Activated Alumina Works To Lower Fluoride to Safe Levels in Drinking Water

Water fluoridation is a double edged sword.  In the U.S. fluoride has been added to most water streams in order to help prevent tooth decay.  (I should mention fluoride naturally occurs in a lot of drinking water sources throughout the world).  However fluoride can be damaging to bones at higher doses and it can even be fatal if you take in large quantities of it.

The desiccant activated alumina plays a very important role in reducing fluoride levels in water.  By doing this activated alumina leaves enough fluoride in water for people to receive its potential health benefits while at the same time it makes sure that health damaging amounts of fluoride do not remain in drinking water.

In 1994 the World Health Organization recommended that fluoride levels in water should be contained from 0.5-1mg/L.  Fluoride levels above 1mg should undergo defluoridation, which can be done three different ways: with chemicals and precipitation, with membrane based technologies, or with ion exchange and adsorption.

A lot of times these methods are used in combination.  For example, when fluoride levels are above 15ppm using lime which falls under the chemicals and precipitation category should be used because they can handle the high levels of fluoride.  Once that level is lowered using lime, activated alumina, which falls under the adsorption category, should be used to reduce to the fluoride content to below 1ppm since activated alumina can purify water up to 99%.

How does Activated Alumina work in removing fluoride from water?

Activated alumina adsorbs fluoride because fluoride is attracted to alumina.  It wants to make aluminum fluoride which it does once it comes into contact with activated alumina.  The alumina fluoride will remain stuck to the alumina beads so long as the pH level of the water remains below 6.  If water’s pH remains lower than a 6 the effectiveness of activated alumina starts to be reduced.  It can also allow aluminum to get in your water, although aluminum does not typically dissolve in water.

Note: Reverse osmosis is used to remove aluminum from water and so can certain distillers.  Aluminum does not typically get into water because water has a difficult time dissolving it.

It’s recommended that you pre-treat activated alumina with aluminum sulfate before you use it in order to improve the first adsorption runs.  After pretreatment it’s important to remember that adsorption reactions with activated alumina are  flow-rate dependent.  Although activated can handle high flow rates and still work, its adsorption capacity is reduced and this could lead to having to do additional cycles.  Doubling the flow rate allowed 33% more fluoride through the activated alumina beds, thus reducing activated alumina effectiveness in adsorbing fluoride.

There is a possibility of other ions interfering with the adsorption process when working with activated alumina, this is due to water in the U.S. containing other ions.  These ions are usually sodium chloride, sodium sulfate, and sodium bicarbonate.  Sodium chloride and sodium sulfate do not interfere with the adsorption process, but sodium bicarbonate can reduce the capacity of activated alumina between 33% and 70%.

Activated alumina like most desiccants can be regenerated.   Sodium hydroxide, aluminum sulfate, or sulfuric acid are applied to a lye solution with the activated alumina, allowing the adsorbent to be regenerated.  Once regenerated activated alumina can continue to be re-used, and when used properly activated alumina can last years.

Activated alumina is essential in removing fluoride in water up to 99% and making water safe for people to drink.

Sources:

http://www.watersanitationhygiene.org/References/EH_KEY_REFERENCES/WATER/Water%20Quality/Fluoride/Defluoridation%20Using%20Activated%20Alumina%20%28UNICEF%29.pdf

http://www.bibliotecapleyades.net/salud/salud_fluor23.htm

World Health Organization: http://whqlibdoc.who.int/trs/WHO_TRS_846.pdf

http://www.tramfloc.com/tf133.html

Double Flow Rate = 33% decrease in adsorption capacity, reverse osmosis, and ions.  http://www.purewateroccasional.net/newnewsletter8.html

Water below pH of 6 reduces effectiveness of Activated Alumina http://greenlivingqa.com/content/fluoride-filtration-using-alum

Drying Oxygen in Aerobic Digestion

Waste water needs to be treated before it can be released back into the environment.  There are many different processes that are used to accomplish this.  This article focuses on how aerobic digestion works and why drying oxygen with desiccants can be beneficial to you.

Aerobic digestion requires the use of bacteria to digest the sludge, which is the collective contaminate in a water supply.  The key component to making this process work is oxygen and there are two reasons why.

One reason why oxygen is needed is because large volumes of bacteria used in this process quickly eat up all of the oxygen the bacteria need to live, so without oxygen the process would not work and all the bacteria would die.

The second reason again revolves around aerobic bacteria eating up all of the oxygen.  This presents another problem when releasing waste water back into a river or stream.  When all the bacteria is released back into a stream for example they end up taking all of the river’s oxygen, which makes it impossible for plants and fish that are dependent on the stream to survive.  The added oxygen ensures that there is enough for the wildlife.

Aerobic digestion works by using either a PSA oxygen generator or a cryogenic compressor/oxygen generator to aerate oxygen into the bacteria and sludge mixture.  Before oxygen can be added the digestion process moisture needs to be removed from the air.  This is done by using either silica gel or activated alumina in an air dryer.  Once the air has been dried the aerobic bacteria can remove the sludge.

Drying the oxygen with silica gel and activated alumina can increase the efficiency of a PSA unit or cryogenic compressor, and help purify the oxygen thus giving you a higher concentration of it to insert into the digestion process.  More oxygen equals more bacteria, and more bacteria makes sludge go away faster.

The primary advantage of aerobic digestion is that the process is quick and produces a high quality result.  The downside is it uses a lot of energy and the potential to kill the bacteria if you do not use enough energy.  The restart process once the bacteria are killed is very time consuming, and the extra cost from using too much energy to run this operation requires skilled workers and constant supervision.

