employed. Similar phenomena have been observed when expanded clays have been used in clean up of sewerage, when LECA-like clays were used with sand filters(11). Natural organic matter removal using LECA is a well established area, with studies showing effective removal of these from water in the 10 to 50 mg L-1 contamination regime. LECA was shown to be efficient across the range of contaminants with efficiencies on a similar order to heavy metal coagulants, with a noticeable improvement over sand(12). Complete filtration was achieved when coagulation using chitosan was combined with LECA filtration(13). Results consistently outperformed the control experiments.
A viable comparison to LECA would be activated molecular sieves, as used frequently in the chemistry laboratory; as LECAs can be regenerated, are effective at low ‘loadings’ and are themselves insoluble. Furthermore, LECA is incompressible under pressurised or gravity type loads. These
Anthracite is one of the most ubiquitous forms of coal. Occurring naturally and easily mined, it is inexpensive. Having many uses aside from as fuel, its use as a filtration medium is widely known(14). One of the key environmental benefits of employing anthracite as a filter and not as a fuel is that as it is not combusted, there is no emission of carbon dioxide to the atmosphere. It should be noted, however, that anthracite is seldom used alone as a pre-RO filter. Rather, it is used as part of a dual media filter set-up alongside sand(15) or alongside sand and garnet as a mixed-media filter,
providing superior coarse-to-fine filtration. Typically, anthracite is present to remove suspended solids and some dissolved organics(16). Anthracite’s relative hardness means that it is not easily decomposed or crushed in filtration scenarios.
In non-RO settings, anthracite filtration has proven its worth by being effective at the removal of organic traces from aqueous solution(17). Simple anthracite and sand filters have been shown to be effective at removal of organic carbon from contaminated water at flow rates as high as 14 m3 h-1, removing more than two thirds of the organics present in a single pass(18).
A common theme throughout the literature is that anthracite is extensively used as a pre-RO filter as part of a comprehensive “pretreatment strategy”. When used as a filter for pre-RO in an in-line flocculation-filtration scheme, anthracite is used alongside sand and it was found that turbidity reduction was high (i.e. fewer dissolved or partly dissolved solids were allowed through the filter) with excellent performance in even a short time on the order of five hours, with high flow rates of up to 10 m3 h-1(19).
Anthracite-led dual media filters have been used in tropical settings in RO plants, proving highly effective at treating raw, polluted seawater pre-RO, eventually producing in excess of 50 m3 per day (50,000 L) of drinking water that is fully compliant with WHO standards(20). This particular example provided over 35% water recovery over the course of one year. Alongside granular activated carbon (a material with which anthracite shares many similarities) anthracite was used as a highly effective component in a dual filter to remove biological material from a pre-RO experimental set up. This small scale set-up showed no decline in performance even after 55 days of constant activity, in excess of 5 metres per hour through a 20 cm filter. In both examples, minimal processing of the anthracite was required - limited to only grinding to suitable particle sizes. As part of a multi-media filtration set up, anthracite alongside greensand and activated charcoal was able to remove significant quantities of iron from aqueous solution prior to a RO process(21).
Across filtration applications for RO, anthracite grain sizes vary between 0.35 and 0.8 mm, with a minimum bed depth of 0.8 m. The most modern plants using anthracite/sand dual media filters can handle in excess of 40 m3 per hour(22). Most of anthracite’s filtration utility stems from its excellent size exclusion functionality and porosity. After use, spent anthracite is typically discarded.
Anthracite already commonly finds broad filtration use, particularly as a vital component of a dual media filtration set up for solvent extraction and electrowinning processes for metal production.
One study in northern Greece took anthracite and LECA filters for RO and ran them in parallel, aiming to find a discernible difference between the two approaches(23). The research - both on plant and laboratory scales - found that LECA and anthracite performed largely the same across testing for organics concentration in the filtrate, silt density and turbidity. Notable, however, is that both methods suffered performance wise in the colder winters, but not to an unacceptable level. The LECA material and the anthracite were in the size range of 1.5 to 2.5 mm.
Both types of filtration - when in a dual media filter set-up - are highly effective against the exposure of algal blooms to the RO membrane(24). Bacteria are much more difficult to remove using dual filter processes than solid contaminants and organic materials, however, dual media filters containing anthracite have been shown to remove some bacteria in one study, but the majority were removed by subsequent filtration processes(25). In a further study, LECA filters were shown to entrap - but also in some cases to develop - bacterial life(26).
Naturally, filtration is not the only weapon in the RO plant designer’s arsenal. Oftentimes, seawater prior to filtration will be chemically treated to precipitate out expected contaminants - these are then collected in the sand filter or are allowed to separate out with gravity. Sodium hexametaphosphate can be added to induce precipitation of calcium carbonate and calcium sulfate. Acidification (as seawater is slightly basic) can be performed and aluminium sulfate added to make filtration easier(27). Post-filtration treatments are not uncommon, depending on the scale of contamination.
Whilst both materials (clay and anthracite) are abundant and easily accessible in both mining and economical terms, anthracite holds the advantage of being able to be used ‘out of the box’ with only minimal processing required. Processing is typically limited to grinding the raw anthracite to suitable particle sizes. Clay on the other hand must first be heated in a rotary kiln, and then depending on application, doped/impregnated with another chemical substance - adding expense, time and complexity to the process. As mentioned, a key advantage to using anthracite as a filter is diverting it away from fuel.
- Access to safe, potable water is of ever increasing importance for both domestic and industrial supply, particularly in the developing world and those places affected by seasonal acute drought
- Reverse osmosis is one of the processes by which drinking water can be produced from sea water
- RO relies on a partly permeable membrane to function, and if this becomes blocked or fouled, the process efficiency drops significantly
- Filtration, often using a dual media set-up, is thus used to ensure a better quality supply to the membrane to prevent fouling
- Anthracite is one of the superior forms of coal, naturally existing as one of the purest sources of carbon. It has long been used as a filtration medium.
- Lightweight expanded clay aggregate (LECA) is a material made from the treatment of clay in a rotary kiln to produce a porous, low density material that is well suited to filtration applications
- Anthracite and LECA are two leading materials used as filters - both are highly effective at removing contaminants from water (heavy metals, organic residues, etc.) in both conventional water clean up and pre-RO situations
- With seas getting ever more polluted, and water scarcity is becoming more of an issue in some regions, effective RO processes must be established - such processes cannot work without efficient filtration from the likes of anthracite and LECA
African Pegmatite is the leading supplier of highly pure anthracite and superior quality lightweight expanded clay aggregate for a variety of uses including water filtration - especially for desalination through reverse osmosis processes. Providing wide reach and broad experience, African Pegmatite is the go-to partner for vital filtration applications.
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