Filtration

Advantages of Glass Filter Media over Sand and Silica Filter Media

Better water filtration can be achieved with glass-based filtration media and Desolidex, available from African Pegmatite’s comprehensive catalogue of water purification products, for long term, lower cost and higher performing filtration.

Ensuring a clean and stable water supply is often critical to industrial and manufacturing processes, not to mention for foodservice and domestic water consumption. As part of sequential treatment of water, a key step is the filtration and removal of solid contaminants from the water supply. Potential ingest of undesirable solids can have serious ramifications downstream, regardless of the application or process. As such, the use of a long-lasting and robust solid filtration platform is crucial to ongoing system success.

Traditional in-line filtration manifolds have often been composed of sand and/or silica. Whilst these are both effective media, both in terms of performance and cost, long term use is hampered by the need for backwashing and other issues (see below). Capitalising on the proliferation of waste glass, cullet has in recent years found successful usage as a filtration medium, offering excellent solid removal properties whilst being robust and reliable, at a low cost. Building on this, African Pegmatite offers the highly optimised glass-based filtration medium, Desolidex, well suited for long-term, in-line filtration of contaminated water.

Glass Advantages vs Sand and Silica

The use of glass as filtration media began in the late 1990s and is gaining popularity as an alternative to sand and silica. Glass came naturally as filtration media as a result of the unceasing search for the best media. This is because it offers superior performance and it is in generous supply in the form of broken pieces of glass from a waste/recycling stream, and thus is very cost effective.

There are lots of pieces of broken glasses in different colours. In the recycling facilities, they are not always pre-separated by colour, meaning, the product of the crushing process has a mixture of colours - predominantly clear, green and brown, collectively referred to as cullet.

Many bottle manufacturing companies don't use cullet to produce new bottles because beverage companies want pretty bottles with uniform color. Cullet is also not used in plate glass production, such as for windows. This leaves the broken glass to a limited market, making it the best option for the water filtration industry and the best alternative to the sand filter.

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The Concept of a Sand Filter

There are various kinds of sand filters being used in water treatment plants. The performance of such filters not only depends on the sand characteristics but also the operating procedures and the type of filter. A filter may have only one type of sand or several types and the lower limit of sand particle size is typically about 20 μm.

During the filtration process, tiny solid particles get trapped in the sand bed. This means that with continuous operation, the bed will be able to capture finer and finer particles up to a point that it gets saturated. Once that happens, the bed will need to be regenerated by backwashing.

An effective filter bed should have sand whose grains are about the same size - about 0.4 to 0.6 mm. The 20-grade silica is the most common sand filter media because it creates an effective mesh that can sieve very small-sized particles, as small as 20 μm.

However, sand/silica media filters have some disadvantages. For example, the particles of sand can clump together and eventually calcify, which creates channels where water passes through without being filtered. This is why frequent backwashing is necessary for commercial settings - some do it once every week which increases the operating costs.

Secondly, filtration only takes place through the depth of 15 cm, and ‘gunk’ builds up there which necessitates the replacement of the filter system every five to eight years as grains eventually break down.

The Concept of a Glass Filter

The glass filter is gaining much popularity as the preferred medium for water filtration in both water treatment plants and swimming pools. The glass has some unique characteristics that makes it uniquely suitable for filtration purposes.

The surface of the glass is slightly negatively charged giving it the ability to attract very fine particles. These charges are weak and will release the particles during backwashing, which results in more efficient and cost-effective cleaning.

Glass is less dense than sand and, therefore, doesn’t pack as tight, meaning it will not leave any gaps for unfiltered water to pass. It also has a tighter turbid catch giving it the ability to filter particles as tiny as 9 μm, with some brands going as low as 3 μm. This density differential has other positive impacts such as a lesser propensity to form a ‘filter cake’. Filtration efficiency is greatly decreased by ‘filter cake’ formation.

This high-quality filtration can be achieved even without shock treatments or chloramine-producing flocculants which is why glass finds application in many indoor pools where air and water quality are key priorities.

Glass filters can last for ten years or longer but after that duration, a simple regeneration with acid or chlorine and a replacement of the top layer with fresh glass pieces is all that is required. Such simple maintenance procedures render glass filters much more suited to long-term, high-output filtration settings. This is because glass does not physically or chemically break down easily from the cycles of filtration and backwashing, and this further minimises any development of any fractures in the filter bed where bacteria could build up and resist flushing through backwashing.

