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The use of zeolites as a filtering medium

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Are all zeolites suitable for use as filter media?

In recent years, the use of this promising mineral as a filter medium has become more popular, as its benefits and advantages have been tested against sand and anthracite and Garnet multilayer filters. But are all zeolites the same and, most critically, can they all be used as a filter medium?

The answer is NO.

There are 2 characteristics that most zeolites have that make them unsuitable for use as filter media:

Characteristic 1: The percentage (%) of clay present in the samples.
Most of the natural zeolites contain concentrations of 5 to 40% of clay adhered to their structure, which makes it a serious inconvenience to be used as a filter medium, since they disintegrate and form colloidal particles that pass through filters clouding and fouling all subsequent filter elements.

EVEN IF YOU BACKWASH THESE FILTERS WITH INTENSITY, THESE FINE PARTICLES MAY CONTINUE TO APPEAR DURING OPERATION.

From numerous studies and laboratory tests, we have found that a concentration greater than 10% of clay in the material can cause serious problems, both in the initial backwash and during the operation.

How can you be sure that the zeolite you are going to use is suitable for the application?

The most rigorous way is through the acid solubility test as specified by the ANSI / AWWA B100–89 standard, which consists of the following:

1.- A 100-gram sample of material is prepared, which must previously be dehydrated in a drying oven at 200 degrees until ensuring that the moisture has been completely removed.

2.- A one-liter solution composed of 500 ml of hydrochloric acid and 500 ml of distilled water is prepared.

3.- The inverse mixture is left in the water for the duration of the effervescence, occasionally it must be shaken slightly to guarantee an adequate reaction.

4.- Once the effervescence stops, the material must be washed with distilled water in a sieve or percolator with a sieve of equal or more than 40 mesh, until the material stops adding turbidity to the water.

5.- Subsequently, the material must be dried in a drying oven at 200 degrees Celsius until 100% of its humidity is removed.

6.- Once the material has cooled, it must be weighed on an analytical balance.

Formula: acid solubility (%) = (Weight loss x 100) / Original weight.

The American Water Works Association recommends doing the test at least twice, and if there is a discrepancy of more than 2% between the two readings, then the test should be carried out a couple more times.

The AWWA B100–89 standard sets 5% acid solubility as the upper limit.

For practical purposes, a material with a maximum of 10% is suitable for use as a filter medium.

 

However, in order to achieve either of these two parameters, it is essential for the original zeolite to have excellent quality and that it be subjected to a chemical washing process.

Unfortunately, most zeolite providers do not have the appropriate raw material, quality controls, and processes necessary to manufacture reliable filter media.

Characteristic 2: Density of heavy metals

In most natural zeolites, the presence of heavy metals such as arsenic, chromium, lead, iron and manganese has been detected in concentrations of up to 5%. This represents a real risk to public health, since these metals tend to leach during the filter operation, contaminating the influent many times destined for human consumption.

The only zeolite-based filter media that can guarantee that they do not add heavy metals to the influent are those certified by NSF ANSI 61.

In order for NSF to certify a filter medium, it is subjected to a washing process with distilled water, where after a certain number of filter cycles, the filtered water is analyzed to determine if there are heavy metals in them in higher concentrations to those acceptable according to standard 61.

Having an NSF certification means having strict controls, both in the process and in the quality of the final product––it is also the best way to guarantee that the zeolite you buy gives you the result you are looking for.

What factors influence the performance and quality of one zeolite versus another?

There are three main factors that influence the performance of a zeolite as a filter medium:

1.- The apparent density

The apparent density is related to the porosity of the material and therefore its permeability.

Various studies have shown that the lower the density, the higher the permeability of the filter medium is obtained and, therefore, its load capacity increases, as well as its operating cycle.

2.- The surface area

Zeolites filter both mechanically and through an adsorption phenomenon. This is the main difference and advantage over filters and anthracite.

Since the surface area determines the adsorption surface of the filter medium, it becomes a fundamental element since it is what gives it the ability to adsorb colloidal particles and dissolved organic material.

3.- The particle size

The particle size of a filter medium is normally classified based on the effective particle range.

There is a relatively proportional correlation:

The higher the particle range, the longer the length of the filtration stroke, but the lower the ability to retain smaller particles.

In the case of the Zeomedia 12–20 this can provide a longer filtration stroke than a multilayer filter, but a particle retention of up to 5 microns, while Zeomedia 14–40 provides a much shorter filtration stroke, but is capable of retaining 1 micron particles.

For this reason, it is important that, depending on the type of water to be filtered, and the required quality, the zeolite with the appropriate grain size is properly selected.

Comparative table

PARAMETER

ZEOMEDIA
Lavada 16-40

ZEOLITA
NUEVO MEX

ZEOLITA
IDAHO

ZEOLITA
MEXICANA

Volumetric weight (kg/m3)

720,00

906

1,000,00

950

Specific weight (g/cm2)

1,8

2,32

2,2

2,2

Effective size range of meshes

14-40, 12-20

14-40

14-40

8-20

Uniformity Coefficient

1,27

1,29

1,97

2.1

% of material below mesh 40 (weight)

1,36%

1%

1,10%

3.10%

% of soluble in acid (weight)

4,30%

9,50%

6,84%

17,34%

Wheat Firmeza (Newtons)

77,4

74

74,6

Compression work (seconds)

29

25

26,5

Poisson Coefficient (Seconds/Newtons)

0,375

0,338

0,335

Surface area m2/ gram

34,84

25

26,5

20,5

Mineralogical composition

% of clinoptilolite

81%

62%

73%

50-70%

Mortnolinolite %.

2%

8%

0%

20-30%

Certification

NSF/AMSI 61

NSF/AMSI 61

NSF/AMSI 61

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