Purifying and drying oxygen can help to prevent this from happening.

http://www.wastewaterhandbook.com/

An Overview of How Desiccants Are Involved in Sulfur Removal

Sulfur Stinks…

Literally, in nature some naturally occurring sulfur and sulfur compound smells include skunk spray and rotting eggs.  Sulfur’s reputation as a horrible smelling element has led the natural gas industry to calling any natural gas with sulfur in it, sour gas.  In addition to its horrible smell sulfur can also be deadly to humans when it is potent enough, and it is also corrosive.  Thus the removal of it from natural gas streams is an essential process, which is called natural gas sweetening.

In order for natural gas to be considered sour, hydrogen sulfide must  exceed 5.7 milligrams per cubic meter of natural gas.  Natural gas sweetening usually uses a process called amine treatment in order to remove sulfur from gas streams.  Sulfur has an affinity for amine and when it passes through the tower that contains amine, the sulfur sticks to the amine and is removed from the stream.  Amine treatment works similarly to glycol treatment because both use a liquid solution to remove unwanted elements and compounds from natural gas streams.

Despite my sulfur bashing earlier, and yes it does smell, it can also be useful.  For example sulfur is frequently used in fertilizer, matches, and insecticides and it is used to make sulfuric acid which has many industrial applications.  The sulfur that is removed from natural gas streams can be recovered using the Claus Process and sold as a separate product from natural gas.

The Claus Process has two steps: the thermal step and the catalytic step.

The thermal step is designed to turn the majority of the hydrogen sulfide removed from the gas stream into regular sulfur.  This is done by oxidizing hydrogen sulfide with air at high temperatures.  Approximately 60-70% of the sulfur is produced during this step.

The catalytic step is designed to take the remaining hydrogen sulfide and the newly created sulfur to make even more sulfur by heating them together over a catalyst, usually activated alumina and a specialty titania.  Specialty titania helps to convert the remaining hydrogen sulfide into sulfur and activated alumina helps to protect the titania from sulfation poisoning due to oxygen breaking through.

Approximately 97% of sulfur that is removed from natural gas streams is recovered using the Claus Process.  In the United States approximately 15% of all produced sulfur is extracted from natural gas streams.

Sources:

http://www.naturalgas.org/naturalgas/processing_ng.asp#sulphur

http://minerals.er.usgs.gov/minerals/pubs/commodity/sulfur/

…and Oxygen and Carbon Compound Adsorbents?

Absorption and adsorption are two natural occurring processes that are similar, but are not the same.  Here is a basic breakdown of how they are different:  absorption occurs when one material’s physical state is absorbed into another material’s physical state, while adsorption occurs when one material physically sticks to another material without changing it’s physical state.

Absorption occurs when a gas turns into a liquid, or a liquid into a solid, etc.  This is what separates it from adsorption, the physical state of the molecules have changed.  For example if you were to drink a glass of milk, your body would absorb it into your digestive system and eventually into your bloodstream.  The earth absorbs the suns rays and has converted its energy into the life sustaining planet we live on today.  The roots of plants absorb water when it rains converting into the energy it needs to survive.  All of these examples feature one material’s phase being turned into another.

Adsorption occurs when liquid or gas molecules stick to the side of surface, preserving their physical state.  This is useful for separating certain molecules from one another.  Adsorbents are most commonly found as carbon compounds or oxygen compounds.

Oxygen compound adsorbents are used to make products like silica gel which works to absorb moisture and reduce humidity levels or zeolites which can be tailored to specifically remove certain molecules from the air like carbon dioxide.

Carbon compound adsorbents like activated carbon can be effectively used to treat waste water and gas.  Contaminates will get stuck to the pores that are found all over the surface area of activated carbon while the water filters through.

Absorption and adsorption are both sorption processes, they both take in a substance or hold it in place and that is how they are related and why the are so similar, the process, however, is different.

How are they different?

Activated alumina and molecular sieve, they look similar (they both come in a small spherical beaded shape),  they perform the same process (they adsorb material at a molecular level), and they both have regenerative properties (you can re-use the material once the desiccant has reached capacity) yet they are two completely different products.  So the question is, with so many similarities, how are they different?

What they are made of is a good starting point.  Activated alumina is made out of aluminum oxide that is highly porous, while molecular sieve is made out crystalline metal alumino-silicates.  What this means is the pores on molecular sieve can be shaped into specific sizes most commonly seen as 3A, 4A, 5A, and 13X, where as activated alumina’s pores do not have specifically measured sizes.  This means molecular sieve can be used to separate certain molecules of specific sizes from one another, for example removing ammonia from natural gas streams.

From an application standpoint, here is how they differ.  Activated alumina has a real strong water adsorption capacity, it can adsorb a lot more water than molecular sieve, this makes it a very useful material in air compressors or for certain natural gas processing applications. The durability of the material allows it withstand a lot of pressure along with high levels of humidity.

Activated alumina can’t adsorb the large variety of materials or separate certain molecules from one another like molecular sieves can, making it ineffective in a process like ethanol dehydration.  This is because activated alumina would be able to adsorb both ethanol and water molecules and thus no separation would occur.

Molecular sieve may not be able to adsorb as much water but if you needed to reduce water to very low amount, up to 0.1ppm, molecular sieve would be your absorbent of choice because this is something other adsorbents besides molecular sieve have been incapable of doing.

Molecular sieve can also be used to separate specific molecules from one another, due to the customization of their pore sizes.  For example you can separate water from ethanol, and carbon dioxide, ammonia, and larger hydrocarbons from natural gas streams, which is something activated alumina can’t do, or won’t do with same efficiency.