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Desolidex Glass Filter Outperforms Sand Silica Filter

Desolidex is a non-crystalline (amorphous) filter media which is made from one hundred percent crushed and recycled glass. It is preferred over sand because of its lower density and its smoothness which facilitates better performance. Its manufacturing process is environmentally friendly and is sustainable, as such it is a desirable material from an ecological standpoint. Desolidex has a bulk density of 1.14 g cm-3, with an effective size of 0.60 to 0.65 mm; and a uniformity coefficient of 1.40 to 1.65 meaning that consistent filtration is practically guaranteed.

Desolidex lasts up to five times longer than sand and requires 16 percent less material compared to sand. All these make it a cheaper alternative to silica sand. And while using this media, filtration takes place from the top to the bottom of the bed and this allows for better filtration because of the higher pollutant load capacity. This can be attributed to the low density of glass which makes it not to pack as tightly as sand. An impressive service flow rate of between 10 and 30 cubic metres of water per square metre of Desolidex per hour is comfortably tolerated at a working pressure of 100 to 180 kPa. Bed depts are effective from as low as 500 mm.

Filtration efficiency, perhaps the most important characteristic of a filtration media, is excellent for in-line Desolidex filters. In-house testing has revealed excellent performance data for flocculated pool water, with clean up efficiencies as high as 98%, as follows:

Water conditionSingle pass (%)Double pass (%)Triple pass (%)
92 NTU turbidity889598
87 g mL-1 suspended solids859498

These data are testament to Desolidex’s strong performance in highly turbid and high concentration contaminated water sources. In addition to conventional solid filtration, dissolved heavy metal ions such as manganese and iron can be removed from water owing to Desolidex’s highly negative surface charge. This charge results in the attraction of positively charged dissolved ions, and during testing has been shown to remove 66% of iron species present in raw water in a concentration greater than 2 mg L-1. Additionally, 100% of manganese has been shown to be removed by Desolidex when manganese was present in raw water in concentrations as low as 1 mg L-1.

Desolidex doesn’t support the growth of algae and fungus because it filters down to a lower micron level than sand and is, therefore, able to remove up to 30 percent more material compared to sand silica. It is worth mentioning that it can trap more pollutants while using only about 80 percent of the required media when compared to sand filters.

In terms of operational considerations, attritional loss of the filter per annum is highly dependent upon the backwashing regime used, but is typically in the region of 5 to 6%, making Desolidex an attractive choice for long-term filtration setups. Standard backwashing rates for Desolidex peak at 5 m3m-2h-1 at 100 kPa.

And as Desolidex is more efficient, it doesn’t need as much backwashing as sand filters. This also means the average time taken for backwashing will also be reduced and this will result in up to 23 percent less water usage, giving rise to longer filter run times and thus improved system productivity. Furthermore, when backwashing is required, Desolidex’s glass components readily release contaminants, further enhancing global efficiency.

Acti-Desolidex

A related product to Desolidex, acti-Desolidex is also composed of predominantly recycled glass cullet, it affords similar filtration behaviors to standard Desolidex but is produced at a larger particle size making it better suited to less fine filtration applications and best suited to industrials suspended solids removal. Acti-Desolidex is as useful in terms of flow rate, significantly reduced required backwashing and regeneration requirements as Desolidex, when compared with traditional sand or silica filters. Acti-Desolidex can, however, operate at higher backwash rates than Desolidex if needed, and also boasts an impressive 8 to 12 cubic metres per square metre per hour operating rate.

Final thoughts

Desolidex has revolutionised the water treatment industry and its use is becoming more widespread in many countries. Adding to its already wide applicability and high popularity is that it is made from broken pieces of glass which are considered as waste which would otherwise go to landfill. This makes it environmentally friendly and less costly. Therefore, it can be said that Desolidex, and related Acti-Desolidex use reduces energy usage for the overall system, all without harbouring contaminants.

African Pegmatite supplies a wide range of filtration media suitable for high performing water filtration applications, including glass cullet ground to exacting specifications, as well as the Desolidex filtration system that offers superior in-line filtration capability for long term purification settings.

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